Ultralow noise microwaves with free-running frequency combs and electrical feedforward

TL;DR

This paper introduces a simplified method for generating ultralow noise microwaves using electronic feedforward noise cancelation, improving robustness and manufacturability of optical frequency comb-based systems.

Contribution

It replaces feedback control with electronic feedforward in optical frequency division, relaxing comb source requirements and enabling fully chip-scale microwave generation.

Findings

Achieved phase noise as low as -153 dBc/Hz at 10 kHz offset.

Demonstrated femtosecond timing jitter.

Eliminated large servo bump noise at high offset frequencies.

Abstract

Optically generated microwave signals exhibit some of the lowest phase noise and timing jitter of any microwave-generating technology to date. The success of octave-spanning optical frequency combs in down-converting ultrastable optical frequency references has motivated the development of compact, robust and highly manufacturable optical systems that maintain the ultralow microwave phase noise of their tabletop counterparts. Two-point optical frequency division using chip-scale components and ~1 THz-spanning microcombs has been quite successful, but with stringent requirements on the comb source's free-running noise and feedback control dynamics. Here we introduce a major simplification of this architecture that replaces feedback control of the frequency comb in favor of electronic feedforward noise cancelation that significantly relaxes the comb requirements. Demonstrated with both a high repetition rate solid-state mode-locked laser and a microcomb, feedforward on a 10 GHz carrier results in more robust operation with phase noise as low as -153 dBc/Hz at offsets >10 kHz, femtosecond timing jitter, and elimination of the large "servo bump" noise increase at high offset frequency. The system's compatibility with a variety of highly manufacturable mode-locked laser designs and its resilience and straightforward implementation represents an important step forward towards a fully chip-scale implementation of optically generated microwaves, with applications in radar, sensing, and position, navigation and timing.