Mode selection technique for a laser
A mode selection technique in a laser is described wherein a recess is formed in a surface of a waveguide in the laser. The recess provides a region of free space propagation within the waveguide which preferentially selects the lowest order mode. A mode selective RF excited CO 2 slab laser, having a stable resonator in the waveguide dimension and a negative branch unstable resonator in the non-waveguide dimension, is constructed and the position and size of the recess is considered to provide low order mode selection.
1. A laser comprising:
first and second parallel surfaces separated to form a waveguide on a first axis between said surfaces;
a discharge region contained within at least a portion of said waveguide;
a resonant cavity having a propagation axis orthogonal to said first axis;
characterised in that:
a recess is located on at least one of said surfaces, said recess being configured to create a region within said waveguide in which free space propagation occurs in the direction of said first axis.
2. A laser according to claim 1 wherein said recess has a length along said propagation axis in the range 0.1 to 1 Rayleigh range.
3. A laser according to claim 1 wherein a height of said waveguide on said first axis is in the range 1 mm to 4 mm.
4. A laser according to claim 3 wherein said height of said waveguide is in the range 1.3 mm to 2.8 mm.
5. A laser according to claim 3 wherein said recess has a depth greater than one twentieth of said height.
6. A laser according to claim 5 wherein said depth is infinite.
7. A laser according to claim 1 wherein said recess is provided on said first surface and a further recess is provided on said second surface.
8. A laser according to claim 1 wherein said resonant cavity is an unstable resonator in an axis orthogonal to said first axis giving a forward wave mode and a reverse wave mode, and wherein said reverse wave mode has a beam waist.
9. A laser according to claim 8 wherein said recess is located upon an optical axis of said unstable resonator.
10. A laser according to claim 8 wherein said recess is located substantially upon said beam waist.
11. A laser according to claim 8 wherein said recess has a width such that over a majority of a length of said recess, between one half and all of said reverse wave mode is overlapped by said recess.
12. A laser according to claim 3 wherein said discharge region is created between first and second elongate electrodes with an inter-electrode gap equal to said height of said waveguide.
13. A laser according to claim 1 wherein said discharge is an RF excited CO 2 laser gas.
14. A method of providing a mode selective laser, comprising the steps:
(a) providing first and second parallel surfaces which can be arranged to form a waveguide therebetween;
(b) machining a recess in at least one of said surfaces of said waveguide;
(c) providing a resonant cavity around said waveguide; and
(d) creating a gas discharge within at least a portion of said waveguide.
15. A method according to claim 14 , wherein said laser is an RF excited CO 2 slab laser with a negative branch resonator.