Fiber-Optic Interferometry Using Narrowband Light Source and Electrical Spectrum Analyzer: Influence on Brillouin Measurement

TL;DR

This paper investigates interference effects in fiber-optic Brillouin measurements using a simple interferometer setup, revealing noise sources and proposing suppression techniques to improve measurement accuracy.

Contribution

It demonstrates the influence of interference patterns on Brillouin measurements and introduces methods to suppress related noise for enhanced sensing performance.

Findings

Interference patterns extend beyond the Brillouin frequency shift range.

Noise in Brillouin measurements originates from interference between reference and reflected light.

Applying a delay line longer than the light source's coherence length suppresses noise.

Abstract

We observe an interference pattern using a simple fiber-optic interferometer consisting of an electrical spectrum analyzer and a narrowband light source, which is commonly employed for observing the Brillouin gain spectrum. This interference pattern expands well beyond the frequency range corresponding to the Brillouin frequency shift in silica fibers (approximately 11 GHz at 1550 nm). Using both silica single-mode and polymer optical sensing fibers, we then experimentally prove that the distinctive noise in a self-heterodyne-based Brillouin measurement with an unoptimized polarization state originates from the interference between the reference light and the Fresnel-reflected light. This noise can be almost completely suppressed by employing a delay line that is longer than the coherence length of the light source and by artificially applying a high loss near the open end of the sensing fiber.