Power spectral density analysis of relative phase jitter in a twin-soliton molecule

Abstract

Investigation on the relative phase evolution between two bounded optical solitons is essential for its potential applications in development of larger telecommunication capacity of optical fiber transmission lines, resolution improvement in advancing ultrafast characterization approaches and all-optical information storage. In this paper we characterized relative phase jitter power spectral density (PSD) of a soliton molecule pair generated from a passively mode-locked Er:fiber laser (DOI: 10.1364/CLEO_SI.2019.SW3H.6). Through tracking fast shifts of one certain spectral interference fringe, the relative phase noise PSD is obtained by balanced detection. The estimated measurement resolution is at 10^(-13) rad2/Hz level and the integrated phase noise from 10 MHz to 100 Hz is only 3.5 mrad. The estimated relative linewidth is far below 1 mHz. Comparison between phase noise PSD and intensity noise PSD indicates that AM-PM conversion plays an important role in relative phase jitter dynamics between the two solitons. It should be pointed out that, the calibration of PSD at high Fourier frequency is not rigid, due to the reason that interpolating process is not a strict calibration method and thus may cause error on the whole noise spectrum. This would bring uncertain to the determination of measurement resolution and integrated phase noise. Despite of this, our spectral interference fringe tracking technique is still attractive for its simplicity and shows potential in ultra-high resolution in phase noise measurement.