Soliton dynamics of ring quantum cascade lasers with injected signal

Abstract

Nonlinear interactions in many physical systems lead to symmetry breaking phenomena in which an initial spatially homogeneous stationary solution becomes modulated. Modulation instabilities have been widely studied since the 1960s in different branches of nonlinear physics. In optics, they may result in the formation of optical solitons, localized structures that maintain their shape as they propagate, which have been investigated in systems ranging from optical fibres to passive microresonators. Recently, a generalized version of the Lugiato–Lefever equation predicted their existence in ring quantum cascade lasers with an external driving field, a configuration that enables the bistability mechanism at the basis of the formation of optical solitons. Here, we consider this driven emitter and extensively study the structures emerging therein. The most promising regimes for localized structure formation are assessed by means of a linear stability analysis of the homogeneous stationary solution (or continuous-wave solution). In particular, we show the existence of phase solitons – chiral structures excited by phase jumps in the cavity – and cavity solitons. The latter can be deterministically excited by means of writing pulses and manipulated by the application of intensity gradients, making them promising as frequency combs (in the spectral domain) or reconfigurable bit sequences that can encode information inside the ring cavity.

Laser Frequency Combs with Fast Gain Recovery: Physics and Applications

“…This work pointed to a deep connection between parametric oscillation in optically pumped Kerr microresonators and the single‐mode instability of lasers—an analogy that will be later corroborated in ring QCLs. [ 16,26,62,70 ] In fact, one of the most intriguing features of this state is its spectral distribution, which features an intense central peak with a family of nearly symmetrical sidebands, which become progressively weaker the further they lie from the center of the spectrum (Figure 4b). Beside this spectral indication of a parametric process underlying the formation of the harmonic state, a rigorous demonstration of its frequency comb nature was obtained by multiheterodyne measurements, [ 71 ] based on the beating between two QCLs—one operating in the harmonic state and the other in a fundamental frequency comb state with a spacing of 1 FSR.…”

“…These insights have been consolidated in recent theoretical works that generalize the LLE by introducing a new single equation simultaneously capturing the physics of two distinct classes of frequency comb generators based on nonlinear active and passive optical media. [ 16,70 ] Their formal unification makes it possible to establish a rigorous connection between the similar types of frequency comb spectra that can be observed in these systems. In addition, the generalized LLE also allows to study a hybrid device that combines several features of active and passive systems: it is a ring QCL driven by an external optical signal like a passive Kerr microresonator.…”

Exploring frequency comb generation in optically injected ring quantum cascade lasers

“…To gain further insights into the process of comb formation in ring QCLs, we conducted extensive numerical simulations of our structure. Similar simulations 32 have previously been undertaken in both mid-IR [33][34][35] and THz QCL 14,22 studies to enhance our comprehension of the underlying dynamics of QCL systems. In this particular investigation, we utilized a master equation formalism previously derived from a two-level Maxwell-Bloch formalism 15,36,37 .…”

Self-emergence of robust solitons in a microcavity

“…A similar scenario now confronts microresonator-based optical frequency combs, or microcombs, which have enabled notable breakthroughs in metrology, telecommunications, quantum science and many other areas [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] . A robust, repeatable approach for initiating and reliably maintaining the microcomb into the same type of soliton state, particularly the single-soliton state, is widely acknowledged as critical, with recent notable progress [8][9][10][11][12][13][14][15][16][17][18][19][20] .…”