Laser source frequency drift compensation in Φ-OTDR systems using multiple probe frequencies

Fully distributed fiber sensors, such as phase sensitive optical time domain reflectometry (Φ-OTDR) systems, have drawn significant attention from researchers, especially for use in geophysical applications. Distributed sensing, cost efficiency, wide dynamic range, good spatial resolution, and high accuracy make these sensors ideal for industrial use and for replacing traditional geophones. However, inevitable drifts in the central frequency of laser sources always cause low frequency noise in the output, which could easily be mistaken with real sub-Hertz environmental vibrations. This deteriorates the data accuracy, especially when dealing with low frequency seismic waves. In this study, we propose a method in which adding an extra probe frequency to a Φ-OTDR setup provides a reference frequency. This reference frequency provides information regarding changes in the laser source and other environmental noises, such as humidity and temperature, helping to refine extracted results from low frequency noise. This feature is also very useful for frequency domain analysis, where we may lose the near DC band information during mathematical measurements. Regarding the adjustable properties of this reference frequency, it can be implemented in various Φ-OTDR applications and commercial devices.