Conceptual model of a measuring fiber-optical sensing system

DOI: 10.31673/2412-9070.2025.024101

Abstract

The article discusses the use of backscattering of light waves from an optical fiber used in a distributed fiber-optic sensing system to measure various parameters. Rayleigh, Brillouin, and Raman backscattering provide different sensitivity to different measured quantities and attract the attention of researchers. Moreover, using a fiber optic cable in real time can provide information about the environment of various objects, such as: telecommunications and rail transport systems, large bridges and buildings, and assess their technical condition, which is important when building a smart city concept based on the Internet of Things.
Considering the capabilities of a system that combines the three principles mentioned above, it can effectively separate the measurement of strain and temperature, and provide measurements of both dynamic and static parameters. However, the combined system is extremely complex if the three systems are independent of each other.
In order to simulate the concept of the measurement system, the study used two consecutive pulses to implement Brillouin amplification of Rayleigh backscattering of light waves to combine the Rayleigh and Brillouin methods. For the demodulation of the Raman scattering of the two pulses, a 3-bit pulse coding method was used, which allows integrating optical Raman reflectometry in the time domain into the hybrid system.
Herein, we propose a hybrid distributed fiber-optic sensing system that combines systems using three different scattered lightwaves via a single-end scheme. Two successive pulses are launched into the system to realise Brillouin amplification of the Rayleigh backscattering lightwave: a Rayleigh probe pulse of φ-OTDR for vibration measurement, followed by a Brillouin pump pulse of a single-end Brillouin reflectometry system for strain/temperature measurement. The Rayleigh backscattering lightwave of the probe pulse serves as a continuous probe wave for the single-end Brillouin reflectometry. For Raman reflectometry, a wavelength division multiplexer (WDM) is used to obtain Stokes and anti-Stokes Rayleigh scattering lightwaves. Because a pulse pair is used in the system and the frequency deviation is not sufficiently large to be distinguished in the Raman reflectometry system, a 3-bit pulse coding method is employed to demodulate the Rayleigh scattering lightwave of the two pulses.
The results of the work show the advantages of using a combined measuring scheme, how it implements simultaneous measurement of several parameters and achieves favorable measurement accuracy.

Keywords: fiber optic measurement, fiber optic sensing system, distributed fiber optic measurement, Raman scattering, fiber sensor.