# Comb-rooted multi-channel synthesis of ultra-narrow optical frequencies of few Hz linewidth

## TL;DR

This paper presents a multi-channel optical frequency synthesizer that generates ultra-narrow linewidth lasers with high stability and tunability, suitable for ultra-precision applications like optical clocks and spectroscopy.

## Contribution

The development of a comb-rooted synthesizer producing multiple ultra-narrow linewidth frequencies with individual channel selectability and high power amplification without stability loss.

## Key findings

- Achieved 1.0 Hz linewidth at 1 s stability
- Produced multiple stable channels with tens of mW power each
- Demonstrated potential for ultra-precision optical applications

## Abstract

We report a multi-channel optical frequency synthesizer developed to generate extremely stable continuous wave lasers directly out of the optical comb of an Er-doped fiber oscillator. Being stabilized to a high-finesse cavity with a fractional frequency stability of $3.8\times10^{-15}$ at 0.1 s, the comb-rooted synthesizer produces multiple optical frequencies of ultra-narrow linewidth of 1.0 Hz at 1 s concurrently with an output power of tens of mW per each channel. Diode-based stimulated emission by injection locking is a key mechanism that allows comb frequency modes to sprout up with sufficient power amplification but no loss of original comb frequency stability. Channel frequencies are individually selectable with a 0.1 GHz increment over the entire comb bandwidth spanning 4.25 THz around a 1550 nm center wavelength. A series of out-of-loop test results is discussed to demonstrate that the synthesizer is able to provide stable optical frequencies with the potential for advancing diverse ultra-precision applications such as optical clocks comparison, atomic line spectroscopy, photonic microwaves generation, and coherent optical telecommunications.

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Source: https://tomesphere.com/paper/1903.10719