Optical fibre laser

1. A fibre laser comprising a single mode or low-order-mode cladding pumped fibre laser oscillator or preamplifier, and a cladding pumped fibre laser power amplifier adapted to guide multiple transverse modes, wherein an output from the oscillator or preamplifier is applied through a mode mixing means to the power amplifier to provide that a laser signal of the cladding pumped fibre laser power amplifier is emitted from the fibre laser power amplifier in the multiple transverse modes.

2. A fibre laser as claimed in claim 1 , wherein the mode mixing means includes a mode mixing fibre and a cladding mode stripper.

3. A fibre laser as claimed in claim 2 , wherein the cladding mode stripper comprises a capillary mounted coaxially and adjacent the mode mixing fibre and has an end proximal to the single or low-order-mode oscillator or preamplifier which is adapted to use total internal reflection to redirect radiation in the stripper away from the fibre.

4. A fibre laser as claimed in claim 1 , wherein the oscillator or preamplifier includes a core, an inner cladding and an outer cladding, and the power amplifier includes a core and an inner cladding defining a waveguide for the laser signal, and an outer cladding defining a waveguide for pumping energy, wherein the core and inner cladding diameter of the power amplifier are respectively larger than the core and inner cladding of the oscillator or preamplifier.

5. A fibre laser as claimed in claim 1 , wherein the mode mixing means includes a mode mixing fibre having a core and cladding, the core having a non-circular cross-section.

6. A fibre laser as claimed in claim 1 , wherein the mode mixing means is a fibre having at least one coil or bend along its length to induce mode mixing.

7. A fibre laser as claimed in claim 1 , wherein the oscillator or preamplifier comprises a fibre having a core, an inner cladding and an outer cladding defining separate waveguides for the laser signal and pump light respectively, the inner cladding having a non-circular cross-section to promote mixing of pump modes.

8. A fibre laser as claimed in claim 7 , wherein a fused fibre pump combiner is used to introduce multimode pumped light into the inner cladding of the fibre.

9. A fibre laser as claimed in claim 1 , wherein pump energy is introduced in a forward, reverse or in both forward and reverse directions.

10. A fibre laser as claimed in claim 1 , in which the optical fibre in the power amplifier includes an inner cladding with a non-circular cross-section to promote mixing of pump modes.

11. A fibre laser as claimed in claim 1 , wherein a fused fibre pump combiner having a multimode core feed through is used to multiplex and demultiplex pump light and the laser signal.

12. A fibre laser as claimed in claim 1 , wherein the power amplifier has an output which is applied through an optical fibre having a larger core diameter than that of the power amplifier.

13. A fibre laser as claimed in claim 1 , wherein the oscillator has an output which is modulated by direct modulation of pump laser diodes.

14. A fibre laser as claimed in claim 1 , which is a pulsed laser amplified in a chain of one or more preamplifiers before a master amplifier.

15. A fibre laser as claimed in claim 1 , wherein said fibre laser further includes a power amplifier adapted to guide multiple-transverse modes formed by an active fibre having a core and an inner cladding defining a waveguide for the laser signal, and an outer cladding defining a waveguide for pumping energy; wherein the core and inner cladding diameter of the power amplifier are substantially larger than the core and inner cladding of the oscillator.

16. A fibre laser comprising a single mode or low-order-mode cladding pumped fibre laser oscillator or preamplifier, and a cladding pumped fibre laser power amplifier adapted to guide multiple transverse modes, wherein an output from the oscillator or preamplifier is applied through a mode mixing means to the power amplifier in a forward direction, and any light entering the laser oscillator or preamplifier from a reverse direction that is substantially opposite the forward direction, is removed by a cladding mode stripper.

17. The fibre laser as claimed in claim 16 , wherein said mode mixing means includes a mode mixing fibre.

18. The fibre laser as claimed in claim 16 , wherein the light entering the mode mixing means from the reverse direction is re-distributed to fill the aperture of the mode mixing fibre prior to being removed by the cladding mode stripper.

19. A fibre laser comprising a fibre having a first portion, a second portion and a third portion, said first portion including a single mode or low-order-mode cladding pumped fibre laser oscillator or preamplifier, said second portion including a mode mixing fibre, and said third portion including a cladding pumped fibre laser power amplifier including multiple transverse modes, wherein said mode mixing fibre distributes a single mode or low-order mode signal among the multiple transverse modes of the cladding pumped fibre laser power amplifier.

20. The fibre laser as claimed in claim 19 , wherein said second portion further includes a cladding mode stripper for preventing any light entering the laser oscillator or preamplifier in a reverse direction from the laser oscillator or preamplifier.