Coherent light generators – Particular beam control device – Q-switch
Patent
1983-12-21
1985-10-08
Sikes, William L.
Coherent light generators
Particular beam control device
Q-switch
372 93, 372 98, H01S 311
Patent
active
045464774
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a pulse transmission mode laser that may also be used in a pulse reflection mode.
The output from conventional Q-switched lasers drops when induced birefringence is present in the laser rod. Briefly the reason for this is that conventional lasers contain a polariser that rejects any radiation not correctly polarised, hence the depolarisation that occurs due to birefringence leads to lost energy and poor efficiency. The birefringence could have several sources, a common one is due to thermal stresses as occurs, for example in high repetition rate lasers. Another source is due to imperfectly grown laser rods as commonly occurs, for example, in calcium tungstate.
Techniques exist for maintaining efficiency when birefringence is present but these have some deficiencies. For example the method of Scott and De Wit employs two separate laser rod-flashlamp assemblies and is quite complex. The crossed porro laser, as described for instance in the Specification of Australian Pat. No. 466,196, (British Pat. No. 1,358,023) Ferranti Limited or the Specification of U.S. Pat. No. 3,924,201, International Laser Systems, is simpler but the outcoupling available is restricted to near 50% which may not be suitable for lasers generating very high or very low peak powers. One method of eliminating the effects of birefringence is to generate an unpolarised beam. Electro-optic devices able to switch unpolarised radiation have been fabricated but these are not readily available.
The object of this invention is to provide a laser geometry able to generate unpolarised, Q-switched radiation using commonly available electro-optic Pockels cells.
The invention generally comprises a laser operating either on the pulse transmission mode or the pulse reflection mode and comprises a laser cavity containing on one side of the laser rod a mirror and on the other side, in sequence, a polariser and electro-optical cell and first fold-back reflector means and through a second reflector to the polariser to form a loop around the optical cell. The mirror can be a partial mirror to allow output from the cavity through the mirror in pulse reflection mode, or it can be a total mirror in which case the output can be arranged to be from the polariser in pulse transmission mode.
The loop so formed is thus independent of the laser rod, energy rejected out the side of the polariser is directed back into it via the loop around the electro-optical cell.
To enable the invention to be readily understood, the following description will be made with reference to the accompanying drawings in which:
FIG. 1 shows a typical laser assembly embodying the invention, using a partially reflecting mirror to allow output through the mirror. If the mirror is totally reflecting the output is from the polariser as shown by the dotted arrow.
FIG. 2 shows the ray paths through the polariser and the electro-optical cell when there is no phase shift in the electro-optical cell, and
FIG. 3 shows the ray paths when a .lambda./2 phase shift occurs in the electro-optical cell.
Referring first to FIG. 1, the laser rod 1 is positioned in the laser cavity which is defined between the partially transmitting mirror 2 and the fold-back reflector 3, in this case a double reflecting prism, the cavity including, between the laser rod 1 and the fold-back reflector 3, the polariser 4 and the electro-optical cell 5.
The fold-back reflector 3 passes radiation back outside of the electro-optical cell to be reflected by a second reflector 6, in this case a prism, back to the polariser 4.
The laser operates by redirecting any energy rejected out the side of the polariser 4 back into the polariser via the loop composed of the two totally internal reflectors 3 and 6. Similarly energy passing through the polariser 4 is returned to it by the loop, although in this case the energy circulates in the opposite direction. Of course, as illustrated in the Figures and as would be inherently known to those skilled in the art, reflectors 3 and 6 must be positioned with respect to pola
REFERENCES:
patent: 3719414 (1973-03-01), Wentz
patent: 3914710 (1975-10-01), Young
patent: 4331387 (1982-05-01), Wentz
patent: 4408334 (1983-10-01), Lundstrom
patent: 4441186 (1984-04-01), Erickson
Ernest et al., "Giant Optical Pulse Shortening Through Pulse-Transmission Mode Operation of a Pulse Laser"; Phys. Lett., vol. 22, No. 2, Aug. 1, 1966.
Jr. Leon Scott
Sikes William L.
The Commonwealth of Australia
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