Observation of Fundamental Thermal Noise in Optical Fibers down to Infrasonic Frequencies

TL;DR

This study measures the fundamental thermal noise in optical fibers at infrasonic frequencies, confirming theoretical predictions and showing that coating modifications can reduce noise levels, with implications for fiber-based systems.

Contribution

First direct measurement of thermal noise in optical fibers at infrasonic frequencies, validating theoretical models and exploring noise reduction via coating changes.

Findings

Thermal noise exhibits 1/f spectral density down to 0.2 Hz.

Measured noise levels align with theoretical predictions between 0.2 Hz and 20 kHz.

Changing fiber coating reduces the 1/f thermal noise level.

Abstract

The intrinsic thermal noise in optical fibers is the ultimate limit of fiber-based systems. However, at infrasonic frequencies, the spectral behavior of the intrinsic thermal noise remains unclear so far. We present the measurements of the fundamental thermal noise in optical fibers obtained using a balanced fiber Michelson interferometer. When an ultra-stable laser is used as the laser source and other noise sources are carefully controlled, the 1/f spectral density of thermal noise is observed down to infrasonic frequencies and the measured magnitude is consistent with the theoretical predictions at the frequencies from 0.2 Hz to 20 kHz. Moreover, as observed in the experiment, the level of 1/f thermal noise is reduced by changing the coating of optical fibers. Therefore, a possible way to reduce the thermal noise in optical fibers at low Fourier frequencies is indicated. Finally, the inconsistency between the experimental data on thermomechanical noise and existing theory is discussed.