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.

Research highlights
► We use evanescent-wave pumping scheme to excite Whispering-Gallery-Mode fibre laser.
► We fabricate a coupled fibre laser by binding two bare optical fibres together.
► The free spectral range (FSR) of the fibre laser has been effectively broadened.
► The measured FSR of the fibre laser is consistent with theoretical prediction.