WO1996028867A1

WO1996028867A1 - Solid-body-laser device

Info

Publication number: WO1996028867A1
Application number: PCT/DE1996/000505
Authority: WO
Inventors: Adam Dombi
Original assignee: Las - Laser Analytical Systems Gmbh
Priority date: 1995-03-15
Filing date: 1996-03-14
Publication date: 1996-09-19
Also published as: DE19510713C2; DE19510713A1

Abstract

A solid-body laser device with a substantially rod or plate-shaped laser body (1) with a longitudinal axis (A), a resonator (4, 5) fitted axially parallel to the longitudinal axis of the laser body and a pump light source to excite the laser body, in which means (2, 3) for setting a predetermined temperature profile along the longitudinal axis are allocated to the laser body in such a way that, when the device is operating, there are periodically alternating regions of higher and lower laser body temperature.

Description

Solid state laser device

Description

The invention relates to a solid-state laser device of the type specified in the preamble of claim 1.

Solid-state lasers, which contain crystals or glasses as the laser-active medium, belong because of their relatively simple Chen construction and the high achievable pulse power to the most important laser types in practice.

They are pumped with lamps of different gas fillings and designs, light-emitting diodes or with lasers - for example semiconductor lasers - the pumping light source being chosen so that the main part of the emitted radiation lies in the spectral range of the absorption bands of the laser medium. For this reason, the use of near-infrared (NIR) -emitting semiconductor lasers for pumping YAG (ttrium aluminum garnet) solid-state lasers, which can be precisely matched to the main absorption bands, can be very efficient from an energy point of view. In this way, a pump light utilization (expressed by the so-called "slope efficiency" - cf. the explanations further below) of 70 to well over 90% is achieved. These pump light sources are therefore becoming increasingly widespread in the so-called DPSS (diode pumped solid-state) laser arrangements.

The increasing use and the excellent market prospects to be expected in the future of the miniaturized DPSS-YAG laser also result from the possibility, with simple construction, of high performance in pulse operation with a high repetition frequency as well as in continuous operation and an almost ideal Gaussian To achieve beam profile.

Here, however, a serious problem arises with the so-called "thermal lensing", the uncontrolled formation of lens-like refractive index gradient areas and possibly (in the case of high volume-specific pumping power) also birefringent areas in the laser body leads to an uncontrolled refraction of the laser beam and thus to a severe deterioration in the beam quality.

The effects of "thermal lensing" are particularly unacceptable in connection with a frequency multiplication (especially doubling) of the laser radiation generated by non-linear effects, because the frequency doubling efficiency and thus the achievable output power with decreasing beam quality at the desired final frequency are strong sinks.

Attempts have been made to use laser crystals with a trapezoidal longitudinal section (so-called slab geometry), in which the beam does not verge straight along a crystal longitudinal axis, but after entering the crystal at a Brewster angle zigzag in a plane between two opposite circumferential surfaces ¬ runs to compensate for the disturbing influence of "thermal lensing". A further development of this principle is described in EP 0 301 526 B1, according to which the beam propagates helically in the crystal with sequential reflection in each case on all circumferential surfaces. However, the processing of the crystals is very complex, the compensation effect has proven unsatisfactory and the laser threshold increases noticeably.

In the unpublished German patent application Az.

P 44 02 688.4, the inventor of the present application proposed that, in the case of a transversely pumped DPSS laser, the pumping light in the direction of the longitudinal axis of a rod-shaped to modulate the intensity of the laser crystal, whereby alternately arranged regions with high and low energy density are to be produced in the crystal. This procedure is not feasible in the case of front-end pumped arrangements and requires complex control and control electronics for reliable implementation over a longer operating period (during which parameter changes or failures of pump light sources must be expected).

The invention is therefore based on the object of creating a solid-state laser device of the type mentioned at the outset, with which a simplified suppression of the uncontrolled "thermal lensing" which can be flexibly adapted to different device configurations is achieved.

This object is achieved by a solid-state laser device with the features of claim 1.

The invention includes the idea of alternating areas with higher and lower temperatures by targeted local control of efficient heat dissipation from the laser medium and / or local cooling and / or additional heating in the laser body (rod or plate) in the longitudinal direction, ie in the beam travel direction produce. As a result, refractive index gradients with different signs are built up alternately in the material. After passing through it, the beam shape of a beam that propagates in the longitudinal direction of the crystal is opposite the beam shape of a beam as it is in the material without thermal effects there were no significant differences because the lensing-related beam deflections or deformations at the individual refractive index gradients essentially cancel each other out over the entire length of the laser body.

This idea has many possible forms in special solid-state laser configurations, also beyond the transverally pumped miniaturized DPSS laser.

As a means for setting a predetermined temperature profile, an arrangement can be provided on at least one peripheral surface of the laser body for highly effective heat dissipation from the laser body to the environment (especially the air), which alternates periodically in the direction of the longitudinal axis of the laser rod Has sections with higher and lower heat transfer resistance.

This arrangement can be formed in a simple and inexpensive manner, essentially by at least one metallic heat-conducting body with periodically alternating projections and recesses, in which the projections face the laser body and rest in good thermal contact on the peripheral surface thereof. while the cutouts each form a spacing area with poor thermal contact with the laser body. Instead of the spacing areas, heat-insulating areas can also be provided between the areas with high thermal conductivity. Instead of a specially designed heat dissipation arrangement or in addition to this, an active heating and / or cooling device can also be provided on the circumferential surface of the laser body, which is designed such that it periodically alternates areas in the direction of the longitudinal axis of the laser body generated with a higher and lower initial temperature and thus also temperature gradients inside the material.

For this purpose, on the one hand, an active, in particular electrically operated, cooling device with cooling elements or areas arranged at intervals in the direction of the longitudinal axis of the laser body can be provided. This can be, for example, a row arrangement of Peltier elements.

On the other hand, the alternating T gradients can also be generated by an, in particular electrically operated, additional heating device with heating elements or regions arranged at intervals in the direction of the longitudinal axis of the laser rod - of course in combination with an effective heat dissipation device.

A combination of a heating device with a cooling device with alternating contact areas to the laser body is also possible.

Finally, an electrically operated, combined heating / cooling device with cooling and heating areas arranged alternately in the direction of the longitudinal axis of the laser rod can be provided as a particularly simple, effective and inexpensive arrangement. The special arrangements mentioned can each be arranged individually or in combination - on two mutually opposite side surfaces of the laser body.

The laser body is in particular a miniature laser crystal made of Nd- or Yb-doped YAG with a cuboid shape or at least one flat surface facing the heat dissipation body or the cooling and / or heating element. In the case of a cuboid crystal, the dimensions can be approximately 20x20x5 mm, for example.

In a - practically preferred - transversely pumped arrangement, at least one pump light source is arranged laterally from the laser body (transversely) and radiates into it essentially perpendicular to the direction of the longitudinal axis. Preferably, however, a plurality of NIR semiconductor lasers, in particular on both sides of the laser body, are provided as pump light sources. Optical fibers for guiding the pump light are preferably provided between the light exit surfaces of the laser diodes and the solid-state laser body. This enables a very compact design of the pump unit as a laser diode-optical fiber block.

In order to realize a compact and at the same time geometrically highly stable laser device and for technological simplification, at least the laser body and the resonator with the arrangement for heat dissipation and / or the heating and / or cooling device are preferably constructed as a coherent compact unit. Using commercially available cooling arrays, the laser body per and the resonator advantageously on a - in particular double cascaded - Peltier element arrangement with Al ₂ 0 ₃ _ base plate, mounted as a support by means of SMD (surface mounting) technology, especially soldered onto the support or glued on in a heat-conductive manner.

The side surfaces of the laser body, on which no pump light is coupled in, are metallized for efficient use of the pump light, but are not polished to suppress parasitic modes. The pump light coupled in via the other side surface (s) can be diffusely reflected here several times, but parasitic modes cannot be reflected and amplified. The arrangement can therefore be simplified due to the elimination of a mode diaphragm (which would also disadvantageously block out part of the beam power generated and thus destroy it).

The arrangement according to the invention can be used particularly advantageously for solid-state laser devices with frequency multiplication, in particular doubling, in which a frequency multiplier element, in particular a frequency doubler crystal (KDP = potassium diposphate, KTP = potassium titanyl phosphate, additionally) in the laser cavity (ie between the resonator mirrors). Lithium niobate or the like) with nonlinear optical properties is provided.

Since the stability of the dimensions and the relative position of the components are particularly important in such an arrangement, the above-mentioned design as a compact unit is particularly advantageous here, with the Frequency doubler crystal is integrated in the coherent compact unit. The end face of the frequency doubler crystal facing away from the laser body can in particular at the same time be designed as a fully reflecting resonator mirror surface and the end face of the laser body facing away from the frequency doubler crystal can be designed as a coupling-out mirror. This arrangement is - although it places high demands on the quality of the surface treatment of the crystal end faces - technologically relatively easy to implement, and the amount of complex adjustment work is minimized.

The laser device constructed as a compact assembly expediently has a temperature sensor for detecting the temperature, a control unit connected to its output and a heating and / or cooling device connected to its output for keeping the temperature of the assembly constant in order to avoid changes in geometry caused by temperature fluctuations would lead to a deterioration in the laser parameters.

The arrangement according to the invention can expediently be implemented in connection with a pump arrangement with which the laser crystal is illuminated as homogeneously as possible. For this purpose, diffuser elements can be incorporated in the laser crystal on the side surfaces, via which the pump light is coupled in, which act in the manner of the integrating sphere.

On the other hand, it can advantageously also be implemented with a pump arrangement with which in the direction of Areas of higher and lower luminance are periodically alternately generated in the laser body longitudinal axis. It is important in any case that the temperature field in the laser crystal can be set to be stable over a long period.

Optimized operation of such a laser device is possible if a processing device is provided which is connected on the output side to a control input of the heating and / or cooling device and / or the control device for the pumping light source (s) and via which a predetermined temperature profile and / or a predetermined luminance distribution in the direction of the longitudinal axis of the laser body is set or maintained.

In an advantageous embodiment, means for shaping the beam cross section can be provided in the laser resonator, with which an essentially circular cross section can be formed from the beam cross section which is usually not a priori circular in the YAG laser. In the simplest case, this can be a so-called mode diaphragm (in which, however, the optimization of the beam cross section is purchased with a loss of output power that can be coupled out), or a suitably shaped refractor element can be used.

An even improved frequency or mode characteristic can be achieved by the additional provision of a so-called "seeder", ie a laser diode with an optical arrangement for the longitudinal coupling of the radiation into the (solid-state) laser body through the rear-view mirror of the resonator. So that Radiation from the solid-state laser, which without such an arrangement has a plurality of modes within a bandwidth of approximately 4.4 GHz, is essentially locked to one mode or frequency ("injection locking").

Advantageous developments of the invention are characterized in the subclaims or are shown in more detail below together with the description of the preferred embodiment of the invention with reference to the figures. Show it:

FIG. 1 a shows a schematic diagram (in longitudinal section) of an arrangement of laser body and heat dissipation body in accordance with an embodiment of the invention,

FIG. 1b shows a basic illustration (in longitudinal section) of an arrangement comprising a laser body and a cooling arrangement according to a further embodiment of the invention,

FIG. 1 c shows a basic illustration (in longitudinal section) of an arrangement comprising a laser body, additional heating and heat dissipating body according to a further embodiment of the invention,

FIG. 1d shows a schematic diagram (in longitudinal section) of an arrangement comprising a laser body and a combined heating and cooling arrangement according to a further embodiment of the invention,

FIG. 2 shows a schematic diagram to explain the “thermal lensing” effect in the prior art,

3a shows a simplified longitudinal sectional illustration of a structural shape of the arrangement according to FIG. FIG. 3b shows a simplified top view of the design according to FIG. 2a,

FIG. 3c shows a simplified cross-sectional illustration of the design according to FIG. 2a,

FIG. 4 shows a simplified overall perspective view of a solid-state laser device according to an embodiment of the invention,

FIG. 4a shows a detailed illustration of a modification of the device shown in FIG. 4,

FIG. 5 shows a simplified overall perspective view of a solid-state laser device according to a further embodiment of the invention (with frequency doubling),

FIG. 5a shows a detailed illustration of a modification of the device shown in FIG. 5,

FIG. 6 shows a basic illustration of a modification of the embodiments shown in FIG. 4 or FIG. 5,

FIG. 7 shows a simplified block diagram of a measuring and control circuit for a solid-state laser device according to an embodiment of the invention and

FIG. 8 shows a graphic illustration to illustrate an advantageous effect of the device according to the invention. FIG. 1 a shows a basic illustration (in the longitudinal section) of an Nd-doped YAG laser crystal 1 which is rectangular in the longitudinal section, each with a metal body 2 and 3 arranged on the upper side la and the lower side 1 b - shown only in part in the figure made of solid aluminum for heat dissipation from the laser crystal to the surrounding atmosphere.

The heat dissipation bodies 2, 3 of the same construction each have a profiled surface in the laser crystal surfaces la or lb facing the surface with slots 2a or 3a which are rectangular in cross section, between which regions 2b and 3b of the heat dissipation bodies 2 which are in contact with the laser crystal or 3 are arranged. In the contact areas 2a *, 3a 'of the slits 2a, 3a there is no physical contact and thus there is a greatly reduced heat conduction between the laser crystal and the compared to the contact areas 2b', 3b 'of the projecting sections 2b, 3b with the laser crystal 1 Heat dissipation bodies 2, 3.

Between the rear end face 1c (left in the figure) provided with a completely reflective coating - the first resonator mirror - 4 and the front end face (right in the figure) with a partially reflective - forming the second resonator mirror - Coating 5 provided end face ld of the laser crystal forms a laser beam (bundle) with a wavelength in a known manner above a threshold power when pump light is irradiated - for example from InAlAs semiconductor laser diodes arranged laterally (not shown in this figure) in the figure from 1064 nm. In the figure, a partial beam R ₀ lying slightly above the longitudinal axis A of the crystal, ie having a so-called offset, is shown.

The irradiated pump light heats the laser crystal 1 above the ambient temperature, so that heat is dissipated to the environment via the upper and lower side surfaces 1 a, 1 b and the heat dissipation bodies 2, 3 adjacent to them.

As a result of the very different heat conduction between the laser crystal and the heat dissipation bodies in the areas 2a ', 3a' on the one hand and 2b ', 3b ¹ on the other hand, the heat dissipation bodies are modulated in the direction of the longitudinal axis A of the laser crystal 1 in accordance with the geometric shape of the surfaces Dissipation of heat energy. As a result, a temperature field of the shape shown in dashed lines in the figure is formed in the laser crystal 1, which is characterized by a periodicity of isotherms 1 ^, I ₂ , I ₃ which are approximately elliptical in cross section in the direction of the longitudinal axis A.

As a result of the temperature dependency of the refractive index of the laser medium and - at least with higher pump powers - the occurrence of birefringence effects, a spatial distribution of the refractive indices corresponding to the shape of the temperature field and having areas R _lr R ₂ , R _{3 of the} same refractive index is formed (a kind of " Refractive index field ") in the laser crystal.

As is illustrated schematically in the figure by the course of the ray R ₀ , the spatial distribution shown causes - 15 -

The refractive indices periodically alternate deflections of the beam in alternating directions with respect to the longitudinal axis A, so that with a corresponding shape of the refractive index field within the laser crystal 1 on the end faces 1c, 1d, a beam orientation parallel to the longitudinal axis A results. If one considers the axially offset beams in their entirety, this leads to the formation of a beam bundle with a high degree of parallelism and phase coherence and an advantageously almost ideal Gaussian energy distribution in the beam cross section.

The specified arrangement thus suppresses the disadvantageous effect of "thermal lensing" in conventional solid-state laser arrangements, as is sketched in FIG. 2.

Fig. 2 shows a laser crystal 1 ^! with conventional, along the side surfaces la ', lb' essentially uniform heat dissipation and the course of the central jet R _m ι and an off-axis jet R _Q 'of isotherms ^I I'_' ^I 2 ¹ ' - ^ 3 '' which also here Areas R ', R', R ₃ 'correspond to the same refractive index. The schematically illustrated beam course illustrates the deviation of the course of the off-axis beam from the direction of the longitudinal axis A "of the laser crystal as a result of the uncontrolled thermal lens, which is also associated with phase shifts and thus a deterioration in the coherence of the laser radiation.

FIG. 1b shows (again in longitudinal section) a schematic diagram of a further arrangement made of a laser crystal l and a cooling arrangement, which is formed here from a row of Peltier elements 6.1 to 6.4 or 6.5 to 6.8 arranged at predetermined intervals above and below the laser crystal.

The Peltier elements cool the laser crystal at the contact surfaces with its upper or lower side surface la or lb, so basically they functionally take the place of the projections of the heat dissipation bodies 2, 3 facing the laser crystal in FIG. To simplify the figure, it is assumed that the resulting temperature field and thus the spatial distribution of the refractive index also correspond to the conditions in FIG. In the case of the device according to FIG. 1b, of course, if the cooling elements 6.1 to 6.8 are controlled separately, if necessary, by means of a suitable choice of the voltages of the individual cooling elements, a more differentiated and, above all, time-dependent control of the shape of the T field can be achieved and thus the beam path of the off-axis laser beam R ₀ .

This makes it possible to readjust the beam path and the phase relationships in the laser device in an electrical-thermal way without changes to the structure. This enables the elements to be fixed in a technologically advantageous manner immediately during the manufacture of the device.

This advantage also has the arrangement shown in FIG. 1c - again as a basic illustration in longitudinal section - of laser body 1, heat dissipation bodies 2 'and 3' and one Additional heating from five in the heat dissipation body 2 'and five in the heat dissipation body 3' arranged electrical heating elements 7.1 to 7.10 according to a further embodiment, as far as the heating elements can be controlled individually.

In this embodiment, the surface of the heat dissipation bodies 2 'and 3' is not profiled on the side facing the laser crystal, but can be completely flat, the heating elements being able to be inserted from the side facing away from the laser crystal.

Otherwise, it is assumed in Fig. 1c that the expansion and the spacing of the heating elements correspond to those of the slots in Fig. La and the thermal effect of the heating elements, i.e. the influence on the temperature field in the laser crystal, which is equivalent to the slots of the heat dissipation bodies 2, 3. These simplifying assumptions were, of course, only made with regard to the clarity of the representations. The designer of the laser device will undertake the specific implementation with regard to the geometry and the thermal parameters according to the desired performance parameters. The dimensions and distances of the areas with a higher and those with a lower cooling effect need not all be the same.

FIG. 1d shows a further basic illustration of an arrangement comprising a laser body 1 and a combined heating and cooling arrangement each consisting of four cooling elements 6.1 to 6.4 or 6.5 to 6.8 and five heating elements 7.1 to 7.5 or 7.6 to 7.10 in an upper or one lower heat sink 2 "or 3". Here too is in the drawing provided that the thermal conditions in the laser crystal resulting from the use of the cooling and heating elements correspond to those in the arrangement according to FIG.

However, it is clear that in particular this arrangement with active cooling and heating elements is a far more pronounced differentiation of the T field, i.e. the realization of significantly larger T gradients than the arrangement according to FIG. Here, too, individual control of the active elements enables a suitable shaping of the T field and thus of the refractive index field and thus also readjustment in an electrical-thermal way.

In the devices according to FIGS. 1b to 1d, however, the active elements can also be controlled in groups or all together - operated in series, for example. This simplifies the control and reduces the manufacturing costs, but reduces the possible variations in the formation of the refractive index field in the laser crystal.

FIG. 3a shows a simplified longitudinal sectional illustration of a technologically advantageously realizable type of construction of the basic arrangement according to FIG. La, FIG. 3b a simplified top view and FIG. 3c a simplified cross-sectional illustration (perpendicular to the longitudinal axis of the laser crystal) of this type.

An aluminum oxide substrate 10 with surface metallization (in-plating) 10a serves as a carrier, on which a lower rer (passive) heat sink 11 and an upper (passive) heat sink 12, each made of solid copper, are soldered. The heat sinks 11, 12 enclose between them the flat cuboidal Nd: YAG or YbrYAG laser crystal 1 ". The surfaces of the heat sinks 11, 12 facing the laser crystal are, in the manner explained above, also in the longitudinal axis direction of the crystal (x direction ) profiled slots 11a and 12a and projections 11b and 12b which extend perpendicularly to it (in the y direction) and protrusions 11b and 12b on each surface The slots 12a of the upper heat sink 12 have a smaller depth here than that of the lower heat sink 11.

3a to 3c, the resonator mirrors are not attached to the end faces of the laser crystal 1 ", but it is assumed that there are externally arranged mirrors (not shown in these figures). Accordingly, both end faces of the laser crystal penetrating axis-offset beam R ₀ "is shown, which enters and exits through special openings 12d and 12e of the upper heat sink 12.

The upper and lower peripheral surfaces la "and lb" of the laser crystal 1 "are not polished, but are provided with a reflective metallization la / M" or lb / M ".

3b and 3c, the ends of the optical fibers facing the laser crystal 1 "are also two lateral ones

Pump light supply arrangements 13 and 14 shown, and the beam path of the individual pumping light beams is sketched (with dashed lines). 3c clearly shows that the pump light between the top and bottom metallization la / M "or lb / M" of the laser crystal is often reflected and is therefore used extremely efficiently. As mentioned above, the non-polished, optically imperfect surface, however, at the same time prevents modes of higher order and in particular parasitic modes from being amplified and being able to impair the beam quality.

In the example shown, the dimensions of the YAG crystal 1 "are approximately (L / W / H) 5/4 / 0.5 mm, those of the lower heat sink 11 are approximately 7/5 / 1.5 mm and those of the upper heat sink 12 (in the bent state) approx. 13/6 / 1.5 mm and the slot widths in the heat sinks approx. 0.5 mm.

FIG. 4 shows a simplified overall perspective view of a solid-state laser device in which the cooling arrangement in the design according to FIGS. 3a to 3c is implemented. The reference numerals used in these figures for the parts shown there are also used in FIG. 4 and these elements are not described again here.

4 - at three pumping light sources - shows more precisely that a laser diode array 15a, 15b, 15c and 16a, 16b, respectively, is at the end of the light guides 13a, 13b, 13c and 14a, 14b, 14c facing away from the laser crystal. 16c is coupled with 1 to 4 W pump power each.

This figure also shows a rear, fully reflecting resonator mirror 17 and a front, semi-reflecting render resonator mirror 18 shown. Both mirrors are soldered to the local metallization 10a ^{1 of} the carrier 10 'via connecting surfaces ("pads" or "legs") 17a, 17b or 18a, 18b which are angled twice and are bendable for adjustment. Furthermore, between the mirrors 17, 18 and the laser crystal 1 ", an element 19 or 20 is provided for beam shaping (z-axis expansion) of the generated laser beam and for reducing transverse modes of higher order, with which a circular beam cross These optical elements are also soldered onto metallization areas of the carrier 10 '.

The carrier 10 'is made of three Al ₂ O ₃ carrier plates 10.1' by a double cascaded cooling arrangement. 10.2 'and 10.3' and an intermediate array 10.4 'and 10.5' of Peltier elements. Such an arrangement is geometrically highly stable in the event of changing ambient temperatures or changing energy input, since the cascading of the Peltier arrangements achieves an automatic compensation of temperature gradients. A temperature sensor 21 is arranged on the surface of the upper plate 10.1 ', by means of which the temperature of the laser device is sensed and with the aid of which this can be additionally regulated by suitable control of the Peltier elements.

4a is a detailed illustration of a configuration of the laser crystal which is particularly advantageous with regard to the use of the pump radiation in the arrangement according to FIG. 4. Here, the laser crystal 1 "'has, in addition to the base and top surface coating lb / M"' or la / M "', for further homogenization of the pump light distribution in the interior of the crystal on the long side surfaces lc"' or ld "', which face the optical fiber groups 14 and 13 and are parallel to the direction of the laser beam R, each have approximately hemispherical depressions lc / S"' or ld / S "'with respect to the ends of the optical fibers. These are incorporated in the laser medium According to the principle of the integrating sphere, depressions act as scattering surfaces with respect to the pumping light beam bundles emerging from the ends of the adjacent optical fibers to adjust the refractive index ("index matching gel"), into which the ends of the optical fibers 14 and 13 are immersed.

FIG. 5 shows a (again simplified) overall perspective view of a further solid-state laser device according to an embodiment of the invention, with which the frequency of the laser beam generated is additionally doubled. The design and arrangement of the laser crystal is the same as in the device described above, so that the same reference numbers are used in this regard.

An essential difference consists in the structure of the pump arrangement, which here is formed with two compact assemblies 22, 23 from an integrated laser diode arrangement with beam focusing. (Such an arrangement is in the The earlier application of the inventor mentioned at the beginning is described in more detail and is incorporated by reference into the subject matter of the present application. ) The radiation from the laser diodes, which are lined up in rows, is bundled into a focal point by the elements for beam focusing; however, the laser crystal 1 "is placed outside the focal length of the pump arrangements 22, 23, so that the pump light radiation is approximately linear over the length of the crystal.

A further essential difference from the laser device according to FIG. 4 is the presence of a KTP (potassium titanyl phosphate) frequency doubler crystal 24, which is arranged beyond the rear element 20 (in the figure on the right) for beam shaping with respect to the laser crystal. The frequency doubler 24 is also soldered onto the upper carrier plate 10.1 'via bendable connection surfaces 24a, 24b, via which an initial adjustment of the position relative to the other optical components is possible. Its end face 24c facing away from the laser crystal has a convex shape with a geometry which is very precisely adapted to the special laser arrangement and has a mirror coating 25, so that it also acts as a rear resonator mirror.

In a similar way, the front resonator mirror is formed here by a partially reflecting mirror coating 26 of the front end face of the laser crystal 1 ″.

With a temperature control of the KTP crystal 24, one can be carried out in a simple manner without mechanical intervention Matching the effective resonator length of the arrangement can be achieved without the need for additional controllable refraction elements (such as a temperature-controlled quartz plate or a piezomechanical tuning device). Alternatively, an element of this type can also be used, which is not shown in the figure. Depending on the specific design, a corresponding control unit must be provided for the thermal or piezo-mechanical tuning of the laser arrangement.

5a is a detailed illustration of a configuration of the laser crystal which is particularly advantageous with regard to the use of the pump radiation in the arrangement according to FIG. 5, only the details which differ from FIGS. 3a to 3c or FIG. 5 being explained below.

Here, the laser crystal 1 "" has a further homogenization of the pump light distribution inside the crystal on the long side faces 1c "" and 1d "", which face the laser diode blocks 22 and 23 (see FIG. 5), each have an approximately semi-cylindrical groove lc / N "" or ld / N "". These depressions worked into the laser medium act - analogously to the hemispherical depressions according to FIG. 4 - with regard to the pump light beam bundles (focused outside the side surfaces of the laser crystal 1 "") according to the principle of the integrating sphere as scattering surfaces.

Features of the arrangements shown as examples in FIGS. 4 and 5 can also be combined with one another; For example, a device with a frequency doubler can also Having one or two separate resonator mirrors, or in the case of a device without frequency doubling, at least one of the resonator mirrors can be formed on an end face of the laser crystal. Both the laser crystal and the frequency doubler can also be made from another material known for the respective purpose: Nd- or Yb-doped glass can be used as the laser material and KDP (potassium diphosphate) or lithium niobate for the frequency doubler be used.

A simple Peltier element arrangement or, if necessary, also simply a good heat-conducting support plate can also be used as the support - to increase the cooling effect also with ribbing or possibly. with liquid cooling - serve. The components can be fixed on the carrier, for example, using a heat-conducting adhesive.

FIG. 6 is a schematic diagram for explaining a further modification of the embodiments of a solid-state laser device explained above, with which a narrowing of the spectral bandwidth and a unification of the phase position of the laser beam generated can be achieved.

The solid-state laser body with a laser wavelength of 1064 nm, designated YAG in this figure, is assigned two resonator mirrors ML1 and ML2 in the usual way. The front, semitransparent mirror ML2 is formed on the convex end face facing away from the solid-state laser body YAG of an intracavitary frequency doubler crystal, referred to here as KTP, the rear mirror ML1 separately. R1 and r2 denote the mirror radii of the resonator mirror, r3 the radius of the front KTP end face. Typically rl is in the range of 10 cm, r2 and r3 are in the order of a few mm. The presence of an antireflective coating and "HR ..." of a highly reflective coating for a specific wavelength is identified on the mirror or end faces by "AR ...".

A collimator lens COL2 is shown extracavitary (in the figure to the left of the front mirror ML2). Other optical elements and the pump arrangement are omitted here for the sake of simplicity. The relationships with regard to the beam frequencies are illustrated by specifying the wavelengths (1064 nm = primary wavelength, 532 nm = wavelength after frequency doubling) at various points in the arrangement. The adjustment after the adjustment is carried out in the manner described above, for example by T-control of the frequency doubler, which is illustrated in the figure by an arrow with the reference symbol TC.

An InGaAs laser diode LAD is arranged to the side of the solid-state laser arrangement and likewise emits at a wavelength of 1064 nm with an output power of approximately 50 mW. After passing through a collimator lens COL1 and two deflecting mirrors M1 and M2, their radiation is coupled longitudinally into the solid-state laser arrangement with a transmission coefficient of approximately 1%. There, this narrow-band radiation effects a frequency and phase locking ("injection locking") of the radiation emitted by the solid-state laser YAG to the radiation of the semiconductor laser LAD, the so-called "seeder". The principle explained in FIG. 6 on the basis of a linear resonator arrangement can in principle also be implemented analogously in the case of a ring resonator arrangement (known per se and therefore not shown separately here), in which case the coupling of the laser diode radiation into the solid-state laser is at an angle its longitudinal axis takes place, which is matched to the geometry of the ring resonator.

With this arrangement, the frequency can be tuned even better, and for the simultaneous tuning of the effective resonator length and phase length of the seeder, in particular a single (not shown) piezomotor-longi-tudinally displaceable plane-parallel quartz plate can be inserted into the beam path Seeder and laser radiation are penetrated obliquely at a small angle.

FIG. 7 is a simplified block diagram of an electrical-thermal measuring and control circuit 100 for a solid-state laser device according to an embodiment of the invention for controlling the beam parameters. This circuit - to be understood in the sense of an example for a large number of possible control circuits - is particularly suitable for temporary use with the laser arrangement, for example for initial adjustment or readjustment after prolonged operation.

The starting point is a cooling arrangement for the laser crystal according to FIG. 1b with four individually controllable pelletizing elements 6.1 to 6.4 above the laser crystal 1 and four likewise individually controllable Peltier elements 6.5 to 6.8 below the crystal. The measuring and control circuit 100 comprises a flat optical sensor arrangement in the form of a CCD matrix 101, which is arranged opposite the front light exit surface of the laser 1 in such a way that an image of the laser beam is generated on it when the laser is in operation. The number of image recording elements in the matrix is selected in accordance with the quality requirements for the laser beam.

The output of the CCD matrix 101 is connected to the input of a preprocessing unit 102 for interference suppression and signal level adjustment. Its output is connected to an input of a comparator unit 103, the other input of which is connected to the data output of a sample image memory 104, in which at least one predetermined laser beam cross section is stored, which is to be used as a basis for the control or setting of the laser device . In the comparator unit 103, the real beam cross-section recorded by the CCD matrix 101 is compared with the pattern beam cross-section and a signal sequence which characterizes the comparison result is provided at the output. Processing width and speed of the units 102 and 103 and the storage capacity of the memory 104 are selected in accordance with the number of image recording elements of the CCD matrix 101.

The signals specifying the result of the beam comparison are fed to the data input of a processing unit 105, which is formed, for example, by a microprocessor and is connected to the output of the comparator unit 103, and which furthermore has a data memory (RAM) 106 and a program memory in the usual way (such as an EPROM) 107. The processing unit can optionally - which is shown by dashed lines - also be connected to the output of a temperature sensor (according to the figure of the sensor 21 from FIG. 4 or 5), so that the temperature of the laser arrangement also enters into the control of the cooling arrangement.

In the processing unit 105, a set of control data for the voltage supply of the individual Peltier elements, which is finally a chip, is calculated by calling up a program and pre-stored data for correcting the T field in the laser crystal 1 by changing the operating voltage of the Peltier elements 6.1 to 6.8 - Power supply unit 108 is supplied for the cooling elements and causes a corresponding setting of the operating voltages and thus the cooling capacities of the Peltier elements. This leads to a shape of the T-field in the laser crystal with which the desired laser beam parameters are achieved. The setting of the voltage supply unit can be locked after the setting process has ended.

In the arrangement described, the processing program for the measured beam data can be an iterative program, the changed beam parameters being continuously detected during the setting and a subsequent iteration step can be used as a basis. A fuzzy logic algorithm is particularly suitable for this.

To display the recorded data and processing results for an operator and to influence the processing In the processing process, the components mentioned are each further connected to a display and input unit 109, for example the screen and keyboard of a conventional PC.

A basically completely analog circuit can be used to control or set an active heating arrangement according to FIG. 1c or a combined cooling and heating arrangement according to FIG. 1d. Furthermore, in a similar manner, as an alternative or in addition to the control of the temperature field, the luminance distribution of the pump light in the solid-state laser can be controlled via the separate control of several pump light sources.

FIG. 8 shows a graphic representation to illustrate an advantageous effect of the device according to the invention.

The laser output power P _o t as a function of the pump power P _j _ _n is shown in a logarithmic representation using typical curve profiles. The dashed line shows a curve profile for a Yb-YAG laser device according to the prior art, while the solid curve shows the Characteristic curve of a device constructed according to an embodiment of the invention. It can be seen that in the latter case, both the increase in the linear region (“slope”) and the achievable maximum output power (based on the threshold power) are significantly higher. The embodiment of the invention is not limited to the preferred embodiment described above. Rather, a number of variants are conceivable which make use of the solution shown, even in the case of fundamentally different types.

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Claims

Expectations

1. Solid-state laser device with an essentially rod-shaped or plate-shaped laser body (1; 1 ") with a longitudinal axis (A), a resonator (4, 5; 17, 18; 25, 26) arranged parallel to the longitudinal axis of the laser body and one Pump light source (15a to 15c, 16a to 16c; 22, 23) for excitation of the laser body,

d a d u r c h g e k e n n z e i c h n e t that

the laser body means (2, 3; 6.1 to 6.8; 7.1 to 7.10; 2 ", 3"; 11, 12) for setting a predetermined temperature profile in the direction of the longitudinal axis in such a way that, in operation of the device, areas of higher height alternate periodically ¬ Herer laser body temperature and areas of lower laser body temperature are assigned.

2. Solid-state laser device according to claim 1, since ¬ characterized in that the means for setting a predetermined temperature profile on a peripheral surface (la, lb; la ", lb"; la / M ", lb / M") of the laser body (1; 1st ") intended arrangement (2, 3; 11, 12) for heat dissipation from the laser body to the environment, which periodically alternate sections (2a, 2b, 3a, 3b; 11a,) in the direction of the longitudinal axis (A) of the laser body, 11b, 12a, 12b) with higher and those with lower heat transfer resistance. 3. Solid-state laser device according to claim 2, since ¬ characterized in that the arrangement for heat dissipation at least one metallic heat conducting body (2,

3; 11, 12) with periodically alternating projections (2b, 3b; 11b, 12b) and recesses (2a, 3a; 11a, 12a), the projections facing the laser body (1; 1 ") and in there is good thermal contact on its peripheral surface, while the recesses each form a spacing area with poor thermal contact with the laser body.

4. Solid state laser device according to one of the preceding claims, characterized in that the means for setting a predetermined temperature profile are arranged on the peripheral surface (la, lb) of the laser body (l), active heating and / or cooling device (6.1 to 6.8; 7.1 to 7.10) which, in the direction of the longitudinal axis (A) of the laser body, periodically alternate areas with higher and lower peripheral temperature.

5. Solid-state laser device according to claim 4, characterized in that an active, in particular electrically operated, cooling device with cooling elements or regions (6.1 to 6.8) arranged at intervals in the direction of the longitudinal axis of the laser body (1) is provided. - 34 -

6. Solid-state laser device according to claim 4, so that a, in particular electrically operated, heating device with heating elements or regions (7.1 to 7.10) arranged at intervals in the direction of the longitudinal axis of the laser body (l) is provided.

7. Solid-state laser device according to claim 4, d a - d u r c h g e k e n n e e c h n e t that an electrically operated, in particular according to the Peltier effect, heating / cooling device is provided with alternately arranged in the direction of the longitudinal axis of the laser body cooling and heating areas.

8. Solid-state laser device according to one of claims 2 to 7, characterized in that on two opposite side surfaces (la, lb) of the laser body (1; 1 ") each have an arrangement for Wärmeabi¬ device (2, 3; 2", 3 ", 11, 12) and / or a heating and / or cooling device (6.1 to 6.8; 7.1 to 7.10) is arranged.

9. Solid-state laser device according to one of the preceding claims, characterized in that the laser body is a, in particular cuboid, miniature laser crystal (1; 1 ") made of Nd- or Yb-doped YAG.

10. Solid-state laser device according to one of claims 2 to 9, characterized in that at least the laser body (1 ") with the resonator (17, 18; 25, 26) and the arrangement for heat dissipation (11, 12) and / or the active heating and / or the cooling device is constructed as a coherent compact assembly on a carrier element (10; 10 ').

11. Solid-state laser device according to claim 10, since ¬ characterized in that the laser body (1 ") with the resonator (17, 18; 25, 26) on a double cascaded Peltier element arrangement (10 '), in particular with Al ₂ 0 _3rd -Groundplate (10.1 ^! ), Mounted as a support element, in particular soldered or glued on thermally conductive, is.

12. Solid-state laser device according to one of the preceding claims, characterized in that a control device for the pump light source (s) for the optional setting of a homogeneous in the direction of the longitudinal axis of the laser body or a periodically alternating areas of higher and lower luminance having luminance profile is provided.

13. Solid-state laser device according to claim 12, since ¬ characterized in that a processing device (105) is provided, the output side with a control input of the heating and / or cooling device. Direction (8.1 to 6.8) and / or the control device for the pump light source (s) is connected and via which a predetermined temperature profile and / or a predetermined luminance distribution in the direction of the longitudinal axis of the laser body (1; 1 ") is set or is maintained.

14. Solid-state laser device according to claim 13, since ¬ characterized in that a with an input of the processing device (105) connected light receiver unit (101) for detecting the beam profile of the laser beam generated in the laser body (1) as a control variable for the temperature profile and / or the luminance distribution is provided.

15. Solid state laser device according to one of claims 10 to 14, characterized in that the compact assembly has a temperature sensor (21) for detecting the temperature, a control unit (21) connected to its output and a heating and / or cooling device connected to its output (6.1 to 6.8) to maintain the temperature profile of the assembly.

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