Phase Noise Measurement of Mode-Locked Lasers Based on a Dual-Wavelength Delayed Self- Heterodyne Interferometer Without Dispersion Management

We propose and demonstrate a novel method for precisely quantifying the repetition-rate phase noise of mode-locked lasers (MLLs) based on a dual-wavelength delayed self-heterodyne interferometer (DSHI), without the need for a precision dispersion management unit. Different from the conventional approach based on dual-wavelength DSHI for measuring the MLL phase noise, which involves multiple comb lines in each of two selected wavelength regions, our approach is based on the combination of the cascaded Mach-Zehnder interferometer-based pulse repetition-rate multiplication with optical injection locking technique to extract and amplify a single comb line for each of two wavelength regions. In this case, an additional precise dispersion management unit is not required. The impact of the wavelength spacing and fiber delay length on the measurement sensitivity and the measurement bandwidth of the repetition rate phase noise of MLL is detailed investigated. In the experiment, a dual-wavelength DSHI system with 2000 m fiber delay and 13 nm wavelength spacing is utilized to accurately characterize the phase noise of the home-built mode-locked laser, which exhibits −135 dBc/Hz at a carrier frequency of 10 GHz and an offset frequency of 10 kHz. The proposed approach enables the MLL repetition-rate phase noise measurement in a simple, high-sensitivity, and precise way, which could be used for a variety of MLLs with low repetition rates.