Fig. 6
Optomechanical time-domain reflectometry of a polyimide-coated fibre. a Optomechanical time-domain reflectometry map of the nonlinear coefficient γ(Ω, z) as a function of frequency detuning Ω/(2π) between two co-propagating optical fields and position z along a fibre under test. The nonlinear coefficients are normalized so that \({\int} {\gamma \left( {\Omega ,z} \right){\mathrm{d}}\Omega }\) is the same for all positions z. The standard single-mode fibre under test is coated with polyimide with outer diameter of 140 µm. The first 850 m of the fibre were immersed in water, and the rest was kept in air. Optomechanical coupling with a resonance frequency of 369.5 MHz due to mode R0,11 of the coated fibre is observed throughout its length. The coupling spectrum in air is narrower with a stronger peak. b Calculated normalized transverse profile of the material displacement of the radial mode R0,11 that is guided by the silica-polyimide structure, as a function of Cartesian coordinates x and y. c Spectra of optomechanical coupling in fibre sections 750 m from input end (blue, in water), and 2.5 km from the input end (red, in air). Markers present measurement data, and lines show fitted Lorentzian line-shapes. d Linewidths Γ11(z)/(2π) as a function of position. The ranges of experimental uncertainty in the linewidth measurements, for fibre in water and air, are noted by horizontal dotted lines. The uncertainties are estimated by the standard deviation of the measured linewidths in each segment. e Resonance frequencies Ω11(z)/(2π) as a function of position
