Commercially available erbium-doped mode-locked fiber lasers are compact, robust, and suitable to be the frequency divider of an ultra-low phase noise photonic microwave generator. However, for a mode-locked fiber laser with repetition rate of a few hundred megahertz, the phase noise floor of the photodetected 10 GHz harmonic is mainly limited by the combined effects of photodiode saturation, shot noise, and thermal noise. In this paper, we demonstrate a 10-15 dB reduction in the 10 GHz phase noise floor by multiplication of the laser repetition rate. Starting with a 250 MHz fundamentally mode-locked Er:fiber laser, we compare two different approaches to repetition rate multiplication: Fabry-Perot cavity filtering and a cascaded, unbalanced Mach-Zehnder fiber-based interferometer. These techniques reduce the phase noise floor on the 10 GHz photodetected harmonic to -158 dBc/Hz and -162 dBc/Hz, respectively, for Fourier frequencies higher than 100 kHz.
Citation: IEEE Photonics Journal
Pub Type: Journals
fiber laser, microwave photonics, mode-filtering cavity, phase noise