DE102013005607A1 - Method and apparatus for optically pumping laser amplifiers for generating laser radiation with defined beam characteristics - Google Patents

Abstract

The task was to generate a fixed energy / power generated by a laser and, if necessary, a specific energy area density determined by the laser-induced destruction threshold of the optical components with the least possible effort and without the necessary beam shaping of the laser input radiation, and thereby the energy / power of the pump radiation used to keep as low as possible. According to the invention, the pump radiation (5) for stimulating the population inversion in the active medium (2) for amplifying a laser input radiation (4) from a laser amplifier (3) as a function of both the energy area density of the laser output radiation (6) of the laser amplifier (3) and as a function of the beam properties , in particular beam cross-sectional profile and / or beam geometry and / or beam size, which surface-modulates the laser input radiation (4) to be amplified.

Description

For generating laser radiation with defined parameters, eg. As a particular spectral bandwidth, power or energy or a particular beam profile, often the overall gain of the laser beam is divided into different amplifier stages. A time-defined laser beam (or laser pulse) generated by an oscillator is amplified here by several amplifiers up to a certain energy or power. This principle is unavoidable in laser systems with high energies or average power. The resulting individual amplifier stages consist of at least one active medium, for example solids (doped glasses or crystals), dyes or gases in which the occupation inversion required in the active medium for the amplification is realized by supplying energy, for example in the form of optical pump radiation becomes.

The invention relates in this case to optically longitudinally pumped, unsaturated individual laser amplifiers. The pump radiation necessary for the population inversion in the active medium and thus for the amplification is introduced here along the optical axis into the active medium of the laser amplifier. Due to the arrangement of the optical components, these amplifiers do not have stable resonators and, consequently, have no mode dependence and no preferred beam cross-sectional profile. This type of amplifier is used in particular in high-power or high-energy lasers, for which efficient amplification and a homogeneous energy density distribution are essential in order to avoid damage or even the destruction of the optical components.

However, the laser amplifiers can also be used to amplify and generate laser radiation of any energy, wavelength, bandwidth, and beam profiles.

A major consideration in designing these amplifiers is energy surface density. The beam profile must be chosen such that the energy surface density is as low as possible to deposit a high amount of energy or power in the beam without exceeding the laser-induced damage threshold of the optical components. Also, the ratio of applied pump energy to extracted energy of the amplified laser beam is of considerable importance. For this purpose, an optimal adaptation of the cross-sectional profile of the pumping radiation introduced into the laser medium to the beam cross-sectional profile of the laser radiation to be amplified is necessary.

These requirements have reinforcing effects such. B. contrary to the "spatial gain narrowing". In the case of unsaturated reinforcement, the beam cross-sectional profile of the laser output radiation is very much dependent on the shape and homogeneity of the cross-sectional profile of the pump radiation, which is why a rectangular intensity distribution over the beam cross section of the pump radiation is frequently used for this purpose. However, due to the propagation characteristics of the active medium and the emission and reabsorption of spontaneous laser radiation (fluorescence) within the active medium, this rectangular cross-sectional profile is washed out at the edges resulting in a super Gaussian cross-sectional profile of the pump radiation within of the active medium. The reduced slope leads to an inhomogeneous reinforcement of the edge regions and ultimately to a constriction of the effective gain profile, which is additionally determined by the number of transmission passages through the active medium. Thus, the special adaptation of the laser input radiation to the pump radiation for the beam profile of the amplified laser radiation is essential.

Previous efforts aimed exclusively at the homogenization of the pump radiation. In this case, a rectangular intensity distribution over the beam cross section of the pump radiation is specified as the target, which no longer has high-frequency interference (for example DE 10 2006 039 074 B4 ). High-frequency non-homogeneous cross-sectional profiles of the pump radiation produce significant increases in the energy surface density of the laser output radiation, which can ultimately lead to the destruction of the optical components of the laser amplifier.

To avoid said risk of destruction, on the one hand, the homogenization was developed by means of a microlens array. This technique based on an optical illumination arrangement (e.g. US 4,619,508 A ) is successfully used in many laser amplifiers with incoherent pump light sources or pump light sources with very short time coherence. On the other hand, techniques have been developed on this basis for pump light sources with very high temporal and spatial coherence, which prevent the dominating Talbot effect with the aid of delays of the individual beams, defined by the subapertures of the microlens array (for example US 2011/0038037 A1 ).

Another method for homogenizing the intensity distribution of the pump radiation is a diffractive optical element. Here, a grating is produced on the basis of diffraction in such a way that the pump radiation diffracted at this element results in a nearly homogeneous rectangular cross-sectional profile in one plane ( H. Kiriyama et al .: "High temporal and spatial quality petawatt-class Ti: sapphire chirped-pulse amplification laser system ", Optics Letters 35, 2010, 1497-1499; Internet presence of Silios Technologies, www.silios.com ). It is likewise possible to realize the homogenization with the aid of a rod-shaped optic, optionally consisting of quartz glass, glass or transparent plastic ( DE 10 2006 039 074 B4 ).

However, with all these techniques developed, the special adaptation of the laser input radiation to the pump radiation is not possible. The above-described effect of the "spatial gain narrowing" and the beam cross-sectional profile of the laser input radiation of the laser amplifier determine the properties of the beam cross-sectional profile of the laser output radiation here. To minimize the energy surface density of the laser output radiation, additional techniques for direct shaping of the laser input radiation must be used here. For example, by installing a telescope from two specially manufactured aspheres, an energy surface density minimized beam cross-sectional profile can be generated ( US 3,476,463 A ).

This is also possible with specially made mirrors. However, these techniques for directly influencing the laser radiation have considerable disadvantages. Due to the either transmissive or even reflective construction of the components for influencing the beam cross-sectional profile of the laser radiation, disturbances and aberrations of the spatial, temporal and spectral parameters of the laser output radiation can occur. These may be, for example, wavefront and beam profile deformations, a deterioration in the temporal intensity contrast of pulsed laser systems or else a narrowing or modulation of the spectral bandwidth. It can also lead to a partially significant increase in the accumulated nonlinear phase, which can significantly degrade the quality of a time-limited laser pulse by non-linear effects. Likewise subject to optics for forming the beam cross-sectional profile very high requirements with respect to the spectral reflectance or transmission, the damage threshold and the surface quality, which makes the use of these techniques, also due to the special design of the aspheres or mirrors, very expensive and also very easy to tune. Minimal deviations of the lenses from the ideal position result in poor focusability of the laser output beam due to wavefront aberrations. For these reasons, the use of beamforming techniques in high-energy or high-power laser systems must be avoided at all costs.

The invention is based on the object with the least possible effort and without required beam shaping of the laser input radiation to generate a fixed energy generated by a laser / power and possibly with a determined by the laser-induced damage threshold of the optical system components energy density and thereby the energy / power the laser pump radiation used to be kept as low as possible.

This object is achieved by a method for generating a laser radiation with defined beam properties, in which a pump radiation for exciting the population inversion of the active medium of a laser amplifier for amplifying a laser input radiation via an optical system is coupled into the active medium of the laser amplifier, wherein the pump radiation, if necessary a homogeneous and / or preferably rectangular intensity distribution over the beam cross-section fixed, according to the invention solved in that the pump radiation in addition to a possible Fixbeeinflussung depending on both the energy surface density of the laser output radiation of the laser amplifier and in dependence of the beam properties, in particular beam cross-sectional profile and / or beam geometry and / or beam size, which is surface modulated to be amplified laser input radiation.

This area modulation of the pump radiation is either controlled dynamically as a function of the currently detected beam properties of the laser input and output radiation or implemented statically on the basis of predetected or predetermined beam properties of the laser input and output radiation.

In a corresponding device in which the radiation of a pumping light source for exciting the population inversion of the active medium of a laser amplifier is coupled via an optical system in the active medium of the laser amplifier, wherein in the beam path of the pump radiation, a beam influencing stage for fix setting a homogeneous and / or preferably rectangular Intensity distribution can be located over the beam cross-section, is dependent on the beam characteristics of the laser input and output radiation of the laser amplifier Strahlflächenmodulierendes optical element within the optical system for coupling the pump radiation into the active medium with this for the purpose of their Strahlflächenmodulierung in connection.

Such a beam-surface-modulating element can be, for example, an actively controlled optical modulator whose inputs are each coupled to at least one detector for detecting the current beam characteristics of the laser input and output radiation of the laser amplifier.

It is also possible to provide a transmissive and / or reflective filter as a radiation-beam-modulating element, the filter properties of which are matched to predetected beam properties of said laser input and output radiation of the laser amplifier.

Further expedient implementation possibilities for the proposed area modulation of the pump radiation are specified in the subclaims.

The spatial surface modulating static or dynamic modulator to be used according to the invention is positioned within the pump radiation. Here, the requirements for the optical elements with respect to the bandwidth, destruction resistance, transmission or reflectivity and also the surface quality due to the properties of the pump radiation are comparatively low. Thus, the proposed special area modulation of the pump radiation to an advantageous low effort, cost and procedurally well implemented method that can be implemented without modification of existing beam architectures with low adjustment effort into existing systems. The lower requirements are also advantageous in terms of life. The area modulator used in no way affects the laser input radiation to be amplified, since it is not transmitted at any time to the spatially modulating modulator or reflected at it. It follows that due to the modulator possibly to be expected interference and aberrations of the spatial, temporal and spectral parameters of the laser output radiation, such as wavefront and beam profile deformations, a deterioration of the temporal intensity contrast of pulsed laser systems or a narrowing or modulation of the spectral bandwidth, as well the influence of the accumulated nonlinear phase can be completely avoided. Due to the often very small modulation depths of the area modulation according to the invention in the percentage range, the loss of the gain, and thus the output energy or output of the laser output beam, is also limited to a minimum. Furthermore, the production of at least the transmissive or reflective filters in high quantities is very reproducible and possible in almost any size. This makes this method inexpensive, very reliable and almost arbitrarily scalable.

The invention will be explained below with reference to exemplary embodiments illustrated in the drawing.

Show it:

1 : Block diagram for laser amplification with inventive beam surface modulation using an actively controlled surface modulator within the optical system for coupling the pump radiation into the active medium

2 : Block diagram according to 1 with additional and known per se fixed influencing of the pump radiation

• a) Darstellung der Querschnittprofile der Pumpstrahlung innerhalb des aktiven Mediums 1.1) bzw. 1.2) Querschnittprofil ohne Modulation 2.1) bzw. 2.2) Querschnittprofil mit Modulation
• b) Darstellung der Querschnittprofile der Laserein- und ausgangsstrahlung 1.) Querschnittprofil der Lasereingangsstrahlung 2.1) bzw. 2.2) Querschnittprofil der Laserausgangsstrahlung ohne Modulation 3.1) bzw. 3.2) Querschnittprofil der Laserausgangsstrahlung mit Modulation

3 : Representation of beam properties / cross-sectional profiles of the laser and pump radiation

• a) Representation of the cross-sectional profiles of the pump radiation within the active medium 1.1 ) respectively. 1.2 ) Cross section profile without modulation 2.1 ) respectively. 2.2 ) Cross-section profile with modulation
• b) Representation of the cross-sectional profiles of the laser input and output radiation 1 .) Cross-sectional profile of the laser input radiation 2.1 ) respectively. 2.2 ) Cross-sectional profile of the laser output radiation without modulation 3.1 ) respectively. 3.2 ) Cross-section profile of the laser output radiation with modulation

4 : Block diagram for laser amplification with inventive beam surface modulation using a transmissive and / or reflective filter within the optical system for coupling the pump radiation into the active medium.

In 1 creates a pump light source 1 one for the population inversion within the active medium 2 a laser amplifier 3 for amplifying a laser input radiation 4 necessary pump radiation 5 , The pump light source 1 In addition, it has an integrated homogenizing unit, which is not explicitly shown in this figure, which contains the pump radiation 5 Homogenized to a super-Gaussian-shaped cross-sectional profile of high order in a jet plane.

By means of an optical system 6 is the homogenized in a plane pump radiation 5 into the active medium 2 of the laser amplifier 3 coupled. The optical system 6 contains a not explicitly shown telescope, which the pump radiation 5 the pump light source 1 from the plane in which the pump radiation 5 homogenized into the active medium 2 maps. The resulting cross-sectional profile of the pump radiation 5 within the active medium 2 corresponds to a super-Gaussian profile low order (see. 3a - 1.1 respectively. 3a - 1.2 ). The size of the super-Gaussian cross-section profile of the pump radiation 5 in the active medium 2 as well as the size of the Gaussian laser input radiation 4 (see. 3b - 1 ) is chosen so that assuming an equally large super Gaussian beam cross-sectional profile of a laser output radiation 7 the maximum energy surface density, determined by the laser-induced Damage threshold of the optical components of the laser amplifier 3 , is not exceeded due to the fixed laser output energy to be reached. The size of the cross-sectional profile of the pump radiation 5 in the active medium 2 is due to the magnification of the imaging telescope-containing optical system 6 set. Subsequently, the laser input radiation 4 through the laser amplifier 3 and by the pump radiation 5 generated population inversion in the active medium 2 to the laser output radiation 7 strengthened.

Following are both those in the laser amplifier 3 coupled laser input radiation 4 as well as its laser output radiation 7 by means of detectors ( 8th . 9 ), such as CCD cameras, spatially measured. The beam cross-section profile of the amplified laser output radiation 7 corresponds in the center of the Gaussian laser input radiation 4 , but at the edge of the super-Gaussian pump radiation 5 (see. 3b - 2.1 respectively. 3b - 2.2 ). This resulting beam cross-sectional profile of the laser output radiation 7 does not have the lowest possible energy surface density, which limits the output energy to be achieved. The energy surface density is calculated from the fixed laser output energy to be achieved and the measured beam cross-sectional profile of the laser output radiation 7 calculated and minimized in the following.

Both by the detector 8th measured beam properties of the laser input radiation 4 , which in this example is assumed to be Gaussian (cf. 3b - 1 ), as well as by the measurement of the beam properties of the laser output radiation 7 by means of a detector 9 certain energy area density is applied to a modulation calculation stage 10 sent, which by means of suitable algorithms to be adjusted area modulation for the pump radiation 5 certainly. The modulation calculation stage 10 is the output side with a light modulator 11 which is within the optical system 6 for coupling the pump radiation 5 into the active medium 2 and the calculated area modulation to be set in the plane of the homogenized pump radiation 5 impresses.

In the case of a Gaussian-shaped input beam profile and a super-Gaussian target output beam profile, the area modulation to be set corresponds to a parabolic attenuation of the pump radiation 5 in the center of the cross section profile (cf. 3a - 2.1 respectively. 3a - 2.2 ).

Subsequently, the laser input radiation 4 again with the laser amplifier 3 and by the pump radiation 5 generated population inversion in the active medium 2 according to the modulated cross-sectional profile of the pump radiation 5 to the laser output radiation 7 strengthened. Again, in the modulation calculation stage 10 the energy surface density from the through the detector 9 measured beam profile of the laser output radiation 7 , from which (already known) through the detector 8th measured beam profile of the laser input radiation 4 and the fixed laser energy to be determined as well as a controlled variable to the light modulator 11 Posted. If there is a change in the beam parameters of the laser input radiation 4 before, so this must also again by means of detector 8th be measured.

The light modulator 11 again modulates the cross-sectional profile of the pump radiation 5 based on the currently output data of the modulation calculation stage 10 , These steps are repeated until the value of the energy surface density of the laser output radiation 7 reaches a minimum value and does not change anymore.

If the minimum value of the energy surface density is reached, the beam cross-sectional profile of the laser input radiation is 4 on the pump radiation 5 optimally adapted. The resulting beam cross section of the laser output radiation 7 corresponds in this case to the required super Gaussian beam profile (cf. 3b - 3.1 respectively. 3b - 3.2 ), which corresponds to an optimal energy surface density at a fixed energy.

Following this area modulation of the pump radiation 5 becomes the magnification of the optical system consisting of the imaging telescope 6 chosen such that the energy surface density of Laseraungangsstrahlung 7 close to, but not above, the damage threshold of the optical components of the laser amplifier 3 lies. This adjustment of the magnification takes place via the selection of the focal length of the second lens of the imaging telescope from the optical system 6 and affects the adaptation of the laser input radiation 4 to the pump radiation 5 not for small changes.

For larger changes in the focal length of said second telescope lens, for example due to a faulty initial estimate of the size of the cross-sectional profile of the pump radiation 5 or the laser input radiation 4 , the determination of the area modulation, however, must be made again. By minimizing the area of the pump radiation 5 in the active medium 2 At constant energy density, the energy of the pump radiation is thus 5 Minimized according to the invention

2 shows a block diagram for laser gain according to 1 with the difference that the means known per se for the fixed influence of the pump radiation 5 For example, to produce a homogeneous rectangular cross-sectional profile the pump radiation 5 , not directly into the pump light source 1 integrated, but outside of this as an optical element 12 in the beam path of the pump radiation 5 are arranged. The means and procedure for the inventive surface modulation of the pump radiation 5 are as in the exemplary embodiment 1 described.

It is also possible to use the procedure described for surface modulation of the pump radiation 5 not with an actively controlled optical light modulator 11 but the beam characteristics of the laser input radiation 4 as well as the laser output radiation 7 to determine in the described type of the aforementioned embodiments and in the beam path of the pump radiation 5 as surface modulating element according to the invention a transmissive and / or reflective filter 13 in the optical system 6 (please refer 4 ).

The filter properties of this transmissive and / or reflective filter 13 for the area modulation of the pump radiation 5 are due to the said predetected beam characteristics of the laser input and output radiations 4 . 7 of the laser amplifier 3 adjusted previously by the modulation calculation stage 10 were determined (by dashed arrow 14 symbolized).

As surface modulating element (transmissive and / or reflective filter 13 ) is recommended during the iterative algorithm for minimizing the energy surface density of the laser output radiation 7 in particular, a filter variant which can be produced quickly, for example a printable optical film. With the produced by the fast-to-produce printed optical film area modulation of the pump radiation 5 Again, the energy surface density of the laser output radiation 7 after that in the embodiment too 1 minimized described iterative process. For this purpose, a repeated production of the surface modulating element (filter 13 ), which explains the advantages of the fast printable film. Was, as described above, a minimum reached, it can be used as the finally used surface modulating element z. B. a transmission filter by means of a layer system applied to a substrate (not explicitly shown in the drawing) corresponding to that in the modulation calculation stage 10 determined modulation produced and used as a filter 13 into the plane of homogenized pump radiation 5 be inserted.

Again, by adjusting the magnification of the optical system including the imaging telescope 6 the size of the cross-sectional profile of the pump radiation 5 in the active medium 2 and thus, at a constant energy density, the energy of the pump radiation 5 be minimized.

LIST OF REFERENCE NUMBERS

1

Pump light source

2

active medium of the laser amplifier 3

3

laser amplifier

4

Laser radiation input

5

pump radiation

6

optical system

7

Laser output

8, 9

detector

10

Modulation calculation stage

11

light modulator

12

optical element

13

filter

14

arrow display

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102006039074 B4 [0006, 0008]
• US 4619508 A [0007]
• US 2011/0038037 A1 [0007]
• US 3476463 A [0009]

Cited non-patent literature

• H. Kiriyama et al .: "High temporal and spatial quality petawatt-class Ti: sapphire chirped-pulse amplification laser system", Optics Letters 35, 2010, 1497-1499; Internet presence of Silios Technologies, www.silios.com [0008]

Claims (10)

A method for generating a laser radiation with defined beam properties, in which a pump radiation for exciting the population inversion of the active medium of a laser amplifier for amplifying laser input radiation via an optical system is coupled into the active medium of the laser amplifier, the pump radiation optionally with respect to a homogeneous and / or Preferably rectangular shaped cross-sectional profile is influenced, characterized in that the pump radiation in addition to a possible Fixbeeinflussung depending on both the energy surface density of the laser output radiation of the laser amplifier and depending on the beam properties, in particular beam cross-sectional profile and / or beam geometry and / or beam size, the laser input radiation to be amplified surface is modulated , A method according to claim 1, characterized in that the area modulation of the pump radiation is controlled dynamically as a function of the currently detected beam properties of the laser input and output radiation. A method according to claim 1, characterized in that the area modulation of the pump radiation is statically effected by an optical filter whose filter properties are made on the basis of pre-detected or predetermined beam characteristics of the laser input and output radiation. A method according to claim 1, characterized in that the area modulation of the pump radiation is iteratively repeated in response to the beam characteristics of the laser input radiation and the laser output radiation generated by the laser amplifier and the respective area modulation influenced pumping laser radiation until the value of the energy surface density of the laser output radiation reaches a minimum value , Device for generating a laser radiation with defined beam properties, in which the radiation of a pump radiation source for pump excitation of the population inversion of the active medium of a laser amplifier is coupled via an optical system in the active medium of the laser amplifier, wherein in the beam path of the pump radiation, a beam influencing stage for fix setting a homogeneous and / or preferably rectangular cross-sectional profile, characterized in that one of the beam properties of the laser input and output radiation ( 4 . 7 ) of the laser amplifier ( 3 ) influenced surface modulating optical element ( 11 . 13 ) with the pump radiation ( 5 ) for the purpose of their surface modulation is related. Device according to claim 5, characterized in that as the pump radiation ( 5 ) surface modulating optical element an actively controlled optical light modulator ( 11 ) is provided, the input side via a modulation calculation stage ( 10 ) with at least one detector ( 8th . 9 ) for detecting the current beam properties of the laser input and output radiation ( 4 . 7 ) of the laser amplifier ( 3 ) is coupled. Device according to claim 5, characterized in that as the pump radiation ( 5 ) surface modulating optical element a transmissive and / or reflective filter ( 13 ) is provided whose filter properties on voretektierte beam properties of the laser input and output radiation ( 4 . 6 ) of the laser amplifier ( 3 ) are adjusted. Device according to claim 7, characterized in that as the pump radiation ( 5 ) surface modulating optical element ( 13 ) is provided a printed optical film, the pressure pattern representing the determined for the Strahlflächenmodulierende effect of the pump radiation filter properties of the optical film. Device according to claim 7, characterized in that as the pump radiation ( 5 ) surface modulating optical element ( 13 ) is provided on a substrate applied dielectric layer system. Device according to claim 7, characterized in that as the pump radiation ( 5 ) surface modulating optical element ( 13 ) is provided with a gray filter adapted with a filter properties determined for the Strahlflächenmodulierende effect of the pumping radiation gray filter.

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