The optically generated microwave oscillator with long-term stability locked at hydrogen maser

Shao-Yang Dai,Tian-Chu Li,Shi-Ying Cao,Fang Fang,Nian-Feng Liu,Kun Liu,Wei-Liang Chen

doi:10.1063/5.0003839

Abstract

We present an oscillator of optically generated microwaves (OGMs) based on an optical frequency comb (OFC) and an ultrastable laser (USL). The USL provides short-term stability, and the OFC transfers the short-term stability to radio frequencies. Frequency synthesis is used to transfer the stability of the OGM oscillator to an oven-controlled crystal oscillator (OCXO). To improve the long-term stability of the OCXO, an additional compensating loop was applied to lock the long term stability of OGM at a hydrogen maser by compensating the frequency drift of the USL. The phase noise of the OCXO is maintained sufficiently low through these locked loops. The OCXO’s phase noise at 1 Hz is about −116 dBc/Hz. The short-term stability (1–100 s) of the OCXO with different compensating periods is better than 4 × 10−15. By replacing the H-maser (BIPM code: 1404850) with the OGM as the local oscillator, the stability of the NIM6 Cs fountain clock is improved from (1.2 × 10−13)τ−1/2 to (5.1 × 10−14)τ−1/2. This work should help optimize fountain clocks for better type A uncertainty or faster frequency offset evaluations.

Highlights

The cesium (Cs) fountain frequency standards1–9 have served as primary frequency standards for nearly two decades
A customized phase-locked dielectric resonator oscillator (PDRO) produces the 45th harmonic of fr, which is divided into two signals: One signal is divided by a 1/12 frequency divider and mixed with a direct digital frequency synthesizer (DDS) signal to produce a 792 MHz radio frequency signal
To measure the stability and phase noise of the optically generated microwaves (OGMs) oscillator, another independent OGM oscillator with similar ultrastable laser (USL) and optical frequency comb (OFC) without the compensating loop is used as the reference of the phasenoise analyzer (Symmetricom 5125A)

Summary

INTRODUCTION

The cesium (Cs) fountain frequency standards have served as primary frequency standards for nearly two decades. To get beyond the limitation of the LO, an ultrastable cryogenic sapphire oscillator or an optically generated microwave (OGM) oscillator has been used as LO, resulting in a frequency stability as low as ∼10−14τ−1/2. In 2009, a dielectric resonator oscillator (DRO) was locked to an ultrastable laser (USL) by using an optical frequency comb (OFC) as the transfer oscillator.. In 2009, the Laboratoire National de Metrologie et d’Essais-Systemes de Reference Temps-Espace (LNE-SYRTE, France) generated microwaves by locking an OFC to an ultrastable laser (USL).. In 2009, the Laboratoire National de Metrologie et d’Essais-Systemes de Reference Temps-Espace (LNE-SYRTE, France) generated microwaves by locking an OFC to an ultrastable laser (USL).25,26 In this way, the USL’s stability is transferred to the OFC’s repetition rate fr. The LNE-SYRTE optimized their fountain clock to the quantum noise limit. The OCXO signal serves as the LO of a NIM6 Cs fountain clock, which improves the stability of the NIM6 fountain clock

Optical generation of microwaves

Stability transfer

Long-term frequency-drift compensation

EXPERIMENTAL RESULTS AND DISCUSSION

APPLICATION TO ATOMIC FOUNTAIN CLOCK

SUMMARY