A LASER

Abstract

A laser system having an optical ring resonator, a beam splitter that directs a first beam travelling in a first direction around the ring resonator, out of the resonator towards a reflector that reflects the first beam back into the resonator to travel in an opposite direction around the optical ring resonator that is in the same direction as a second beam travelling around the optical ring resonator; and a beam modifier configured and adapted to modify a spatial distribution of phase across an aperture of the first beam such as to cause it to become more similar or substantially match that of a spatial distribution of phase across an aperture of the second beam.

Claims

1. A laser system comprising: an optical ring resonator; a gain medium configured and arranged to output: a first beam that will circulate around the optical ring resonator in a first direction, and a second beam that will circulate around the optical ring resonator in a second direction, the second direction being opposite to the first direction; and means for redirecting the first beam so as to cause the first beam to travel in the second direction around the optical ring resonator; the means for redirecting including: means for directing at least a portion of the first beam out from the optical ring resonator; and means for reintroducing the at least a portion of the first beam back into the resonator such that it will travel in the second direction around the ring resonator; the system including: a beam modifier configured and adapted to modify a spatial distribution of phase across an aperture of the at least a portion of the first beam coupled out of the optical ring resonator, such as to cause it to become more similar or substantially match that of a spatial distribution of phase across an aperture of the second beam.

2. A laser system according to claim 1, wherein the beam modifier comprises: a surface upon which the first beam is incident, the surface being profiled to modify a wavefront of the first beam such that the spatial distribution of phase across the aperture of the first beam will become more similar or substantially match that of the spatial distribution of phase across the aperture of the second beam.

3. A laser system according to claim 2, wherein the means for redirecting the first beam comprises: a beam splitter that directs at least a portion of the first beam out of the resonator towards a reflector that reflects the portion of the first beam back into the optical ring resonator.

4. A laser system according to claim 3, wherein the beam modifier is configured to be transmissive to the first beam and arranged between the beam splitter and the reflector.

5. A laser system according to claim 3, wherein a reflecting surface of the reflector is profiled to modify the wavefront of the portion of first beam such that the spatial distribution of phase across the aperture of the first beam will become more similar or substantially match that of the spatial distribution of phase across the aperture of the second beam.

6. A laser system according to claim 5, wherein the beam splitter is configured to output a portion of the second beam and the re-directed first beam from the optical ring resonator.

7. A laser system according to claim 6, comprising: a wavefront distortion rectifier configured and adapted to rectify wavefronts of both the second beam and the redirected modified first beam to correct wavefront aberrations of said second beam and redirected modified first beam.

8. A laser system according claim 7, wherein the wavefront distortion rectifier is arranged to rectify the wavefront of the second beam and redirected first beam following output from the optical ring resonator.

9. A laser system according to claim 7, wherein the wavefront distortion rectifier is configured and arranged intra-cavity within the optical ring resonator.

10. A laser system according to claim 8, wherein the wavefront distortion rectifier comprises: a body that is configured to be transmissive to the first beam and the second beam, the body having an outer surface profiled such that the body has a thickness that varies across the apertures of the first and second beams.

11. A laser system comprising: an optical ring resonator and a wavefront distortion rectifier, the wavefront distortion rectifier being configured and adapted to modify a wavefront of an optical beam incident thereon to correct wavefront aberration of said optical beam.

12. A laser system according to claim 11, wherein the wavefront distortion rectifier is arranged to modify the wavefront of an optical beam that has been outputted from an optical ring resonator.

13. A laser system according to claim 11, wherein the wavefront distortion rectifier is arranged intra-cavity within the optical ring resonator to modify a wavefront of an optical beam within the optical ring resonator.

14. A laser system according to claim 13, wherein the wavefront distortion rectifier is configured and arranged to correct wavefront aberration of a first optical beam travelling in a first direction around the optical ring resonator.

15. A laser system resonator according to claim 14, wherein the wavefront distortion rectifier is configured and adapted to modify the wavefront of an optical beam transmitted through the wavefront distortion rectifier.

16. A laser system resonator according to claim 15, wherein the wavefront distortion rectifier comprises: a surface profiled to modify the wavefront of the optical beam incident on said surface.

17. A method of operating a laser, the method comprising: providing an optical ring resonator; activating a gain medium in order to cause it to output: a first beam that circulates around the optical ring resonator in a first direction, and a second beam that circulates around the optical ring resonator in a second direction, the second direction being opposite to the first direction; directing at least a portion of the first beam out from the optical ring resonator; reintroducing the at least a portion of the first beam back into the resonator such that it travels in the second direction around the ring resonator; and modifying a spatial distribution of phase across an aperture of the at least a portion of the first beam coupled out of the optical ring resonator, such as to cause it to become more similar or substantially match that of a spatial distribution of phase across an aperture of the second beam.

18. A laser system according to claim 3, wherein the beam splitter is configured to output a portion of the second beam and the re-directed first beam from the optical ring resonator.

19. A laser system according to claim 4, wherein the beam splitter is configured to output a portion of the second beam and the re-directed first beam from the optical ring resonator.

20. A laser system according to claim 1, comprising: a wavefront distortion rectifier configured and adapted to rectify wavefronts of both the second beam and the redirected modified first beam to correct wavefront aberrations of said second beam and redirected modified first beam.

Description

[0033] The invention will now be described by way of example with reference to the Figures in which

[0034] FIG. 1 is a schematic of a prior art ring resonator;

[0035] FIG. 2 is a schematic of a ring resonator adapted to redirect one of the counter propagating beams and modify the wavefront of the redirected beam to substantially match that of the other beam; and

[0036] FIG. 3 is a schematic illustration the measurement of the clockwise beam I.sub.c and anticlockwise beam I.sub.a.

[0037] FIG. 2 illustrates a laser system 1 comprising a laser gain medium 2, in this example Nd:YAG, and reflectors 3, e.g. mirrors arranged to create a closed optical path to provide a ring resonator. In this case the optical path is rectangular though the shape is unimportant. The laser system 1 further includes a beam splitter 4, e.g. a partially reflective mirror or possibly a polarising beam splitter, arranged to lie in the optical path of the ring resonator, and a further reflector 5, in this example a corner cube retroreflector though a flat mirror could be used instead.

[0038] The laser gain medium 2 when activated by an external energy source (not shown) outputs counter propagating laser beams: a clockwise beam I.sub.c and anticlockwise beam I.sub.a. For the reasons described in the introduction, there are likely to be differences in the phase profile (i.e. the phase profile in a plane perpendicular to the direction of propagation) of beam I.sub.a compared with beam I.sub.c.

[0039] The beam splitter 4 couples out a portion of I.sub.c from the resonator towards a further reflector 5. The further reflector 5, which in this example is a corner cube retroreflector though could be a flat mirror, reflects the portion of I.sub.c back to the beamsplitter 4 which couples I.sub.c back into the resonator so as to travel in the anticlockwise direction. Between being coupled out and back into the resonator, the wavefront of I.sub.c is modified by a beam modifier 6. The anticlockwise travelling modified clockwise beam I.sub.c is shown in FIG. 2 as I.sub.ca.

[0040] The beamsplitter 4 also functions to couple out from the resonator the combined beam of I.sub.a and I.sub.ca to provide the used output of the laser system 1.

[0041] The beam modifier 6 is positioned in the optical path between the beam splitter 4 and the further reflector 5. The beam modifier 6 comprises a body that is transmissive to beam I.sub.c and the beam modifier 6, beam splitter 4 and further reflector 5 arranged such that beam I.sub.c passes through the body twice, once towards the further reflector 5 and again on its return towards the beam splitter 4.

[0042] The body (e.g. of plate formthe beam modifier may take a form commonly referred to as a phase plate) has an outer surface 6A that is profiled using conventional techniques such as laser etching, such that the thickness of the body varies across the aperture of beam I.sub.c. The profile of the surface 6A is formed to modify the wavefront of I.sub.c such that the wavefront of I.sub.ca at any point about optical path of the ring resonator substantially matches the phase front of I.sub.a.

[0043] In order to profile the surface 6A of the modifier 6, to provide the required modification of beam I.sub.c the wavefront of both clockwise beam I.sub.a and anticlockwise beam I.sub.a need to be measured.

[0044] The wavefront of I.sub.c is measured, with an interferometer, at or close to the position that the surface 6A of the modifier 6 will lie when the laser system 1 is in use. The source beam I.sub.sa of the interferometer arranged to coincide with I.sub.c to form a fringe pattern used to carry out this measurement is shown in FIG. 3.

[0045] The separation x between the measurement point (i.e. where the surface 6A will lie) and the coupling point into the resonator provided by the beam splitter 4 is identified.

[0046] The wavefront of the portion of the beam I.sub.a coupled out of the resonator by the beam splitter 4 is similarly measured. The source beam I.sub.sa of the interferometer used to carry out this measurement is also illustrated in FIG. 3.

[0047] The distance y between the measurement point of the wave front of I.sub.a and the coupling point of I.sub.a provided by the beam splitter 4 out of the resonator is also identified.

[0048] Using the wavefront measurements of I.sub.a and I.sub.c, a profile of the surface 6A is determined that will modify I.sub.c so that its wavefront after travelling a distance x+y following its transmission through the surface the second time matches, as much as possible, that of the measured wavefront of I.sub.c. The various methods of carrying out such a determination will be familiar to those skilled in the art. The surface 6A of the modifier 6 is then profiled to the specification determined using conventional techniques such as, for example, laser etching, ion beam etching, chemical etching or photolithography.

[0049] With reference to FIG. 2, the laser system 1 further comprises a beam rectifier 7 that comprises a body (e.g. of plate form) that is transmissive to I.sub.a and I.sub.ca. The beam rectifier 7 is arranged to rectify the wavefront of the combined beam following its coupling out of the resonator. The body defines a surface 7A profiled, using conventional techniques, so that the thickness of the body varies across the aperture of combined beam of I.sub.a and I.sub.ca, to reduce the divergence, i.e. make more collimated, the laser system's output beam 1. If the surface 7A of the beam rectifier is to be separated from the output of the beam splitter by distance y then the same wavefront measurements of I.sub.a to profile the modifier surface 6A can be used to profile rectifier surface 7A.

[0050] In a variant arrangement, the beam rectifier 7 may be positioned within the resonator as illustrated by ghosted representation 7 in FIG. 2. Where this is so, the surface profiles 6A and 7A of the beam modifier 6 and beam rectifier 7 respectively will need to differ compared with when the beam rectifier is outside of the resonator to account for unintentional modification of the wavefront of I.sub.c by the beam rectifier as it circulates around the resonator.

[0051] It will be appreciated that the laser system 1 may be implemented using gain mediums other than Nd:YAG.

[0052] The optical path of the clockwise beam I.sub.c may be modified, possibly through provision of additional optical components, such that it only passes through modifier 6 once (or more than twice) between the beam splitter 4 and the further reflector 5.

[0053] The beam rectifier 7 may take other forms. For example it may be provided by a reflective surface and this may, for example, be implemented where one or more of the reflectors 3 take the form of a mirror, through profiling one or more of the reflectors' reflective surfaces.

[0054] The resonator may comprise a Q-switch in addition to the laser gain medium.

[0055] Variant ring resonator designs can incorporate components such as corner cubes and folding prisms instead of mirrors, as well as components with focal power e.g. lenses, or curved mirrors.

[0056] It is preferred that reflector 5 is used in order that the first beam is reintroduced into the resonator at substantially the same point that it is ejected. This is preferably achieved by using the same beamsplitter 4 to both eject and reintroduce the first beam. However in a variant, different beam splitters may be used to respectively eject and reintroduce the beam. In another, those probably less preferred variant, it may be possible to dispense with the reflector 5 and instead use an additional beam splitter to reintroduce the first beam into the ring at a different point (e.g. directly opposite the first beam splitter) 4 in the resonator.