Advanced noise reduction techniques for ultra-low phase noise optical-to-microwave division with femtosecond fiber combs

TL;DR

This paper demonstrates the lowest reported phase noise in optical-to-microwave division using fiber-based femtosecond combs, along with a detailed analysis of fundamental noise sources and advanced noise suppression techniques.

Contribution

It introduces a novel ultra-low noise measurement system and compares control schemes to significantly reduce phase noise in optical-to-microwave division.

Findings

Residual phase noise of -120dBc/Hz at 1 Hz offset

10 dB additional noise reduction with fed-forward technique

Effective power stabilization reduces low-frequency phase noise

Abstract

We report what we believe to be the lowest phase noise optical-to-microwave frequency division using fiber-based femtosecond optical frequency combs: a residual phase noise of -120dBc/Hz at 1 Hz offset from a 11.55GHz carrier frequency. We furthermore report a detailed investigation into the fundamental noise sources which afflicts the division process itself. Two frequency combs with quasi-identical configurations are referenced to a common ultrastable cavity laser source. To identify each of the limiting effects we implement an ultra-low noise carrier-suppression measurement system, which circumvents the detection and amplification noise of more conventional techniques. This technique now allows the suppression of these unwanted sources of noise to very low levels. In the Fourier frequency range of ~200 Hz to 100 kHz, a fed-forward technique based on a voltage-controlled phase shifter delivers a further noise reduction of 10 dB. For lower Fourier frequencies, optical power stabilization is implemented in order to reduce the relative intensity noise which causes unwanted phase noise through power to phase conversion in the detector. We implement and compare two possible control schemes based on an acousto-optical modulator and comb pump current. We also present wideband measurements on the relative intensity noise of the fiber comb.