Phase modulation analysis for optical fiber strain caused by raindrop collision

Abstract

Optical fiber vibration sensing technology is based on the phase modulation of the transmitted light caused by the external vibration to achieve vibration measurement. A lot of researches have reported a variety of schemes for optical fiber vibration sensing and set up several application systems, achieving the functions of oil field safety monitoring, perimeter security, pipeline safety monitoring, etc. The phase modulation caused by raindrops is mixed with the sensing signal, however, when the sensing fiber cable is exposed to the atmospheric environment and the rainfall directly acts on the sensing cable. It is difficult to distinguish the valid signal which can cause the false alarms, which thereby seriously affects the normal operation of the sensing system. To our knowledge, to date, there has been no report on the phase modulation of the transmitted light in the optical fiber caused by raindrops. Based on the theory of cloud dynamics, the transformations of refractive index and shape in the core of optical fiber and the phase modulation of light caused by raindrop collision with optical fiber cable are analyzed. The model of optical phase modulation caused by raindrop collision with optical fiber cable is established, and the relationship between phase modulation and rainfall intensity is obtained. With the increase of rainfall intensity, the phase modulation increases. When the length of the optical fiber cable is fixed, the larger the cable diameter, the larger the phase modulation is. The larger the length of the cable, the greater the phase modulation is, with the cable diameter fixed. The phase modulation caused by raindrops has a positive correlation with the cable diameter and the cable length, which is related to the rainfall intensity received on the cable surface, and increases monotonically with the rainfall intensity. A laboratory verification system for phase modulation caused by raindrop collision with optical fiber cable is established, and the relationship between the phase modulation caused by raindrops and the output signal is obtained. The experimental results are compared with the simulation results at the rainfall intensities of 3, 5, 7, 10, 15, 18, 22, and 30 mm/h. The experimental and simulated results are consistent with each other under different rainfall intensities and the error is less than 9%. The results show that the model can be used to simulate the phase modulation caused by rainfall under different rainfall intensities. It provides a theoretical basis for studying the effect of rainfall on the vibration sensing system, based on which the application system can be optimized and the feasible rainfall compensation scheme can be found.