GB2102193A - Lasers resistant to thermal expansion - Google Patents

Abstract

A laser (10) comprises an elongate framework (1) supporting a chamber (2) containing a gaseous lasing medium. Mirrors (3) are fitted to mounts (9) located by leaf springs (7) at the ends of the framework (1) and the cavity so formed is rendered insensitive to thermal expansion by three inextensible flexible members (8), made of carbon fibre or invar, extending between and secured to the opposing mounts (9). Coil springs (6) bias the mounts (9) mutually apart and in so doing tension the members (8). Adjustable tension elements (14) may be provided for the members (8). <IMAGE>

Description

SPECIFICATION Lasers This invention relates to lasers.

In order to produce optimum performance from a laser it is necessary that the mirrors which define the resonator cavity be precisely maintained in angular alignment. For example the tolerance may be as little as one arc second. In use most lasers operate at elevated temperatures and the resulting thermal gradients which arise tend to misalign the cavity mirrors and also to vary the spacing between the mirrors both of which factors degrade the performance of the laser.

A known form of resonator cavity which has been designed to overcome these problems is described in U.S. Patent Specification No. 3,783,407 and comprises a framework on which is mounted three quartz rods to the distal ends of which the resonator mirrors, adjustably carried by mounts, are compressively fitted via springs attached to the framework. The quartz rods have a low co-efficient of thermal expansion and extend parallel to but displaced from the optical beam path. By virtue of the fact that the quartz rods are rigid, fragile and expensive and held in compression other components of the laser require to be so designed as to avoid obstructing the quartz rods.

It is an object of the present invention to provide an improved form of laser resonator cavity.

According to the present invention there is provided a laser resonator cavity comprising a rigid elongate framework at the opposing ends of which mirror mounts are located, spring means being provided to resiliently bias the mounts apart, wherein the resonator cavity is rendered thermally insensitive by the provision of inextensible flexible members made of low thermal expansion material extending between and attached to the mirror mounts and held rigid by tension.

Preferably the flexible members are made of carbon fibre or invar (NiXo 36). The flexible members may be of circular or square or rectangular cross-section.

Conveniently the flexible members are of the same length as the resonator cavity and tensioned by the aforesaid spring means. Adjustable tensioning means may be provided separate from said spring means, for example by adjustable bridges arranged to deflect the flexible members intermediate the mirror mounts.

Embodiments of the present invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which: Fig. 1 illustrates a laser having a resonator cavity according to the present invention; Fig. 2 is a view on arrow A of Fig. 1; Fig. 3 is a view on arrow B of Fig. 1; Fig. 4 illustrates three alternative arrangements of a detail in Fig. 1; Fig. 5 illustrates a modified resonator cavity according to the present invention; Fig. 6 is a view on arrow A of Fig. 5; and Fig. 7 is a view on arrow B of Fig. 5.

In Fig. 1 the laser 10 comprises an elongate framework 1 of L-shaped cross-section on which is mounted a chamber 2 containing a gaseous lasing medium. The chamber 2 has Brewster angled windows 2A aligned with the optical axis 11. Mounted at the ends of the framework 1 by means of leaf springs 7 are mirror mounts 9, and mirrors 3 are adjustably secured to these mounts by a spring loaded three-point mounting having adjustment screws 4 in a manner known per se for fine adjustment. To render the cavity insensitive to thermal expansion a plurality of inextensible flexible members 8 extend between and are secured to opposing mounts 9, the members 8 also being secured centrally at clamp 5 to the framework to permit differential expansion. Coil springs 6 bias the mounts 9 mutually apart and in so doing tension the members 8.Of course, differential expansion can alternatively be provided by eliminating clamp 5 and affixing one mount 9 directly to the framework 1.

Fig. 2 illustrates that the Fig. 1 laser has three members 8 orientated to define two orthogonal planes in a manner similar to that disclosed in U.S.

Patent Specification 3,783,407, but any number may in fact be used.

Fig. 3 illustrates that the leaf springs 7 are arranged with two sets horizontal and two sets vertical which allow movement between the mounts 9 and the framework 1 only in the longitudinal direction (i.e. along axis 1 1). The high lateral stiffness of the springs 7 prevents lateral movements. The framework 1 can therefore expand longitudinally without affecting the spacing of the mirrors 3 and their mounts 9 because their spacing is principally determined by the members 8 which are made of low expansion material and are therefore inextensible.

Fig. 4 illustrates three separate ways in which the ends of the members 8 may be secured to the mounts 9 in a fixed manner. In Fig. 4A a ferrule or ball 12 forms a termination on the member 8 and abuts the mount 9 which is slotted to receive the member 8. In Fig. 4B the mount 9 has a through hole for the member 8 and a threaded transverse bore for a grub screw 1 5 to effect clamping directly on the member 8. In Fig. 4C the member 8 terminates in a loop which surrounds a peg 1 6 secured to the mount 9.

Figs. 5-7 show an alternative form of resonator cavity wherein individual members 8 are intro equal parts terminating by being wrapped round a respective pin 1 3 which is adjustable in angular orientation by means of a machine screw 14, the two pins 13 pertaining to one member 8 being closely adjacent. Thus, each member 8 is effectively varied in length by variation of the angular position of pin 13 so that tensioning of the members 8 is varied. It will be evident that this arrangement provides very sensitive control of member 8 in that the length adjustment achieved is very much less than the angular adjustment of screw 14 due to the gearing between screw 14 and pin 13.In this embodiment four leaf spring arrangements 7' space the mounts 9 from the framework 1 and function as previously described.

Additionally however these arrangements 7' serve to tension the members 8 and in this respect incorporate the function of coil springs 6 previously described.

In an alternative construction pins 1 3 may be fixed and screws 14 arranged to adjust the height of a bridge over which members 8 extend. In a still further alternative screws 14 may be hollow and interposed between the end terminations 1 2 (see Fig. 4A) of members 8 and mounts 9, being threadedly received therein so as to provide for axial adjustment of tension. These constructions retain the mechanical advantage afforded by the gearing referred to above.

It will now be appreciated that in the laser described the mirror mounts are spaced apart by tensioned flexible members which may be of any cross-sectional shape. Tensioning of these members may be by any known means and adjustment of the length of the tensioned members may be by devices located at the ends thereof or by bridges which deflect the members from a truly linear path. The members 8 can be located in slots in the framework 1 to prevent lateral movement or held in position by a series of slotted pegs and need not extend in a linear manner but may extend around obstacles without impairing their function. Preferably the members 8 are protected at least along the greater part of their length from local heating effects caused by operation of the laser when chamber 2 or other lasing medium is at elevated temperature.

Because the members 8 are flexible they are light in weight and utilise a minimum quantity of material which being of low thermal expansion is expensive. The framework 1 although illustrated as a separate component from chamber 2 could be integrated with chamber 2 and the mounts 9 and members 8 then carried by that combination.

If members 8 exhibit a tendency to vibrate due to their slenderness this could be overcome by adhering the members 8 to the framework 1 whilst permitting differential expansion, for example by a sealastomer.

Although not described in any detail the mirrors 3 are preferably carried on mounts 9 via holders with orthogonal adjusting screws 4 to permit fine alignment of the mirrors 3 as is known per se.

Claims (7)

1: A laser resonator cavity comprising a rigid elongate framework at the opposing ends of which mirror mounts are located, spring means being provided to resiliently bias the mounts apart, wherein the resonator cavity is rendered thermally insensitive by the provision of inextensible flexible members made of low thermal expansion material extending between and attached to the mirror mounts and held rigid by tension.

2. A laser resonator cavity as claimed in claim 1, wherein each mirror mount is spring mounted on the framework and each flexible member is centrally clamped by clamp means to the framework to permit differential expansion of the framework with respect to the mirror mounts.

3. A laser resonator cavity as claimed in either preceding claim, wherein adjustable tensioning means are provided for each flexible member.

4. A laser resonator cavity as claimed in claim 2, wherein each clamp means incorporates adjustable tensioning means for the pertaining flexible member.

5. A laser resonator cavity as claimed in claim 4, wherein each flexible member is formed in two equal parts the adjoining ends of which are individually clamped and tensioned by said clamp means.

6. A laser resonator cavity as claimed in any preceding claim, wherein each flexible member is made of carbon fibre or invar.

7. A laser resonator cavity substantially as hereinbefore described with reference to any one of the embodiments.