Self-injection locking
Narrowing a semiconductor laser by coupling it, with no isolator, to a high-Q resonator whose backscattered light feeds back into the laser and pulls it onto the resonance. The mechanism behind hertz-level chip lasers and turnkey microcombs.
In ordinary injection locking a separate master laser forces a slave onto its frequency. Self-injection locking removes the master. A laser diode, typically a DFB or Fabry-Perot chip, is butt-coupled or lens-coupled without an isolator to a high-Q resonator: a crystalline whispering-gallery disk, or a silicon nitride ring on the same chip. Light entering the resonator is scattered weakly backwards by surface and sidewall roughness (Rayleigh scattering), which couples the forward-circulating mode to the backward one. The backward mode leaks out toward the laser only near resonance, so the laser receives a narrowband reflection filtered by the resonator's full quality factor, and it locks to it.
The effect is regime III of the optical feedback regimes put to use, with a resonator in place of a mirror. Inside the locking range the laser frequency follows the resonance rather than its own cavity, and frequency noise is suppressed by a stabilization coefficient that, in the simplest model, grows as the square of the ratio of the resonator's Q to the laser cavity's Q (Kondratiev et al. 2017). With resonator Q in the to range against a diode cavity Q of to , the fundamental linewidth of a laser that free-runs at hundreds of kilohertz to megahertz falls to the hertz level: Jin et al. reported hertz-level fundamental linewidth from a DFB laser locked to a CMOS-fabricated silicon nitride ring, and the Kondratiev 2023 review describes sub-hertz results with ultrahigh-Q crystalline resonators.
Two practical parameters decide whether the lock works. The first is the feedback phase, set by the optical path between laser and resonator; it shifts the locking range and the achievable narrowing, so the gap or a phase section has to be tuned and then held. The second is the backscatter strength itself, which is uncontrolled in a plain ring and is sometimes engineered with a deliberate reflector or a coupled drop port. The lock suppresses fast noise, but the resonator's own thermorefractive fluctuations and drift remain, so the integral linewidth over milliseconds to seconds is usually much larger than the fundamental figure; published results should be read with the distinction made in How to measure laser linewidth.
The same mechanism makes soliton microcombs turnkey: with the pump self-injection-locked to the ring, switching the laser on lands the system in the soliton state without the frequency sweep that free-running pumps require (Shen et al. 2020). Compared with an external-cavity laser, the result is smaller and needs no grating or mechanical tuning, at the price of a locking condition that must be maintained against temperature.
References: N. M. Kondratiev et al., Opt. Express 25, 28167 (2017); N. M. Kondratiev et al., Front. Phys. 18, 21305 (2023); W. Jin et al., Nat. Photonics 15, 346 (2021); B. Shen et al., Nature 582, 365 (2020).