High-sensitivity distributed dynamic strain sensing by combining Rayleigh and Brillouin scattering

Introduction

The fluctuations of optical properties result in light scattering in the optical fiber, the mechanisms of which can be classified into three major categories, known as the Rayleigh scattering, Brillouin scattering and Raman scattering¹. Rayleigh scattering is a typical quasi-elastic scattering resulting from the non-propagating density fluctuations, introducing no frequency shift to the scattering wave. Brillouin scattering is the light scattering from the density waves (i.e. acoustic phonons), adding a Doppler shift to its backscattering wave. Raman scattering is generated by the transition of medium molecules in vibrational and rotational modes, which can be described as the scattering of light from the optical phonons. The optical properties in the fiber fluctuate with the environmental disturbances, which influence the intensity or frequency shift of scattering waves¹. In the past 40 years, the scattering based optical fiber sensors have been widely investigated, where the common communication fiber allows for distributed measurements with low loss and high sensitivity. In 1976, the first distributed optical fiber sensing scheme was reported, known as the optical time-domain reflectometry (OTDR), which has been well commercialized and widely used to investigate the fiber attenuation characteristics². In 1980s, a distributed optical fiber temperature sensor was demonstrated based on the Raman scattering³, while the Brillouin frequency shift (BFS) has been measured for distributed temperature and strain sensing^{4, 5}. At present, both the Rayleigh and Brillouin based sensors allow for the strain and temperature measurements, and their applications range from structure health monitoring to geophysical exploration.

The phase-sensitive OTDR (φ-OTDR) and Brillouin optical time domain analysis (BOTDA) are typical time-domain systems, in which the optical pulse is employed for distributed sensing. In recent twenty years, great advances in devices and schemes have promoted the improvements on the performances of distributed optical fiber sensors, especially in the aspects of long range^6-8, high spatial resolution^{9, 10}, high accuracy^{11, 12}, dynamic measurements^13-17 and multi-parameter sensing^{18, 19}. For the simplest implementation of φ-OTDR, a highly coherent light source is pulse modulated to interrogate the sensing fiber²⁰. The intensity trace of the Rayleigh scattering signal is measured with a jagged shape, as a consequence of the coherent interference among...