Recently, Rayleigh scattering-based distributed fiber sensors have been widely used for measurement of static and dynamic phenomena such as temperature change, dynamic strain, and sound waves. In this review paper, several sensing systems including traditional Rayleigh optical time domain reflectometry (OTDR), Φ -OTDR, chirped pulse Φ -OTDR, and optical frequency domain reflectometry (OFDR) are introduced for their working principles and recent progress with different instrumentations for various applications. Beyond the sensing technology and instrumentation, we also discuss new types of fiber sensors, such as ultraweak fiber Bragg gratings and random fiber gratings for distributed sensing and their interrogators. Ultimately, the limitations of Rayleigh-based distributed sensing systems are discussed.

Distributed time-domain Brillouin scattering fiber sensors have been widely used to measure the changes of the temperature and strain. The linear dependence of the temperature and strain on the Brillouin frequency shift enabled the distributed temperature and strain sensing based on mapping of the Brillouin gain spectrum. In addition, an acoustic wave can be detected by the four wave mixing (FWM) associated SBS process, in which phase matching condition is satisfied via up-down conversion of SBS process through birefringence matching before and after the conversion process. Brillouin scattering can be considered as the scattering of a pump wave from a moving grating (acoustic phonon) which induces a Doppler frequency shift in the resulting Stokes wave. The frequency shift is dependent on many factors including the velocity of sound in the scattering medium as well as the index of refraction. Such a process can be used to monitor the gain of random fiber laser based on SBS, the distributed acoustic wave reflect the distributed SBS gain for random lasing radiation, as well as the relative intensity noise inside the laser gain medium. In this review paper, the distributed time-domain sensing system based on Brillouin scattering including Brillouin optical time-domain reflectometry (BOTDR), Brillouin optical time-domain analysis (BOTDA), and FWM enhanced SBS for acoustic wave detection are introduced for their working principles and recent progress. The distributed Brillouin sensors based on specialty fibers for simultaneous temperature and strain measurement are summarized. Applications for the Brillouin scattering time-domain sensors are briefly discussed.

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 12.5 µ ε 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 12.5 µ ε represents a factor of 3 improvement. This number is limited by a pre-recorded frequency scanning range of RFGA response in the database.

Chalcogenide fibers are currently being used widely in nonlinear optical signal processing, as they exhibit ultrahigh nonlinearity. Here, we propose a sensor based on stimulated Brillouin scattering for simultaneous temperature and strain measurement in a dual-core tapered A s 2 S e 3 -polymethyl methacrylate fiber using a Brillouin optical time-domain analysis system. Different Brillouin frequency responses under temperature and strain variations and the separation of Brillouin frequency shifts (BFSs) in two principal polarization axes are demonstrated experimentally over a 50-cm-long tapered dual-core hybrid microfiber. The temperature coefficients are − 3.8272 M H z / ∘ C and − 3.3302 M H z / ∘ C , and the strain coefficients are − 0.06143 M H z / µ ε and − 0.03463 M H z / µ ε . Due to the different temperature and strain dependences of Brillouin frequency peaks in two polarizations, temperature and strain resolutions of 1°C and 33 µ ε are realized, respectively. Numerical simulations are also reported to account for the BFS difference in two polarization axes.

Driven by the strong need for distributed high-frequency dynamic strain sensing, ultra-fast Brillouin optical time-domain analysis is rapidly becoming a vital technique. Thus, in this Letter, we propose and demonstrate a novel method by using a chirped pulse as a pump signal to extract the relative Brillouin frequency shift (BFS) changes through the real-time delays between adjacent Brillouin traces; this enables static and dynamic strain measurement without time-consuming frequency sweeping process. Benefiting from single-shot measurement based on Brillouin traces, the system has a high acquisition rate that is only limited by the sensor length without averaging, and is immune to the polarization fading problem, thanks to electrical delay time measurement. The pump source is a 1 MHz linewidth laser without a phase-locking loop; the laser frequency drifting noise could be compensated by the signal from the non-disturbed fiber section. In the experiments, BFS measurement resolution of 0.42 MHz with 4.5 m spatial resolution is demonstrated over a 5 km non-uniform fiber.

Dynamic strain sensing over a frequency range from 0.01 to 20 Hz can be used for monitoring earthquakes and volcanoes, charting rock and petroleum formations beneath the earth. However, significant laser frequency drifting (LFD) has limited the detection in this frequency range, especially for distributed frequency detection with phase optical time domain reflectometry (OTDR), where the LFD will introduce a time dependent noise destroying the dynamic strain reconstruction. In this study, a simple and effective method that utilizes the referenced random fiber grating to monitor the variation of laser frequency has been both theoretically analyzed and experimentally demonstrated. During the maximum up to 200 s data acquisition time, the frequency variation of a distributed feedback (DFB) laser with MHz linewidth is obtained from the referenced portion of sensing signal, and then the 1 Hz and 0.01 Hz dynamic strain variations with amplitude of 30 µ ε are reconstructed with strain measurement standard deviation of 66 n ε . Due to signal-to-noise ratio (SNR) enhanced Rayleigh traces from random fiber gratings, a minimum detectable frequency drifting of 7.28 MHz could be achieved over the optical frequency of 2 × 10 14 H z .