A phase sensitive optical time domain reflectometry based distributed acoustic sensing system eliminating the degradation in sensing performance encountered due to the finite extinction ratio of the optical elements, provided with: • a sensing cable used for remote sensing applications, • a coherent laser source configured to generate CW laser with narrow linewidth for detecting vibration sources near a sensing cable, • first gain and filter block configured to amplify the coherent CW laser to adjust the power and filtering the laser to be injected into the cable to eliminate possible undesired frequency components introduced during power adjustment, • an optic pulse generator configured to shape the CW laser into the narrow optic pulses with the desired pulse shape generated by RF signal pulse generator, • a circulator configured to inject the optic signal to the sensing cable and forwarding back reflected Rayleigh scattering signals coming from cable to third gain and filter block, • the third gain and filter block configured to adjust the power level of the signal and filtering out the undesired frequency components on the returned Rayleigh backscattered signal, • a receiver block configured to measure the optic power of the returned Rayleigh backscattered signal, digitizing the measured analog optic power and processing the digitized optic signal power, the system characterised by comprising; • an incoherent laser source configured to generate CW laser with large linewidth for degrading the coherence interference effect or coherence length for the light injected into the cable when optic pulse generator is in OFF state, • second gain and filter block configured to adjust the power of the incoherent light and filtering it such that the resulting incoherent light has a wide linewidth with its power is similar to the power of the coherent leakage light injected when the optic pulse generator is in OFF state, • a combiner configured to combine the optic signal coming from optic pulse generator and second gain and filter block and forwarding the combined optic signal to the circulator. • a sensing cable used for remote sensing applications, • a coherent laser source configured to generate CW laser with narrow linewidth for detecting vibration sources near a sensing cable, • first gain and filter block configured to amplify the coherent CW laser to adjust the power and filtering the laser to be injected into the cable to eliminate possible undesired frequency components introduced during power adjustment, • an optic pulse generator configured to shape the CW laser into the narrow optic pulses with the desired pulse shape generated by RF signal pulse generator, • a circulator configured to inject the optic signal to the sensing cable and forwarding back reflected Rayleigh scattering signals coming from cable to third gain and filter block, • the third gain and filter block configured to adjust the power level of the signal and filtering out the undesired frequency components on the returned Rayleigh backscattered signal, • a receiver block configured to measure the optic power of the returned Rayleigh backscattered signal, digitizing the measured analog optic power and processing the digitized optic signal power, the system characterised by comprising; • an incoherent laser source configured to generate CW laser with large linewidth for degrading the coherence interference effect or coherence length for the light injected into the cable when optic pulse generator is in OFF state, • second gain and filter block configured to adjust the power of the incoherent light and filtering it such that the resulting incoherent light has a wide linewidth with its power is similar to the power of the coherent leakage light injected when the optic pulse generator is in OFF state, • a combiner configured to combine the optic signal coming from optic pulse generator and second gain and filter block and forwarding the combined optic signal to the circulator.