Fig. 7: Water outside the cavity.

Measured optical power of the laser output signal as a function of the optical pump power. Results are shown for forward SBS lasing through the 68 MHz acoustic mode. The bare fibre cavity was kept in air (red), immersed in water for 40% of its length (blue), or fully immersed in water (black). Markers show data points and dashed traces represent trend lines. Partial immersion elevated the lasing threshold by 1.5 dB, since the forward SBS coefficient in the immersed section is reduced by an order of magnitude. Lasing could not be reached with the fibre was wet for its entire length.