Microwave generation with low residual phase noise from a femtosecond fiber laser with an intracavity electro-optic modulator

William C Swann,Nathan R Newbury,Esther Baumann,Fabrizio R Giorgetta

doi:10.1364/oe.19.024387

Abstract

Low phase-noise microwave generation has previously been demonstrated using self-referenced frequency combs to divide down a low noise optical reference. We demonstrate an approach based on a fs Er-fiber laser that avoids the complexity of self-referenced stabilization of the offset frequency. Instead, the repetition rate of the femtosecond Er-fiber laser is phase locked to two cavity-stabilized cw fiber lasers that span 3.74 THz by use of an intracavity electro-optic modulator with over 2 MHz feedback bandwidth. The fs fiber laser effectively divides the 3.74 THz difference signal to produce microwave signals at harmonics of the repetition rate. Through comparison of two identical dividers, we measure a residual phase noise on a 1.5 GHz carrier of -120 dBc/Hz at 1 Hz offset.

Highlights

Microwave signals with high spectral purity are of interest in diverse fields such as clocks, communications, radar, fundamental measurements, and other applications
We examine the suitability of our scheme for low phase noise microwave generation exclusive of any reference cavity noise and, as such, we compare two separate microwave generating systems locked to a common cavity
The relative phase of the 15th harmonic at 1.5 GHz of the two photodetected microwave signals was adjusted so that the signals were in quadrature at the mixer

Summary

Introduction

Microwave signals with high spectral purity are of interest in diverse fields such as clocks, communications, radar, fundamental measurements, and other applications. It is challenging to generate microwave signals with low phase noise, at Fourier offset frequencies close to the carrier, because of upconversion of the ever-present 1/f noise in experimental systems. Cryogenic sapphire microwave oscillators can successfully generate microwaves with extremely low close-to-carrier phase noise [1, 2], but they are physically large systems that require cryogenic temperatures. A Ti:Sapphire comb frequency divider has produced microwaves with an absolute phase noise rivaling, and at some offset frequencies surpassing, cryogenic microwave oscillators [3]. The absolute phase noise of a fiber comb-based system has been shown to be comparable to the Ti:Sapphire comb-based system [11]

Methods

Results

Conclusion