LASER RESONATOR

Abstract: A laser resonator of the type containing in a cavity a laser rod (6) and polariser (1) and a Q-switch (4) arranged about the axis of a laser cavity wherein total reflectors (2, 3) of the prism type are used at the ends of the cavity, the reflectors (2, 3) being TIR prisms with four reflecting surfaces in each angled one to the other by 45<o> and the roof lines (A) parallel to the said laser cavity axis.

Description: "LASER RESONATOR" This invention relates to an improved laser and in particular it relates to a laser of the general type which uses prism reflectors of the total reflection type, such as Porro prisms. Lasers employing Porro prism reflectors, for example the crossed Porro laser, are finding favour due to their insensitivity to mechanical shocks. Such lasers are the subject of patents, such as Ferranti Australian: Patent No. 466,196 and International Laser Systems Inc. (ILS), USA Patent No. 3,924,201. These patents describe systems having a total reflector at each end of the laser cavity and take their output from a polariser. We have now found that a more advantageous laser of the type using a prism-type total reflector at each end is possible using a type of prism described in Opto-Electronics, Volume 5, page 255 (1973) under the title "Compound TIR prism for polarisation- selective resonators". Such a prism consists of four flat surfaces, three of which are inclined at 90° to each other and the fourth inclined at 45 to one of the roof edges formed by the other three sides. A ray entering the surface will undergo four reflections, each at 45° incidence, before being reflected along its original path. Provided the refractive index of the prism exceeds 2 each reflection will be total and no energy will be lost. The four reflections are so arranged that the parallel and perpendicularly polarised components at one reflection are interchanged at the next reflection. This causes the phase shift that occurs at one total internal reflection to be reversed at the next reflection and because there are an even number of reflections the overall ° phase shift due to total internal reflection is zero. However, due to image reversal a phase shift of 180 occurs in one component, hence the prism is equivalent to a half wave plate. A plane polarised beam, entering the prism with its plane of polarisation * 5 inclined at an angle θ with the X axis,.will emerge from the prism with its plane of polarisation rotated by 2Φ. A Porro prism operates differently from the compound TIR prism in that only two total internal reflections occur, and the phase shift occurring 0 at each reflection add rather than cancel. The total resultant phase shift between S and P components from a Porro prism is given by the following expression phase shift = * + 4tan~ 1 /[l - 2/(n 2 )] where n is the refractive index of the Porro prism. The fact that the phase shift is not it or a multiple thereof leads to some performance limitations in lasers using Porro prisms. In the drawings forming part of this specification FIG. 1 is a perspective view of a compound 0 TIR prism, FIG. 2 illustrates schematically an in-line laser according to this invention, and FIG. 3 illustrates similarly a folded design. Referring first to FIG. 1 which illustrates a TIR prism it will be seen that the surface A which forms the entry and exit surface for the rays and which is normal to the laser cavity axis when in use and we thus term the "normal" surface has the surfaces B and C extending rearwardly from it at 90° while the surfaces B and C themselves are at 90° the one to the other, the surface D extending rearwardly from the surface A at a 45 angle with the surfaces B, C and D merging at the roof line R. As referred to earlier herein, provided the refractive index of the prism exceeds /2, each reflection will be total. The four reflections are such that the S and P components of the beam are interchanged as the beam traverses the prism as shown in the illustration. This allows the phase shifts that occur between S and P components on total internal reflection to cancel, giving no phase shift due to this cause. There is a second effect that produces a phase shift and it is the image reversal that occurs when the beam is reflected by the prism. It causes an effective phase shift of 180 in that component of the radiation that is polarised parallel to the Y - axis, hence the phase shift produced by the prism is the same as that produced by a half wave plate. & V . i- _ OMPI ' s A plain polarised beam, entering the prism with its plane of polarisation inclined at an angle θ is the X axis shown in FIG. 1, will emerge from the prism with its plane of polarisation rotated by 2θ. Referring now to FIGS. 2 and 3, in which similar reference characters are used for corresponding parts, a pair of compound TIR prisms 2 and 3 are used which are placed one at each end of the resonant cavity, and as in the case of the crossed Porro laser either an in-line configuration as shown in FIG. 2 or a folded configuration as shown in FIG. 3 can be used. The compound TIR prism 2 at the Pockels cell end of the laser is orientated at 45° so that the reflected beam receives a 90° rotation in its plane of polarisation. The Q-switch is designated 4 and its voltage generator 5. This rotation leads to perfect hcld-off in the Q-spoiled state without the need for bias to be applied to the Pockels cell Q-switch. This allows a fairly simple driving circuit for the Pockels cell, especially when compared to the Porro case in which bias is required in order to achieve perfect hold-off. Further, the elimination of the bias from the Pockels cell is likely to lead to more reliable operation since possible electrical leakage problems will be avoided. The compound TIR prism at the other end is disposed adjacent to the laser rod 6 and is used to control the output coupling by rotation about an axis parallel to the laser beam. At an orientation of Φ, the effective reflectivity is given by R = COS 2 (2 ) which can be varied between 0 and 1007 o . This range exceeds that obtainable with a Porro and leads to some performance advantages for the compound TΪR prism e.g. in very high gain lasers where very low reflectivities are desired or where the intracavity power levels are required to be kept very low. The laser rod 6 is powered in any normal manner such as by a flash lamp 7 powered by a supply 8. The output is at 9. In each case the laser rod is designated 6 and the polariser 10 but in FIG. 3, because of the folding, a mirror 11 is included in the cavity. The 45 face of the compound TIR prisms 2 nd , 3 is shown dotted to readily diff rentiate that reflective face. From the foregoing it will be realised that by replacing the Porro prisms of earlier systems, with TIR prisms, an improved laser results which has low energy loss, firstly because the TIR prisms have the four required reflections internally of the prism and secondly because the perpendicularly polarised components at one reflection are interchanged at the next reflection. The system also lends itself to very rugged mechanical construction because the resonator is relatively unsensitive to alignment errors provided that the retro-reflecting planes of the prism are arranged to be substantially perpendicular to each other. Also, use of compound TIR prisms in polarisation coupled lasers introduces further advantages over the use of Porro prisms. Foremost are the performance advantages of perfect hold-off without the need to bias the Pockels cell and of extending the range of output coupling available. Another advantage 5 arises due to the independence of performance on refractive index, a feature that allows the choice of prism material to be made on such grounds as thermal stability, optical quality, low absorption coefficient, high damage thresholds, etc, rather ^ u than on refractive index as In the case of Porro prisms. Improved reliability and efficiency will result from this freedom in choosing prism properties.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A laser resonator wherein the laser cavity contains at least a laser rod and a polariser and a Q-switch arranged about the axis of the said laser cavity, and wherein the said cavity is defined between a pair of total reflectors of the prism type, characterised in that the said prisms are compound TIR prisms arranged to have -their "normal" surfaces face the cavity but orientated to be one at an angle of 45° to the other about the said laser cavity axis and wherein the roof lines of the said TIR prisms are parallel to the said laser cavity axis. 2. A laser resonator according to claim 1 wherein the said cavity is folded and includes a mirror to fold the said cavity in conjunction with the said polariser. > - _ zA C. ' PI

Inventors: RICHARDS JAMES
Applicants: COMMW OF AUSTRALIA, RICHARDS JAMES, COMMW AUSTRALIA
Application year: 1982
Main classification: H01S3
IPC codes: H01S3