# Phase noise characterization of sub-hertz linewidth lasers via digital cross correlation

## TL;DR

This paper introduces a digital cross correlation method to accurately measure the phase noise of sub-hertz linewidth lasers, overcoming previous reference limitations and enabling detailed noise analysis.

## Contribution

A novel digital cross correlation scheme for absolute phase noise characterization of ultra-stable lasers with sub-hertz linewidths.

## Key findings

- Successfully measured phase noise from 0.5 Hz to 0.8 MHz.
- Demonstrated no reference limitation in phase noise measurement.
- Analyzed the phase noise performance of ultra-stable lasers.

## Abstract

Phase noise or frequency noise is a key metrics to evaluate the short term stability of a laser. This property is of a great interest for the applications but delicate to characterize, especially for narrow line-width lasers. In this letter, we demonstrate a digital cross correlation scheme to characterize the absolute phase noise of sub-hertz line-width lasers. Three 1,542 nm ultra-stable lasers are used in this approach. For each measurement two lasers act as references to characterize a third one. Phase noise power spectral density from 0.5 Hz to 0.8 MHz Fourier frequencies can be derived for each laser by a mere change in the configuration of the lasers. This is the first time showing the phase noise of sub-hertz line-width lasers with no reference limitation. We also present an analysis of the laser phase noise performance.

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Source: https://tomesphere.com/paper/1703.05983