Phase noise measurement of semiconductor optical amplifiers

TL;DR

This paper presents a new method for measuring phase noise in optical amplifiers, demonstrating low noise floors comparable to stabilized lasers, with implications for precision optical applications.

Contribution

A novel phase noise measurement technique for optical amplifiers based on a modified Mach-Zehnder interferometer, providing lower noise floor measurements than previous methods.

Findings

Phase noise floor decreases with input laser power, reaching -125 dBrad^2/Hz at 100 kHz.

Upper bound of amplifier flicker noise is -32 dBrad^2/Hz at 1 Hz.

Measured noise levels are lower than most stabilized lasers.

Abstract

We introduce a novel measurement method for the phase noise measurement of optical amplifiers, topologically similar to the Heterodyne Mach-Zehnder Interferometer but governed by different principles, and we report on the measurement of a fibered amplifier at 1.55 $\mu\mathrm{m}$ wavelength. The amplifier under test (DUT) is inserted in one arm of a symmetrical Mach-Zehnder interferometer, with an AOM in the other arm. We measure the phase noise of the RF beat detected at the Mach-Zehnder output. The phase noise floor of the amplifier decreases proportionally to the reciprocal of the laser power at the amplifier input, down to $-125$ $\mathrm{dBrad^2/Hz}$ at $f=100$ $\mathrm{kHz}$. The DUT flicker noise cannot be measured because it is lower than the background of the setup. This sets an upper bound of the amplifier noise at $-32$ $\mathrm{dBrad^2/Hz}$ at $f=1$ $\mathrm{Hz}$, which corresponds to a frequency stability of $5.2{\times}10^{-17}/\tau$ (Allan deviation), where $\tau$ is the integration time. Such noise level is lower than that of most Fabry-Perot cavity-stabilized lasers. These results are of interest in a wide range of applications including metrology, instrumentation, optical communications, or fiber links.