Extinction ratio free phase sensitive optical time domain reflectometry based distributed acoustic sensing system

A phase sensitive optical time domain reflectometry based distributed acoustic sensing system eliminating a degradation in a sensing performance encountered due to a finite extinction ratio of optical elements, comprising:

• a coherent laser source generating a coherent Continuous Wave (CW) laser beam with a first linewidth for detecting vibration sources near a sensing cable,
• a first gain and filter block amplifying the coherent CW laser beam to adjust a power and filtering the coherent CW laser beam to be injected into the sensing cable to eliminate at least some frequency components that are introducible during a power adjustment,
• an optical pulse generator shaping the coherent CW laser beam into optical pulses with a pulse shape generated by a Radio Frequency (RF) signal pulse generator,
• a circulator injecting an optical signal to the sensing cable and forwarding back reflected Rayleigh scattering signals coming from the sensing cable to a third gain and filter block,
• the third gain and filter block adjusting a power level of the optical signal and filtering out the at least some frequency components on a returned Rayleigh backscattered signal,
• a receiver block measuring an optical power of the returned Rayleigh backscattered signal, digitizing a measured analog optical power and processing a digitized optical signal power,
• the phase sensitive optical time domain reflectometry based distributed acoustic sensing system further comprising;
• an incoherent laser source generating an incoherent CW laser beam with a second linewidth, which is larger than the first linewidth, for degrading a coherence interference effect or a coherence length for a light injected into the sensing cable when the optical pulse generator is in an OFF state,
• a second gain and filter block adjusting a power of an incoherent light and filtering the incoherent light, wherein a resulting incoherent light has a linewidth with a power of the resulting incoherent light that mimics a power of a coherent leakage light injected when the optical pulse generator is in the OFF state,
• a combiner combining the optical signal coming from the optical pulse generator and the second gain and filter block and forwarding the optical signal to the circulator.