Abstract

Phase-sensitive optical time domain reflectometry ( $\Phi$ -OTDR) is often required in many large-scale monitoring applications. However, the frequency response of the system is restricted by the round-trip time transmitted along the sensing fiber. In this article, we propose a new system which can realize ultrasound sensing with a very long sensing distance. The new system uses a weak reflector array as the sensing part to realize a quasi-distributed measurement. An orthogonal frequency division multiplexing (OFDM) method is used to multiplex multiple probe pulses into one interrogating period. This method has a high spectrum utilization efficiency of the transmission bandwidth. Vernier effect is also used to detect the frequency higher than the repetition rate of the probe pulses. With the combination of these two proposed methods, a quasi-distributed measurement of ultrasound signal with a response frequency up to 25 kHz can be realized with a very long sensing distance up to 51 km.

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