We propose a Brillouin optical correlation-domain reflectometry (BOCDR), which can measure the distribution of strain and/or temperature along an optical fiber from a single end, by detecting spontaneous Brillouin scattering with controlling the interference of continuous lightwaves. In a pulse-based conventional Brillouin optical time-domain reflectometry (BOTDR), it is difficult in principle to achieve a spatial resolution less than 1 m, and the measurement time is as long as 5-10 minutes. On the contrary, the continuous-wave-based BOCDR can exceed the limit of 1-m resolution, and realize much faster measurement and random access to measuring positions. Spatial resolution of 40 cm was experimentally demonstrated with sampling rate of 50 Hz.

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.

Brillouin scattering properties in a perfluorinated graded-index polymer optical fiber (POF) with $120\mu \text{m}$ core diameter were experimentally investigated using a laser with an operating wavelength of $1.55\mu \text{m}$ . The Brillouin frequency shift and the Brillouin bandwidth were 2.83 GHz and 105 MHz, respectively. The calculated Brillouin gain coefficient of $3.09\times {10}^{-11}\text{m}/\text{W}$ was comparable to that of fused silica fibers. The Brillouin threshold power of the 100 m POF was estimated to be 24 W, which we believe can be reduced by employing POFs with smaller cores.

Aging degradation and seismic damage of civil infrastructures have become a serious issue for society, and one promising technology for monitoring their conditions is optical fiber sensing. Glass optical fibers have been predominantly used for the past several decades to develop fiber sensors, but currently polymer or plastic optical fibers (POFs) have also been used extensively to develop advanced fiber sensors because of their unique features, such as high flexibility, large breakage strain, and impact resistance. This review focuses on recently developed distributed and quasi-distributed POF-based sensing techniques based on Rayleigh scattering, Brillouin scattering, and fiber Bragg gratings.

We newly develop a temporal gating scheme to enlarge the measurement range of Brillouin optical correlation-domain reflectometry (BOCDR) for a fiber-optic distributed strain measurement. In this scheme, the trade-off problem between the measurement range and the spatial resolution can be dissolved. 66-cm spatial resolution and 1-km measurement range were simultaneously obtained with 50-Hz sampling rate.

We investigated the dependences of Brillouin frequency shift (BFS) on strain and temperature in a perfluorinated graded-index polymer optical fiber (PFGI-POF) at $1.55\mathrm{\mu m}$ wavelength. They showed negative dependences with coefficients of $-121.8\mathrm{MHz}/\%$ and $-4.09\mathrm{MHz}/\mathrm{K}$, respectively, which are $-0.2$ and $-3.5$ times as large as those in silica fibers. These unique BFS dependences indicate that the Brillouin scattering in PFGI-POFs has a big potential for strain-insensitive high-accuracy temperature sensing.