Fig. 3: Characterisation of forward Brillouin gain and the lasing cavity.

a Measured (dashed red) and calculated (dash-dotted green) normalised spectra of inter-modal forward SBS coupling of power between the two principal axes of a 30 metres-long, bare PM fibre under test. The blue trace shows a beating spectrum between forward SBS lasing and a local oscillator replica of the pump wave, and the black trace shows a corresponding beating spectrum of spontaneous inter-modal forward Brillouin scattering from the laser cavity (see Fig. 5 below and later in the text). b Measured reflectivity spectra of the cavity formed by two FBGs, inscribed at both ends of the fibre. Red (blue) traces correspond to light polarised along the fast (slow) principal axes. The black arrow denotes the wavelength of intended forward SBS lasing. At that wavelength, the gratings provide effective reflectivity feedback to light polarised along the slow axis only. c Cavity ring down measurement of slow axis polarised light at the wavelength of peak gratings reflectivity. d Magnified view of part of the panel c trace. The measurements suggest a lifetime of 12 μs, corresponding to round-trip losses of 2.5%.