WO2013093503A2

WO2013093503A2 - Mount for optical element of a laser

Info

Publication number: WO2013093503A2
Application number: PCT/GB2012/053240
Authority: WO
Inventors: Paul Mason; John Hill; Saumyabrata BANERJEE; Andrew LINTERN
Original assignee: The Science And Technology Facilities Council
Priority date: 2011-12-22
Filing date: 2012-12-21
Publication date: 2013-06-27
Also published as: GB201410963D0; CZ27652U1; GB2512521B; KR20140107556A; GB201122181D0; DE212012000230U1; WO2013093503A3; KR101989136B1; GB2512521A

Abstract

A mounting arrangement for an optical element, such as gain medium, for a laser, optical amplifier or other optical systems is disclosed. The mounting arrangement is applied to a vane assembly for cooling by a gas or liquid stream. The vane assembly comprises: an optical element; a vane plate having an aperture, the optical element mounted in the aperture; and a retainer arranged to fit between the optical element and the vane plate to provide a compressive force at the edge of the optical element to hold the optical element in the aperture. The retainer may be a resilient element or spring element. The retainer may be held in position using retaining rings.

Description

MOUNT FOR OPTICAL ELEMENT OF A LASER

Technical Field

The present invention relates to mounting an optical element such as gain medium in a laser, optical amplifier, and other types of optical systems. The mount may be

configured for cooling of the optical element by a gas or liquid stream such as in a cryogenic-gas cooled laser amplifier. The optical element may be an element that becomes heated in operation such as an optical gain medium.

Background Art

Lasers with a high output power are required for a number of applications, such as materials processing, particle acceleration, military applications, and laser induced fusion for energy production. Lasers for these applications are required to provide high energy, high repetition rate pulses. One of the challenges associated with obtaining stable and reliable pulse generation is managing the heat generated in optical elements of the laser. Heating may occur in a variety of components such as optical gain media, Pockels cells, Faraday isolators, frequency conversion stages where some optical absorption occurs and many other components in which absorbed energy is converted to heat. Conventional lasers producing high-energy pulses use rods with water cooling or slabs without active cooling as a gain medium. The pulse energy and/or the pulse repetition rate provided by such lasers is not high enough for laser induced fusion and other applications, such as laser-driven particle accelerators.

Thermal management in optical gain media arranged as slabs has been investigated under a US Department of Energy contract, the results of which have been published as

"Thermal Management in Inertial Fusion Energy Slab

Amplifiers", Sutton and Albrecht, Lawrence Livermore

National Laboratory, 1^st International Conference on Lasers for Inertial Confinement Fusion, Monterey, Canada, May 30- June 2 1995 and as Sutton, S. B. & Albrecht, G. F. (1995), "Thermal management in inertial fusion energy slab

amplifiers", Proceedings of SPIE 2633, 272-281. These papers describe the use of gas-cooling of large aperture slabs where the beam propagates through the cooling medium. The consequences of poor thermal management are thermally induced aberrations and thermally induced birefringence, both leading to a degradation of the quality of the

transmitted beam. Thermally induced deformation or expansion of the gain material can cause beam steering. In the extreme case, the thermally induced stresses can lead to cracking of the gain medium. The arrangement described by Sutton and Albrecht uses an end-pumped configuration with the slabs of gain medium oriented normal to the pump laser beam. The pump laser beam was provided from semiconductor laser diodes. The end-pumped arrangement used gain media segmented into a series of thin slabs with cooling channels there between. A gas is pumped at high velocity through the channels to remove heat from the slabs. As mentioned above, the pump laser beam and emitted beam pass through the cooling medium.

A later project, known as the Mercury Laser, is

described in "Activation of the Mercury Laser: A diode- pumped solid-state laser driver for inertial fusion",

Bayramian et al . , Advanced Solid-State Lasers 2001 Topical Meeting and Tabletop Exhibit, Seattle, Washington, January 29-31, 2001. The project is also described in A. Bayramian et al . (2007), "The mercury project: A high average power, gas-cooled laser for inertial fusion energy development", Fusion Science and Technology 52(3), 383-387. The project goal was to design a laser capable of producing 100J pulses having a pulse length of 2-10ns and a repetition rate of 10Hz. Figure 1 is a schematic diagram of the system for cooling the slabs of gain medium, based on details provided in the various Mercury Laser project publications. The cooling system 5 comprises a heat exchanger 10, channels for routing the gas stream 30, a fan unit 20, and laser

amplifier 50 which includes vane plates 60 with slabs of optical gain media mounted therein. Pump beam and output beam 40 are perpendicular to the slabs. The heat exchanger 10 cools the gas after it has passed by the gain media. The fan unit 20 pumps the gas around the system towards the laser amplifier. Figure 2a shows the vane plates 60 mounted in the amplifier. The vane plates 60 are stacked such that the slabs of gain material lie adjacent to each other and adjacent to windows 82 in the amplifier manifold. A single vane plate is shown in figure 2b. Around the edge of each slab 62 is edge cladding 84 to locate and support the slabs in the vanes. Each slab of gain media 62 is mounted in the vane plate 60 using a polyurethane polymer 85 to pot the slab in the vane plate. The optical gain medium was Yb:S- FAP, which was mounted into aluminium vane plates.

Small gaps 86 between the vane plates, and between the vane plates and the manifold, provide channels through which the cooling gas flows. Gas is pumped through the amplifier manifold .

The optical gain medium is optically pumped by a beam arranged normal to the plane of the slabs, as shown in figure 2a. The output beam generated is also normal to the slabs. The measured wavefront output from the amplifier includes a wavefront distortion due to heating of the gas as it traverses the amplifier. The gas used in the cooling system was helium at a gas pressure of 4 bar within the straight channels between the vane plates. The gas velocity within the channels is Mach 0.1 and the mass flow rate is ~lg/sec. The Mercury Laser operated the gas at around room temperature .

Operating the cooling system at lower temperatures and higher pressures than those used in the Mercury Laser project brings about certain benefits. The increased

temperature range and higher pressure places a greater burden on the system design to prevent excessive vibration of the slab of gain medium and avoid interruption of the gas flow. The mounting method should hold the optical gain medium firm even on cooling from room temperature to

cryogenic temperatures, and minimise thermally-induced stress on the slab due to thermal expansion differences between the vane plate and optical gain medium during such cooling. Furthermore, a vibration free method of mounting optical elements other than gain medium in other areas than vane plates is required that is stable over large

temperature ranges.

Summary of the Invention

The present invention provides a vane assembly, comprising: an optical element of a laser or optical

amplifier, the optical element for cooling by a fluid stream; a vane plate having an aperture, the optical element mounted in the aperture; and a retainer arranged to fit between the optical element and the vane plate to provide a compressive force at the edge of the optical element to hold the optical element in the aperture. This arrangement results in edge clamping of the optical element which avoids obscuration of the optical element. Edge clamping also provides a uniform compressive clamping force around the edge of the optical element such that wavefront distortions as may be caused by face clamping are minimised. By edge of the optical element we mean the surface which is not an optical surface. This surface is usually around the

perimeter, and normal to, the optical surface. For example, for a circular disc shaped optical element the edge is a curved surface. Furthermore, the retainer of the present invention is configured for operation from room temperature down to cryogenic temperatures, because the compressive force allows for differences in contraction or expansion between the vane plate and optical element. In particular, upon cooling the optical element may contract more than the aperture in the vane plate and tend to become loose but the retainer continues to apply the compressive forces holding the optical element in place. Depending on the materials used the optical element may contract less than the aperture in the vane plate and may be subject to cracking, but the retainer which is preferably resilient will accommodate the stress while maintaining sufficient compressive force to hold the optical element in position. The retainer of the present invention is also adapted such that the optical element is removable by removal of the retainer.

The retainer may fit between an outer edge of the optical element and an inner edge of the aperture, the retainer adapted to provide the compressive force at the edge of the optical element. A corresponding compressive force may also be exerted against the vane plate at the edge of the aperture. The retainer may comprise a resilient element or spring element. The compressive force may be provided substantially uniformly around the edge of the optical element.

The edge of the aperture in the vane plate may have a groove for location of the retainer. The edge of the optical element may have a groove for location of the retainer. The grooves provide the advantage of aligning the optical element in the vane plate such as centrally across the thickness of the vane plate.

The retainer may extend substantially completely around the outer edge of the optical element. The retainer is shaped to match the perimeter of the optical element but may be dimensionally smaller.

The retainer may have a c-shaped cross-section. The retainer may have a shape corresponding to that of the perimeter of the optical element, the retainer having a smaller diameter than the optical element so as to provide the compressive force. The retainer may be cleaved or cut transversely to its length.

The retainer may be an extension spring. The extension spring maybe formed into a loop such that it is endless. The extension spring may have a shape corresponding to that of the edge of the optical element, the retainer having a smaller diameter than the optical element so as to provide the compressive force.

The retainer may be a bracelet having a plurality of sprung contact fingers. The bracelet may comprise a back plate. In the assembly the bracelet may be arranged with the spring fingers in contact with the optical element.

The vane assembly may further comprise one or two retaining rings removably coupled to the vane plate to hold the retainer in the aperture of the vane plate. The one or two retaining rings may have an internal diameter at least that of the inner diameter retainer but less than the outer diameter of the retainer.

The optical element and vane plate may be substantially the same thickness. This prevents disturbing the coolant flow across the vane plate and optical element thereby reducing instabilities which may cause vibration.

The optical element may be optical gain medium, such as Yb:YAG, Nd : YAG or other solid-state laser medium suitable for optical pumping.

The present invention also provides a vane plate for mounting an optical element of a laser or optical amplifier for cooling by a fluid stream, the vane plate having an aperture for receiving the optical element, the vane plate having a groove in the edge of the aperture for receiving a retainer for holding the optical element in the aperture.

The present invention further provides a laser or optical amplifier comprising the vane assembly or vane plate described above. The optical element of the laser or optical amplifier may be optical gain medium. A pump beam may be incident on the optical gain medium. The pump beam and/or output beam may be transverse to the plane of the vane assembly. The pump beam and/or output beam may propagate through a cooling fluid stream.

The laser or optical amplifier may be a cryogenic gas cooled laser amplifier.

Although the embodiments above mount the optical element in a vane plate to form a vane assembly, other embodiments of the present invention use corresponding techniques to mount an optical element in mounts other than a vane plate. In this regard the present invention provides an optical assembly, comprising: an optical element; a mount having an aperture, the optical element mounted in the aperture; and a retainer arranged to fit around the edge of the optical element between the optical element and the mount to provide a compressive force at the edge of the optical element to hold the optical element in the aperture.

The retainer may comprise a resilient element or spring element .

The retainer may be one of the following: an annulus having a c-shaped cross-section; an extension spring; and a bracelet having a plurality of sprung contact fingers.

Other techniques and arrangements mentioned above relating to the mounting of the optical element in the vane plate may also be applied to mounting the optical element more generally.

Brief description of the Drawings

Embodiments of the present invention, along with aspects of the prior art, will now be described with

reference to the accompanying drawings, of which:

figure 1 is a schematic diagram of the cooling system of the prior art for cooling optical elements for a laser amplifier;

figure 2a is an illustration of a cross section through the gas cooled amplifier according to the prior art;

figure 2b is an illustration of a vane plate with gain medium potted therein according to the prior art;

figure 3 shows in plan view and cross-section of an optical element mounted in a vane plate with a retainer according to the present invention;

figure 4 is a detailed cross-sectional view of three embodiments of retainer holding the optical element in the vane plate; figure 5a is a perspective view of a vane assembly with optical element mounted in a vane plate according to the present invention; and

figure 5b is an exploded view of the vane assembly of figure 5a showing the individual components.

Detailed Description

Figure 3 shows an embodiment of the present invention in which optical element 162 is held in vane plate 160 by retainer 170. The vane plate 160 may be shaped such that it has a cross-section similar to the prior art as shown in figures 2a and 2b. The tail section of the vane plate 160 gently tapers away to recombine the gas flow from between the vane plates. Other cross-sections may be used. The representation in figure 3 shows a rectangular cross-section but cross-sections such as those described above regarding the prior art may be used.

The optical element 162 of figure 3 is a circular disc shape and is received in a circular aperture in the vane plate 160. Other shapes of optical element 162 may be used such as square, rectangular or elliptical. The optical element 162 is held in the aperture by retainer 170. As shown in the cross-section X-X of figure 3, the vane plate 160 is the same thickness as the optical element 162.

Figures 4a-4c show in more detail how the retainer 170 is located between the vane plate 160 and optical element 162. Three alternatives for the retainer 170 are shown in figure 4. In each case the edge of aperture 160a includes a groove 160b for receiving the retainer 170. The groove 160b may be shaped to match the retainer used. The optical element 162 is also shown to include a groove 162b in edge 162a of the optical element disc. The groove in the optical element 162 is shallower than the groove in the vane plate 160. In other embodiments the groove in the optical element and/or groove in the vane plate edge is not provided.

However, inclusion of the grooves is preferred to align the optical element 162 centrally in the vane plate 162.

The arrangement of retainer 170 between the edge 160a of the aperture of the vane plate 160 and the edge 162a of optical element 162 provides edge clamping of the optical element 162. This is preferable to face clamping because the aperture of the optical element 162 is not obscured. The vane plate 160 and optical element 162 are substantially the same thickness such that the cooling fluid flows smoothly over the surface of the assembled optical element 162 and vane plate 160. A difference in thickness would provide a step obstruction to the cooling fluid. This could result in instabilities in the fluid and vibration. Work has shown that the optical element 162 should be aligned in the vane plate 160 such that the alignment tolerance between the surface of the optical element and the vane plate is less than ΙΟΟμπι to ensure that any disruption to the coolant flow is minimal. This alignment tolerance between the surfaces of the optical element 162 and the vane plate 160 also sets a maximum permissible difference in the thickness between the two such that when assembled the alignment of the surfaces meets the less than lOOpm offset criterion.

The edge mount technique provides a small, uniform compressive force around the edge of the optical element 162 at room temperature. The retainer 170 is sufficiently flexible to accommodate the difference in contraction between the optical element 162 and vane plate 160. For example, the optical element 162 is assembled into the vane plate 160 with the retainer 170 at room temperature, and is then cooled to cryogenic temperatures with helium gas under pressure as coolant, for example at around 150K. The

temperature drop is around 150K. For a difference in thermal expansion coefficient of 2xlCT⁶ K^-1 and a 55mm diameter disc, the resulting difference in contraction would be around 10- 20pm assuming thermal expansion coefficients are constant over the temperature range. In practice the coefficients will change with temperature. Assuming the above difference in contraction is of the correct order of magnitude, this could be sufficient to crack the optical element 162.

Compliance in the retainer 170 allows for this difference. Furthermore, depending on the materials used the difference in thermal expansion coefficients could cause the optical element to become loose. In this case the compressive force of the retainer maintains rigid and firm support of the optical element.

To provide the small, uniform compressive force the retainer 170 is preferably resilient such as provided by a sprung element. Figures 4a-4c show three embodiments of the retainer. Although figure 3 shows a circular disc shaped optical element 162 other shapes may be used, such as square or rectangular. Of the three embodiments some are more suited to certain shapes of optical element 162 as discussed below .

The first embodiment of the retainer 170 is the C-ring, as shown in figure 4a. This is comprised of a ring matching the shape of the optical element 162. The internal

circumference of the ring is slightly smaller than the circumference of the ring once any groove has been taken into account. This slightly smaller size provides the compressive force on the optical element. The cross-section of the ring is a C-shape. The ring is preferably stainless- steel but other materials may be used. The ring may also be cut through radially at a circumferential location to provide stress relief on cooling. Having a cut in the ring also allows the ring to be expanded over the optical element 162 and to locate in the groove 162b. As the C-ring is in contact with the optical element 162 at the groove

continuously around the circumference (except at the cut) the C-ring provides the compressive force uniformly around the edge of the optical element 162. This type of ring is best suited to circular shaped optical elements, but

appropriately shaped rings may also be used for optical elements of other shapes that do not have sharp corners, such as a rectangle with rounded corners.

The second embodiment of the retainer 170 is the spring retainer as shown in figure 4b. This consists of an

extension spring whose length is slightly less than the circumference of the optical element 162. The extension spring is formed into a ring. The resilience of the spring allows it to be expanded over the optical element 162 to locate in the groove 162b. The internal circumference of the ring is slightly less than the circumference of the optical element at the groove. The slightly smaller dimension provides a compressive force on the optical element 162. A compressive force is applied radially at each contact point of the spring with the optical element. Thus, the multiple points around the edge of the optical element uniformly distribute the compressive force around the edge of the optical element 162. The spring is suited to many shapes of optical element and will deform to match the shape of the optical element 162. The spring can also accommodate

irregularities in optical element shape. The spring is not well suited to optical elements having sharp corners. Figure 4c shows a third embodiment of the retainer which takes the form of a bracelet having a backplate ring with a series of spring fingers along the inner edge. The bracelet may be a continuous ring or more preferably is formed of a length of fingerstock. The fingerstock may be Be : Cu fingerstock. The spring fingers can be seen in figure 5b. The series of spring fingers provide multiple contact points on the edge of the optical element such as in the groove of the optical element. The fingers are curved so as to locate in the groove 162b of the optical element 162 and centre the optical element in the aperture of the vane plate 160. Alternatively the fingers may be ridge shaped. The ridge locates in the groove and centres the optical element. Other shapes of fingers may also be used. The spring fingers provide a compressive force on the optical element. The multiple contact points distribute the compressive force evenly around the circumference of the optical disc. The multiple contact points can accommodate irregularities. The bracelet may be formed of a length of fingerstock,

optionally with the ends joined together to form a ring. The fingerstock maybe formed into shapes to fit the shape of the optical element. The fingerstock can be used to mount arbitrarily shapes discs such as circular, square,

rectangular etc.

The cross-section of the C-ring and spring retainers is such as to fit grooves with curved or circular side walls. As shown in figures 4a and 4b the C-ring and spring

retainers locate in a groove in the edge of the vane plate and the edge of the optical element. The groove in the edge of the vane plate is deeper than the groove in the edge of the optical element. The groove in the edge of the vane plate is semi-circular or deeper such as a channel or trough with a semi-circular base. The groove in the optical element 162 is a much shallower recess.

The cross-section of the bracelet with contact fingers shown in figure 4c includes a flat backplate with curved fingers. Thus the groove in the edge of the vane plate takes the form of a rectangular trench. The groove in the optical element 162 is the same shallow curved recess used for the C-ring and spring retainer. Other shapes of bracelet or fingerstock are available and therefore the grooves may be shaped accordingly. For example, a curved backplate

fingerstock may be used with a curved cross-section groove of figure 4.

For the three embodiments of retainer 170 described above, the retainer is held in the groove in the vane plate by a separate retaining ring 180. Figure 5a shows the vane assembly with optical disc 162 mounted in the vane plate 160 and the retainer 170 held in place by the retaining ring 180. Figure 5b is an exploded view showing each of the parts of the vane assembly. The embodiment shown includes the bracelet embodiment of the retainer with circular disc shaped optical element 162. The retaining ring is also shown in the cross-sectional views of figure 4. The retaining ring 180 fits in a recess 161 in the vane plate so that it does not protrude from the surface of the vane plate and disturb the coolant flow. The retaining ring 180 is mounted to the vane plate 160 using screws or other fasteners. The outer diameter of the retaining ring 180 is larger than the outer diameter of the retainer 170 to allow fixing to the vane plate. The inner diameter of the retaining ring 180 extends at least part way across the retainer 170. For example, as shown in figure 4c the retaining ring 180 extends at least half way across the retainer such that the majority of the retainer is supported and held in position by the retaining ring. Depending on geometry and type of retainer used, the retaining ring may extend more or less far across the retainer. Although the above described embodiment relates to circular disc shaped optical element and annular retaining ring, other shapes of optical element and retaining ring may be used.

As mentioned above, the compressive force on the optical element 162 is influenced by the internal diameter of retainer 180. The compressive force on the optical element is also influenced by the diameter of the aperture or groove 160b in which the retainer 180 is seated. By manufacturing vane plates with different groove depths or apertures the compressive force on the optical element can be tailored to the dimensions of individual optical

elements. Preferably the diameter of the aperture, including the depth of the groove in the vane plate is slightly smaller than the diameter of the optical element with retainer fitted there around. This configuration results in the groove 160b of the vane plate 160 aperture pushing against the retainer compressing it slightly. The

combination of this compressive force and that described above resulting from the smaller diameter of the retainer compared to the optical element holds the optical element securely in the vane plate.

The retaining ring 180 and retainer arrangement do not form a permanent fixing because the retaining ring 180 can be removed by removing fixings 182.

In some embodiments the retaining ring 180 may not be necessary because the retainer 170 is a push fit into grooves in the optical element 162 and vane plate 160. In such a case the vane plate does not have a recess for mounting the retaining ring 180.

The retainer 170 is preferably metallic such as

stainless steel, or in the case of the bracelet it may be Be : Cu . The three embodiments of retainer 170 shown in figures 4a-4c provide either multiple contact points such as for the spring and bracelet options, or a continuous line of contact, for the C-ring option. The multiple contact points or lines provide a thermal conduction path between the optical element 162 and the vane plate 160. Heat in the optical element 162 may be conducted away to the vane plate 160. Both the vane plate 160 and optical element 162 are cooled by the fluid coolant flow. Thus, the conduction path through the retainer 170 provides increased effective area for convention cooling of the optical element 162, which helps to reduce thermal gradients in the optical element 162. For example, if the optical element 162 is gain medium which is optically pumped, the thermal gradient produced by absorption of the pump radiation will be reduced.

In an alternative embodiment the retainer, such as the spring retainer, may be made from an electrically resistive material. This allows the retainer to be used as a resistive heater which may be used to balance external heating of the optical element. For example, during pumping of gain media a central region of the optical element 162 will absorb pump radiation and may get hotter than the edges. By applying edge heating in this way thermal gradients across the optical element 162 may reduced. In turn, reduced thermal gradients across the optical element will provide greater uniformity in the wavefront of the beam optical passing through the device. Conversely, resistive heating in the retainer may also be used to provide beam steering by thermally inducing changes to the wavefront passing through the optical element 162.

The above embodiments have been discussed in relation to an optical element 162, which may be a disc of optical gain medium such as YAG, Yb:S-FAP, Yb : YAG, Nd : YAG, or other solid state laser material that lends itself to optical pumping. However, the arrangements described above may be applied to other optical elements that generate heat, for example other components in which absorbed any energy is converted to heat. Additionally the arrangements may be applied to other components which are mounted in the gas stream and are required to be firmly held in position.

In addition to optical gain media, examples of

components which may be held using the above arrangements include Pockels cells, Faraday isolators, frequency

conversion stages where some optical absorption occurs and many other components in which absorbed energy is converted to heat.

Although aluminium may be used for the vane plate 160 titanium is a preferred alternative because of its better thermal matching to gain media mentioned above. For example, a preferred embodiment uses a Yb-doped ceramic YAG amplifier disc mounted in a titanium vane plate.

Strain tests using an edge-edge mounting jig at 150K with a circular titanium disc mounted in an aluminium vane plate 160 have been performed. These tests suggest that the bracelet retainer is the more versatile option of the three embodiments described herein. Furthermore, interferometric assessment of a crystalline YAG disc mounted using the bracelet retainer showed minimal optical distortion using this edge clamping technique compared to an undamped disc. In a further alternative embodiment the optical element may be held in a mount that is not a vane plate, such as a plate or casting. In such a case the optical element is held in an aperture in the mount using any of the techniques described above. The mounting technique may be applied to any optical element that is subjected to extreme changes in temperature and requires a compliant and durable mounting technique to allow for differential expansion or contraction of the mount and optical element. For example, the optical element may be an optical window that is subjected to significant temperature excursions, such as from room temperature to cryogenic temperatures. The retainer does not provide a hermetic seal but known techniques such as o-rings may be used to additionally provide a seal between one side of the optical element and the other. In other embodiments the optical element may be optical gain medium which becomes heated but cools without forced cooling.

The person skilled in the art will readily appreciate that various modifications and alterations may be made to the above described vane assembly without departing from the scope of the appended claims. For example, different shapes, dimensions and materials may be used. The optical element may be an optical gain medium or other heat generating optical element.

Claims

CLAIMS :

1. A vane assembly, comprising:

an optical element of a laser or optical amplifier, the optical element for cooling by a fluid stream;

a vane plate having an aperture, the optical element mounted in the aperture; and

a retainer arranged to fit between the optical element and the vane plate to provide a compressive force at the edge of the optical element to hold the optical element in the aperture .

2. The assembly of claim 1, wherein the retainer fits between an outer edge of the optical element and an inner edge of the aperture, the retainer adapted to provide the compressive force at the edge of the optical element.

3. The assembly of claim 1 or claim 2, wherein the

retainer comprises a resilient element or spring element.

4. The assembly of claim 2, wherein the compressive force is provided substantially uniformly around the edge of the optical element or at a plurality of contact points arranged substantially uniformly around the edge of the optical element .

5. The assembly of any preceding claim, wherein the edge of the aperture in the vane plate has a groove for location of the retainer.

6. The assembly of any preceding claim, wherein the edge of the optical element has a groove for location of the retainer .

7. The assembly of any preceding claim, wherein the retainer extends substantially completely around the outer edge of the optical element.

8. The assembly of any preceding claim, wherein the retainer has a c-shaped cross-section.

9. The assembly of claim 8, wherein the retainer has a shape corresponding to that of the perimeter of the optical element, the retainer having a smaller diameter than the optical element so as to provide the compressive force.

10. The assembly of any of claims 7 to 8, wherein the retainer is cleaved radially.

11. The assembly of any of claims 1 to 7, wherein the retainer is an extension spring.

12. The assembly of claim 11, wherein the extension spring is endless .

13. The assembly of claim 11 or 12, wherein the extension spring has a shape corresponding to that of the perimeter of the optical element, the retainer having a smaller diameter than the optical element so as to provide the compressive force.

14. The assembly of any of claims 1 to 7, wherein the retainer is a bracelet having a plurality of sprung contact fingers .

15. The assembly of any preceding claim, comprising one or two retaining rings removably coupled to the vane plate to hold the retainer in the aperture of the vane plate.

16. The assembly of claim 15, wherein the one or two retaining rings have an internal diameter at least that of the inner diameter retainer but less than the outer diameter of the retainer.

17. The assembly of any preceding claim, wherein the optical element and vane plate are substantially the same thickness .

18. The assembly of any preceding claim, wherein the retainer is metallic.

19. The assembly of any preceding claim, wherein the retainer is electrically resistive for thermal heating of the optical element.

20. The assembly of any preceding claim, wherein the optical element is a heat generating element.

21. The assembly of any preceding claim, wherein the optical element is optical gain medium.

22. The assembly of claim 21, wherein the optical gain medium comprises Yb:YAG, Nd : YAG or other solid-state laser medium suitable for optical pumping.

23. The assembly of any preceding claim, wherein the aperture and optical element are circular.

24. The assembly of any of claims 1 to 23, wherein the aperture and optical element are square or rectangular.

25. A vane plate for mounting an optical element of a laser or optical amplifier for cooling by a fluid stream, the vane plate having an aperture for receiving the optical element, the vane plate having a groove in the edge of the aperture for receiving a retainer for holding the optical element in the aperture .

26. A laser comprising the assembly of any claims 1 to 24 or the vane plate of claim 25 and wherein the optical element is optical gain medium.

27. An optical amplifier comprising the assembly of any claims 1 to 24 or the vane plate of claim 25 and wherein the optical element is optical gain medium.

28. The laser of claim 26 or the optical amplifier of claim 27 arranged such that a pump beam is incident on the optical gain medium.

29. The laser or optical amplifier of claim 26, wherein the pump beam and/or output beam is transverse to the plane of the vane assembly.

30. The laser or optical amplifier of any of claims 28 or

29, wherein the pump beam and/or output beam propagates through a cooling fluid stream.

31. The laser or optical amplifier of any of claims 28 to

30, wherein the laser is a cryogenic gas cooled laser amplifier .

32. An optical assembly, comprising:

an optical element;

a mount having an aperture, the optical element mounted in the aperture; and

a retainer arranged to fit around the edge of the optical element between the optical element and the mount to provide a compressive force at the edge of the optical element to hold the optical element in the aperture.

33. The assembly of claim 32, wherein the retainer

comprises a resilient element or spring element.

34. The assembly of claim 32 or 33, wherein the retainer is one of the following: an annulus having a c-shaped cross- section; an extension spring; and a bracelet having a plurality of sprung contact fingers.

35. A laser or optical amplifier comprising the assembly of any claims 32 to 34.

36. The laser or optical amplifier of claim 35, wherein the laser or optical amplifier is a cryogenic gas cooled laser amplifier .