Subharmonic Optical Injection Locking of a Mode-locked Laser Diode

Optical injection locking is a versatile technique which allows the control of semiconductor lasers and new operational regimes [1], [2]. In dual-tone optical injection, the light of a tunable CW laser is modulated by an electro-optic modulator, creating optical sidebands distanced by the frequency of the RF signal applied
$(\mathrm{f}_{\text {gen}})$
, before being injected into a slave laser - in our case, a mode-locked laser diode (MLLD). In this way,
$\mathrm{f}_{\text {gen}}$
can be tuned so that the longitudinal modes of the slave laser become locked to two of the optical sidebands. A type of subharmonic locking can therefore be achieved for
$\mathrm{f}_{\text {gen}}=\mathrm{f}_{\text {rep}}/\mathrm{n}$
, where n is a natural integer, and f
rep
is the locked pulse repetition rate of the MLLD. This method has shown significant promise for the high-speed control of the repetition rate of MLLD within a wide locking range [2], although most studies have only used n=2 [2–4].