Method of operating a laser and laser apparatus using intra-cavity digital holograms
Laser apparatus includes an output coupler, a gain medium for generating laser light, a rear optical element, and an input coupler. The input coupler is arranged to direct a pump beam to the gain medium and to define an optical path between the output coupler and the rear optical element. The rear optical element is a spatial light modulator arranged to act as an intra-cavity digital holographic mirror which can be digitally addressed. The spatial light modulator displays selectively a gray-scale image of a hologram thereby to phase-modulate laser light in the cavity, thus making it possible to generate an output laser beam having a desired characteristic. The apparatus includes a computer arranged to generate at least one hologram corresponding to a desired output beam characteristic, and a driver circuit responsive to an output from the computer to generate a corresponding gray-scale image of the hologram on the spatial light modulator.
1. Laser apparatus including:
an output coupler;
a gain medium for generating laser light;
a rear optical element;
an input coupler arranged to direct a pump beam to the gain medium and to define an optical path between the output coupler and the rear optical element, wherein the input coupler comprises a folding mirror that forms an L-shaped cavity, the output coupler comprises a flat curvature, and the rear optical element is a spatial light modulator arranged to act as an intra-cavity digital holographic mirror and to display selectively a gray-scale image of a hologram thereby to phase-modulate laser light in the cavity, to generate an output laser beam having a desired shape characteristic; and
an intra-cavity Brewster window to assist the spatial light modulator to operate with a desired polarization, vertical or horizontal.
2. The laser apparatus of claim 1 wherein the spatial light modulator is a phase-only reflective device arranged to operate as a digitally addressed holographic mirror.
3. The laser apparatus of claim 2 wherein the spatial light modulator is arranged at normal incidence in the cavity.
4. The laser apparatus of claim 2 wherein the spatial light modulator has a reflectivity as a function of phase with a variance of less than 1%.
5. The laser apparatus of claim 4 wherein the spatial light modulator is a device comprising parallel-aligned nematic liquid crystal.
6. The laser apparatus of claim 1 wherein the input coupler comprises a first mirror coated for high transmission at the pump beam wavelength and high reflectance at a lasing beam wavelength, and which is oriented to direct the pump beam to the gain medium, and to direct a lasing beam to the spatial light modulator.
7. The laser apparatus of claim 6 wherein the output coupler comprises a second mirror coated for high transmission at the pump beam wavelength and high reflectance at the lasing beam wavelength, and which is oriented to separate the pump beam and the output laser beam.
8. The laser apparatus of claim 1 including a computer arranged to generate at least one hologram corresponding to a desired output beam characteristic, and a driver circuit responsive to an output from the computer to generate a corresponding gray-scale image of the hologram on the spatial light modulator.
9. The laser apparatus of claim 8 wherein the computer is operable to generate a plurality of different predetermined holograms corresponding to respective desired output beam characteristics, to enable on-the-fly selection of a desired output laser beam.