Characterisation of a Nd-doped fibre laser incorporating an etalon formed between the fibre facet and dielectric mirror

Lasing on up to 15 simultaneous wavelengths is demonstrated experimentally with the use of an air-spaced etalon formed between the dichroic mirror and the adjacent fibre facet. A maximum measured frequency shift of 25.4 GHz/μm was observed, and there was only an 11% decrease in the output power at the maximum mirror–fibre separation of 400 μm. A theoretical analysis of the etalon effect in relation to the performance of the fibre laser is also included. The simulation includes the dependence of the effective reflectivity of the mirror and spectral peak spacing on the mirror–fibre separation. The experimental results are consistent with the simulation.