Dynamics of soliton self-injection locking in optical microresonators

Abstract

Soliton microcombs constitute chip-scale optical frequency combs, and have the potential to impact a myriad of applications from frequency synthesis and telecommunications to astronomy. The demonstration of soliton formation via self-injection locking of the pump laser to the microresonator has significantly relaxed the requirement on the external driving lasers. Yet to date, the nonlinear dynamics of this process has not been fully understood. Here, we develop an original theoretical model of the laser self-injection locking to a nonlinear microresonator, i.e., nonlinear self-injection locking, and construct state-of-the-art hybrid integrated soliton microcombs with electronically detectable repetition rate of 30 GHz and 35 GHz, consisting of a DFB laser butt-coupled to a silicon nitride microresonator chip. We reveal that the microresonator’s Kerr nonlinearity significantly modifies the laser diode behavior and the locking dynamics, forcing laser emission frequency to be red-detuned. A novel technique to study the soliton formation dynamics as well as the repetition rate evolution in real-time uncover non-trivial features of the soliton self-injection locking, including soliton generation at both directions of the diode current sweep. Our findings provide the guidelines to build electrically driven integrated microcomb devices that employ full control of the rich dynamics of laser self-injection locking, key for future deployment of microcombs for system applications.

Dual-laser self-injection locking to an integrated microresonator

“…The result for the linear case needs to be modified to take into account the nonlinear shift 𝛿𝜁 ± of the microresonator frequencies; specifically, the normalized free-running laser detunings 𝜉 ± = 2(𝜔 d ± − 𝜔 ± )/𝜅 must be adjusted by the same amount: ξ± = 𝜉 ± + 𝛿𝜁 ± . The relations between ξ± and ζ± then take the following form [25]:…”

“…To date, SIL to integrated Si 3 N 4 resonators have been used for the formation of microcomb solitons, and the development of sub-Hz lasers [20][21][22][23][24]. Furthermore, butt-coupling of a DFB diode to an integrated high-Q Si 3 N 4 microresonator was used as a compact SIL-based soliton source [25].…”

A hybrid integrated dual-microcomb source

“…In addition, high-level compatibility with CMOS fabrication processes [35][36][37] and also with the III-V semiconductor platform [38][39][40] have been demonstrated. Recent progress in silicon photonics, combined with the self-injection locking (SIL) effect [41][42][43][44][45][46][47][48][49], enabled optical microcomb generation using semiconductor laser diodes (LDs) instead of bulky narrow-linewidth lasers, thereby greatly simplifying the process of microcomb generation and paving the way for the development of fully integrated chip-scale single microcomb sources based on high-Q microresonators (MRs) [38,39,[50][51][52]. Proof-of-concept experiments based on passive high-Q MRs achieved broadband optical spectrum down-conversion to the RF range, generating ultrawide frequency combs with widely variable (from GHz to THz) line spacing in the near-infrared (NIR), SWIR, and visible-wavelength ranges in bulk and on-chip structures [53][54][55][56][57][58][59][60][61][62][63].…”

In-Band Pumped Thulium-Doped Tellurite Glass Microsphere Laser

“…Whispering gallery mode (WGM) resonators with high Q-factors and a small mode volume providing efficient light-matter interaction are very attractive photonic elements for obtaining optical frequency combs for different applications [1][2][3][4][5][6][7][8][9], creating optical filters, switches, microlasers, sensors, and other miniature optoelectronic devices [10][11][12][13][14][15][16]. Microlasers in the two-micron range are potentially interesting for extensive application in remote chemical sensing, biomedicine, and atmosphere monitoring.…”