RF Frequency Synthesizer Based on Self-Mode-Locked Multimode Lasers

Low-noise local oscillators are very important in modern communication systems. To realize a stable solution with a limited size, symmetric multimode multi-section semiconductor laser pairs are reported in this article for building a chip-level RF synthesizer. The symmetric laser structure utilizes a shared distributed Bragg reflector (DBR) and counterpropagating laser output with the help of an external feedback circuit to achieve self-mode-locking. The achieved phase noise performance is -92 dBc/Hz at a 10 kHz offset frequency for intermodal operation at 26.4 GHz using 5 locked modes, which is a 57 dB improvement compared to the free-running case. Moreover, an RF synthesizer covering frequencies of 11-13 GHz is realized based on the achieved mode-locked system using current tuning. The related phase noise of the laser pair beat-note output is -100 dBc/Hz at a 10 kHz offset of carrier frequencies of 11-13 GHz, which is 58 dB better than the phase noise of the beat-note output without mode-locking over 11-13 GHz.