GB2293266A

GB2293266A - Folded cavity laser with improved temperature stability

Info

Publication number: GB2293266A
Application number: GB9518255A
Authority: GB
Inventors: Trevor James Cook
Original assignee: Thales Optronics Taunton Ltd
Current assignee: Thales Optronics Taunton Ltd
Priority date: 1994-09-07
Filing date: 1995-09-07
Publication date: 1996-03-20
Anticipated expiration: 2015-09-07
Also published as: GB9417964D0; GB9518255D0; FR2724266B1; FR2724266A1; GB2293266B

Abstract

An optical resonance cavity contains a rod 16 of optically active material which is excited to emit light, and a resonance path is formed between an prismatic end reflector 12 and a partially reflective output reflector 14. The resonance path is folded by an intermediate corner cube reflector 10, and contains a Q-switch 18. The output reflector 14 comprises a block of transparent material having a semi-reflective coating, and the end reflector 12 is adhesively bonded to the output reflector, resulting in a very stable configuration (e.g. with respect to temperature). The hypotenuse faces of the end reflector 12 and corner cube reflector 10 are tilted to prevent the formation of secondary cavities. <IMAGE>

Description

LASER GENERATOR TECHNICAL FIELD OF THE INVENTION This invention relates to laser generators of the folded cavity type.

BACKGROUND GB 1 515 985 discloses a laser generator of the folded cavity type, in which an optical resonance cavity contains a body of optically active material, means for exciting the active material to generate oscillating radiation in the cavity, a resonance path formed between an end reflector and a partially reflective output reflector, and at least one intermediate reflector disposed in the path between the end reflector and the output reflector.

In this prior arrangement, the two reflectors are in the form of mirrored surfaces which may be carried on a common support plate. However, this arrangement can result in low output efficiency, and the output can change considerably with variations in temperature causing distortion of the plate and movement of the reflectors.

An aim of the present invention may be viewed as being to provide a form of folded cavity which is capable of giving improved efficiency over a wide temperature range.

SUMMARY OF THE INVENTION The present invention proposes a folded cavity laser generator comprising an optical resonance cavity containing a body of optically active material, means for exciting the active material to generate oscillating radiation in the cavity, a resonance path formed between an end reflector and a partially reflective output reflector, and at least one intermediate reflector disposed in the path between the end reflector and the output reflector, in which the output reflector comprises a body of transparent material having a coating which is semi-reflecting at the laser wavelength, and the end reflector comprises a prismatic reflector which is fixed with the output reflector.

Ideally the end reflector and the output reflector would be formed from a single block, but since there is no known machine which can form complex shapes of the kind which may be required, the two reflectors will generally be formed as separate bodies. The two bodies are preferably bonded together by an adhesive rather than being clamped together for example.

The invention also provides a folded cavity laser generator comprising a prismatic reflector having a hypotenuse face which is inclined with respect to a plane to which the resonance path is perpendicular on entering and leaving the prism. Thus, any reflection from this face cannot reduce the output by contributing to the lasing threshold due to the formation of secondary resonance cavities.

BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a schematic representation of the cavity of a laser generator of the invention, Figure 2 is a front view of a reflector assembly included in the cavity, looking from the left in Fig. 1, and Figure 3 is an end view of the reflector assembly.

DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 shows a folded optical resonance cavity, in which the optical resonance path is formed into two parallel linear sections A and B by a prismatic corner cube reflector 10. One end of the path is defined by an end reflector 12 and the opposite end is defined by a partial reflector 14, both of which will be described in more detail below. A rod 16 of optically active material such as YAG (ytrium aiuminium garnate, Y3AlsO,2), is positioned in the optical path (in this case in section B) such that the material can be optically excited by known means (e.g. a flash lamp, not shown) to emit light radiation into the optical path. The optical path also contains an optional dye Q-switch 18 in the form of a wafer of material having an absorption band which covers the lasing wavelength.The Q-switch does not allow oscillation to occur until it becomes saturated, and thus acts to concentrate the energy into a very short duration pulse (typically 8 to 15 nanoseconds) having a high peak power (e.g.

2 megawatts). An alternative method of Q-switching could be used, requiring a polariser plate and an electro-optic crystal cell. In the present case the dye Q-switch is located between the rod 16 and the corner cube 10, although it could be positioned anywhere in the optical path. A pair of alignment wedges 18 and 20, which are arranged to be rotated relative to each other in known manner, are included for making fine adjustments to the optical path (see below). These could be mounted anywhere in the path.

The output reflector 14, which is shown in more detail in Fig.s 2 and 3, comprises a glass bar 22 of rectangular transverse section. When viewed from the front as in Fig. 2, it will be seen that one end face 24 of the bar is rounded, and an oval area of the front face 25 adjacent to the rounded end 24 is mirrored with a multi-layer coating 26, which is 500/0 reflective at the lasing wavelength. The opposite, parallel face 27 (Fig. 1) has a coating which is anti-reflecting at the lasing wavelength. Thus, when photons are incident on the mirror area 26, 50% are reflected back into the optical path to contribute to the amplification while 50% pass perpendicularly through the bar 22 to emerge as a laser output pulse C.

The opposite end 28 of the bar 22 is ground perfectly flat, optically smooth, and perpendicular to the four side faces of the bar.

The end reflector 1 2 is of a prismatic form, the roof angle between faces 30 and 32 (Fig. 3) being 90". It will also be noted that the hypotenuse face 38 is ground at 5 to the image through the prism, and hence is set at 5 to both the front face 25 of the output reflector 14 and with respect to a plane to which the resonance path is perpendicular on entering and leaving the prism.

The reason for this will be explained below. An end face 40 of the end reflector 1 2 (Fig. 2) is ground flat and optically smooth, and is bonded to the end face 28 of the bar 22 using suitable cements.

Like the reflector 12, the hypotenuse face 21 of the corner cube 10 is tilted, during manufacture, at an angle of 5 with respect to the internal angles.

Again, the reason for this will shortly be explained.

Light entering the prism 12 from the optical path A undergoes total internal reflection and reemerges back into the light path parallel to the entering beam. Since the end reflector 12 is fixed relative to the output reflector 14 the relative alignment of the two reflectors is accurately determined and cannot change due to mechanical shock, changes in temperature, or the like.

The wedges 18 and 20 permit fine adjustments to be made to correct any slight shift in alignment between the reflectors 12 and 14 after the cements are cured.

When the flash lamp is fired continuously it often tends to heat the rod 16 unevenly due to one side of the rod being closer to the lamp. This results in the rod bending away from the lamp. To compensate for this, and for any other small mechanical changes with temperature, the prism 1 2 is orientated such that the compensating axis of the prism eliminates this shift. The hypotenuse faces 38 and 21 of the prism 12 and of the corner cube 10 are thus tilted, during manufacture, with respect to the incoming and outgoing beams, to prevent these surfaces from forming a secondary cavities with the output coupler 14. If these faces were truly perpendicular to the incident beams a resonant mode could become active between the mirrored surface 26 and the respective face 38, 21, even if these faces have very low reflectivity. This could contribute to the lasing threshold, so that any slight shift in these faces with temperature would affect the lasing threshold and beam profile. The deliberate tilting of these surfaces ensures that only the reflective face 26 of the output coupler 14 and the internal prism surfaces 30 and 32 of the end reflector 12 can define the path of the photons. This results in an extremely efficient and stable resonator.

Claims

1. A folded cavity laser generator comprising an optical resonance cavity containing a body of optically active material, means for exciting the active material to generate oscillating radiation in the cavity, a resonance path formed between an end reflector and a partially reflective output reflector, and at least one intermediate reflector disposed in the path between the end reflector and the output reflector, in which the output reflector comprises a body of transparent material having a coating which is semi-reflecting at the laser wavelength, and the end reflector comprises a prismatic reflector which is fixed with the output reflector.

2. A laser generator according to Claim 1, in which the end reflector and the output reflector are formed as separate bodies.

3. A laser generator according to Claim 2, in which the end reflector and the output reflector are bonded together by an adhesive.

4. A laser generator according to any preceding claim, in which the end reflector has a hypotenuse face which is inclined with respect to a plane to which the resonance path is perpendicular on entering and leaving the prism.

5. A laser generator according to any preceding claim, in which the intermediate reflector comprises a prismatic reflector having a hypotenuse face which is inclined with respect to a plane to which the resonance path is perpendicular on entering and leaving the prism.

6. A folded cavity laser generator substantially as described with reference to the drawings.