DE102015001673A1 - Device for cooling optical elements - Google Patents

Abstract

The object was to realize a low-cost and efficient cooling device for optical elements, in particular for the active medium of a laser amplifier, with the lowest possible mechanical low-voltage recording of cooling and / or cooling elements and at the same time to produce a high cooling effect, with optical elements in transmission and the device can be operated even at low temperatures. According to the invention the object is achieved by a device for cooling optical elements according to claim 1. Here, for a mechanically low-stress heat transfer from the at least one optically transmissive cooling element (3) to a heat sink (9) between the at least one optically transmissive cooling element (3) and the heat sink (9) about the optical axis (5) of the at least one optically transmissive cooling element (3) for the purpose of heat transfer position and / or shape changeable, heat-conducting compensating elements (7).

Description

The invention relates to a device for cooling optical elements to be operated in transmission, without affecting the light path (for example, the active medium of a laser amplifier, nonlinear crystals or Faraday rotators), which also allows use at low temperatures.

Many laser amplifiers today use cooled laser materials at temperatures of about 100K. Under these conditions, better thermomechanical properties of the laser materials and, in some cases, better laser properties, such as those of active media doped with trivalent ytterbium ions, are usually achieved (cf. DC Brown, "The Promise of Cryogenic Solid-state Lasers", Ieee J Sel Top Quant 11, 587-599 (2005) ; TY Fan, DJ Ripin, RL Aggarwal, JR Ochoa, B. Chann, M. Tilleman and J. Spitzberg, "Cryogenic Yb3 + Doped Solid-state Lasers", Ieee J Sel Top Quant 13, 448-459 (2007) ).

The achievable output power of a laser or a laser amplifier is in this case essentially limited by the fact that the heat generated in the active medium must be dissipated. The direct dissipation of the heat through the active medium is only possible to a limited extent in particular for large-volume laser materials, such as are required for generating high pulse energies. The heat conduction generally degraded for doped materials in comparison to the pure host material makes this more difficult (cf. PA Popov, PP Fedorov, SV Kuznetsov, VA Konyushkin, VV Osiko and TT Basiev, "Thermal conductivity of single crystals of Cal - x Yb x F2 + x solid solutions", Dokl. Phys. 53 198-200 (2008) ; RL Aggarwal, DJ Ripin, JR Ochoa and TY Fan, "Measurement of thermooptic properties of Y3Al5O12, Lu3Al5O12, YAlO3, LiYF4, LiLuF4, BaY2F8, KGd (WO4) 2 and KY (WO4) 2 laser crystals in the 80-300K temperature range "J Appl. Phys. 98 103514 (2005) ).

A direct dissipation of the heat over the edge of the active medium is therefore useful only for small diameters, since both the resulting temperature gradients and the entire temperature swing have a disruptive effect on the laser properties (cf. J. Koerner, C. Vorholt, H. Liebetrau, M. Kahle, D. Kloepfel, R. Seifert, J. Hein, and MC Kaluza, "Measurement of temperature-dependent absorption and emission spectra of Yb: YAG, Yb: LuAG , and Yb: CaF2 between 20 ° C and 200 ° C and their predictions on their influence on laser performance ", J. Opt. Soc. At the. B 29, 2493-2502 (2012) .).

As a general approach for lasers with high output power, therefore, either the minimization of the diameter of the active medium (fiber laser) or the minimization of the thickness of the active medium in conjunction with back-cooling on (disk laser). Both methods, however, lead to problems when high pulse energies are to be generated. In the case of the fiber laser, the intensity is limited due to the optical damage threshold. In the disk laser, the gain factor is limited due to the unfavorable aspect ratio, in particular for large beam diameters, in order to avoid parasitic amplification of spontaneous emission transversely to the beam direction. For solid state lasers with high output energies, therefore, mainly bulk materials come into question.

A major problem in this case is the stress-free contacting of the active medium with the heat sink with simultaneous good thermal contact. A direct mechanical contact between the laser material and heat sink thus fails due to the generally different coefficients of expansion. This is especially true if the use is to be made over a wide temperature range.

One way to achieve the stress-free good thermal bonding is to surround the material by means of a good heat-conducting liquid, generally a liquid metal, and hereby to realize the transition to a rigid heat sink (see. US 2006/024548 A1 . US 7,551,656 B2 ). However, this approach has two significant disadvantages for the intended application. For temperatures less than about 220 K, no liquid metals or alloys are found, with the use of other liquids means a strong reduction in thermal conductivity. In addition, the fluid used is not transparent, so that a cooling of the active medium over the entire beam cross section (as necessary for large beam cross sections or poorly heat-conducting laser materials) prohibits the use in transmission.

Another way of ensuring the thermal contacting of the material is the use of a thin gas gap between the heat sink and the laser material. This is z. In US 2014/0036946 A1 realized. The laser medium is located here at a small distance from the heat sink, but without having direct mechanical contact to this. The heat transfer takes place via the gas-filled intermediate space over the entire cross section of the laser medium. The disadvantage is that only a small thermal contact is achieved and operation in transmission due to the non-transparent heat sink is impossible.

On a similar principle, the in US 5,363,391 presented solution, wherein instead of the optically dense heat sink highly heat-conductive optically transparent materials are used. This allows the cooling of the front and back of the material, as well as the use of a stack, which consists of alternating heat conductor and laser material. The transparent heat conductors are then mechanically connected directly to the side of the actual heat sink. The main problem of this approach is that a stress-free mounting of transparent heat conductors is just as important as the actual laser medium. The direct contact with the heat sink thus leads to stress when used over a wide temperature range, which prohibits the use at low temperatures.

High cooling capacities and simultaneously low-voltage retention of the active medium over a wide temperature range are achieved today by the use of several plates of active material, which are lapped at high speed by a gas stream (cf. Banerjee, K. Eitel, PD Mason, PJ Phillips, M. Siebold, M. Loeser, C. Hernandez-Gomez, and JL Collier, "High-efficiency 10 J diode pumped cryogenic gas cooled Yb: YAG multislab amplifier," Opt. Lett. 37, 2175-2177 (2012) ). However, these systems are technically associated with high costs and thus can only be used in very large laser systems.

The invention has for its object to realize a low-cost and efficient cooling device for optical elements, in particular for the active medium of a laser amplifier, with as mechanically low voltage recording of the cooling and / or cooling elements and at the same time to produce a high cooling effect, wherein optical elements in transmission and the device can be operated even at low temperatures.

According to the invention the object is achieved by a device for cooling optical elements according to claim 1. Here, for a mechanically low-tension heat transfer from the at least one optically transmissive cooling element to a heat sink between the at least one optically transmissive cooling element and the heat sink about the optical axis of the at least one optically transmissive cooling element for the purpose of heat transfer position and / or shape changeable, thermally conductive Compensation elements provided. These compensating elements may be embodied according to claim 2 by metallic fins. They are connected at one end to at least one optically transmissive cooling element and at the other end possibly via a carrier plate to the heat sink. In this case, the compensation elements can be present separately and soldered on the one hand to the at least one optically transmissive cooling element and on the other hand optionally via a support plate to the heat sink. Alternatively, the compensation elements could also be realized from a portion of the support plate by slot-shaped training and the support plate to the heat sink, and the ends of the compensating elements to be connected to the at least one optically transmissive cooling element.

As a result, it is possible for mechanical stresses, which can occur due to the thermal expansion of the at least one optically transmissive cooling element, to be compensated for by the positionally and / or shape-adjustable compensating elements. For example, these may deform and, in the case of a specific arrangement, convert the change in length of the compensating elements that accompanies the thermal expansion into a rotation of the at least one optically transmissive cooling element.

Since this form of low-voltage absorption of the at least one optically transmissive cooling element is not realized by the compressibility of a liquid (eg mercury), it can also be used at very low temperatures. In addition, there are the usual advantages in the prevention of mercury, which is a toxic heavy metal, which emits highly toxic vapors over a wide temperature range and is also expensive to dispose of.

The attachment of the positionally and / or shape-changeable, heat-conducting compensating elements to the at least one optically transmissive cooling element and the carrier plate connected to the heat sink can be effected in different ways, which ensure use even at low temperatures. For example, cohesive connections such as gluing, welding or soldering are possible. The device is suitable for cooling a number of optical elements which are to be operated in transmission without impairing the light path, for example active media in a laser amplifier, nonlinear crystals or Faraday rotators.

The invention will be explained below with reference to an illustrated in the drawing cooling head for active media of a laser (high-power laser) as an example.

Show it:

1 : Cross-section through the cooling head, consisting of a sandwich-type construction of cooling plates and laser materials held in separate plates

2 : Top view of a cooling plate with elements for cooling

3 : Top view of a holder plate for laser material

4 : Top view of a cooling plate with alternative dimensions

As an exemplary embodiment (cross section see 1 ) of the invention is intended here from a plurality of individual laser media 1 existing laser head are described. The individual disks of the active medium are alternately coated with a transparent, highly thermally conductive material (eg sapphire) 3 sandwiched and through a thin gas-filled gap 2 separated. In this case, the waste heat generated by the laser process in the active medium over the gas gap over the entire surface of the transparent heat-conducting material 3 transfer. The distance of 1 and 3 is via a spacer 4 fixed (generally a few microns). The spacer 4 is generally not designed as a heat-conducting transition and can thus be made as a thin circular ring also from poorly heat-conductive materials (eg Kapton). This can be interrupted at one or more points, so that a pressure compensation within the cooling head is possible. As contacting gas here are all gases or gas mixtures in question, which are present in the desired temperature range of about 100 K in the gaseous state, such as He, Ar, Ne, Xe, Kr, N ₂ , H ₂ , O _2, etc. In the example intended for this purpose Helium can be used, since hereby a particularly good heat transfer can be achieved. The pressure in the intermediate space should be less than or equal to 1 bar in the embodiment. Expansion differences between sapphire plate 3 and laser material 1 Do not lead to tension, as there is no fixed mechanical contact.

According to the invention, the sapphire plates are about the optical axis 5 encircling with a thin (a few hundred microns) annular copper sheet 6 contacted. This has radially arranged, inwardly directed, in the same direction curved webs 7 their ends thermally conductive to the outer edge of the sapphire plates in the connection area 8th are soldered (see 2 ). The outer area of the copper sheets 6 is now directly connected to a heat sink, the z. B. by a temperature-stabilized thick copper sheet 9 can be realized.

Does it now come to expansion differences between the copper ring 6 and the sapphire plate 3 , These are connected by a rotational movement of the sapphire plate with a twisting of the copper bars 7 balanced. There is thus no mechanical stress within the optically effective sapphire plate 3 , The composite of sapphire plate, thin copper sheet and heat sink forms a unit, which will be referred to below as a cooling plate.

The laser material itself is resiliently mounted, in order to ensure its centering and on the other to allow unhindered expansion. This is z. B. by means of an arrangement according to 3 achieved in a separate holder plate. The active medium becomes inside the surrounding holder 10 with two fixed support points 11 and a spring plate 12 centered. Such a supported laser medium is referred to below as an amplifier plate.

An airtight sealing of the laser head is achieved by using low temperature suitable sealing rings 13 , For example, made of Teflon, achieved between amplifier and cooling plates. In addition, an end plate 14 installed on each side of the head. This also has at least one implementation, which allows the gas into the interstices of the head (not shown for reasons of clarity). The individual plates of the laser head can finally, for example, by provided in the cooling or amplifier plates holes 16 be bolted together.

In the presented laser head, a high cooling capacity is achieved, with all optically relevant parts are suspended without stiff mechanical connections. As a result, mechanical tension, as you are expected when using over a wide temperature range otherwise avoided.

As a further embodiment, it is conceivable that for some laser arrangements with certain good heat-conducting laser media (for example, titanium sapphire) of the sandwich-like structure (see. 1 ) is deviated. In this case, the optical element to be cooled and the optically transmissive cooling element are identical (see dependent claim 5 ). Then you can use the active laser medium directly with the optical axis 15 pointing ends of the slats 7 in the connection area 8th solder.

For high overall performance to be cooled, it is recommended that instead of the radially symmetrical aperture, an aperture (or beam profile) with a high aspect ratio (cf. 4 ) to use. For example, instead of using a radially symmetric beam profile 15 a rectangular beam profile conceivable 15a , This ensures a good dissipation of the registered heat in the narrow axis and allows a scaling of the total power in the long axis. In 4 an exemplary structure for such a cooling plate is shown. In this example, the slats are 7a so arranged that an extension of the optically transmissive cooling element 3a is translated into a lateral translation.

LIST OF REFERENCE NUMBERS

1

laser medium

2

gas-filled gap

3, 3a

optically transmissive cooling element

4

spacer

5

optical axis

6, 6a

thin copper sheet

7, 7a

Stege

8, 8a

connecting area

9, 9a

heat sink

10

Holder z. B. aluminum

11

support point

12

eder sheet

13

seal

14

endplate

15, 15a

Steel cross-section

16

drilling

QUOTES INCLUDE IN THE DESCRIPTION

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

Cited patent literature

• US 2006/024548 A1 [0007]
• US 7551656 B2 [0007]
• US 2014/0036946 A1 [0008]
• US 5363391 [0009]

Cited non-patent literature

• DC Brown, "The Promise of Cryogenic Solid-state Lasers", Ieee J Sel Top Quant 11, 587-599 (2005) [0002]
• TY Fan, DJ Ripin, RL Aggarwal, JR Ochoa, B. Chann, M. Tilleman, and J. Spitzberg, "Cryogenic Yb3 + Doped Solid-state lasers," Jeee Jel Top Quant 13, 448-459 (2007) [0002 ]
• PA Popov, PP Fedorov, SV Kuznetsov, VA Konyushkin, VV Osiko and TT Basiev, "Thermal conductivity of single crystals of Cal - x Yb x F2 + x solid solutions", Dokl. Phys. 53 198-200 (2008) [0003]
• RL Aggarwal, DJ Ripin, JR Ochoa and TY Fan, "Measurement of thermooptic properties of Y3Al5O12, Lu3Al5O12, YAlO3, LiYF4, LiLuF4, BaY2F8, KGd (WO4) 2 and KY (WO4) 2 laser crystals in the 80-300K temperature range "J Appl. Phys. 98 103514 (2005) [0003]
• J. Koerner, C. Vorholt, H. Liebetrau, M. Kahle, D. Kloepfel, R. Seifert, J. Hein, and MC Kaluza, "Measurement of temperature-dependent absorption and emission spectra of Yb: YAG, Yb: LuAG , and Yb: CaF2 between 20 ° C and 200 ° C and their predictions on their influence on laser performance ", J. Opt. Soc. At the. B 29, 2493-2502 (2012) [0004]
• Banerjee, K. Eitel, PD Mason, PJ Phillips, M. Siebold, M. Loeser, C. Hernandez-Gomez, and JL Collier, "High-efficiency 10 J diode pumped cryogenic gas cooled Yb: YAG multislab amplifier," Opt. Lett. 37, 2175-2177 (2012) [0010]

Claims (8)

Device for cooling optical elements, in particular active laser media, by means of at least one optically transmissive cooling element ( 3 ), in which for a mechanically low-tension heat transfer from the at least one optically transmissive cooling element ( 3 ) to a heat sink ( 9 ) between the at least one optically transmissive cooling element ( 3 ) and the heat sink ( 9 ) about the optical axis ( 5 ) of the at least one optically transmissive cooling element ( 3 ) for the purpose of heat transfer position and / or shape changeable, heat-conducting compensating elements ( 7 ) are provided. Apparatus according to claim 1, characterized in that the position and / or shape-changeable heat-conducting compensating elements ( 7 ) are embodied by metallic lamellae. Device according to claim 2, characterized in that the metallic lamellae ( 7 ) in each case for heat transfer with one end possibly via a carrier plate ( 6 ), in particular of copper, with the heat sink in connection and with its other end to the at least one optically transmissive cooling element ( 3 ) are attached. Device according to claim 2, characterized in that the lamellae ( 7 ) in at least one of the at least one optically transmissive cooling element ( 3 ) facing area of at least one support plate ( 6 ) are formed. Apparatus according to claim 1, characterized in that the optical element to be cooled, the at least one optically transmissive cooling element ( 3 ). Device according to claim 1, characterized in that the at least one optically transmissive cooling element ( 3 ) via an optically transparent gas-filled gap ( 2 ) with at least one optical element to be cooled ( 1 ) is coupled. Device according to Claim 6, characterized by a sandwich-type construction of a plurality of optically transmissive cooling elements ( 3 ) and the optical elements to be cooled ( 1 ). Device according to claim 6 or 7, characterized in that the at least one optically transmissive cooling element ( 3 ) consists of sapphire, yttrium aluminum garnet or diamond.

Images