Correlation Between Double-Pulse Signal Characteristics and Brillouin Backscatter Spectrum in Optical Fibers

Brillouin backscattering is a well-known phenomenon in fiber optics, where the input light in an optical fiber interacts with sound waves caused by thermal vibration, which leads to a frequency-shifted backscattered light. This Brillouin frequency shift of the backscattered light changes with changes in strain and temperature, which allows the fiber to measure these changes for applications like structural monitoring, geotechnical analysis, etc. Traditional Brillouin optical time-domain reflectometry (BOTDR) systems are limited to a ~1-meter resolution due to a trade-off between spatial resolution and backscattered Brillouin frequency shift [1]. A shorter pulse improves this trade-off and provides better resolutions. Koyamada et al. found that this trade-off can be relaxed by using a double pulse (two closely spaced pulses), and they achieved a 20-cm spatial resolution (five times better than traditional) and ±1 MHz Brillouin frequency accuracy [2]. Despite its great potential, no one has studied double pulse signal characteristics (such as signal pulse width and the time interval between pulses) and their impact on the spectrum of Brillouin backscattered light. In this study, we simulated and validated the results of Naruse et al. on single-pulse behavior and extended the analysis to double pulses.