50-GHz Repetition Gain Switching Using a Cavity-Enhanced DFB Laser Assisted by Optical Injection Locking

We demonstrate pulse generation at a repetition rate of 50 GHz by gain switching an injection-locked distributed feedback (DFB) laser. The small-signal BW of the DFB laser was enhanced from an intrinsic BW of 30 GHz to 52 GHz due to the joint effects of photon-photon resonance (PPR) and detuned loading. The detuned loading effect is achieved by exploiting the frequency-dependent cavity loss, which results in an increase of the effective differential gain, and correspondingly, the relaxation oscillation frequency. In addition to small-signal measurements, we show that the PPR and the detuned loading effects can also be utilized to improve large signal gain-switching, despite the dynamic changes of the detuned loading condition due to the large signal chirp. By modulating the laser with 50-GHz RF signals, we obtained 50-GHz repetition rate pulses from the gain-switched laser, confirming that the BW enhancement effects are still valid for large-signal modulation. Subsequently, we optically injection lock the gain-switched laser with strong external seeding light, which suppresses the chirp and creates coherent frequency tones with 50 GHz spacing. The optical injection locking (OIL) further enhances the large-signal BW, resulting in narrower pulse width of 9.5 ps (6.2 ps after deconvolution) in the time domain and three sideband peaks above 70% from the peak in the frequency domain (full width half maximum of 130 GHz). Assuming linear chirp, the pulse could be further compressed to 2.4 ps.