Ultrastable 10 GHz Mode-Locked Laser on Semi-Insulating Substrate Through RF Injection Locking

Abstract

We have developed a passive semiconductor mode-locked laser on a semi-insulating (S.I.) substrate, which significantly reduces parasitic capacitance. This allows for efficient direct modulation and improved performance, making it ideal for RF fundamental and subharmonic injection locking using a Ground-Signal-Ground probe with relatively lower RF power to reduce timing jitter. Hybrid mode-locking was observed at RF powers as low as –20 dBm for the fundamental frequency, with higher power required for subharmonic frequencies. Experimental results demonstrate significant performance improvements, including linewidth reduction from 40 kHz (free-running laser) to sub-hertz levels after RF fundamental injection locking, phase noise reduction by over 40 dBc/Hz at a 10 kHz offset, and a decrease in timing jitter from 7.2 ps to approximately 80 fs with an injected fundamental RF power of –5 dBm. Subharmonic injection locking offers simplified modulation requirements and circuitry, although it requires a higher injection RF power. These significant improvements highlight the potential of our approach for advanced optical communication and optical clock systems.