Ultra-High-Speed Optical Fiber Sensor Can Monitor Structural Health in Real Time

A real-time fiber-optic distributed sensing system for monitoring strain and temperature in physical structures has been developed, which requires light injection from only one end of the fiber. The novel sensing system demonstrated a sampling rate of 100 kHz, an improvement of over 5,000 times the conventional rate.

Optical fiber sensors based on Brillouin scattering show promise as a technology for monitoring the condition of structures because they are highly sensitive and stable and can be used for distributed strain and temperature measurements.

However, real-time distributed strain measurement so far has been achieved only for two-end-access systems. Such systems limit the ability to embed the sensors into structures. Further, using a two-end-access system to measure strain becomes infeasible when extremely high loss or breakage occurs at a point along the sensing fiber.

Brillouin Optical Correlation-Domain Reflectometry (BOCDR), which operates based on the correlation control of continuous lightwaves, is considered to be an intrinsically one-end-access distributed sensing technique with high spatial resolution. However, the highest sampling rate reported for BOCDR is 19 Hz, which would result in a time of distributed measurement totaling from several tens of seconds to several minutes. In conventional BOCDR, the Brillouin frequency shift (BFS), used to derive strain and temperature, is obtained by performing a frequency sweep over the entire Brillouin gain spectrum (BGS) using an electrical spectrum analyzer. The sweep speed of the analyzer limits the sampling rate.

(a) BGS acquisition. By mixing with a frequency-swept microwave, the BGS originally observed in the frequency domain can be obtained in the time domain at high speed. (b) BFS acquisition. The BGS is approximated by a one-period sinusoidal waveform and rectified. Its phase delay, which corresponds to the BFS, is then detected using an exclusive-OR (XOR) logic gate and a low-pass filter (LPF). The output voltage is in one-to-one correspondence with the phase delay in the range from 0 to 180°. Courtesy of Tokyo Institute of Technology.

Researchers from Tokyo Institute of Technology, Japan Society for the Promotion of Science and the University of Tokyo were able to increase the sampling rate of BOCDR from 19 Hz to 100 kHz, or over 5,000 times the previous rate. The researchers demonstrated real-time distributed measurement with an intrinsically one-end-access reflectometry configuration by using a correlation-domain technique. In this method, the BGS is obtained at high speed by sweeping the frequency spectrum using a voltage-controlled oscillator. To speed the system further, the team converted the BGS (which was still limiting the sampling rate) into a synchronous sinusoidal waveform using a bandpass filter, allowing the frequency shift to be expressed as its phase delay. The team then used an exclusive-OR logic gate and a low-pass filter to convert the phase delay to a voltage, which could be directly measured.

When a single-point measurement was performed at an arbitrary position, the researchers experimentally verified a strain sampling rate of up to 100 kHz by detecting a 1-kHz dynamic strain applied at an arbitrary position along the fiber. When distributed measurements were performed at 100 points with 10 times averaging, the researchers verified a repetition rate of 100 Hz by tracking a mechanical wave propagating along the fiber.

Potential drawbacks to this ultrahigh-speed configuration could include reduced measurement accuracy, lowered spatial resolution and limited strain dynamic range.

Schematic structure and photograph of the fiber under test, and the measured temporal variation of the strain distribution. Courtesy of Tokyo Institute of Technology.

The researchers were able to achieve one-end-access real-time distributed Brillouin sensing and anticipate that intrinsic one-end accessibility, an advantage that other techniques do not provide, could compensate for these shortcomings. The researchers believe that their sensing system is a promising technique for distributed dynamic strain and temperature sensing and could be especially practical for relatively short measurement ranges.

The sensing system could be used to monitor the health of various physical structures ranging from buildings and bridges to windmill blades and aircraft wings. The system also has potential applications in robotics, where it could serve as the electronic "nerves" for detecting touch, distortion and temperature change.

The research was published in Light: Science & Applications (doi: 10.1038/lsa.2016.184).