Use of longer soliton pulses to reduce Raman noise in fibre squeezing

A source of quantum noise in a non-linear optical medium is the coupling of photons with thermal phonons, called the Raman effect. This noise source sets the lower limit of squeezing in the polarisation variables for ultrashort pulses. Increasing the soliton pulse width reduces the coupling between photons and phonons by decreasing the overlap of the pulse spectrum and the Raman frequency response, which is specific to the medium. We investigate the dependence of squeezing on the soliton pulse width, to determine if improvements in squeezing result from longer pulses. We numerically solve a stochastic nonlinear Schrodinger equation with extra nonlinear terms for Raman effect (derived within the truncated Wigner approach), for a 3M FS-PM 7811 silica fibre using the pde solver XMDS 2.0. Computational efficiency was improved by use of a single-Lorentzian approximation for the Raman gain profile. Results show that squeezing can be improved for longer pulse widths. However, there is a tradeoff between the Raman effects and the classical phase noise due to guided acoustic wave Brillouin scattering.