Broadening free spectral range of an evanescent-wave pumped Whispering-Gallery-Mode fibre laser by Vernier effect

Evanescent-wave pumping scheme has been successfully used to excite Whispering-Gallery-Mode fibre laser. The free spectral range of the lasing spectrum from a single optical fibre is found to be very narrow, to increase the free spectral range, a coupled cylinder-cavity structure has been fabricated by binding two bare optical fibres together, based on Vernier effect, the free spectral range of the coupled cylinder-cavity structure has been effectively broadened over 15 times compared with that of a single optical fibre. The measured free spectral ranges for both coupled cylinder-cavity structure and the single optical fibre are consistent with theoretical prediction.