Enhanced timing of a 113 km O-TWTFT link with digital maximum likelihood estimation process

TL;DR

This paper introduces a complex least squares method using maximum likelihood estimation to improve timing accuracy and sensitivity in long-distance optical time transfer, achieving record low received power levels.

Contribution

The novel CLS method incorporates amplitude and phase data for enhanced timing extraction in optical links, surpassing previous sensitivity benchmarks.

Findings

Achieved a record minimum received power of 0.1 nW over 113 km link.

Enhanced timing precision approaches quantum limits.

Demonstrated improved detection sensitivity at ultra-low power levels.

Abstract

Optical two-way time-frequency transfer (O-TWTFT), employing linear optical sampling and based on frequency combs, is a promising approach for future large-scale optical clock synchronization. It offers the dual benefits of high temporal resolution and an extensive unambiguous range. A critical challenge in establishing long-distance free-space optical links is enhancing detection sensitivity. Particularly at ultra-low received power levels, the error caused by time extraction algorithms for linear optical sampling becomes a significant hindrance to system sensitivity, surpassing the constraints imposed by quantum limitations. In this work, we introduce the Complex Least Squares (CLS) method to enhance both the accuracy and sensitivity of time extraction. Unlike most previous methods that relied solely on phase information, our scheme utilizes a maximum likelihood estimation technique incorporating both amplitude and phase data. Our experiments, conducted over a 113 km free-space link with an average link loss of up to 100 dB, achieved a record minimum received power of 0.1 nW, which is over ten times lower than previous benchmarks. The precision also approaches the quantum limitation.