Ultra-scalable clock and carrier sychronisation for optical and wireless networks using sequentially-locked optical frequency combs

_All telecommunications systems, which transmit data from device to device, whether through optical fibre between continents or through the air between mobile phones and radio masts, fundamentally rely on 1\. carrier synchronisation, determining the frequency used to send the data (whether that be visible, microwaves, or radio waves), and 2\. clock synchronisation, determining the data transmission rate. Consequently, both types of synchronisation are critical to modern telecommunications system performance.

Additionally, clock and carrier synchronisation is key to accurate time synchronisation - essential for synchronising the UK's critical national infrastructure (CNI), including power stations in the National Grid, our railways and our mobile and broadband networks._

_Our proposal aims to address a key issue impacting the UK's CNI: our CNI is currently clock synchronised by global satellite navigation systems (GNSSs), such as GPS and Galileo. This is a major vulnerability: synchronisation provided by GNSSs may be lost due to solar storms, cyberattacks, jamming or volcanic ash obstruction. An alternative is to distribute highly accurate clocks through our existing optical fibre infrastructure.

However, this brings two major research challenges: 1\. scalability: a single highly accurate clock can currently only reach up to about 1000 endpoints, 2\. optical fibre variation: distribution of clocks through optical fibre introduces inaccuracy due to variation of the fibre medium due to e.g. temperature change. There is also a major commercial challenge: how to address research challenges 1 & 2 at low cost._

_To address these challenges, we propose using coupled optical frequency combs, which each output a 'comb' of light of different frequencies, all synchronised together. In this approach, thousands of comb frequencies from a central extremely high clock accuracy but expensive comb each synchronise a downstream inexpensive optical frequency comb through \>100 km optical fibre.

These downstream optical frequency combs each have thousands of comb frequencies of their own, each of which clock synchronise an endpoint, allowing synchronisation of millions of endpoints from the central highly accurate frequency comb, addressing the scalability challenge. We address the optical fibre variation challenge by exploring new digital methods of measuring and compensating for the optical fibre medium variation.

We address the cost challenge by exploring the miniaturisation of ultra-fast laser-based optical frequency combs. We would demonstrate our approach in a field trial optical fibre link between an extremely accurate optical frequency comb hosted by BT and low-cost ultra-fast laser-based frequency combs hosted at UCL and developed by Menhir Photonics._