Injection locking
Forcing one laser to adopt the frequency and phase of light injected from another. Inside the locking range the slave inherits the master's coherence, enabling noise cleanup, chirp suppression, and colorless sources.
Inject enough light from a master laser into a running slave laser, close enough in frequency, and the slave stops oscillating on its own terms: it locks, emitting at the master's frequency with a fixed phase relationship. Outside that window it free-runs (with interference artifacts); at the boundary it passes through unstable dynamics.
The locking range grows with the injection ratio (injected power over slave intracavity power) and with the slave's cavity linewidth; in semiconductor lasers the α-factor skews the range asymmetrically toward the red side. Ratios from −40 to −10 dB give practical ranges from sub-GHz to tens of GHz.
What locking buys:
Coherence transplant. The slave's output takes on the master's linewidth and low-frequency phase noise. A powerful but noisy slave (a high-power FP, a gain-switched comb line) inherits the purity of a weak but clean master: amplification without an amplifier's ASE.
Modal discipline. Injection suppresses side modes (SMSR improvement in FP lasers), stabilizes one polarization in VCSELs, and selects a single line from a comb. This is the mechanism behind comb-line "filtering by locking."
Dynamics reshaping. A locked slave's resonance stiffens: relaxation-oscillation frequency and damping increase, extending modulation bandwidth well beyond the free-running value, while chirp under direct modulation collapses because the master pins the frequency. This combination (direct-modulation simplicity with external-modulation spectral behavior) is a recurring research and product theme.
Colorless transmitters. In WDM-PON architectures, a seed wavelength distributed from the head end injection-locks (or injection-seeds) reflective devices (R-SOA, FP) at each subscriber, so every terminal is identical hardware yet emits the assigned wavelength.
Bench realities: an isolator-free path from master into slave is required by definition, so protect the master with an isolator or circulator; polarization alignment matters; and temperature-tune the slave's free-running frequency into the locking window before expecting stability.