Fig. 1
The optomechanical time-domain reflectometry principle. Light at the input of a fibre under test consists of two optical tones (marked yellow and dark blue) within a common pulsed envelope (a). The two tones are detuned from a central optical frequency ω0 by radio-frequency offsets \(\pm {\textstyle{1 \over 2}}\Omega\). The difference in frequencies Ω is chosen near the resonance frequency Ωm of the stimulation of a particular guided acoustic mode of the fibre R0,m. The two pulsed tones are launched into a fibre under test (yellow and dark blue pulses, propagating left to right). The stimulation of acoustic waves along the fibre under test is accompanied by the coupling of optical power between the two tones: the lower-frequency pulses (dark blue) are amplified whereas the higher-frequency ones are attenuated (yellow). The exchange of power is monitored based on the Rayleigh backscatter of the two field components (red and light blue). The two backscatter contributions are separated by narrowband, backwards stimulated Brillouin scattering (SBS) amplifiers (b). The waveforms are then detected and sampled for further offline processing. The difference between the two backscatter traces ΔP(Ω, z) (c) is used to calculate the nonlinear optomechanical coefficient γ(m)(Ω, z), as a function of position z, for the particular choice of Ω (d). The measurement sequence is repeated for multiple values of radio-frequency offsets Ω (e), to reconstruct a three-dimensional map of optomechanical coupling as a function of both position and frequency (f). A cross-section of that map describes the local spectrum of the optomechanical coefficient γ(m)(Ω, z0) at location z0 (g). The optomechanical coupling spectrum γ(m)(Ω, z0) varies with the mechanical impedance of the medium outside the fibre at z0. The spectrum is narrow with a sharp peak for an uncoated section of fibre in air (illustrated in blue curve). It becomes broader when the same section is immersed in liquid (red curve)
