Optical fibre sensing shows promise for monitoring the condition of structures. By embedding long optical fibres into a structure, strain and temperature distributions along the fibres can be detected. Until now, however, time of distributed measurement took from several tens of seconds to several minutes.
Now, a team from the Tokyo Institute of Technology and the University of Tokyo has increased the sampling rate by more than 5000 times, which enables real-time distributed measurement.
The approach uses BOCDR – Brillouin optical correlation-domain reflectometry – which is a distributed sensing technique with high spatial resolution, high sensitivity and stability.
In all Brillouin sensors, the strain and temperature dependence of the Brillouin frequency shift (BFS) is exploited to derive strain and temperature. In conventional BOCDR, the BFS is obtained by performing a frequency sweep over the whole Brillouin gain spectrum (BGS) using an electrical spectrum analyser. The sweep speed of the spectrum analyser limits the sampling rate to 19Hz.
By sweeping the frequency spectrum using a voltage-controlled oscillator, the researchers were able to achieve a higher speed acquisition. However, deriving the BFS from the BGS still limited the sampling rate. To speed up the system further, the BGS was converted into a synchronous sinusoidal waveform using a band-pass filter, allowing the BFS to be expressed as its phase delay. Then, using an exclusive-OR logic gate and a low-pass filter, the phase delay was subsequently converted into a voltage, which was directly measured.
The sensing system also has potential applications in robotics – acting as electronic ‘nerves’ for detecting touch, distortion, and temperature change.
Author
Peggy Lee
Source: www.newelectronics.co.uk