Injection locking

Injection locking is the engineering term for frequency entrainment in electronic oscillators — particularly in laser systems, microwave circuits, and clock distribution networks. When a small periodic signal is injected into a free-running oscillator, the oscillator locks to the injected frequency within a capture range that depends on injection strength and quality factor. The phenomenon is exploited in optical communications to phase-lock slave lasers to a master reference, eliminating phase noise and enabling coherent detection.

The mathematical description of injection locking is identical to that of phase locking in biological oscillators, but the engineering literature has developed its own vocabulary and design criteria. The Adler equation, derived in 1946 for vacuum-tube oscillators, remains the standard model for predicting locking range and transient behavior. Modern applications include injection-locked frequency dividers in CMOS circuits and optically injection-locked lasers for LiDAR systems.