Coherent light generators – Particular beam control device – Q-switch
Patent
1982-11-26
1985-03-26
Sikes, William L.
Coherent light generators
Particular beam control device
Q-switch
372105, 372100, H01S 311
Patent
active
045077851
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an unpolarised electro-optically Q-switched laser.
The output from conventional Q-switched lasers decreases when induced birefringence is present in the laser rod. The explanation 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 that occur in high repetition rate solid state lasers.
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 asssemblies and is quite complex. The crossed-Porro laser is simpler but the output coupling available at high repetition rates is restricted to near 50% which may not be suitable for lasers generating very high or very low peak powers. Further, at high input power levels the crossed-Porro laser suffers from hot spots that may be difficult to overcome. One method of eliminating all 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 the present invention is to provide an improved laser which is relatively free of the problems referred to and the present invention achieves this by a laser geometry able to generate unpolarised, Q-switched radiation using commonly available electro-optic Pockels cells and including a birefringent prism.
According to this invention an improved electro-optically Q-switched laser is provided wherein a laser cavity is formd between a totally reflective mirror and a partially reflective mirror and contains a laser rod and a Q-switching Pockels cell, characterised by a birefringent prism of a material selected to give a low-angle walk-off so arranged in the said cavity that, when a quarter-wave voltage is applied to the Pockels cell, the first pass will be cancelled by an equal and opposite walk-off during the return path.
The following description will refer to the drawings numbered respectively FIGS. 1 to 5 in which:
FIG. 1 shows the laser geometry,
FIG. 2 shows the principle of operation, showing at A the condition of zero volts on the Pockels cell and at B the condition when the cell is energised.
FIG. 3 demonstrates the method of producing a plane polarised output, and
FIGS. 4 and 5 are graphs showing un-Q-switched output against repetition rate and Q-switched output against input at 168 Hz.
The schematic diagram of the laser as shown in FIG. 1 shows that it operates on a somewhat different principle to that commonly used. The novel feature of this laser is that the normal polarising device is, as said, replaced by a birefringent prism 1 of special design. This prism must have the property of providing approximately 1 degree of angular walk-off between orthogonal polarisations after passing through the prism.
In FIG. 1 the semi-opaque mirror 2 forms one end of the laser cavity, the other end being defined by the total mirror 3. The laser rod 4 is pumped by any suitable source. The Pockels cell or Q-switch is designated 5.
The principle of operation of the laser is shown in FIG. 2. Radiation, after passing through the birefringent prism 5 is split up into two orthogonally polarised components, the extraordinary E and the ordinary O rays, that propagate in two slightly different directions. If these two rays are reflected by the mirror 3 back through the prism 1 a further separation of the two rays will occur. Provided this separation is of the order of 1 degree the losses in the cavity will be very high and laser action will be suppressed. However, if a voltage is suddenly applied to the Pockels cell 5 to induce a quarter wave difference between the E and O rays returning to the prism from the mirror 3 will be interchanged. When this happens the walk-off occurring in the first pass will be cancelled by
REFERENCES:
patent: 3243724 (1966-03-01), Vuylsteke
patent: 3564450 (1971-02-01), Immarco et al.
patent: 3740663 (1973-06-01), Andringa
Richards; "Unipolarized EO Q-Switched Laser"; Appl. Optics vol. 22, No. 9; 1 May, 1983.
Bespalov et al; "Single Crystal, Electro-Optic Shutter for Q-Switching Lasers Emitting Unpolarized Radiation"; Opt. Tech. (38), No. 12, Dec. 1971.
Kamach et al; "Electro-optically Q-Switched Pulse-periodic YAG:Nd.sup.3+ Laser"; Sov. Jour. Quant. Electron. 8(7); Jul. 1978.
Jr. Leon Scott
Sikes William L.
The Commonwealth of Australia
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