50-GHz Repetition Gain Switching Using A Cavity-enhanced DFB Laser Assisted By Optical Injection Locking

• Main Authors: Liu, Z, Matsui, Y, Schatz, R, Khan, F, Kwakernaak, M, Sudo, T
• Format: Article in Journal/Newspaper
• Language: English
• Published: Institute of Electrical and Electronics Engineers (IEEE) 2020
• Subjects: Directly Modulated Lasers; Optical Injection Locking; Photon-Photon Resonance; Gain Switching; DML
• Online Access: https://discovery.ucl.ac.uk/id/eprint/10091134/1/JLT-25012-2019_R2_final_nohighlight.pdf; https://discovery.ucl.ac.uk/id/eprint/10091134/

We demonstrate pulse generation at a repetition rate of 50 GHz by gain switching an injection-locked distributed feedback (DFB) laser with large modulation bandwidth (BW). The small-signal BW of the DFB laser was extended from an intrinsic BW of 30 GHz to 52 GHz due to the joint effects of photon-photon resonance (PPR) and detuned-loading, which exploits the frequency-dependent cavity loss to enhance the effective differential gain and thereby the relaxation oscillation frequency. The results show that PPR and 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. After that, we optically injection lock the gain-switched laser with strong external seeding light, which suppresses the chirp and creates frequency tones with 50 GHz spacing. The optical injection locking (OIL) further improves the large-signal BW, resulting in narrower pulse width. The spectrum of the gain-switched OIL directly modulated laser (OIL-DML) showed three sideband peaks above 70% from the peak (full width half maximum of 130 GHz), and a pulse width of 9.5 ps (6.2 ps after deconvolution) was obtained. The results indicate that the pulse could be compressed to 2.4 ps, assuming linear chirp.