Search

To capture vibration waveforms whose frequencies exceed the limitation set by sensing fiber length in OTDR distributed vibration sensing (DVS), we propose a sampling rate enhancement technique based on frequency-division-multiplexing (FDM). Our proposal, unlike conventional FDM-based sampling rate enhancement methods, directly eliminates inherent distortion by using complementary frequency concept with new probe pulse sequence. The proposed method can suppress the distortion of arbitrary waveforms in regular fiber without prior knowledge of the vibration. The proposal can accurately measure vibration waveforms and fully characterize the vibration on a standard single-mode fiber; it determines not only the frequency but also the correct amplitude and the exact shape of the vibration. Furthermore, our distortion compensation scheme incorporates a state-of-the-art FDM-based fading suppression technique, yielding high sampling rates and high sensitivity simultaneously. We demonstrate the validity of the proposal by proof-of-concept experiments in which the number of multiplexed frequencies is set to three for both sampling rate enhancement and fading suppression. In creating the experimental setup, we take the great care with the optical pulse shape and the carrier frequencies to avoid crosstalk. We successfully measure vibration waveforms with frequencies ranging from 1 kHz to 9 kHz on a sensing fiber with length of about 12 km.

To suppress coherent fading in Φ-OTDR distributed vibration sensing (DVS) with frequency-division-multiplexing (FDM), we propose an efficient methodology named the double averaging approach that processes complex vectors of Rayleigh scattered light on IQ plane. The double averaging approach rotates and averages complex vectors across the time domain as well as the optical frequency domain; this yields the final phase representing the vibration with less phase noise than the conventional method. This allows the proposal to prevent the phase unwrapping failures caused by the noise and improve the vibration detection sensitivity without incurring penalties in detection bandwidth or system complexity. The proposal is also effective in suppressing the vibration waveform distortion permitted by the conventional method. We confirm the effectiveness of the proposal through experiments in which we apply nϵ-level vibrations to an over 5 km sensing fiber and perform Φ-OTDR DVS by using FDM pulses composed of three optical frequencies.