Raman Self-Frequency Shift of Dissipative Kerr Solitons in an Optical Microresonator

Abstract

The formation of temporal dissipative solitons in continuous wave laser driven microresonators enables the generation of coherent, broadband and spectrally smooth optical frequency combs as well as femtosecond pulses with compact form factor. Here we report for the first time on the observation of a Raman-induced soliton self-frequency shift for a microresonator soliton. The Raman effect manifests itself in amorphous SiN microresonator based single soliton states by a spectrum that is hyperbolic secant in shape, but whose center is spectrally red-shifted (i.e. offset) from the continuous wave pump laser. The shift is theoretically described by the first order shock term of the material's Raman response, and we infer a Raman shock time of 20 fs for amorphous SiN. Moreover, we observe that the Raman induced frequency shift can lead to a cancellation or overcompensation of the soliton recoil caused by the formation of a (coherent) dispersive wave. The observations are in excellent agreement with numerical simulations based on the Lugiato-Lefever equation (LLE) with a Raman shock term. Our results contribute to the understanding of Kerr frequency combs in the soliton regime, enable to substantially improve the accuracy of modeling and are relevant to the fundamental timing jitter of microresonator solitons.Introduction. -Microresonator based optical frequency combs (Kerr combs) [1, 2] enable optical frequency comb generation from a continuous wave (CW) laser, with repetition rates in the microwave domain (>10 GHz), and broad spectral bandwidth [3][4][5]. Recently, a qualitatively new operation regime has been discovered [6], in which the parametrically generated comb seeds the formation of a temporal dissipative (cavity) soliton [7][8][9]. Such temporal dissipative solitons, first externally induced in fiber cavities [10], have been observed in Kerr frequency comb experiments using crystalline microresonators [6,11] and have recently also been generated in photonic chip based silicon nitride (SiN) integrated resonators [5,12]. Soliton based microresonator frequency combs have several attractive features, in particular being fully coherent, having smooth envelopes and giving access to femtosecond pulses. Indeed, the short duration of temporal solitons in crystalline microresonators have been used for external fiber broadening [6,13] and have allowed to achieve self-referencing (i.e. determination of the comb's carrier envelope frequency) enabling to count the cycles of light [13]. Moreover, taking advantage of dispersion engineering [3,14], the presence of third (and higher) order dispersion allows (coherent) dispersive waves (DWs) to be generated [12] via the effect of soliton induced Cherenkov radiation [15]. This process has been used to create a photonic chip based (coherent) frequency comb in a SiN microresonator spanning 2/3 of an octave at electronically detectable mode spacing. However, while advances in theoretical simulations of the soliton regime have occured [14,16,17] a key underlying question is what ot...

Microresonator-based solitons for massively parallel coherent optical communications

“…A detailed sketch of the experimental setup is given in Section 1 of the SI. The resonance frequencies of the cavity can be tuned at a rate of approximately −2.5 GHz/K with an accuracy of approximately 200 MHz, limited by the resolution of the heater of approximately 0.1 K. In addition, as a consequence of intra-pulse Raman scattering [47], the center frequency of the comb can also be tuned by slowly changing the pump frequency during operation at a constant external temperature. The associated tuning range is limited to approximately ±500 MHz before the comb state is lost; the tuning resolution is given by the pump laser and amounts to approximately 10 MHz for our devices (TLB-6700, New Focus; TSL-220, Santec).…”

Octave-spanning dissipative Kerr soliton frequency combs in Si_3N_4 microresonators

“…Although dispersive wave features are also present in the uMI state spectra, in the DKS state their width reduces with a simultaneous increase in the individual comb tooth power and a shift of the dispersive wave maximum [19]. Other spectral signatures of the DKS regime include the soliton Raman self-frequency shift, that leads to a redshift of the soliton's sech 2 envelope with respect to the pump [30]. The Raman self-frequency shift is absent in the uMI comb state, but can be masked in the DKS state by the soliton recoil due to dispersive wave formation.…”

Observation of Fermi-Pasta-Ulam Recurrence Induced by Breather Solitons in an Optical Microresonator

“…3. SRS is generally significant for SiN microresonators [29,39]. However, in fluoride microresonators, SRS is much weaker [21]; hence, symmetric breathing of solitons can be expected.…”

“…Soliton behavior in microresonators is governed by the LLE [38,47]. For SiN microresonators, SRS is important in determining the property of solitons [29,36,39,40]. Hence, we use the generalized LLE, with SRS included, to describe the soliton generation,…”