Regular ArticlesDistributed measurement of acoustic vibration location with frequency multiplexed phase-OTDR

- Position, frequency and dynamic change of a high-frequency vibration is measured.
- The vibration position and frequency are measured simultaneously and fully distributed.
- The fully distributed simultaneous measurement is conducted with FDM-OTDR using only a modulator.
- The cycle of measured vibration frequency is shorter than the repetition time of FUT.
- Measurement setup has flexibility and measurement condition can be changed with no hardware change.

All-fiber distributed vibration sensing is attracting attention in relation to structural health monitoring because it is cost effective, offers high coverage of the monitored area and can detect various structural problems. And in particular the demand for high-speed vibration sensing operating at more than 10 kHz has increased because high frequency vibration indicates high energy and severe trouble in the monitored object.Optical fiber vibration sensing with phase-sensitive optical time domain reflectometry (phase-OTDR) has long been studied because it can be used for distributed vibration sensing in optical fiber. However, pulse reflectometry such as OTDR cannot measure high-frequency vibration whose cycle is shorter than the repetition time of the OTDR. That is, the maximum detectable frequency depends on fiber length. In this paper, we describe a vibration sensing technique with frequency-multiplexed OTDR that can detect the entire distribution of a high-frequency vibration thus allowing us to locate a high-speed vibration point. We can measure the position, frequency and dynamic change of a high-frequency vibration whose cycle is shorter than the repetition time. Both frequency and position are visualized simultaneously for a 5-km fiber with an 80-kHz frequency response and a 20-m spatial resolution.