Stokes microcombs in silicon nitride microresonators

TL;DR

This work demonstrates on-chip stimulated Raman scattering in silicon nitride microresonators operating under normal GVD, yielding Stokes frequency combs with a ≈$9$ THz shift and a gain bandwidth of ≈$5$ THz that span >$100$ nm. By employing two pump schemes—an amplified tunable laser with an isolator and a self-injection-locked diode laser—the authors observe cascaded Stokes combs, dark-pulse (platicon-like) Kerr states, and robust Kerr–Raman switching governed by detuning and locking phase. A comprehensive numerical model based on coupled-mode equations reproduces the experimental dynamics, showing that Raman gain can seed and facilitate coherent comb formation at both pump and Stokes frequencies under normal GVD. The results advance silicon nitride photonics by enabling compact, reconfigurable hybrid Kerr–Raman comb sources with potential for Raman-based spectroscopy and coherent communications, and underline the practical impact of SIL in achieving tunable comb states, albeit with Raman-induced linewidth broadening that can be mitigated by reducing pump power.

Abstract

Silicon nitride microresonators have become an ubiquitous platform for cutting-edge photonics applications. Improvement in silicon nitride fabrication techniques, providing ultra-high quality-factor values up to $10^7$, has opened up new possibilities for nonlinear effects realizations in such structures. Here we report for the first time to our knowledge on the observation of the Stokes microcombs in silicon nitride on-chip microresonators exhibiting normal group velocity dispersion. Moreover, using different pump schemes, namely, a tunable laser with an isolator and a stabilized diode laser, we demonstrate on-chip stimulated Raman frequency combs including dark-pulse Raman states. We reveal a complex interplay between Kerr and Raman nonlinearities and elaborate effective method of controllable switching between predominantly Kerr-comb and predominantly Raman-comb operation. We prove the Raman-induced platicon formation by numerical model which shows perfect agreement with experimental results. These findings are of special importance for silicon nitride photonics and provide a basis for novel photonic devices.

Figures (10)

• Figure 1: The conceptual illustration of the process of Stokes frequency comb generation. The stimulated Raman scattering initiates four-wave mixing.
• Figure 2: (a) Experimental setup. Two types of laser sources are used as a pump: an amplified tunable laser with isolator and a DFB laser diode. A high-Q silicon nitride on-chip microresonator with normal GVD is investigated. An OSA, an ESA and an oscilloscope are used for measurements. (b) Photo of the chip. (c) Schematic energy diagram of the stimulated Raman scattering process, where $\omega_p$ and $\,\,\Omega_R$ are the pump and Raman frequencies, and $\,\, \omega_S \,\,$ is the Stokes shift. (d) Experimental spectra of Stokes generation. The left spectrum corresponds to the onset of generation, while the lower spectrum corresponds to higher coupled power into the microresonator.
• Figure 3: Characterization of initial SRS. (a)-(c) Initial SRS spectra at different pump wavelength. (d) Dependence of the Stokes and anti-Stokes frequency shifts (blue and red dots, respectively) on the pump wavelength in the critically coupled microring. Yellow dots represent the difference between these two frequency shifts, which appears to be equal within the resolution of the OSA. (e) Dependence of the Stokes frequency shift on the pump wavelength for critically coupled and undercoupled microrings. The frequency shift is approximately 9 THz, and the Raman gain bandwidth is about 5 THz.
• Figure 4: Raman comb evolution with the increase of the intracavity power. (a)-(e) The spectra of the Raman comb generation with the increasing of the coupled to microresonator energy. The pump line power decreases by 6 dB from (a) to (e). (f) The spectrum obtained at pump power of 60 mW coupled into the chip.
• Figure 5: The Stokes frequency comb obtained in case of pumping with stand alone amplified laser source. (a)-(e) The spectra obtained with pumping at 1526, 1536, 1545, 1567, and 1564 nm consequently are presented. Both Stokes and anti-Stokes components are observed. In the inset in panel (e) The sub-comb is observed near 1600 nm.