Strain measurement range enhanced chirped pulse φ-OTDR for distributed static and dynamic strain measurement based on random fiber grating array

A high-performance distributed sensing system based on a random fiber grating array (RFGA) and multi-frequency database demodulation (MFDD) method for strain induced delay time measurement is demonstrated. It enables a wide measurement range for both static and dynamic strain sensing. The proposed MFDD method can enlarge the strain measurement range, since the large strain variation induced time domain trace distortion could be compensated for by laser initial frequency changes. Furthermore, a random fiber grating made with embedded large random refractive index changes along the single-mode fiber could provide a stable reflection with a wide reflection spectrum range. Such a structure successfully improves the time delay measurement precision and achieves a large tuning range, as demonstrated by the database in which a set of pre-recorded undisturbed reflected Rayleigh traces form RFGA at various laser frequencies. Ultimately, a dynamic strain with a peak-to-peak value of 12.5µε at a vibration frequency of 50 Hz is accurately reconstructed when the pulse repetition rate is 1 kHz, which was not detected using a conventional chirped pulse phase-sensitive optical frequency domain reflectometers. The maximum measurable strain variation of about 12.5µε represents a factor of 3 improvement. This number is limited by a pre-recorded frequency scanning range of RFGA response in the database.