Forward Stimulated Brillouin Scattering Analysis of Optical Fibers Coatings

Hilel Hagai Diamandi,Avi Zadok,Yosef London,Gil Bashan,Keren Shemer

doi:10.1109/jlt.2020.3041352

Abstract

The proper function of protective coating layers is essential for the handling and application of brittle optical fibers. The elastic parameters of polymer coatings can be studied through off-line analysis of test samples. However, the monitoring of these properties on a working fiber during service is challenging. In this work, we use forward stimulated Brillouin scattering processes in standard single mode fibers to measure the acoustic velocity in several types of coating layers. Pump light launches short acoustic pulses outward from the core of the fiber. Multiple reflections at the boundaries between cladding and coating, and between coating and air, form a series of delayed acoustic echoes across the core. These echoes are monitored, in turn, by photo-elastic phase modulation of probe light. Data are collected at temperatures between 25-120 °C. The thermal dependence of the acoustic velocities in several coatings and of the F-SBS resonance frequencies is investigated. Observations are corroborated by calculations. The proposed technique is well suited for research and development of coating materials, production line quality control, reliability studies and preventive maintenance of working fibers.

Highlights

T HE coating layers of brittle silica optical fibers are essential for their handling and deployment outside the research laboratory
The two metrics are closely correlated, as expected: Modes that are largely confined in the silica cladding exhibit comparatively smaller values of αT, whereas those with higher confinement in the coating layer are characterized by stronger thermal sensitivity of their resonance frequencies
We have proposed and demonstrated the measurement of acoustic velocity in the coating layer of standard working fibers using Forward stimulated Brillouin scattering (F-SBS) processes

Summary

INTRODUCTION

T HE coating layers of brittle silica optical fibers are essential for their handling and deployment outside the research laboratory. The oscillations of the acoustic modes, and the entire F-SBS process, are sensitive to the elastic properties of media beyond the fiber cladding. This characteristic serves as the basis for a new concept for optical fiber sensing of liquids outside the cladding boundary [19]–[26], which was first proposed by our group in 2016 [19]. By the Thevenaz group at EPFL and by ourselves [27], [28], have formulated F-SBS processes in coated standard fibers Both works demonstrated the quantitative analysis of liquids beyond the thin polyimide coating layer of commercially available fibers [27], [28].

FORWARD STIMULATED BRILLOUIN SCATTERING IN STANDARD COATED FIBERS

EXPERIMENTAL SETUP AND PROCEDURES

RESULTS

Findings

DISCUSSION