Ultrahigh-speed distributed Brillouin reflectometry

Optical fibre sensors based on Brillouin scattering have been vigorously studied in the context of structural health monitoring on account of their capacity for distributed strain and temperature measurements. However, real-time distributed strain measurement has been achieved only for two-end-access systems; such systems reduce the degree of freedom in embedding the sensors into structures, and furthermore render the measurement no longer feasible when extremely high loss or breakage occurs at a point along the sensing fibre. Here, we demonstrate real-time distributed measurement with an intrinsically one-end-access reflectometry configuration by using a correlation-domain technique. In this method, the Brillouin gain spectrum is obtained at high speed using a voltage-controlled oscillator, and the Brillouin frequency shift is converted into a phase delay of a synchronous sinusoidal waveform; the phase delay is subsequently converted into a voltage, which can be directly measured. When a single-point measurement is performed at an arbitrary position, a strain sampling rate of up to 100 kHz is experimentally verified by detecting locally applied dynamic strain at 1 kHz. When distributed measurements are performed at 100 points with 10 times averaging, a repetition rate of 100 Hz is verified by tracking a mechanical wave propagating along the fibre. Some drawbacks of this ultrahigh-speed configuration, including the reduced measurement accuracy, lowered spatial resolution and limited strain dynamic range, are also discussed. An optical fibre sensing scheme that measures strain with a high spatial resolution and a very high sampling rate has been developed. Optical fibre sensors based on Brillouin scattering are promising for monitoring structural health. The system built by Yosuke Mizuno of Tokyo Institute of Technology and colleagues measures the frequency shift induced in the fibre’s Brillouin gain spectrum on stretching the fibre. This frequency shift is converted into a phase delay of a sinusoidal waveform, which enables the direct detection of the frequency shift. The approach allows single-point strain measurements to be performed at a rate of up to 100 kilohertz at any point along the fibre. Distributed measurements at multiple points along the fibre are also possible, although at lower repetition rates. Importantly, the scheme only requires access from one end of the fibre.