Forward Transmission Based Ultra-Long Distributed Vibration Sensing With Wide Frequency Response

A novel ultra-long distributed vibration sensor using forward transmission, coherent detection, and a frequency-shifted optical delay line is proposed and experimentally demonstrated. In the proposed scheme, a pair of multi-span optical fibers are deployed for sensing. A frequency-shifted optical delay line consisting of an acousto-optic modulator and a time delay fiber is used at the far end of these two fibers. Coherent detection is used to retrieve the vibration-induced phase fluctuations of the base-band signals as well as the intermediate frequency signals generated by the frequency-shifted optical delay line. Two differential phase signals can be calculated from the obtained phase fluctuations, which can then be used to localize the vibration events by correlation operations. Localization of a few hundred Hz, around 1 kHz and tens of kHz vibrations has been experimentally demonstrated over a total length of 1230 km sensing fiber. Less than 125 m spatial resolution can be obtained over the 615 km sensing range for vibrations with larger than 1 kHz frequency by using averaging of 30 times tests due to the nature of asynchronous operation. Frequencies from infrasound to ultrasound can be detected. The proposed scheme has advantages of ultrabroad frequency response, ultra-long sensing range and simple sensing structure due to the nature of forward transmission and coherent detection. The sensing length can be further extended using more fiber spans and erbium-doped fiber amplifiers, making it a promising candidate for vibration event detection and localization in long-haul and ultra-long-haul fiber links.