DE19510713C2 - Solid-state laser device with means for setting a temperature profile of the laser body - Google Patents

Description

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

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

They come with lamps of different gas filling and design, light emitting diodes or pumped with lasers - such as semiconductor lasers, whereby the pump light source choose is that the majority of the emitted radiation in the spectral range of the Absorption bands of the laser medium. Because of this, the use of near infrared (NIR) emitting, by appropriate doping the main absorption bands of tunable semiconductor lasers for pumping YAG (Yttrium aluminum garnet) solid-state lasers are very efficient from an energy perspective  his. Hereby a pump light utilization (expressed by the so-called "slope efficiency" - cf. see the explanations below) from 70 to well above 90% reached. These pump light sources are therefore found in the so-called DPSS (diode Pumped solid-state) laser arrays are becoming increasingly common.

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

Here, however, occurs with the so-called "thermal lensing", the uncontrolled Formation of lens-like refractive index gradient areas and if necessary (at due to high volume-specific pumping capacity) also of birefringent Areas in the laser body pose a serious problem that leads to an uncontrolled Refraction of the laser beam and thus a severe deterioration in the Beam quality leads.

The effects of "thermal lensing" are particularly related to a frequency multiplication (specially doubling) of the laser radiation generated unacceptable due to nonlinear effects because of frequency doubling efficiency and thus the achievable output power with decreasing beam quality at desired end frequency drops sharply.

Attempts have been made to use trapezoidal longitudinal laser crystals cut shape (so-called slab geometry), in which the beam is not rectilinear along a crystal longitudinal axis, but after entering the crystal under the Brewster angle zigzag in a plane between two opposite Circumferential surfaces extend to the disruptive influence of "thermal lensing" compensate. A further development of this principle is in EP 0 301 526 B1 described, according to which the beam in the crystal screw-like under each  sequential reflection spreads on all peripheral surfaces. Editing the However, crystals are very complex here, the compensation effect has proven to be proven unsatisfactory and the laser threshold increases noticeably.

In the post-published document DE 44 02 688 A1, the inventor of the proposed application proposed for a transversely pumped DPSS (diode pumped solid state) laser the pump light in the direction of the longitudinal axis of a to modulate rod-shaped laser crystal in its intensity, thereby alternating arranged areas with high and low energy density in the crystal should be. This is not the case with front-end pumped arrangements feasible and requires reliable implementation over long periods of operation last (during which with parameter changes or failures of Pump light sources must be expected) a complex control and Control electronics.

From the document U. J Greiner et al., "Diode-Pumped Nd: YAG Laser Using Reflective Pump Optics", Applied Physics B 58 ( 1994 ), pages 393 to 395, it is known to use a transversely pumped laser rod in a solid-state laser device to surround a tubular, water-permeable cooling device in order to avoid the occurrence of "thermal lensing". A disadvantage of such a device, however, is that such cooling cannot completely prevent the formation of a temperature gradient perpendicular to the longitudinal axis of the laser body and also that the thermal lensing which reduces the beam quality is not completely eliminated.

DE 39 30 328 A1 describes a generic solid-state laser device, in the case of "thermal lensing" in a plate-shaped laser body with fluid-cooled th side rails is encountered, which the laser body in a longitudinal section contact two opposite side surfaces. With the help of the side rails in this longitudinal section, the temperature profile of the laser body is close to the Side surfaces influenced. This device also has the disadvantage that  "Thermal lensing" cannot be completely eliminated.

The object of the invention is therefore to provide a solid-state laser device at the outset mentioned type with the least possible impairment of the beam quality due to thermal lensing.

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

The invention includes the idea of targeted local control of one 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, d. H. in Beam direction, alternating areas with higher and with lower Generate temperature. This results in an alternating refractive index in the material Gradients built up with different signs. After going through them has the beam shape of a beam propagating in the longitudinal direction of the crystal compared to the beam shape of a beam as it is without thermal effects in the material no significant differences due to the "lensing" -related Beam deflections or deformations at the individual refractive index gradients essentially over the entire length of the laser body.

This idea has many different forms of expression in special Solid-state laser configurations, also via the transverally pumped miniaturizer DPSS laser.

On the one hand, as a means for setting a predetermined temperature profile an arrangement 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) may be provided periodically in the direction of the longitudinal axis of the laser rod alternating sections with higher and lower heat transfer resistance having.  

This arrangement can be produced in a simple and inexpensive manner essentially with at least one metallic heat-conducting body periodically alternating projections and recesses are formed in the the projections face the laser body and are in good thermal contact rest on its peripheral surface, while the recesses each one Form a distance area with poor thermal contact to the laser body. Instead of the spacing areas you can choose between the areas with high Thermal conductivity also heat-insulating areas can be provided.

Instead of or in addition to a specially designed heat dissipation arrangement can also be an active arranged on the peripheral surface of the laser body Heating and / or cooling device may be provided, which is designed so that it in Direction of the longitudinal axis of the laser body periodically alternating areas with higher and lower peripheral temperature and thus also temperature gradients generated inside the material.

On the one hand, an active, in particular electrically operated, cooling device can be used for this direction with at intervals arranged in the direction of the longitudinal axis of the laser body Neten cooling elements or areas may be provided. This can be about one Row arrangement of Peltier elements.

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

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

Finally, can be particularly simple, effective and inexpensive available arrangement an electrically operated, according to the Peltier effect  Working, combined heating / cooling device with in the direction of the longitudinal axis of the Laser rod alternately arranged cooling and heating areas can be provided.

The special arrangements mentioned can each - individually or in combination - Arranged on two opposite side surfaces of the laser body his.

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 of the heat discharge body or the cooling and / or heating element facing flat surface. At a cuboid crystal, the dimensions can be, for example, approximately 20 × 20 × 5 mm.

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

To realize a compact and at the same time geometrically highly stable laser device and to simplify the technology, 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 and the resonator are advantageously mounted on a - in particular double-cascaded - Peltier element arrangement with Al ₂ O ₃ base plate, 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, for suppression parasitic modes, however, are not polished. That over the other side surface (s) Coupled pump light can be diffusely reflected here several times, parasitic However, fashions cannot be reflected and amplified. The arrangement can therefore by the elimination of a fashion diaphragm (which also adversely affects one Would hide part of the generated beam power and thus destroy it) be simplified.

The arrangement according to the invention is particularly advantageous for solid-state lasers directions with frequency multiplication, special doubling, can be used in which additionally a frequency in the laser cavity (i.e. between the resonator mirrors) multiplier element, in particular a frequency doubler crystal (KDP = Potassium diposphate, KTP = potassium titanyl phosphate, lithium niobate or similar) with not linear optical properties is provided.

Since it is particularly with such an arrangement on the stability of the dimensions and the relative position of the components is particularly important here mentioned embodiment as a compact unit advantageous, the frequency doubling crystal is integrated into the coherent compact unit. The End face of the frequency doubler crystal facing away from the laser body especially at the same time as a fully reflecting resonator mirror surface and the End face of the laser body facing away from the frequency doubler crystal as coupling-out be formed mirror. This arrangement is - although it places high demands on the quality of the surface treatment of the crystal faces - technolo It is relatively easy to implement, and the scope of complex adjustment work becomes minimized.

The laser device constructed as a compact assembly expediently has se a temperature sensor to record the temperature, one with its output connected control unit and a heating and / or connected to its output  Cooling device to keep the temperature of the assembly constant Changes in geometry caused by temperature fluctuations, which lead to deterioration minimization of the laser parameters.

The arrangement according to the invention can be conveniently related be realized with a pump arrangement with which the laser crystal is possible homogeneously illuminated. This can be done in the laser crystal on the sides surfaces over which the pump light is coupled, additional diffuser elements be incorporated, which act in the manner of the Ulbricht sphere.

On the other hand, it can advantageously also be implemented with a pump arrangement be periodically alternating with that in the direction of the longitudinal axis of the laser body Areas of higher and lower luminance are generated. It is important in everyone Case that the temperature field in the laser crystal can be set long-term stable.

Optimized operation of such a laser device is possible if one Processing device is provided on the output side with a control input of the heating and / or cooling device 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 is adjusted or maintained.

In an advantageous embodiment, means for shaping the Beam cross section can be provided, with which usually from the YAG lasers are not a priori circular beam cross section essentially can form a circular cross section. In the simplest case, this can be a so-called mode aperture (in which the optimization of the beam cross-section is purchased with a loss of output power that can be extracted), or a suitably shaped refractor element can be used.

An even improved frequency or mode characteristic can be achieved through the  additional provision of a so-called "seeder", d. H. a laser diode with optical arrangement for the longitudinal coupling of the radiation into the (Solid-state) laser body through the rear mirror of the resonator, can be achieved. This allows the radiation from the solid-state laser to be generated without a such arrangement multiple modes within a bandwidth of about 4.4 GHz has to be locked essentially to a mode or frequency ("injection locking").

Advantageous developments of the invention are characterized in the subclaims Draws or are preferred below along with the description of the th embodiment of the invention with reference to the figures. Show it:

FIG. 1a is a schematic representation (in longitudinal section) an array of laser body and heat-dissipating device according to an embodiment of the invention,

FIG. 1b shows a schematic representation (in longitudinal section) an array of laser body and cooling arrangement according to another execution form of the invention,

Fig. 1c shows a schematic representation (in longitudinal section) an array of laser body, additional heating and heat-dissipating device according to another embodiment of the invention,

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

Fig. 2 is a schematic diagram for explaining the "thermal lensing" effect in the prior art,

Fig. 3a is a simplified longitudinal sectional view of a design of the Anord voltage according to Fig. 1a,

FIG. 3b shows a simplified plan view of the design according to Fig. 1a,

Fig. 3c is a simplified cross-sectional view of the design according to Fig. 1a,

Fig. 4 is a simplified overall perspective view of a Festkörperla servorrichtung according to an embodiment of the invention,

FIG. 4a is a detailed view of a modification of the embodiment shown in Fig. 4 Before direction,

Fig. 5 is a simplified overall perspective view of a Festkörperla servorrichtung according to another embodiment of the invention (with frequency doubling),

FIG. 5a is a detailed view of a modification of the device in Fig. 5 shown before,

Fig. 6 is a schematic diagram of a modification shown in FIG. 4 or FIG. 5 embodiments shown,

Fig. 7 is 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 is a graph illustrating an advantageous effect of the apparatus according to the invention.

FIG. 1a shows in a schematic representation (in longitudinal section) has a shape in the longitudinal section of rectangular Nd-doped YAG laser crystal 1 with one on the top 1a and the bottom 1b arranged - only partially shown in the figure - metal bodies 2 and 3 Made of solid aluminum for heat dissipation from the laser crystal to the surrounding atmosphere.

The heat dissipation bodies 2 , 3 constructed in the same way each have a profiled surface in the laser crystal surfaces 1 a or 1 b facing the surface with slots 2 a or 3 a with a rectangular cross section, between which areas 2 b and 3 are in contact with the laser crystal b the heat sink 2 or 3 are arranged. In the contact areas of the slots 2 a, 3 a, there is no physical contact, and thus a relation to the contact areas of the projecting portions 2 b, 3 b with the laser crystal 1 greatly reduced thermal conduction between the laser crystal and the Wärmeableitkörpern 2; 3.

Between the rear (in the figure, left), with a fully reflective coating - the first resonator mirror - 4 provided end face 1 c and the front (in the figure right-hand), a partially reflective - forming the second resonator mirror - coating 5 is provided end face 1 d of the laser crystal is formed when irradiating pump light - for example from InAlAs-semiconductor laser diodes arranged to the side of the laser crystal 1 (not shown in this figure) - a laser beam (bundle) with a wavelength of 1064 nm is formed in a known manner above a threshold power. The figure shows a partial beam lying somewhat above the longitudinal axis A of the crystal, ie having a so-called offset.

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 greatly different heat conduction between the laser crystal and the heat dissipation bodies in the regions 2 a, 3 a on the one hand and 2b, 3b on the other hand, the heat energy is dissipated in a modulated manner in accordance with the geometric shape of the surfaces of the heat dissipation bodies in the direction of the longitudinal axis A of the laser crystal 1 . 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 I 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 dependence of the refractive index of the laser medium and - at least with higher pump outputs - the occurrence of birefringence effects, a spatial distribution of the refractive indices corresponding to the shape of the temperature field and having areas R ₁ , R ₂ , R _{3 of the} same refractive index is formed (to a certain extent a " Refractive index field ") in the laser crystal.

As is shown schematically in the figure in the course of the beam, the spatial distribution of the refractive indices shown periodically alternately deflects 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 1 c, 1 d results in a beam orientation parallel to the longitudinal axis A. If you consider the off-axis 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 1 a', 1 b 'substantially uniform heat dissipation and the course of the center beam Rm' and an off-axis beam RO 'of isotherms I ₁ ', I ₂ ', I ₃ ', which also correspond to areas R ₁ ', R ₂ ', R ₃ ' of 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 accompanied by phase shifts and thus a deterioration in the coherence of the laser radiation.

FIG. 1b shows (again in the longitudinal cross-section) is a schematic view of another arrangement of a laser crystal 1 and a cooling arrangement, which is formed here from a respective row of spaced at predetermined intervals Peltier elements 6.1 to 6.4 or 5.6 to 8.6 above and below of the laser crystal.

The Peltier elements cool the laser crystal at the contact surfaces with its upper or lower side surface 1 a or 1 b, so basically they functionally replace the projections of the heat dissipation bodies 2 , 3 facing the laser crystal in FIG. 1 a. In the figure, it is assumed in a simplified manner that the resulting temperature field and thus the spatial distribution of the refractive index also correspond to the conditions in FIG. 1a. In the device according to FIG. 1b, of course, if the cooling elements 6.1 to 6.8 are controlled separately, if necessary, by suitable selection of the voltages of the individual cooling elements, a more differentiated and, above all, time-dependent control of the shape of the T-field and thus the beam path of the laser beam _{Ro can be made} respectively.

This is an electrical-thermal adjustment of the beam path and the phase relationships in the laser device without changes to the structure possible. This enables a fixation of the in a technologically advantageous manner Elementes immediately when the device is manufactured.

This advantage also has the arrangement shown in FIG. 1c - again as a basic illustration in longitudinal section - consisting of laser body 1 , heat dissipation bodies 2 'and 3 ' and an additional heater comprising five electrical heating elements 7.1 to 7.10 arranged in heat dissipation body 2 'and five in heat dissipation body 3 ' a further embodiment, provided that 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 spacing of the heating elements correspond to those of the slots in Fig. 1a and the thermal effect of the heating elements, ie the influence on the temperature field in the laser crystal, which is equivalent to the slots of the heat dissipation body 2 , 3 . Of course, these simplifying assumptions were only made with a view 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 higher and those with lower cooling effect need not all be the same.

Fig. 1d shows a further schematic diagram of an arrangement of a laser body 1 and a combined heating and cooling arrangement of four cooling elements 6.1 to 6.4 or 5.6 to 8.6 and five heating elements 7.1 to 7.5 or 7.6 top in a to 7.10 or a lower heat sink 2 "or 3 ". Here, too, it is assumed in the drawing 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. 1a.

However, it is clear that in particular this arrangement with active cooling and heating elements can enable a far more pronounced differentiation of the T field, ie the realization of substantially larger T gradients than the arrangement according to FIG. 1a. 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 - for example operated in series. 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 is a simplified longitudinal sectional view 3b 3c shows a technologically advantageous realizable design of the basic arrangement shown in Fig. 1a, Fig. Is a simplified plan view and Fig. Is a simplified cross-sectional view (perpendicular to the longitudinal axis of the laser crystal) of this design.

An aluminum oxide substrate 10 with surface metallization (in-plating) 10 a serves as a carrier, onto which a lower (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 Yb: YAG 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 ) successive and perpendicular to this (in the y-direction) extending slots 11 a and 12 a and projections 11 b and 12 b, respectively. On this surface they each have an in-coating 11 c and 12 c, which are used for The slots 12 a of the upper heat sink 12 have a smaller depth here than that of the lower heat sink 11 .

In the arrangement according to FIGS . 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).

Correspondingly, an axially offset beam R _o "penetrating both end faces of the laser crystal is shown, which enters and exits through special openings 12 d and 12 e of the upper heat sink 12 .

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

In Fig. 3b and 3c, the laser crystal 1 'facing ends of optical fibers of two lateral pumping light supply arrangements are shown 13 and 14, and the beam path of the individual pump light beams is outlined (in dashed lines). In Fig. 3c is in this case easy to recognize that the pump light between the top and bottom metallization 1 a / M "or 1b / M" of the laser crystal is often reflected and is therefore used extremely efficiently. As mentioned above, the non-polished, optically imperfect surface prevents that higher order modes and especially parasitic modes are amplified and can affect 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 (when bent) approx. 13/6 / 1.5 mm and the slot widths in the heat sinks approx. 0.5 mm.

Fig. 4 shows a simplified perspective overall view of a solid-state laser device, in which the cooling arrangement is realized in the design according to Fig. 3a to 3c. 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.

In FIG. 4, three pump light sources show in more detail that a laser diode array 15 a, 15 b, 15 at the end of the light guides 13 a, 13 b, 13 c and 14 a, 14 b, 14 c facing away from the laser crystal c or 16 a, 16 b, 16 c is coupled with 1 to 4 W pump power each.

A rear, fully reflecting resonator mirror 17 and a front, semi-reflecting resonator mirror 18 are also shown in this figure. Both mirrors are soldered via double angled, bendable connection surfaces during assembly ("pads" or "legs") 17 a, 17 b or 18 a, 18 b to the local metallization 10 a 'of the carrier 10 '. Furthermore, an element 19 or 20 is provided between the mirrors 17 , 18 and the laser crystal 1 "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 section is generated. This optical elements are also soldered to metallization areas of the carrier 10 '.

The carrier 10 'is a double cascaded cooling arrangement made of three Al ₂ O ₃ carrier plates 10.1 '. 10.2 'and 10.3' and an intermediate array 10.4 'and 10 , 5 ' of Peltier elements. Such an arrangement is geometrically highly stable with changing ambient temperatures or changing energy input, since an automatic compensation of temperature gradients is achieved by cascading the Peltier arrangements. 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 can be sensed and with the aid of which it can be additionally regulated by suitable control of the Peltier elements.

FIG. 4a is a detailed representation of a with respect to the utilization of the pumping radiation particularly advantageous embodiment of the laser crystal in the arrangement of Fig. 4. Here, the laser crystal 1 '''in addition to the basic and Deckflächenbe coating 1 b / M "or 1a / M "for further homogenization of the Pumplichtver distribution inside the crystal on the long side faces 1 c""and 1 d""facing the optical fiber groups 14 and 13 , respectively, and parallel to the direction of the laser beam R, in each case opposite the ends of the optical fibers approximately hemispherical depressions 1 c / S '''or 1d / S'''. These recesses worked into the laser medium act with respect to the pumping light beam emerging from the ends of the adjacent optical fibers according to the principle of the integrating sphere as a scattering surface. In order to reduce reflection losses of the pump radiation on the side surfaces and thus to improve the coupling, it is additionally provided to fill the depressions with a gel 13 a or 14 a to adjust the refractive index ("index matching gel") into which the ends of the optical fibers 14 or immerse 13 .

Fig. 5 shows a (simplified turn) overall perspective view of a further solid-state laser apparatus according to an embodiment of the invention, with the additional frequency doubling of the laser beam generated is carried out. The formation and arrangement of the laser crystal is the same as in the device described above, so that the same reference numerals are used in this regard.

An essential difference is the construction 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 described in more detail in the earlier application of the inventor mentioned at the beginning.) The radiation from the laser diodes arranged 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 crystal length.

Another significant 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 on the other side (in the figure on the right) of the rear element 20 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 24 a, 24 b, via which an initial adjustment of the position relative to the other optical components is possible. Its end face 24 c facing away from the laser crystal has a convex shape with a geometry that 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 mirroring 26 of the front end face of the laser crystal 1 ″.

With a temperature control of the KTP crystal 24 , the effective resonator length of the arrangement can be adjusted in a simple manner without mechanical intervention, without additional controllable refraction elements (such as a temperature-controlled quartz plate or a piezomechanical tuning element) being necessary. Alternatively, such an element 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 piezomechanical tuning of the laser arrangement.

FIG. 5a is a detailed representation of a with respect to the utilization of the pumping radiation particularly advantageous embodiment of the laser crystal in the arrangement of Fig. 5 wherein hereinafter only will be explained by Fig. 3a to 3c or Fig. 5 distinguishing details.

Here, the laser crystal 1 "" for further homogenization of the pump light distribution inside the crystal on the long side faces 1 c "" and 1 d "" facing the laser diode blocks 22 and 23 (see FIG. 5), respectively an approximately semi-cylindrical groove 1 c / N "" or 1d / N "". These recesses worked into the laser medium act - analogously to the hemispherical recesses according to FIG. 4 - with respect 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 have 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 of another material known for the respective purpose: Nd- or Yb-doped glass, for example, and KDP (potassium diphosphate) or lithium niobate for the frequency doubler can be used as the laser material.

A simple Peltier element arrangement or possibly also can be used as the carrier simply a good heat-conducting carrier plate - to increase the cooling effect too with ribbing or u. U. with liquid cooling - serve. The fixation of the Components on the carrier can be, for example, by a thermally conductive Be made of glue.

FIG. 6 is a schematic diagram to explain 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.

Two resonator mirrors ML1 and ML2 are assigned in the usual way to the solid-state laser body, designated YAG in this figure, with a laser wavelength of 1064 nm. 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. R ₁ and r ₂ denote the mirror radii of the resonator mirrors, r ₃ the radius of the front KTP end face. Typically r1 is in the range of 10 cm, r ₂ and r ₃ are in the order of a few mm. "AR..." the presence of an anti-reflective coating and with "HR ..." a highly reflective coating for a certain wavelength.

Extra cavitation (in the figure to the right of the front mirror ML2) is another Collimator lens COL2 shown. Other optical elements and the pump arrangement are omitted here for simplification. The conditions regarding the Beam frequencies are given by specifying the wavelengths (1064 nm = primary Wavelength, 532 nm = wavelength after frequency doubling) at different Make clear the arrangement. The adjustment after the adjustment takes place on the manner described above, that is to say by T-control of the frequency ver dopplers, which is illustrated in the figure by an arrow with the reference symbol TC is.

An InGaAs laser diode LAD is arranged on the side of the solid-state laser arrangement, which also has a width of 1064 nm with an output power of approx. 50 mW emitted. Their radiation is after passing a collimator lens COL1 and two deflecting mirrors M1 and M2 with a transmission coefficient of about 1% longitudinally coupled into the solid-state laser arrangement. Effected there this narrowband radiation locks frequency and phase ("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 a ring resonator arrangement (known per se and therefore not shown separately here), in which case the laser diode radiation is coupled into the solid-state laser at an angle to the longitudinal axis thereof is matched to the geometry of the ring resonator.

With this arrangement, the frequency can be tuned even better simultaneous adjustment of effective resonator length and phase length of the Seeders in particular a single (not shown) piezomotor longitudinal displaceable plane-parallel quartz plate can be inserted into the beam path, that of the seeder and the laser radiation each at a small angle  is penetrated diagonally.

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 Peltier 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 during operation of the laser. The number of image acquisition elements of 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. The real beam cross-section recorded by the CCD matrix 101 is compared in the comparator unit 103 with the sample 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 furthermore in the usual way with a data memory (RAM) 106 and a program memory (for example an EPROM) 107 connected is. The processing unit may optionally be connected to the output of a temperature sensor (according to the figure of the sensor 21 from FIG. 4 or 5), as shown by broken lines, so that the temperature of the laser arrangement is also included in 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 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 , which finally a voltage supply unit 108 for the Cooling elements is supplied and a corresponding setting of the operating voltages and thus the cooling powers 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 power supply unit can be locked after completing the setting process.

The processing program for the measured beam data can be at described arrangement be an iterative program, wherein during the Setting the changed beam parameters are continuously recorded and one next iteration step can be used as a basis. Particularly well suited there is a fuzzy logic algorithm for this.

To display the recorded data and processing results for an operator and for. Influencing the processing process, the components mentioned are also each connected to a display and input unit 109 - such as 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 way, 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 by separately controlling a plurality of pump light sources.

In FIG. 8 is a graph illustrating an advantageous effect of the apparatus according to the invention is given.

It is shown in logarithmic representation using typical curve profiles the laser output power Pout as a function of the pump power Pin. The dashed Line shows a curve for a Yb-YAG laser device according to the prior art the technology, while the solid curve is the characteristic of a Embodiment of the invention constructed device. It can be spotted, that in the latter, both the increase in the linear region ("slope") and the achievable maximum output power (based on the threshold power) is much higher.

The embodiment of the invention is not limited to the above specified preferred embodiment. Rather is a number of Variants conceivable, which of the solution shown also in principle use different types of designs.

Claims (25)

1. Solid-state laser device with an essentially rod-shaped or plate-shaped laser body ( 1 , 1 ") having a longitudinal axis (A), a resonator ( 4 , 5 , 17 , 18 , 25 , 26 arranged parallel to the longitudinal axis (A) of the laser body ), a pump light source ( 15 a to 15 c, 16 a to 16 c, 22 , 23 ) for excitation of the laser body ( 1 , 1 ") and with the laser body ( 1 , 1 ") associated 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 (A), characterized in that the means ( 2 , 3 , 6.1 to 6.8 , 7.1 to 7.10 , 2 ", 3 ", 11 , 12 ) are designed for setting a predetermined temperature profile such that, during operation of the device in the direction of the longitudinal axis (A), periodically alternating areas of higher and lower areas of the laser body temperature are present. 2. Solid-state laser device according to claim 1, characterized in that the means for setting a predetermined temperature profile on a peripheral surface (1a, 1b; 1a ", 1b"; 1a / M ", 1b / M") of the laser body ( 1 ; 1 " ) provided arrangement ( 2 , 3 ; 11 , 12 ) for heat dissipation from the laser body to the environment, which periodically alternately in the direction of the longitudinal axis (A) of the laser body sections ( 2 a, 2 b, 3 a, 3 b; 11 a, 11 b, 12 a, 12 b) with higher and those with lower heat transfer resistance. 3. Solid-state laser device according to claim 2, characterized in that the arrangement for heat dissipation at least one metallic heat-conducting body ( 2 , 3 ; 11 , 12 ) with periodically alternating projections ( 2 b, 3 b; 11 b, 12 b) and recesses ( 2 a, 3 a; 11 a, 12 a), wherein the projections face the laser body ( 1 ; 1 ") and bear in good thermal contact on the peripheral surface thereof, while the recesses each form a spacing area with poor thermal contact to 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 on the peripheral surface ( 1 a, 1 b) of the laser body ( 1 ) arranged active heating and / or cooling device ( 6.1 to 6.8 ; 7.1 to 7.10 ) include, the periodically in the direction of the longitudinal axis (A) of the laser body alternating areas with higher and lower peripheral temperature. 5. Solid-state Laservorrichturtg according to claim 4, characterized in that an active, electrically operated cooling device with in the direction of the longitudinal axis (A) of the laser body ( 1 ) at intervals arranged elements or areas Kühlele ( 6.1 to 6.8 ) is provided. 6. Solid-state laser device according to claim 4, characterized in that an electrically operated heating device in the direction of the longitudinal axis of the laser body ( 1 ) at intervals arranged heating elements or areas ( 7.1 to 7.10 ) is provided. 7. Solid-state laser device according to claim 4, characterized in that an electrically operated one, especially one that works according to the Peltier effect, Heating / cooling device with in the direction of the longitudinal axis of the laser body alternately arranged cooling and heating areas is provided. 8. Solid-state laser device according to one of claims 2 to 7, characterized in that two arrangements for heat dissipation ( 2 , 3 ; 2 ", 3 ", 11 , 12 ) and / or heating and / or cooling devices ( 6.1 to 6 , 8 ; 7.1 to 7.10 ) ( 1 ; 1 ") are arranged opposite each other with respect to the laser body. 9. Solid-state laser device according to one of the preceding claims, characterized in that the laser body is a, in particular quaderför shaped, miniature laser crystal ( 1 ; 1 ") made of Nd- or Yb-doped YAG. 10. Solid state laser device according to one of the preceding claims, characterized in that at least one transversely to the longitudinal axis (A) radiating pump light source ( 15 a to 15 c, 16 a to 16 c; 22 , 23 ) is seen before. 11. Solid-state laser device according to claim 10, characterized in that a plurality of transversely to the longitudinal axis (A), in particular on both sides of the laser body, irradiating semiconductor laser ( 15 a to 15 c, 16 a to 15 c; 22 , 23 ) are provided as pumping light sources. 12. Solid-state laser device according to claim 11, characterized in that between the light exit surfaces of the semiconductor laser ( 15 a to 15 c, 16 a to 16 c) and the laser body ( 1 ") optical fibers ( 13 a to 13 c, 14 a to 14 c) are provided for guiding the pump light. 13. Solid state laser device according to one of claims 10 to 12, characterized in that on the sides facing the pumping light sources surfaces (1c ''',1d'''; 1c "", 1d "") of the laser body ( 1 ''', 1 "") Means (1c / S ''', 1d / S'''; 1c / N "", 1d / N "") are provided for homogenizing the pump light distribution. 14. Solid state laser device according to claim 13, characterized in that the means for homogenizing the pump light distribution in the laser body incorporated recesses with essentially spherical or cylindrical are the boundary surface. 15. Solid state laser device according to one of claims 2 to 14, 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 heater - And / or cooling device is constructed as a coherent compact package on a carrier element ( 10 ; 10 '). 16. Solid-state laser device according to claim 15, 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 O ₃ base plate ( 10.1 '), mounted as a carrier element, in particular soldered or glued on in a heat-conductive manner. 17. Solid state laser device according to one of claims 9 to 16, characterized in that the side surfaces (1a / M ", 1b / M") of the laser body ( 1 "), to which no pump light is coupled, are not polished and preferably metallized. 18. Solid state laser device according to one of the preceding claims, characterized in that a frequency multiplier element ( 24 ), in particular a frequency doubler crystal, is provided. 19. Solid state laser device according to claim 18 and claim 15 or 16, characterized in that the frequency multiplier element ( 24 ) is integrated in the cohesive compact assembly. 20. Solid-state laser device according to claim 18 or 19, characterized in that the end face facing away from the laser body ( 24 c) of the fre quenz doubler crystal as a fully reflecting resonator mirror surface ( 25 ) and the end face facing away from the frequency doubler crystal of the laser body ( 1 ") as decoupling mirror ( 26 ) are trained. 21. Solid state laser device according to one of the preceding claims, characterized in that a control device for the pump light source (s) for optional adjustment of one in the direction of the longitudinal axis (A) of the laser body homogeneous or one periodically alternating areas higher and lower luminance profile is provided. 22. Solid-state laser device according to claim 21, characterized in that a processing device ( 105 ) is provided which is connected on the output side to a control input of the heating and / or cooling device ( 6.1 to 6.8 ) 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 ( 1 ; 1 ") is set or maintained. 23. Solid-state laser device according to claim 22, characterized in that a with an input of the processing device ( 105 ) connected light sensor unit ( 101 ) for detecting the beam profile of the laser beam generated in the laser body by ( 1 ) is provided as a control variable for the temperature profile and / or the luminance distribution . 24. Solid-state laser device according to one of claims 13 to 23, characterized in that the compact assembly has a temperature sensor ( 21 ) for detecting the temperature, a processing device connected to its output for control ( 105 ) and a heating and / connected to its output. or cooling device ( 6.1 to 6.8 ) for maintaining the temperature profile of the assembly. 25. Solid-state laser device according to one of the preceding claims, characterized in that a longitudinal from the back of the full reflecting resonator mirror (MLI) radiating semiconductor laser (LAD) for frequency and phase locking of solid-state laser radiation is seen.