GB2190237A - Folding prism for use between two sections of a folded laser - Google Patents

Abstract

The prism 16 comprises a first face 17 arranged at the Brewster angle to a beam of radiation 18 entering the prism from a first section of the laser, a second face 17 arranged at the Brewster angle to the beam of radiation 19 leaving the prism, and at least one further face 50, 51 providing loss-free total internal reflection of the beam of radiation within the prism. In a further embodiment (Figs. 7 and 8) the total internal reflection within the prism takes place from the back face (72) of the prism. <IMAGE>

Description

SPECIFICATION Folding prism for use between two sections of a folded laser One of the factors affecting the output power of a laser is the length of the optical path through an excited active medium. Hence the length of the optical path is dependent upon the required power output. The physical dimensions of a laser will be determined by other considerations, however, and it is common to "fold" the optical path of a laser to reduce its actual length. Lasers using free space propagation may be folded using inclined reflectors or prisms to achieve the change in direction of the optical path. Folds through angles up to 180" are common.

Waveguide lasers are also common and operate somewhat differently from the free-space laser, and in particular there are difficulties in producing "folds" which do not introduce losses. The problem is that when the laser waveguided radiation leaves the waveguide to strike a folding mirror or to enter a folding prism its mode of propagation changes to that of free space propagation where the beam is divergent. This change in the mode of propagation causes a loss in energy, referred to as the coupling loss, when the diverging radiation attempts to re-enter the waveguide. It is known to reduce losses by coating surfaces though which the laser radiation passes with anti-reflection coatings. However, such coatings are liable to damage by the high power densities produced by modern lasers.

It is an object of the invention to provide a folding prism primarily, though not exclusively, for use with a waveguide laser which provides substantially reduced coupling loss.

According to the present invention there is provided a folding prism for use between two sections of a folded laser, which prism includes a first face arranged at the Brewster angle to a beam of radiation entering the prism from a first section of the laser, a second face arranged at the Brewster angle to the beam of radiation leaving the prism and entering a second section of the laser, and at least one further face providing loss-free total internal reflection of the beam of radiation within the prism.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a side view of a waveguide laser incorporating a folding prism according to a first embodiment of the invention; Figure 2 shows a plan view of the laser and prism of Fig. 1; Figure 3 is a similar view to that of Fig. 1 with the prism separated from the laser; Figure 4 is a similar view to that of Fig. 2 with the prism separated from the laser; Figure 5 is an isometric view of the prism used in the first embodiment; Figure 6 is a diagram illustrating the effect of the prism of Figs. 1 to 5; Figure 7 is a plan view of a waveguide laser incorporating a prism according to a second embodiment of the invention; and Figure 8 is a similar view to Fig. 7 with the prism separated from the laser.

Referring now to Figs. 1 and 2 these show the main features of a waveguide laser incorporating a prism according to the invention.

The waveguide laser comprises a block 10 of dielectric material in which are formed two parallel bores 11 of such shape and dimensions as to result in waveguide propagation of laser radiation. The exciting electrodes necessary to excite a laser gas contained in the bores 11 are not shown, and the two reflectors defining the ends of the laser optical cavity are shown schematically at 12 and 13 respectively. Reflector 12 is 100% reflecting, whilst reflector 13 transmits a small amount of laser radiation to form an output beam 14.

The chain-dotted line 15 denotes the path of the laser radiation.

At the end of the block 10 remote from the reflectors 12 and 13 is a folding prism 16, one face 17 of which is shown in contact with an end face of the block 10. The face 17 of the prism is arranged to be at the Brewster angle to radiation (18) incident upon it from one of the laser bores 11 and also to radiation (19) leaving the prism on its way back into the waveguide bores. Accordingly, the adjacent end face of the block 10 is also arranged at the Brewster angle so that the face 17 of prism 16 may be placed in contact with the end face of the block 10.

Figs. 3 and 4 show the arrangement of Figs. 1 and 2 with the prism separated from the block for the purposes of clarity.

The prism illustrated in Figs. 1 to 4 is of a complex shape which is illustrated in the isometric view of Fig. 5. The front face 17 is, as stated above, arranged at the Brewster angle to incident (18) and returning (19) laser radiation. The rear part of the prism consists of two faces 50 and 51 which together produce a 1800 rotation of the incident beam by means of total internal reflection of the radiation passing through the prism. Radiation incident upon the front face 17 is refracted downwards. The faces 50 and 51 must be perpendicular to the direction of propagation of the radiation in one plane, and accordingly these two faces are inclined to the horizontal, rather than being vertical in order to compensate for the initial refraction.Fig. 6 shows that the angle 0 between either of the faces 50 and 51 is in fact equal to 1800 minus twice the angle whose tangent is the refractive index of the prism material. By way of example, therefore, if the prism is made from a material having a refractive index of 1.5 then the angle 6 between each of the faces 50 and 51 and the horizontal is approximately 67".

The use of such a prism ensures that the radiation leaving the laser as the output beam is plane polarised, due to the presence of the Brewster angle face 17, the plane of polarisation being parallel to the plane of incidence of the radiation on the face 17.

The reduction of losses achieved by the use of the invention is due to two main factors. In the first place, as is well known, transmission of light at the Brewster angle is in theory lossless. In contrast, anti-reflection coatings suffer losses of the order of 0.3t per pass.

As already stated degredation of coatings is evident after exposure to high power density CW radiation and to non-laser light from the gas discharge. Hence the lack of coatings is an advantage.

Secondly, unlike the use of mirrors which experience surface losses, total internal reflection is lossless. There is, however, some slight absorption loss in the prism material but this is considerabiy less than the surface loss arising from the use of a mirror.

A further advantage occurs if the prism is placed in physical contact with an end face of the laser section in that divergence losses are avoided.

Other forms of prism may be used which satisfy the requirements of the invention. Fig.

7 shows a plan view on one other form of prism, with Fig. 8 showing the same view with the prism 16 separated from the block 10 for clarity. In this arrangement all four faces of the prism shown in Figs. 7 and 8 are "vertical". However the two "side" faces 70 and 71 are inclined so as to be at the Brewster angle to radiation from the waveguide cavities 11 of the laser block 10. The total internal reflection within the prism now takes place from the "back" face 72 as shown, the angle a being greater than the critical angle for the prism material used. The prism of Figs. 7 and 8 is clearly of much simpler form than that of Figs. 1 to 6.

The materials used for the prims will be dependent, at least in part, on the wavelength of the radiation produced by the laser. Materials such as zinc selenide (ZnSe), potassium or sodium chlorides (KCI or NaCI) or germanium (Ge) may be suitable at a wavelength of around 10 microns. The best material will be one having the lowest bulk absorption, the highest thermai conductivity. A further consideration is the choice of geometry, which depends upon the refractive index of the material.

The above description has described prisms for producing 1800 folds in a waveguide laser.

Clearly the same principle may be used for folds at other angles. Similarly, although the waveguide sections shown have been stated to have electrodes for exciting a laser gas, the same prisms may be used to produce folds between passive laser sections. It will also be understood that the invention described above may be used in lasers using free-space propagation, though such lasers simpler folding arrangements are available.

Claims (8)

1. A folding prism for use between two sections of a folded laser, which prism includes a first face arranged at the Brewster angle to a beam of radiation entering the prism from a first section of the laser, a second face arranged at the Brewster angle to the beam of radiation leaving the prism and entering a second section of the laser, and at least one further face providing loss-free total internal reflection of the beam of radiation within the prism.

2. A prism as claimed in Claim 1 in which the first and second faces are contiguous, the total internal reflection taking place between two further faces.

3. A prism as claimed in Claim 1 in which total internal reflection takes place at one further face.

4. A prism as claimed in any one of Claims 1 to 3 which provides a fold of 1800.

5. A prism as claimed in any one of the preceding claims for use with a waveguide laser comprising two or more laser sections.

6. A prism as claimed in Claim 5 arranged to have said first and second faces in physical contact with faces of said waveguide laser sections.

7. A prism as claimed in any one of the preceding claims made from one of the materials of the group containing zinc selenide, potassium chloride, sodium chloride and germanium.

8. A folding prism substantially as herein described with reference to the accompanying drawings.