Generation of optical frequency combs in fibres: an optical pulse analysis

Abstract

The innovation of optical frequency combs (OFCs) generated in passive mode-locked lasers has provided astronomy with unprecedented accuracy for wavelength calibration in high-resolution spectroscopy in research areas such as the discovery of exoplanets or the measurement of fundamental constants. The unique properties of OCFs, namely a highly dense spectrum of uniformly spaced emission lines of nearly equal intensity over the nominal wavelength range, is not only beneficial for high-resolution spectroscopy. Also in the low-to medium-resolution domain, the OFCs hold the promise to revolutionise the calibration techniques. Here, we present a novel method for generation of OFCs. As opposed to the mode-locked laser-based approach that can be complex, costly, and difficult to stabilise, we propose an all optical fibre-based system that is simple, compact, stable, and low-cost. Our system consists of three optical fibres where the first one is a conventional single-mode fibre, the second one is an erbium-doped fibre and the third one is a highly nonlinear low-dispersion fibre. The system is pumped by two equally intense continuous-wave (CW) lasers. To be able to control the quality and the bandwidth of the OFCs, it is crucial to understand how optical solitons arise out of the initial modulated CW field in the first fibre. Here, we numerically investigate the pulse evolution in the first fibre using the technique of the solitons radiation beat analysis. Having applied this technique, we realised that formation of higherorder solitons is supported in the low-energy region, whereas, in the high-energy region, Kuznetsov-Ma solitons appear.

Soliton radiation beat analysis of optical pulses generated from two continuous-wave lasers

“…1 [15][16][17][18] . The generation of an OFC starts with two independent and free-running CW lasers that have equal intensity and feature relative frequency stability of 10 -8 over one-hour time frame sufficient for astronomical applications in the low-and medium-resolution range.…”

“…2(B)) [35][36][37][38] The initial condition of the case we are actually interested in (radiation of two CW lasers) is described by (FIG. 2(C)) [15][16][17][18] :…”

“…It consists of two fibre stages where the first stage is a conventional single-mode fibre, the second one is a suitably pumped amplifying Erbium-doped fibre with anomalous dispersion. The initial input field comes from two continuouswave lasers (CW) that generate a deeply modulated bichromatic cosine-wave [15][16][17][18] .…”

Generation of optical frequency combs via four-wave mixing processes for low- and medium-resolution astronomy

“…As the optical pulses propagate through fibres A and B, their intensity experiences periodical modulation over the propagation distance [45]. This periodicity in the peak power occurs due to the formation and the subsequent propagation of higher-order solitons [33,41]. We define the optimum length L opt of a fibre as the propagation distance between the beginning of the fibre and the first pulse intensity maximum.…”

“…We numerically investigate the four-wave mixing cascade approach with the particularity that it involves a long piece of an erbium-doped fibre with anomalous dispersion where strong pulse compression based on the higher-order soliton compression takes place [31,32,33]. We focus the analysis on how the quality of the compression and the pulse pedestal build-up depend on the input power, laser frequency separation, and group-velocity dispersion of the first fibre.…”