WO2017092789A1

WO2017092789A1 - Radiation field generating unit

Info

Publication number: WO2017092789A1
Application number: PCT/EP2015/078111
Authority: WO
Inventors: Marwan ABDOU-AHMED; Andreas Voss
Original assignee: Universität Stuttgart
Priority date: 2015-11-30
Filing date: 2015-11-30
Publication date: 2017-06-08

Abstract

To improve an operation of a radiation field generating unit comprising a first source of a first seed radiation field and an amplifier, in which an introduced radiation field, which comprises said first seed radiation field, is introduced, and said amplifier comprises a laser active medium exhibiting a population inversion and said amplifier amplifies an intensity of said introduced radiation field and thereby generates an amplified radiation field, it is suggested, that said radiation field generating unit comprises a second source of a second seed radiation field and said first and second seed radiation fields contribute to said introduced radiation field.

Description

RADIATION FIELD GENERATING UNIT

The invention relates to a radiation field generating unit comprising a first source of a first seed radiation field and an amplifier, in which an introduced radiation field, which comprises said first seed radiation field, is introduced, and said amplifier comprises a laser active medium exhibiting a population inversion and said amplifier amplifies an intensity of said introduced radiation field and thereby generates an amplified radiation field.

Radiation field generating units showing the aforementioned features are known from the prior art.

It is the object of the present invention to improve an operation of such a radiation field generating unit.

This object is solved by a radiation field generating unit as defined above which comprises a second source of a second seed radiation field and said first and second seed radiation fields contribute to said introduced radiation field.

The advantage of the present invention has to be seen in the fact that within such a radiation field generating unit during a dead time of one of said seed radiation fields, within which dead time of said one of said seed radiation fields an intensity of said one seed radiation field is decreased, in particular is zero or negligible small, the other seed radiation field of said seed radiation fields is introduced into said amplifier and accordingly disadvantageous effects which usually occur during such dead times of said one of said seed radiation fields are avoided .

For example, during such dead times of one of said seed radiation fields damages to said amplifier can occur. Usually damage to said amplifier during such dead times is due to storage of excess energy in said laser active medium, in particular due to said population inversion exceeding a tolerable level, the excess of said population inversion being due to a further pumping without an emission stimulated by said introduced radiation field.

In particular, the invention avoids the problem, which occurs, if said first seed radiation field is not constantly introduced into said amplifier, such that an intensity of the amplified radiation field corresponding to said first seed radiation field is not constant, because during a dead time of said first seed radiation field an accumulated energy, in particular said population inversion, inside said laser active medium is increasing and therefore immediately after said dead time said first seed radiation field, which in particular is introduced into said amplifier as said introduced radiation field, is stronger amplified as said first seed radiation field before said dead time and thereby for example it is even possible that such stronger amplified radiation field, which is generated by the amplification of said first seed radiation field, damages parts of said radiation field generating unit.

For example, said introduced radiation field comprises said first seed radiation field and said second seed radiation field and thereby while one of said seed radiation fields exhibits the dead time, within which the intensity of said one of said seed radiation field is decreased, in particular is zero or negligible small, said introduced radiation field does not possess any dead time, as long as the intensity of the other of said seed radiation fields is adjusted to avoid damages to said amplifier.

According to the present invention said second seed radiation field is selected and adjusted with respect to said first seed radiation field such that said introduced radiation field comprising said first seed radiation field and said second seed radiation field always maintains a certain level of stimulated emission in said laser active medium such that said population inversion does not exceed said tolerable level, above which a damage to said amplifier would occur.

For example, said population inversion within said laser active medium appears at at least one exited state and at at least one non-exited state with said at least one exited state and said at least one non-exited state

corresponding to at least one laser active transition within said laser active medium.

In particular, said population inversion within said laser active medium appears at several exited states and at several non-exited states with said several exited states and said several non-exited states corresponding to several laser active transitions within said laser active medium.

Advantageously, said first seed radiation field stimulates said at least one on laser active transition, in particular stimulates one or more of said several laser active transitions in particular at a wavelength of said first seed radiation field and thereby in particular said first seed radiation field is amplified.

Preferably, said second seed radiation field stimulates said at least one laser active transition, in particular stimulates one or more of said several laser active transitions in particular at a wavelength different from said wavelength of said first seed radiation field and thereby in particular said second seed radiation field is amplified .

With respect to said amplifier no further details have been given so far.

For example, said amplifier is a sequence of several amplifying units, such that said seed radiation fields are amplified within several steps by several amplifying units providing the advantage that for each step of amplification an appropriate amplifying unit can be used. One advantageous solution provides, that said amplifier comprises an external energy source, which introduces pumping energy into said laser active medium, such that for example said amplifier is provided with pumping energy generating said population inversion at said at least one exited state or said several exited states and said at least one non-exited state or said several non-exited states corresponding to said at least one laser active transition or said several laser active transitions.

A broad variety of said laser active media can be used.

According to one preferred solution the laser active medium of said amplifier is YB-doped .

According to an advantageous solution the laser active medium of said amplifier comprises lutetium.

Further it is of particular advantage if the laser active medium of said amplifier comprises yttrium.

In a preferred solution the laser active medium of said amplifier comprises an aluminate, in particular AI0₄, for example yttrium aluminum garnet,

Y3AI₂ (AI0₄)₃ and in another preferred solution the laser active medium of said amplifier comprises lutetium aluminum garnet, Lu₃AI₂ (AI0₄)₃.

In a preferred solution the laser active medium of said amplifier comprises calcium.

In particular it is advantageous if the laser active medium of said amplifier comprises calcium fluoride, CaF₂.

For example the laser active medium of said amplifier comprises lutetium (III) oxide, Lu₂0₃. In a particular advantageous solution the laser active medium of said amplifier comprises CaGdAI0₄.

In another preferred solution the laser active medium of said amplifier comprises, in particular Yb-doped, Scandium oxide, preferably Sc₂Si0₅, for example Yb : Sc₂Si0₅.

In particular an absorption rate of said first and second seed radiation fields, in particular of said introduced radiation field, and for example of a passing radiation field, which passes through said laser active medium, depends on a wavelength of said radiation field.

Accordingly, for example a total absorption of said introduced radiation field, in particular of the passing radiation field, which passes through said amplifier, by said amplifier depends on the wavelength of said radiation field.

With respect to the overall design of said first source no further details have been given so far.

In particular said first source emits said first seed radiation field as pulses, in particular pulses on demand, in particular in the sense that said first source is switchable from emitting said first seed radiation field for a given time interval to not emitting said first seed radiation field for another predefined time interval.

Accordingly a user of said radiation field generating unit is able to create a time variation of said amplified radiation field corresponding to said first seed radiation field as he needs for his application of said radiation field generating unit. In a preferred embodiment said first source comprises an emitter of a first initial seed radiation field and a modulator, which modulates said first initial seed radiation field, in particular its intensity and/or its propagation direction, and thereby generates said first seed radiation field.

Such an embodiment provides an efficient and cost efficient way to create said first seed radiation field possessing pulses on demand by using said emitter for emitting said first initial seed radiation field comprising regularly repeating pulses and said modulator extracts unrequested pulses, for example by absorption of such pulses or by redirection of such pulses.

In another advantageous embodiment, said second initial seed radiation field has a constant intensity in time and said modulator decreases said intensity of said second initial seed radiation field on demand or said modulator extracts said second initial seed radiation field on demand, for example by absorption or by redirection of said second initial seed radiation field.

Said modulator of said first source is for example an optical device.

One advantageous solution provides, that said first source is switchable between a high mode and a low mode, with the intensity of said first seed radiation field, in particular a time averaged intensity of said first seed radiation field, being lower in said low mode than the intensity of said first seed radiation field, in particular the time averaged intensity of said first seed radiation field, in the high mode and thereby it is possible for the user to generate a time variation of said first seed radiation field, in particular a variation in time of the intensity of said first seed radiation field, as he demands.

In a preferable embodiment, the intensity of said first seed radiation field is zero or negligible small in the low mode. In particular dead times of said first seed radiation field correspond to times, in which said first source operates in said low mode.

In principle there are various ways to switch said first source between said high mode and said low mode.

For example said first source is turned off in the low mode.

In a preferred embodiment during operation of said first source in said low mode said modulator modulates, in particular extracts, for example by redirection of said initial seed radiation field or for example by absorption of said first initial seed radiation field, said first initial seed radiation field and thereby said first source emits said first seed radiation field with lower intensity or does not emit said first seed radiation field during operation in said low mode.

In a preferred embodiment said first source emits in the high mode said first seed radiation field and during operation of said first source in said high mode the value of the intensity of said first seed radiation field remains constant.

In a particular advantageous embodiment said first source emits in the high mode said first seed radiation field and during operation of said first source in said high mode the value of the intensity of said first seed radiation field varies periodically in time and thereby said first source emits sequences of radiation field pulses, the temporal order of which is set by the user of said radiation field generating unit by switching said first source between said low mode and said high mode.

Accordingly a broad variety of temporal orders of sequences of radiation field pulses is possible. For example it is of particular advantage, if the temporal order of said first seed radiation field comprises intervals within which the intensity of said first seed radiation field is uniform and in particular larger than zero and said intervals are temporally followed by dead times, within which the intensity of said first seed radiation field decreased or is zero or at least negligible small, such that the arrangement of said first source is simple and a cost efficient embodiment is provided.

In particular, the temporal order of sequences of radiation field pulses is build up by time intervals, in which the value of the intensity of said first seed radiation field varies periodically in time, and dead times, in which the intensity of said first seed radiation field is decreased or no first seed radiation field is present, with the temporal lengths of said intervals and said dead times are adjustable and such can be different at different positions of said

sequence.

For example a repetition rate of said first seed radiation field, for example in said intervals, in which the value of the intensity of said first seed radiation field varies periodically in time, is larger than 1 MHz, preferably larger than 5 MHz, in particular larger than 10 MHz, advantageously larger than 20 MHz.

For example said repetition rate of said first seed radiation field is, for example in said intervals, in which the value of the intensity of said first seed radiation field varies periodically in time, smaller than 150 MHz, preferably smaller than 120 MHz, advantageously smaller than 100 MHz, in particular smaller than 80 MHz, for example smaller than 60 MHz.

In a preferred embodiment said repetition rate of said first seed radiation field emitted by said first source in the high mode is larger than 1 MHz, in particular larger than 5 MHz, preferably larger than 10 MHz, advantageously larger than 15 MHz. Advantageously a duration of one pulse of said first seed radiation field emitted by said first source, in particular emitted by said first source in the high mode, is smaller than one nanosecond, preferably smaller than 500 picoseconds, advantageously smaller than 200 picoseconds, for example smaller than 900 femtoseconds.

For example, said dead times of said first seed radiation field are larger than one nanosecond, in particular larger than 50 nanoseconds, for example larger than 500 nanoseconds, in particular larger than one microsecond, in particular larger than 50 microseconds, for example larger than 500 microseconds.

With respect to the overall design of said second source no further details have been given so far.

In particular said second source emits said second seed radiation field as pulses, in particular pulses on demand, in particular in the sense that said source is switchable from emitting said second seed radiation field for a given time interval to not emitting said second seed radiation field for another predefined time interval .

Accordingly a user of said radiation field generating unit is able to create a time variation of said second seed radiation field or of said amplified radiation field corresponding to said second seed radiation field as he needs for his application of said radiation field generating unit.

In a preferred embodiment said second source comprises an emitter of a second initial seed radiation field and a modulator, which modulates said second initial seed radiation field, in particular its intensity and/or its propagation direction, and thereby generates said second seed radiation field . Such an embodiment provides an efficient and cost efficient way to create said second seed radiation field possessing pulses on demand by using said emitter for emitting said second initial seed radiation field comprising regularly repeating pulses and said modulator extracts unrequested pulses, for example by absorption of such pulses or by redirection of such pulses.

Said modulator of said second source is for example an optical device.

One advantageous solution provides, that said second source is switchable between a high mode and a low mode, with the intensity of said second seed radiation field, in particular a time averaged intensity of said second seed radiation field, being lower in said low mode than the intensity of said second seed radiation field, in particular the time averaged intensity of said second seed radiation field, in the high mode and thereby it is possible for the user to generate a time variation of said second seed radiation field, in particular a variation in time of the intensity of said second seed radiation field, as he demands.

In a preferable embodiment, the intensity of said second seed radiation field is zero or negligible small in the low mode.

In principle there are various ways to switch said second source between said high mode and said low mode.

For example said second source is turned off in the low mode. In a preferred embodiment during operation of said second source in said low mode said modulator modulates, in particular extracts, for example by redirection of said initial seed radiation field or for example by absorption of said second initial seed radiation field, said second initial seed radiation field and thereby said second source emits said second seed radiation field with lower intensity or does not emit said second seed radiation field during operation in said low mode.

In a preferred embodiment said second source emits in the high mode said second seed radiation field and during operation of said second source in said high mode the value of the intensity of said second seed radiation field remains constant.

In a particular advantageous embodiment said second source emits in the high mode said second seed radiation field and during operation of said second source in said high mode the value of the intensity of said second seed radiation field varies periodically in time and thereby said second source emits sequences of radiation field pulses, the temporal order of which is set by the user of said radiation field generating unit by switching said second source between said low mode and said high mode.

Accordingly a broad variety of temporal orders of sequences of radiation field pulses is possible.

For example it is of particular advantage, if the temporal order of said second seed radiation field comprises intervals within which the intensity of said second seed radiation field is uniform and in particular larger than 0 and said intervals are temporally followed by dead times, within which the intensity of said second seed radiation field decreased or is 0 or at least negligible small, such that the arrangement of said second source is simple and a cost efficient embodiment is provided. In particular, the temporal order of sequences of radiation field pulses is build up by time intervals, in which the value of the intensity of said second seed radiation field varies periodically in time, and dead times, in which the intensity of said second seed radiation field is decreased or no second seed radiation field is present, with the temporal lengths of said intervals and said dead times are adjustable and such can be different at different positions of said sequence.

For example a repetition rate of said second seed radiation field, for example in said intervals, in which the value of the intensity of said second seed radiation field varies periodically in time, is larger than 1 MHz, preferably larger than 5 MHz, in particular larger than 10 MHz, advantageously larger than 20 MHz.

For example said repetition rate of said second seed radiation field, for example in said intervals, in which the value of the intensity of said second seed radiation field varies periodically in time, is smaller than 150 MHz, preferably smaller than 120 MHz, advantageously smaller than 100 MHz, in particular smaller than 80 MHz, for example smaller than 60 MHz.

In a preferred embodiment said repletion rate of said second seed radiation field emitted by said second source in the high mode is larger than 1 MHz, in particular larger than 5 MHz, preferably larger than 10 MHz, advantageously larger than 15 MHz.

Advantageously a duration of one pulse of said second seed radiation field emitted by said second source, in particular emitted by said second source in the high mode, is smaller than one nanosecond, preferably smaller than 500 picoseconds, advantageously smaller than 200 picoseconds, for example smaller than 900 femtoseconds. For example, said dead times of said second seed radiation field are larger than one nanosecond, in particular larger than 50 nanoseconds, for example larger than 500 nanoseconds, in particular larger than one microsecond, in particular larger than 50 microseconds, for example larger than 500

microseconds.

With respect to the wavelengths of said seed radiation fields no further details have been given.

In principle it is possible that the values of said seed radiation fields are within a broad range of values, in particular it is possible that the value of the wavelength of said first seed radiation field is larger than the value of the wavelength of said second seed radiation field, however it is also possible that in another embodiment the value of the wavelength of said first seed radiation field is smaller than the value of the wavelength of said second seed radiation field .

It is particular advantageous, if the values of the wavelengths of said seed radiation fields differ by at most a value of a bandwidth of their pulses, in particular by at most 1 nanometer, in particular by at most 3 nanometers.

In a preferred solution the values of the wavelengths of said seed radiation fields differ by less than 100 nanometer, for example by less than

50 nanometers, preferably by less than 20 nanometers, for example by less than 10 nanometers.

It is advantageous, if the wavelength of said second seed radiation field differs from the wavelength of said first radiation field by less than 5 %, in particular by less than 2 %. In a preferred embodiment at least one of said wavelengths of said seed radiation fields correspond to a peak wavelength of an emission spectrum of said laser active medium and thereby the intensity of said seed radiation field is particular advantageously amplified.

One preferred solution provides that at least one of said wavelengths of said seed radiation fields lies within an interval of wavelengths corresponding to a shoulder of the peak of said emission spectrum of said laser active medium.

In principle it is possible that rates of emission of said laser active medium at said wavelengths of said seed radiation fields are different.

For example in a preferred embodiment the rate of emission of said laser active medium at said wavelength of said first seed radiation field is smaller than the rate of emission of said laser active medium at said wavelength of said second seed radiation field, whereas in another, advantageous solution the rate of emission of said laser active medium at said wavelength of said first seed radiation field is larger than the rate of emission of said laser active medium at said wavelength of said second seed radiation field .

In another advantageous embodiment it is provided that the rates of emission of said laser active medium at said wavelengths of said seed radiation fields are about the same, in particular these rates differ by less than 10 % with respect to each other, preferably differ by less than 5 % with respect to each other and for example differ by less than 1 % with respect to each other.

With respect to the selection and adjustment of said second seed radiation fields no further details have been given.

Preferably, said selection and adjustment of said second seed radiation field comprises one or more of the following factors of influence on the stimulated emission generated by said second seed radiation field which are in particular: the intensity of said second seed radiation field, the polarization of said second seed radiation field, the absorption of said second seed radiation field by said amplifier, in particular by said laser active medium, the amplification of said second seed radiation field by said amplifier, in particular by said laser active medium, and an optical path length of said second seed radiation field in said laser active medium, in particular a number of passes through said laser active medium.

Further said selection and adjustment of said second seed radiation field can be improved in order to operate said amplifier in a more stable way and to generate a more stable amplified radiation field.

Preferably, the intensity of said second seed radiation field is selected and adjusted with respect to the arrangement of said amplifier, in particular with respect to the intensity and the wavelength of said first seed radiation field, such, that, when said second seed radiation field is introduced into said amplifier, it keeps the population inversion within said laser active medium at an essentially constant level, in particular on that level, which is present in said amplifier, when said first seed radiation field is introduced.

In an advantageous embodiment, said second seed radiation field is selected and adjusted, in particular during dead times of the first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, said population inversion is kept below an uppermost target level, in particular below said tolerable level.

Preferably, the intensity of said second seed radiation field, for example the temporal variation of said intensity of said second seed radiation field is selected and adjusted, in particular during dead times of the first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, said population inversion is kept below a second uppermost target level, in particular below said tolerable level. For example said second uppermost target level corresponds to said

uppermost target level.

It is particular advantageous, if said second seed radiation field is selected and adjusted, in particular during dead times of the first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, said population inversion is kept within a target band, which is bounded by above from a higher target level and bounded by below by a lower target level corresponding to 40% of said higher target level, in particular corresponding to 60% of said higher target level, preferably corresponding to 70% of said higher target level, for example corresponding to 80% of said higher target level.

Preferably said higher target level is at a level below said tolerable level, for example below said uppermost target level.

Preferably, the intensity of said second seed radiation field, for example the temporal variation of said intensity of said second seed radiation field is selected and adjusted, in particular during dead times of the first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, said population inversion is kept within said target band, which is bounded by above from a second higher target level and bounded by below by a second lower target level corresponding to 40% of said second higher target level, in particular corresponding to 60% of said second higher target level, preferably corresponding to 70% of said second higher target level, for example corresponding to 80% of said second higher target level.

Preferably, said second higher target level is at a level below said tolerable level, in particular below said second uppermost target level. In particular said second higher target level corresponds to said higher target level.

In particular said second lower target level correspond to said lower target level.

In an preferred embodiment, said second seed radiation field is selected and adjusted, in particular during dead times of said first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, said population inversion is kept essentially constant at a desired target level, with said population inversion being essentially constant at said desired target level, if it is at said desired target level with deviations from said desired target level being smaller than ± 10 %, advantageously smaller than ± 5 %, in particular smaller than ± 3 %, preferably smaller than ± 2%, for example smaller than ± 1 %, of said desired target level.

In particular said desired target level is at a level below said tolerable level, for example below said uppermost target level.

Preferably, said desired target level is at a level below said higher target level .

Advantageously said desired target level is at a level above said lower target level.

In particular, said desired target level corresponds to the level of the

population inversion as it appears, when said first seed radiation field in the high mode of said first source is introduced into said amplifier.

Preferably, the intensity of said second seed radiation field, for example the temporal variation of said intensity of said second seed radiation field is selected and adjusted, in particular during dead times of said first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, said population inversion is kept essentially constant at a second desired target level, with said population inversion being essentially constant at said second desired target level, if it is at said second desired target level with deviations from said second desired target level being less than ± 10 %, advantageously less than ± 5 %, in particular less than ± 3 %, preferably less than ± 2 %, for example less than ± 1 %, of said second desired target level.

Preferably, said second desired target level is at a level below said tolerable level, in particular below said second uppermost target level.

In particular said second desired target level corresponds to said target level.

It is advantageous, if said second seed radiation field is selected and adjusted, in particular during dead times of the first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, the intensity of said amplified radiation field corresponding to said first seed radiation field, in particular the intensity of said first final amplified radiation field corresponding to said first seed radiation field, is kept below an

uppermost target intensity. Preferably, the intensity of said second seed radiation field, for example the temporal variation of said intensity of said second seed radiation field is selected and adjusted, in particular during dead times of the first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, the intensity of said amplified radiation field corresponding to said first seed radiation field, in particular the intensity of said first final amplified radiation field corresponding to said first seed radiation field, is kept below a second uppermost target intensity.

Preferably said second uppermost intensity corresponds to said uppermost intensity.

It is particular advantageous, if said second seed radiation field is selected and adjusted, in particular during dead times of said first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, the intensity of said amplified radiation field corresponding to said first seed radiation field, in particular the intensity of said first final amplified radiation field corresponding to said first seed radiation field, is kept within a target interval, which is bounded by above from a higher target intensity and bounded by below by a lower target intensity corresponding to 40 % of said higher target intensity, in particular corresponding to 60 % of said higher target intensity, preferably corresponding to 70 % of said higher target intensity, for example corresponding to 80 % of said higher target intensity.

Preferably, said higher target intensity is at a level below said uppermost target intensity.

Preferably, the intensity of said second seed radiation field, for example the temporal variation of said intensity of said second seed radiation field is selected and adjusted, in particular during dead times of said first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, the intensity of said amplified radiation field corresponding to said first seed radiation field, in particular the intensity of said first final amplified radiation field corresponding to said first seed radiation field, is kept within a second target interval, which is bounded by above from a second higher target intensity and bounded by below by a second lower target intensity corresponding to 40 % of said second higher target intensity, in particular corresponding to 60 % of said second higher target intensity, preferably corresponding to 70 % of said second higher target intensity, for example corresponding to 80 % of said second higher target intensity.

Preferably, said second higher target intensity is at a level below said uppermost target intensity.

In particular, said second higher target level corresponds to said higher target level.

In particular, said second lower target level corresponds to said lower target level.

In a preferred embodiment, said second seed radiation field is selected and adjusted, in particular during dead times of said first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, the intensity of said amplified radiation field corresponding to said first seed radiation field, in particular the intensity of said first final amplified radiation field corresponding to said first seed radiation field, is kept essentially constant at a desired target intensity, with said intensity of said amplified radiation field corresponding to said first seed radiation field being essentially constant at said desired target intensity, if it is at said desired target intensity and deviations from said desired target intensity are smaller than ± 10 %, advantageously smaller than ± 5 %, in particular smaller than ± 3 %, preferably smaller than ± 2 %, for example smaller than ± 1 %, of said desired target intensity.

Preferably, said desired target intensity is at a level below said uppermost target intensity.

Advantageously said desired target intensity is at a level below said higher target intensity.

In particular, said desired target intensity is at a level above said lower target intensity.

Preferably, the intensity of said second seed radiation field, for example the temporal variation of said intensity of said second seed radiation field is selected and adjusted, in particular during dead times of said first seed radiation field, such that, for example in case of variations of said first seed radiation field, in particular in case of temporal variations of the intensity of said first seed radiation field, the intensity of said amplified radiation field corresponding to said first seed radiation field, in particular the intensity of said first final amplified radiation field corresponding to said first seed radiation field, is kept essentially constant at a second desired target intensity, with the intensity of said amplified radiation field corresponding to said first seed radiation field being essentially constant at said second desired target intensity, if it is at said second desired target intensity and deviations from said second desired target intensity are smaller than ± 10 , advantageously smaller than ± 5 %, in particular smaller than ± 3 %, preferably smaller than ± 2 %, for example smaller than ± 1 %, of said second desired target intensity.

Preferably, said second desired target intensity is at a level below said uppermost target intensity. Advantageously, said second desired target intensity is at a level below said second higher target intensity.

In particular, said second desired target intensity is at a level above said second lower target intensity.

In particular, said second desired target intensity corresponds to said desired target intensity.

Preferably, the intensity of said second seed radiation field, in particular during dead times of said first seed radiation field, is adjusted such, that said first seed radiation field is amplified all the time in the same manner, in particular such, that each pulse of said first seed radiation field is amplified in the same manner.

Preferably the intensity of said second seed radiation field is selected and adjusted in consideration of a difference of two values of the total amplification of said amplifier, where one of said values corresponds to the total

amplification at the wavelength of said first seed radiation field and the other of said values corresponds to the total amplification at the wavelength of said second seed radiation field, advantageously such that said difference in amplification is compensated by said selected and adjusted intensity of said second seed radiation field and thereby the population inversion in said amplifier is kept below said uppermost target level, in particular within said target interval, for example essentially at said desired target level as defined in the preceding paragraphs.

Preferably the intensity of said second seed radiation field is adjusted in consideration of a difference of two values of the total absorption of said amplifier, where one of said values corresponds to the total absorption at the wavelength of said first seed radiation field and the other of said values corresponds to the total absorption at the wavelength of said second seed radiation field, advantageously such that said difference in absorption is compensated by said adjusted intensity of said second seed radiation field and thereby the population inversion in said amplifier is the same when said second seed radiation field is introduced in said amplifier as when said first seed radiation field is introduced in said amplifier.

The advantage of said embodiment has to be seen in the fact, that the population inversion within said amplifier is kept constant even during the dead time of said first seed radiation field and consequently said first seed radiation field is amplified after said dead time as much as before said dead time and for example within such a radiation field generating unit each pulse of said first seed radiation field with an irregular sequence of pulses is amplified with the same rate of amplification.

With respect to a relation between said population inversion and the intensity of said amplified radiation field no further details have been given.

In general, the intensity of said amplified radiation field is larger if said population inversion is larger.

For example, if said population inversion is kept below a certain upper level, the intensity of said amplified radiation field is kept below a certain upper intensity, in particular as long as other factors of influence to said

amplification, for example the passes through said laser active medium or the absorption of said one of said seed radiation fields by said amplifier or the intensity of said one of said seed radiation fields or the polarization of said one of said seed radiation fields or, if the other of said seed radiation fields is introduced into said amplifier at the same time, the aforementioned properties of said other of said seed radiation fields, are kept constant.

For example, if said population inversion is kept essentially constant, the amplification of said first seed radiation field, in particular of said introduced radiation field is essentially constant. In particular, if said population inversion is kept essentially constant the intensity of said amplified radiation field is kept essentially constant.

With respect to a coordination of an operation of said first source and an operation of said second source no further details have been given.

It is advantageous, if the operation of said first source and said second source are adjusted, such that said introduced radiation field, which is introduced in said amplifier and which comprises of said first and second seed radiation fields, possesses a regular evolution in time, in particular such that said introduced radiation field possesses no long dead times, in particular said dead times of said introduced radiation field are smaller than one millisecond, preferably smaller than 500 microseconds, advantageously smaller than 100 microseconds.

Preferably said radiation field generating unit comprises a coordinating device, which coordinates the operation of said sources and their emission of said seed radiation fields in order to generate a particular desirable introduced radiation field.

In a preferred embodiment said first source and said second source are connected by the coordinating device, which coordinates, that only one of said seed radiation fields is introduced into said amplifier at the same time.

The advantage thereby is that the value of said introduced radiation field is at most as large as the intensity of one of said seed radiation fields and

accordingly said introduced radiation field is amplified in a more balanced way.

Another advantage is, that, because the extraction of energy from said laser active medium depends on the intensity of said introduced radiation field, said extraction of energy from said laser active medium is bounded by an upper value. In a particular advantageous embodiment said coordinating device switches said second source into said low mode, when said first source is in said high mode and switches said second source into said high mode, when said first source is in said low mode and accordingly at a given time one of said seed radiation fields is introduced as said introduced seed radiation field in said amplifier such that a balanced introduced radiation field is introduced in said amplifier.

Accordingly in this advantageous embodiment at a given time one of said seed radiation fields is introduced in said amplifier thereby generating a balanced introduced radiation field and as a consequence of said balanced introduced radiation field, which in particular at different times comprises either said first seed radiation field or said second seed radiation field, the population inversion within said amplifier remains on a stable level and accordingly a balanced amplified radiation field is generated, where in particular parts of said amplified radiation field corresponding to a particular wavelength, for example the part of said amplified radiation field corresponding to the wavelength of said first seed radiation field, is amplified all time in the same manner.

An advantageous solution provides that said pumping energy which is introduced in said amplifier, while one of said sources of one of said seed radiation fields is in low mode, is extracted by the another one of said seed radiation fields.

Thereby the advantage of said embodiment has to be seen in the fact, that during operation of said source in said low mode another seed radiation field extracts said pumping energy from said amplifier, such that no or hardly energy is accumulated in said amplifier and thereby in particular damages to said amplifier by an increased accumulated energy are prevented and in an advantageous way during a dead time of one of said seed radiation fields no or hardly energy is accumulated within said amplifier, such that a pulse of said seed radiation field following said dead time is amplified in the same manner as preceding pulses and subsequent pulses and thereby advantageously a balanced amplification of said introduced radiation field into said amplified radiation field is achieved.

With respect to a processing and a use of said amplified radiation field no further details have been given.

An advantageous embodiment provides that said radiation field generating unit comprises a separation unit element, which separates a part of said amplified radiation field which corresponds to said wavelength of said first radiation field and a part of said amplified radiation field which corresponds to said wavelength of said second seed radiation field and such said separation element generates a first final amplified radiation field possessing the wavelength of said first seed radiation field and generates a second final amplified radiation field possessing the wavelength of said second seed radiation field.

Thereby it is advantageous if said first final amplified radiation field is separated from said second final amplified radiation field, such that it is possible to use one of said final amplified radiation fields without disturbance by the other of said final amplified radiation fields.

In particular said first final amplified radiation field possesses the same sequence of pulses than said first seed radiation field possesses and accordingly adjusting the sequence of pulses of said first seed radiation field corresponds to adjusting the sequence of pulses of said first final amplified radiation field.

In particular said second final amplified radiation field possesses the same sequence of pulses than said second seed radiation field possesses and accordingly adjusting the sequence of pulses of said second seed radiation field corresponds to adjusting the sequence of pulses of said second final amplified radiation field. An advantageous embodiment provides that said amplified radiation field, in particular one of said final amplified radiation fields, is used for micro- structuring of materials.

A particular advantageous solution provides that said first and second final amplified radiation fields are used for micro-structuring of materials.

The advantage of short pulses, for example pulses the temporal length of which are shorter than 900 picoseconds, in particular shorter than

400 picoseconds is that the time of exposure of said materials, which are micro-structured by said final amplified radiation fields, to said amplified radiation field is limited and accordingly the structuring is more precise and disadvantageous effects by the exposure with said amplified radiation field are limited .

Further features and explanations with respect to the present invention are disclosed in connection with the detailed specification and the drawings.

In the drawings:

Fig. 1 shows a schematic arrangement of a first embodiment of a radiation field generating unit according to the present invention;

Fig. 2 shows a first example of a schema of temporal variation of

intensities of a first and a second seed radiation field of the radiation field generating unit;

Fig. 3 shows a schematic arrangement of an amplifier of the radiation field generating unit according to the present invention;

Fig. 4 shows schematically energy levels of a laser active medium of the amplifier and corresponding transitions; shows an emission spectrum of the laser active medium according to the first embodiment of the present invention; shows an embodiment of an introducing element of a field

generating unit according to the present invention; shows a time variation of a population inversion and a time variation of intensity an amplified radiation field corresponding to the first seed radiation field; shows an emission spectrum of a laser active medium of a second embodiment of the present invention; shows an emission spectrum of a laser active medium of a third embodiment of the present invention; shows an emission spectrum of a laser active medium of a fourth embodiment of the present invention; shows an emission spectrum of a laser active medium of a fifth embodiment of the present invention; shows an emission spectrum of a laser active medium of a sixth embodiment of the present invention; shows a second example of a schema of temporal variation of intensities of the first and the second seed radiation field of the radiation field generating unit; Fig. 14 shows a time variation of the population in version and a time variation of the intensity of the amplified radiation field

corresponding to the first seed radiation field according to an eight embodiment of the present invention and

Fig. 15 shows a time variation of the population in version and a time

variation of the intensity of the amplified radiation field

corresponding to the first seed radiation field according to a ninth embodiment.

A first embodiment of a radiation field generating unit 10 according to the present invention as shown in fig. 1 comprises a first source 12 of a first seed radiation field 14 with wavelength II, a second source 16 of a second seed radiation field 18 with wavelength 12, a coordinating device 22, which coordinates the operation of sources 12 and 16, and an amplifier 24, which amplifies an intensity of an introduced radiation field 26 and thereby produces an amplified radiation field 28, where introduced radiation field 26 comprises first and second seed radiation fields 14 and 18.

Furthermore, said radiation field generating unit 10 comprises an introducing element 32, by which seed radiation fields 14 and 18 are introduced into amplifier 24, and a separation element 34, in which amplified radiation field 28 is introduced and which separates from amplified radiation field 28 a first final amplified radiation field 36, a wavelength of which corresponds to wavelength I I of first seed radiation field 14, and a second final amplified radiation field 38, a wavelength of which corresponds to wavelength 12 of second seed radiation field 18.

An intensity of first seed radiation field 14 varies irregularly in time, in particular the value of the intensity of first seed radiation field 14 either corresponds to an intensity II or is zero, in the sense, that the value of the intensity is negligible small. Therefor first source 12 is switchable between a high mode, in which it emits first seed radiation field 14, and an low mode, in which it does not emit a radiation field. During operation of first source 12 in the high mode, the intensity of first seed radiation field 14 varies periodically, in particular it either corresponds to intensity II or is zero, in the sense of being negligible small, and for example a temporal periodicity tp of the intensity of first seed radiation field 14 during operation of first source 12 in high mode is tp = tl + t2, where in time interval tl the intensity of first seed radiation field 14 corresponds to II and in second time interval t2 the intensity of first seed radiation field 14 is zero, in the sense of being negligible small.

As an example, in fig . 2 a temporal variation of the intensity of first seed radiation field 14 is shown, where during a first period pi the first source 12 operates in the high mode and emits three pulses of first seed radiation field 14 each of the pulses has a time length tl and an intensity II and in between these pulses are time intervals of length t2 in which the intensity of the first seed radiation 14 is negligible small, followed by a period p2 in which first source 12 operates in the low mode, in which the intensity of first seed radiation field 14 emitted by first source 12 is negligible small, in a following period p3 first source 12 is in the high mode and emits nine pulses of first seed radiation field 14, on period p3 follows a period p4 in which first source 12 is in the low mode and the intensity of first seed radiation field 14 emitted by first source 12 is negligible small and in the following period p5 in which first source 12 is in the high mode first source 12 emit pulses of first seed radiation field 14, four pulses of which are shown in fig. 2.

In a variation of the present embodiment, first source 12 emits in the high mode first seed radiation field 14 with constant intensity II .

For example first source 12 comprises an emitter 52 of a first initial seed radiation field 54, an intensity of which varies periodically in time and in particular the intensity of first initial seed radiation field 54 either corresponds to intensity II or is negligible small and first initial seed radiation field 54 has the same wavelength II as first seed radiation field 14.

In addition, first source 12 according to the embodiment as shown in fig . 1 comprises a modulator 56, which modulates first initial seed radiation field 54 into first seed radiation field 14 in particular by transmitting first initial seed radiation field 54 during operation of first source 12 in high mode and redirecting first initial seed radiation field 54 during operation of first source 12 in low mode.

Accordingly, the intensity of first seed radiation field 14 varies irregularly in time in particular without a periodical repetition of sequences corresponding to the same value of the intensity and the same temporal length, when first source 12 is irregularly switched between the high mode and the low mode.

Second source 16 is switchable between a high mode and a low mode, where during operation in the high mode second source 16 emits second seed radiation field 18 with a constant, high intensity I2h, for example a larger intensity than II of first seed radiation field 14, and during operation in the low mode second source 16 emits second seed radiation field 18 with a low intensity 121, which is significantly smaller than I2h.

In a variation of the embodiment, second source 16 does not emit a radiation field during operation in the low mode.

Second seed radiation field 18 has a wavelength 12, which differs from wavelength I I of first seed radiation field 14, and during operation of second source 16 in the high mode an intensity of second seed radiation field 18 corresponds to an intensity I2h, and during operation of second source 16 in the low mode an intensity of second seed radiation field 18 corresponds to an intensity 121. As an example, in fig . 2 a temporal variation of the intensity of second seed radiation field 18 is shown, where in the period pi second source 16 operates in the low mode and accordingly the intensity of second seed radiation field 18 is 121 and in the following period p2 second source 16 operates in the high mode, such that the intensity of second seed radiation field 18 emitted by second source 16 corresponds to I2h, in the following period p3 second source 16 is in the low mode and emits second seed radiation field 18 with intensity 121, on period 3 follows period 4, in which second source 16 is in the high mode and the intensity of second seed radiation field 18 emitted by second source 16 corresponds to I2h and it follows period p5, in which second source 16 is in the low mode and the intensity of second seed radiation field 18 is 121.

Accordingly, second source 16 operates in the low mode, when first source 12 operates in the high mode and second source 16 operates in the high mode, when first source 12 operates in the low mode.

For example second source 16 comprises an emitter 72 of a second initial seed radiation field, which has the same wavelength 12 as second seed radiation field 18, and a modulator 76.

In that embodiment emitter 72 emits continuously second initial seed radiation field 74, which has an intensity which corresponds to intensity 12 and modulator 76 allows second source 16 to emit second initial seed radiation field 74 during operation of second source 16 in the high mode, such that second initial seed radiation field 74 is emitted by second source 16 as second seed radiation field 18 and during operation of second source 16 in the low mode modulator 76 redirects second initial seed radiation field 74, such that second source 16 does not emit a radiation field in the low mode.

Coordinating device 22 is connected to first source 12 and to second source 16 and coordinates the operation of sources 12 and 16 in such a way, that second source 16 operates in the low mode, when first source 12 operates in the high mode, and second source 16 operates in the high mode, when first source 12 operates in the low mode.

Accordingly, coordinating device 22 coordinates, that either first source 12 emits first seed radiation field 14 or second source 16 emits second seed radiation field 18, in particular coordinating device 22 triggers second source 16 to emit second seed radiation field 18, when first source 12 suspends emitting first seed radiation field 14, as can be seen in the schema shown in fig. 2, where in each of the periods pi to p5 one of the sources 12 and 16 operates in the high mode and the other of the sources operates in the low mode.

Amplifier 24 as shown in fig . 3 comprises a laser active medium 102 and an external energy source 104, which introduces pumping energy 106 to laser active medium 102.

External energy source 104 is for example a radiation field source which emits a radiation field, in particular a pumping laser beam, which is introduced into laser active medium 102 and thereby transmits pumping energy 106 into laser active medium 102.

Introduced radiation field 26 is introduced into laser active medium 102 and as a passing radiation field 112 it extracts energy from laser active medium 102 and thereby the intensity of passing radiation field 112 is increased and passing radiation field 112 exits laser active medium 102 as amplified radiation field 28, the intensity of which is increased with respect to the intensity of introduced radiation field 26 as a consequence of extraction of the energy from laser active medium 102.

Laser active medium 102 possesses several states corresponding to different energy levels. For example laser active medium possess at least three states corresponding to energy levels E₀, Ei and E₂, as schematically shown in fig. 4, with energy level E₀ being the lowest energy level of these three energy levels and E₂ being the highest energy level of these energy levels E₀, Ei and E₂ and accordingly the energy corresponding to energy level Ei is larger than the energy corresponding to energy level E₀ and is smaller than the energy corresponding to energy level E₂.

The difference between the energies corresponding to energy levels E₀ and E₂ is such, that pumping energy 106 induces a transition 124 from the state corresponding to energy level E₀ to the state corresponding to energy level E₂ and such for example a population of the state corresponding to energy level E₂ is increased .

The state corresponding to energy level E₂ is an unstable state, such that by a transition 126 from the state corresponding to energy level E₂ to the state corresponding to energy level Ei, for example a relaxation process, an energy 128 is released, which for example as heat diffuses in laser active material 102, and the state corresponding to energy level Ei is populated .

There exists a transition 132 from the state corresponding to energy level Ei to the state corresponding to energy level E₀, and transition 132 is a laser active transition and accordingly, the state corresponding to energy level Ei is an exited state of the laser active transition 132 and the state corresponding to energy level E₀ is a non-exited state of the laser active transition 132.

Laser active transition 132 is stimulated by a stimulating photon 136 of passing radiation field 112, in particular with the energy of photon 136 corresponding to the energy difference between energy levels Ei and E₀, and by this stimulated transition 132 a photon 138 is emitted and emitted photon 138 possesses the same mode than stimulating photon 136, in particular the frequency, the phase, the polarization and the spatial direction of propagation of the two photons 138 and 136 after stimulated transition 132 are the same and thereby passing radiation field 112 is amplified by stimulated transition 132 in a coherent way.

In fig . 4 stimulating photon 136 is drawn twice, once as photon 136b before laser active transition 132 and a second time as photon 136a after stimulated, laser active transition 132 together with emitted photon 138.

Within a more complex laser active medium, there are various possible laser active transitions, such that laser active medium 102 is able to emit radiation fields with a variety of different wavelengths.

According to the present embodiment laser active medium 102 comprises an ytterbium (Yb) doped crystal, in particular Yb doped yttrium aluminum garnet (Yb : YAG).

An intensity of an emitted radiation field by such laser active medium 102 varies continuously as function of a wavelength of the emitted radiation field and the cross section o_em characterizes the rate of emission of laser active medium 102 for a particular wavelength.

An emission spectrum shows the rate of emission of a medium for a particular wavelength and in fig. 5 the emission spectrum of Yb doped YAG is shown in terms of the emission cross section o_em as function of the wavelength of the emitted radiation field.

The emission spectrum as shown in fig. 5 exhibits a peak 202 at a peak wavelength 204, at which the emission cross section reaches its largest value within a broad interval of wavelengths, here within the interval bounded by the wavelengths corresponding to about 900 nm and 1.100 mm, with the value of the emission cross section at peak wavelength 204 is around

2,1 10^"20 cm². Peak 202 possesses a first shoulder 212 within an interval of wavelengths, which are smaller than peak wavelength 204 and are larger than a lower bound wavelength 214, and in the interval of wavelengths corresponding to first shoulder 212 the value of the emission cross section increases

continuously upon increasing the value of the wavelength, for example the value of lower bound wavelength 114 corresponds to 99 % of the value of peak wavelength 204, in particular the value of lower bound wavelength 204 is about 10 nanometer smaller than the value of peak wavelength 204 and for example the value of the emission cross section increases by about a factor of 4 upon increasing the value of the wavelength within the interval

corresponding to first shoulder 212, in particular it increases from a value of about 0,5 10^"20 cm² to a value of about 2,1 10^"20 cm².

Peak 202 of the emission spectrum possesses a second shoulder 222 within an interval of wavelengths, which are larger than peak wavelength 204 and are smaller than a higher bound wavelength 224, and the emission cross section decreases continuously upon increasing the wavelength within the interval of wavelengths corresponding to second shoulder 222, for example the value of the emission cross section decreases to about ¹A of the value of the emission cross section at peak wavelength 204 within the interval of wavelengths corresponding to second shoulder 222, in particular the value of the emission cross section at higher bound wavelength 224 is about 1/4 of the value of the emission cross section at peak wavelength 204 and for example the value of higher bound wavelength 224 is about 101 % of the value of peak wavelength 204, in particular the value of higher bound wavelength 224 is about 10 nanometer larger than the value of peak wavelength 204.

According to the present embodiment wavelength II of first seed radiation field 14 corresponds to peak wavelength 204 and wavelength 12 of second seed radiation field 18 is smaller than wavelength I I of first seed radiation field 14, in particular wavelength 12 of second seed radiation field 18 is smaller than peak wavelength 204 and larger than lower bound wavelength 214,

accordingly wavelength 12 of second seed radiation field 18 lies in the interval corresponding to first shoulder 212 and for example the value of wavelength 12 of second seed radiation field 18 is about 99,5 % of the value of wavelength I I of first seed radiation field 14, in particular the value of wavelength 12 of second seed radiation field 18 is about 5 nanometers smaller than the value of wavelength I I of first seed radiation field 14.

Introducing element 32 is an optical element, the interaction of which with a radiation field depends on the wavelength of the radiation field and thereby introducing element 32 guides first seed radiation field 14 and second seed radiation field 18 to amplifier 24.

According to the embodiment shown in fig. 6 introducing element 32 is for example a dichroic mirror, the reflection and transmission properties of which are wavelength dependent such that first seed radiation field 14 with wavelength I I passes through the dichroic mirror from a first side 302 to a second side 304, which is situated oppositely to first side 302 and second seed radiation field 18, which falls onto second side 304, is reflected by the dichroic mirror.

Propagation directions of incident seed radiation fields 14 and 18 are orientated such, that propagation directions of seed radiation fields 14 and 18 after interacting with introducing element 32, that is in embodiment according to fig. 6 propagation direction of seed radiation field 14 after passing through the dichroic mirror and propagation direction of reflected seed radiation field 18, are orientated in the same direction and thereby seed radiation fields 14 and 18 are introduced as introduced radiation field 26 to amplifier 24.

Separation element 34 is an optical element, the interaction of which with a radiation field depends on the wavelength of the radiation field and thereby separation element 34 separates from amplified radiation field 28, which is introduced into separation element 34, a part with wavelength II

corresponding to first seed radiation field 14 resulting to first final amplified radiation field 36 and a part of amplified radiation field 28 with wavelength 12 corresponding to second seed radiation field 18 resulting to second final amplified radiation field 38 and as consequence of different interactions of these parts of amplified radiation field 28 with separation element 34 the propagation directions of final amplified radiation fields 36 and 38 are different.

For example, separation element 34 is a dichroic mirror similar to introducing element 32 and the part of amplified radiation field 28 with wavelength II corresponding to first seed radiation field 14 passes through the dichroic mirror and leaves separation element 34 as first final amplified radiation field 36 and the part of amplified radiation field 28 with wavelength 12

corresponding to second seed radiation field 18 is reflected by the dichroic mirror and leaves separation element 34 as second final amplified radiation field 38.

Thus radiation field generating unit 10 operates as follows.

First source 12 sends first seed radiation field 14 to introducing element 32, which introduces radiation field 14 as introduced radiation field 26 into amplifier 24 with first seed radiation field 14 being with respect to its temporal variation a sequence of pulses, where on a single pulse or a sequence of temporal periodically repeated pulses a dead time, in which in particular first source 12 is in low mode with emission of first seed radiation field 14 with negligible intensity, temporally follows with the duration of the dead time varying in a non-periodically, irregular manner.

For example, sequences of pulses and dead times in between are set by conditions set by application of first final amplified radiation field 36 for micro structuring parts to be produced. Operation of second source 16 is controlled by coordinating device 22 in accordance with operation of first source 12, such that second source 16 emits second seed rad iation field 18 in the high mode d uring dead times of first seed rad iation field 14, as shown in fig .2.

Seed radiation fields 14 and 18 are introduced into amplifier 24 by introducing element 32 as introd uced radiation field 26 and because second seed radiation field 18 is emitted during dead times of first seed rad iation field 14, in a temporal variation of introd uced radiation field 26 there are no dead times.

According ly, temporal evol ution of introd uced rad iation field 26 corresponds in particular to seq uences of temporally periodical ly occurring pulses and time periods with temporally constant intensity.

In ampl ifier 24 pumping energy 106 is induced by external energy source 104 and the exited state, which corresponds to the state corresponding to energy level Ei, is populated, for example via the state corresponding to energy level E₂ by transitions 124 and 126, and as a conseq uence the exited state of laser active transition 132 is hig her populated than the non-excited state of laser active transition 132, which corresponds to the state corresponding to energy level E₀, and therefore a population inversion occurs in laser active

med ium 102.

Due to the presence of the population inversion at the excited state and the non-exited state of laser active transition 132 the transition 132 is stimu lated by photons 136 of passing field 112 and therefore the intensity of passing field 112 is amplified, and accordingly the intensities of first and second seed rad iation fields 14 and 18 are amplified .

Accord ingly, on the one hand, the excited state of laser active transition 132 is populated as a conseq uence of the introd uction of pumping energy 106 and on the other hand, the excited state of the laser active transition 132 is depopulated by stimulation of laser active transition 132 by the passing radiation field 112.

The level of the population inversion, that is the excess population of the excited state of laser active transition 132 over the population of the non- excited state of laser active transition 132, depends, for constant pumping energy 106 as it is the case in the present embodiment, on the intensity of the passing radiation field 112, because with higher intensity there are more photons 136 of passing radiation field 112 which stimulate laser active transition 132 and on the wavelength of passing radiation field 112, because the rate of stimulated transition 132 depends on the wavelength of passing radiation field 112, as indicated by the emission spectrum.

On the other hand, the amplification of passing radiation field 112 depends on the level of the population inversion, because the larger the excess population of the excited state of laser active transition 132 over the population of the non-excited state of laser active transition 132 is the more often stimulate transition 132 occurs and the stronger passing radiation field 112 is amplified .

Therefore, in order to achieve a uniform amplification of first seed radiation field 14 after a dead time of first seed radiation field 14, in which second seed radiation field 18 is introduced in laser active medium 102 instead of first seed radiation field 14, the population inversion has to be kept constant during the dead time by adopting the intensity of second seed radiation field 18 to laser active medium 102 and by taking into account the amplification of second seed radiation field 102 within amplifier 24, the absorption of second seed radiation field 102 within amplifier 24, for example by laser active medium 102 and by optical devices guiding second seed radiation field 18, and for example the number of passes of second seed radiation field 18 through laser active medium 102 and in particular the polarization of second seed radiation field 18 in such a way, that the level of population inversion is kept constant. That is, the intensity of second seed radiation field 18 has to be adjusted and the chosen value of the intensity of second seed radiation field 18 depends on the wavelengths of first and second seed radiation fields 14 and 18, the intensity of first radiation field 14 and the arrangement of amplifier 24.

Because the population inversion is not measureable, by designing a radiation field generating unit 10 according to the present invention, the intensity of second seed radiation field 18 has to be fined tuned during building up the radiation field generating unit 10 until the condition is reached, that, when during a time interval of a dead time of first seed radiation field 14 second seed radiation field 18 is introduced into amplifier 24 instead of first seed radiation field 14, first seed radiation field 14 is amplified after the dead time as much as before the dead time.

In the present embodiment the second seed radiation field is selected and adjusted, such that the population in version is kept below an uppermost level and thereby the intensity of the amplified radiation field 28 corresponding to the first seed radiation field 14, in particular the intensity of the first final amplified radiation field 36, is kept below an uppermost intensity.

For example, in fig . 7 a time evolution of the population inversion and the intensity of the amplified radiation field 28 corresponding to the first seed radiation field 14 is shown for a time period before, during and after a dead time of the first seed radiation field 14, corresponding to an operation of first source 12 in the high mode, low mode and again in the high mode. Before the dead time first source 12 emits pulses of first seed radiation field 14 and after each pulse the population inversion is decreased due to the stimulated emission 132 by the introduced first seed radiation field 14 and during the pulses the population inversion increases due to the introduction of pumping energy 106. During the dead time of first seed radiation field 14, second seed radiation field 18 is introduced into amplifier 24 such that the population in version is kept below the uppermost level, for example the population in version varies during the dead time of first seed radiation field 14 below the uppermost level, and because the population in version is kept below the uppermost level after the dead time the intensity of the amplified radiation field 28 corresponding to the first seed radiation field 14 is below an

uppermost intensity.

In summary, the advantages of a radiation field generating unit 10 according to the present invention are as follows.

In a dead time of first seed radiation field 14 and if there would be no second seed radiation field 18 there would be a dead time of introduced radiation field 26 and consequently a dead time of passing radiation field 112 such that there would be no stimulated transition 132 and in consequence the additional energy of pumping energy 106 would not be extracted from laser active medium 102 which results firstly to a higher population inversion and a pulse of first seed radiation field 14 and consequently a pulse following the dead time of passing radiation field 112 is more extensively amplified due to more stimulated transitions 132 caused by the higher population of the excited state of laser active transition 132 than earlier pulses of first seed radiation field 14 or subsequent pulses of first seed radiation field 14 are amplified and secondly another consequence would be, that the additional energy of pumping energy 106 damages laser active medium 102.

Such disadvantageous consequences are prevented by second seed radiation field 18 with its intensity being selected and adjusted in accordance with the design of field generating unit 10, which becomes introduced radiation field 26 and passing radiation field 112 during dead times of first seed radiation field 14 and thereby also in dead times of first seed radiation field 14 stimulations of laser active transition 132 occur, in particular with a rate as large as when first seed radiation field 14 would be introduced, and in consequence the balance between the introduced additional energy of pumping energy 106 and extracted energy out from laser active medium 102 by photons 138 which are emitted during transition 132 is kept. Accordingly due to the introduction of second seed radiation field 18 as introduced radiation field 26 in amplifier 24 during dead times of first seed radiation field 14 and the resulting balance of introduced and extracted energy from laser active medium 102, the inversion population within amplifier 24 is kept constant and pulses of first seed radiation field 14 are amplified in a homogeneous manner even after dead times of first seed radiation field 14.

Furthermore, due to the introduction of second seed radiation field 18 during dead times of first seed radiation field 14, the introduced energy of pumping energy 106 is extracted from laser active medium 102 and thereby damages to laser active medium 102 caused by an accumulation of energy are prevented .

According to a second embodiment laser active medium 102' comprises lutetium aluminum garnet (LuAG) and ytterbium (Yb), in particular ytterbium doped lutetium aluminum garnet (Yb : LuAG), the emission spectrum of which is shown in fig . 8, in terms of the emission cross section as function of a wavelength of an emitted radiation field.

For laser active medium 102' the value of peak wavelength 204' is about 1030 nm and the value of lower bound wavelength 214' is about 12 nm smaller than the value of peak wavelength 204' and the value of higher bound wavelength 224' is about 12 nanometer higher than the value of peak wavelength 204'.

According to the present embodiment the value of the emission cross section is at peak wavelength 204' about 2,5 10^"20 cm² and the value of the emission cross section is at lower bound wavelength 214' and at higher bound wavelength 224' about 1/5 of its value at peak wavelength 204'.

The value of wavelength 12' of second seed radiation field 18' is about 5 nanometer smaller than the value of wavelength I I' of first seed radiation field 14'. According to a third embodiment laser active medium 102" comprises lutetium (III) oxide (Lu₂0₃) and ytterbium, in particular Yb-doped lutetium (III) oxide, the emission spectrum of which is shown in fig . 9 in terms of a signal strength in arbitrary units, that its value is normalized to some reference signal strength, as function of a wavelength of an emitted radiation field.

The value of peak wavelength 204" is about 1033 nanometer.

The value of lower bound wavelength 214" is about 15 nanometer smaller than the value of peak wavelength 204" and the strength of the signal reduces within the interval corresponding to first shoulder 212" by a factor of about 2,5.

The value of higher bound wavelength 224" is about 20 nanometer larger than the value of peak wavelength 204", that is the value of higher bound

wavelength 224" is about 102 % of the value of peak wavelength 204". The value of the signal strength decreases from its value at peak wavelength 204" to its value around higher bound wavelength 224" by a factor of about 7, such that in particular the decrease of the value of the signal strength from the value at peak wavelength 204" towards its value at the bound wavelengths 224" or 214" in the interval corresponding to second shoulder 222" is different to this decrease of the value of the signal strength in the interval

corresponding to first shoulder 212".

According to the present embodiment the value of wavelength I I" of first seed radiation field 14" is about 1033 nanometer, which corresponds to the value of peak wavelength 204", and the value of wavelength 12" of second seed radiation field 18" is larger than the value of wavelength I I" and the value of 12" lies in the interval corresponding to second shoulder 222" and is for example about 3 nanometer larger than the value of I I", such that the value of 12" is about 100,3 % of the value of I I". According to a fourth embodiment laser active medium 102"' comprises CALGO (CaGdAI0₄), in particular ytterbium doped CALGO, the emission spectrum of which is shown in fig . 10 in terms of an intensity, which is normalized to some reference value, as function of a wavelength of an emitted radiation field.

Peak 202"' is a broad peak, in the sense that in an interval of wavelengths which are larger than a lower peak wavelength 2041 and which are smaller than a higher peak wavelength 204h the value of the intensity of an emitted radiation field by laser active medium 102"' varies only slightly, for example this value varies only within a range of about 10 % of the value of the emitted intensity at peak wavelength 204"'.

For example the values of lower peak wavelength 2041 and higher peak wavelength 204h differ with respect to each other by around 2 %, in particular these values differ by more than 10 nanometer and less than for example 20 nanometers and in the present embodiment they differ by about 14

nanometer.

The value of the emitted intensity decreases in the interval of wavelengths corresponding to first shoulder 212"' from a value corresponding to peak wavelength 204"' to a value at lower bound wavelength 214"', in particular the value of the emitted intensity reduces by a factor of about 9.

The value of emitted intensity decreases in the interval of wavelengths corresponding to second shoulder 222"' from the value at peak wavelength 204"' to a value at higher bound wavelength 224"' and the reduction is by about a factor of 100.

According to the present embodiment wavelength I I'" and 12"' corresponding to seed radiation fields 14"' and 18"', respectively, lie within an interval bounded by lower peak wavelength 2041 and by higher peak wavelength 204h, such that the intensity of emitted radiation fields at wavelength 12"' and at wavelength I I'" differ by less than 10 % with respect to each other.

According to a fifth embodiment laser active medium 102"" comprises calcium fluoride (CaF₂), in particular yttrium (Yb) and sodium (Na) doped calcium fluoride, the emission spectrum of which is shown in fig. 11 in terms of an intensity of emitted radiation field normalized to an arbitrary reference intensity as function of a wavelength of an emitted radiation field.

Peak 202"" is asymmetric in the sense, that the difference between the values of lower bound wavelength 214"" and peak wavelength 204"" and the difference between the values of higher bound wavelength 224"" and peak wavelength 204"" differ significantly, for example by a factor larger than 2 in particular in the present embodiment by a factor of about 7. In the present embodiment the values of lower bound wavelength 214"" and peak

wavelength 204"" differ by about 8 nanometer and the values of peak wavelength 204"" and higher bound wavelength 224"" differ by about 50 nanometer.

Peak 202"" is asymmetric in the sense that the factors by which the intensity of the emitted radiation field reduces from its value at peak wavelength 204"" to bound wavelengths 214"" and 224"", respectively, differ significantly, for example these factors differ by a factor of more than 2, in particular more than 4 and in the present embodiment these factors differ by about a factor of 10.

According to the present embodiment the intensity of emitted radiation field at lower bound wavelength 214"" is about 80 % of its value at peak wavelength 204"" and the value of the intensity of emitted radiation field at higher bound wavelength 224"' is less than 10 % of its value at peak wavelength 204"". According to the present embodiment the value of wavelength I I"" of first seed radiation field 14"" lies in the interval corresponding to second shoulder 222"" and the value of wavelength 12"" corresponding to second seed radiation field 18"" lies within the interval of wavelengths corresponding to first shoulder 212"".

The values of intensity of emitted radiation field at wavelength 12"" and at wavelength II"" are about the same, in particular the values differ by less than 5 % with respect to each other.

According to a sixth embodiment laser active medium 102 comprises SSO, in particular ytterbium doped SSO, the emission spectrum of which is shown in fig. 12 in terms of gain cross section, which is a measure of how much an introduced radiation field is amplified, as function of a wavelength of an emitted radiation field.

The emission spectrum according to the present embodiment possesses two peaks 202i and 202ii with first peak 202i located at a first peak wavelength 204i and second peak 202ii located at a second peak wavelength 204ii and for example the values of first peak wavelength 204i and second peak wavelength 204ii differ by less than 5 % with respect to each other.

The value of gain cross section decreases monotonically upon decreasing the value of wavelength of emitted radiation field in an interval corresponding to a first shoulder 212i with the interval corresponding to first shoulder 212i being bounded by peak wavelength 204i and a first lower bound wavelength 214i, the value of which is smaller than the value of peak wavelength 204i, and the value of gain cross section decreases monotonically upon increasing the value of wavelength of emitted radiation field in an interval corresponding to a second shoulder 222i with the interval corresponding to second shoulder 222i being bounded by peak wavelength 204i and a first higher bound wavelength 224i, the value of which is larger than the value of peak wavelength 204i. The value of gain cross section decreases monotonically from its value at peak wavelength 204ii of second peak 202ii upon decreasing the value of the wavelength of emitted radiation field in an interval corresponding to a first shoulder 212ii of second peak 202ii, with the interval corresponding to second shoulder 212ii being bounded by peak wavelength 204ii and a lower bound wavelength 214ii, the value of which is smaller than the value of peak wavelength 204ii, and the value of gain cross section decreases monotonically upon increasing the value of the wavelength of emitted radiation field in an interval corresponding to a second shoulder 222ii of second peak 202ii with the interval corresponding to second shoulder 222ii being bounded by peak wavelength 204ii and a higher bound wavelength 224ii.

For example peak wavelength 204ii of second peak 202ii is larger than the value of peak wavelength 204i of first peak 202i.

According to the present embodiment wavelength II of seed radiation field 14 corresponds to peak wavelength 204i of first peak 202i and the value of wavelength 12 of second seed radiation field 18 lies in the interval corresponding to first shoulder 212ii of second peak 202ii, that is the value of 12 according to the present embodiment is smaller than peak wavelength 204ii of second peak 202ii.

In a variation of this embodiment wavelength 12 corresponds to peak wavelength 204ii of second peak 202ii.

According to a seventh embodiment, second seed radiation field 18' is pulsed during high mode of second source 16, shown in fig. 13, where the intensities of first seed radiation field 14 and second seed radiation field 18' during three periods pi', p2', and p3' are shown.

In time period pi', first source 12 is in the high mode and emits pulses of first seed radiation field 14, four of which are shown in fig. 13, and second source 16 is in the low mode and emits no second seed radiation field 18'. In time period p2', first source 12 is in the low mode and emits no first seed radiation field 14 and second source 16 is in the high mode end emits, here for example three, pulses of second seed radiation field 18'.

In time period p3', first source 12 is again in the high mode and emits pulses of first seed radiation field 14, three of which are shown in fig . 13, and second source 16 is in the low mode and emits no second seed radiation field 18'.

According to an eight embodiment the second seed radiation field 18 is selected and adjusted such that the population in version is kept in a target band, which is bounded by above by a higher target level and bounded below by a lower target level, and because the population in version is kept within the target band the intensity of the amplified radiation field 28 corresponding to the first seed radiation field 14 varies within a target interval, which is bounded by above from a higher target intensity and bounded by below by a lower target intensity, as shown in fig. 14.

According to a ninth embodiment the population inversion is kept essentially constant at a desired target level, for example with the deviations from said target level being smaller than a predefined value, and because the population inversion is kept essentially constant at the desired target level the intensity of the amplified radiation field 28 corresponding to the first seed radiation field 14 is kept essentially constant at a desired target intensity, as shown in fig . 15.

Apart from the described elements, the remaining elements of the second embodiment to the ninth embodiment are the same as those elements of one of the other embodiments so that with respect to these elements reference is made to the explanations given in connection with the other embodiments.

Claims

C L A I M S

1. Radiation field generating unit (10) comprising a first source (12) of a first seed radiation field (14) and an amplifier (24), in which an introduced radiation field (26), which comprises said first seed radiation field (14), is introduced, and said amplifier (24) comprises a laser active medium (102) exhibiting a population inversion and said amplifier (24) amplifies an intensity of said introduced radiation field (26) and thereby generates an amplified radiation field (28), c h a r a c t e r i z e d i n t h a t

said radiation field generating unit (10) comprises a second source (16) of a second seed radiation field (18) and said first and second seed radiation fields (14, 18) contribute to said introduced radiation field (26).

2. Radiation field generating unit (10) according to claim 1, characterized in that said amplifier (24) comprises an external energy source (104), which introduces pumping energy (106) into said laser active medium (102).

3. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said first source (12) comprises an emitter (52) of a first initial seed radiation field (54) and a modulator (56), which modulates said first initial seed radiation field (54), in particular its intensity and/or its propagation direction, and thereby generates said first seed radiation field (14).

4. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said first source (12) is switchable between a high mode and a low mode, with the intensity of said first seed radiation field (14) is lower in said low mode than the intensity of said first seed radiation field (14) is in the high mode.

Radiation field generating unit (10) according to one of the preceding claims, characterized in that said first source (12) emits in the high mode said first seed radiation field (14) and during operation of said first source (12) in said high mode the value of the intensity of said first seed radiation field (14) remains constant.

Radiation field generating unit (10) according to one of the preceding claims, characterized in that said first source (12) emits in the high mode said first seed radiation field (14) and during operation of said first source (12) in said high mode the value of the intensity of said first seed radiation field (14) varies periodically in time.

Radiation field generating unit (10) according to one of the preceding claims, characterized in that a repetition rate of said first seed radiation field (14) emitted by said first source (12) in the high mode is larger than 1 MHz.

Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second source (16) comprises an emitter (72) of a second initial seed radiation field (74) and a modulator (76), which modulates said second initial seed radiation field (74), in particular its intensity and/or its propagation direction, and thereby generates said second seed radiation field (18).

Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second source (16) is switchable between a high mode and a low mode, with the intensity of said second seed radiation field (18) is lower in said low mode than the intensity of said second seed radiation field (18) is in the high mode.

10. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second source (16) emits in the high mode said second seed radiation field (18) and during operation of said second source (16) in said high mode the value of the intensity of said second seed radiation field (18) remains constant.

11. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second source (16) emits in the high mode said second seed radiation field (18) and during operation of said second source (16) in said high mode the value of the intensity of said second seed radiation field (18) varies periodically in time.

12. Radiation field generating unit (10) according to one of the preceding claims, characterized in that a repetition rate of said second seed radiation field (18) emitted by said second source (16) in the high mode is larger than 1 MHz.

13. Radiation field generating unit (10) according to one of the preceding claims, characterized in that a wavelength (12) of said second seed radiation field (18) differs from a wavelength (I I) of said first seed radiation field (14) by less than 5 %.

14. Radiation field generating unit (10) according to one of the preceding claims, characterized in that at least one of said wavelengths (II, 12) of said seed radiation fields (14, 18) correspond to a peak wavelength (204) of an emission spectrum of said laser active medium (102).

15. Radiation field generating unit (10) according to one of the preceding claims, characterized in that at least one of said wavelengths (II, 12) of said seed radiation fields (14, 18) lies within an interval of wavelengths corresponding to a shoulder (212, 222) of a peak (202) of said emission spectrum of said laser active medium (102).

16. Radiation field generating unit (10) according to one of the preceding claims, characterized in that rates of emission of said laser active medium (102) at said wavelengths (I I, 12) of said seed radiation fields (14, 18) are about the same.

17. Radiation field generating unit (10) according to one of the preceding claims, characterized in that a selection and adjustment of said second seed radiation field (18) comprises one or more of the following factors of influence on a stimulated emission generated by said second seed radiation field (18) which are in particular: the intensity of said second seed radiation field (18), the polarization of said second seed radiation field (18), the absorption of said second seed radiation field (18) by said amplifier (24), the amplification of said second seed radiation field (18) by said amplifier (24), and an optical path length of said second seed radiation field (18) in said laser active medium (102).

18. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second seed radiation field (18) is selected and adjusted such, that said population inversion is kept below an uppermost target level .

19. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second seed radiation field (18) is selected and adjusted such, that said population inversion is kept within a target band, which is bounded by above from a higher target level and bounded by below by a lower target level corresponding to 40% of said higher target level.

20. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second seed radiation field (18) is selected and adjusted such, that said population inversion is kept essentially constant a desired target level, with said population inversion being essentially constant at said desired target level, if it is at said desired target level with deviations from said desired target level being smaller than ± 10 % of said desired target level.

21. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second seed radiation field (18) is selected and adjusted such, that the intensity of said amplified radiation field (28) corresponding to said first seed radiation field (14) is kept below an uppermost target intensity.

22. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second seed radiation field (18) is selected and adjusted such, that the intensity of said amplified radiation field (28) corresponding to said first seed radiation field (14) is kept within a target interval, which is bounded by above from a higher target intensity and bounded by below by a lower target intensity corresponding to 40% of said higher target intensity.

23. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said second seed radiation field (18) is selected and adjusted such, that the intensity of said amplified radiation field (28) corresponding to said first seed radiation field (14) is kept essentially constant at a desired target intensity, with said intensity of said amplified radiation field (28) corresponding to said first seed radiation field (14) being essentially constant at said desired target intensity, if it is at said desired target intensity and deviations from said desired target intensity are smaller than 10 % of said desired target intensity.

24. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said first source (12) and said second source (16) are connected by a coordinating device (22), which coordinates, that only one of said seed radiation fields (14, 18) is introduced into said amplifier (24) at the same time.

25. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said coordinating device (22) switches said second source (16) into said low mode, when said first source (12) is in said high mode and switches said second source (16) into said high mode, when said first source (12) is in said low mode.

26. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said pumping energy (106) which is introduced in said amplifier (24), while one of said sources (12, 16) of one of said seed radiation fields (14, 18) is in low mode, is extracted by the another one of said seed radiation fields (14, 18).

27. Radiation field generating unit (10) according to one of the preceding claims, characterized in that said amplified radiation field (28) is used for micro-structuring of materials.