Photodetection system immune to amplitude-to-phase conversion noise for ultralow noise photonic microwave generation

Microwave signals generated by photodetection of the pulse train of an optical frequency comb locked to an ultrastable laser have ultrahigh spectral purity and frequency stability. The amplitude-to-phase noise conversion (APC) that occurs in the photodetection, however, degrades the phase noise of the generated microwave signal over the range from tens of hertz to several kilohertz. Here we demonstrate active APC compensation by precisely tuning the bias voltage applied to a photodetector using the APC coefficient measured by comparing with a reference photodetector. In this scheme, the APC was dramatically reduced and a long-term steady rejection of the laser relative intensity noise of more than 60 dB was achieved.