Brillouin optical time-domain analysis (BOTDA) requires frequency mapping of the Brillouin spectrum to obtain environmental information (e.g., temperature or strain) over the length of the sensing fiber, with the finite frequency-sweeping time-limiting applications to only static or slowly varying strain or temperature environments. To solve this problem, we propose the use of an optical chirp chain probe wave to remove the requirement of frequency sweeping for the Brillouin spectrum, which enables distributed ultrafast strain measurement with a single pump pulse. The optical chirp chain is generated using a frequency-agile technique via a fast-frequency-changing microwave, which covers a larger frequency range around the Stokes frequency relative to the pump wave, so that a distributed Brillouin gain spectrum along the fiber is realized. Dynamic strain measurements for periodic mechanical vibration, mechanical shock, and a switch event are demonstrated at sampling rates of 25 kHz, 2.5 MHz and 6.25 MHz, respectively. To the best of our knowledge, this is the first demonstration of distributed Brillouin strain sensing with a wide-dynamic range at a sampling rate of up to the MHz level.

We report on what we believe to be the first truly distributed birefringence measurement of polarization-maintaining fibers (PMFs) based on transient Brillouin grating (TBG). A TBG is created by two short pump pulses in the slow axis of the PMF, and then the birefringence-related TBG spectrum is mapped by scanning a probe pulse launched in the fast axis, where the local birefringence can be calculated using the birefringence induced frequency shift. Two types of widely used PMFs, bow-tie and panda, with a length of 8 m were measured at a spatial resolution of 20 cm, and the results show that the birefringence features a periodic variation, and their variation ranges are approximately 2.4x10(-6) and 1.3x10(-6) along the test fibers, respectively.

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We propose and demonstrate a high-performance and long-range Brillouin optical time-domain analysis (BOTDA) based on time-division multiplexing measurement, where a probe pulse and a pump pulse are used to perform the measurement on a selected sensing section, and the measurement of the entire sensing fiber is realized by combining the series measurements over different sections through changing the delay time between the two pulses. In experiment, a 100 km sensing fiber is divided into 11 sections based on the gain-controlled principle, and spatial resolutions of 0.6 m and 2 m are obtained at the end of 75 km and 100 km, respectively.

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Guided acoustic wave Brillouin scattering has gained considerable interest in recent years because of its capacity to detect mechanical features of materials surrounding the optical fiber. Nevertheless, distributed measurements using this mechanism are rarely taken because of the impracticality of the method’s forward scattering mechanism. Recently, remarkable work using ingenious schemes has managed to address the difficulty, which opens a brand new way to achieve position-resolved substance identification. However, due to the long acoustic wave lifetime and insufficient signal-to-noise ratio (SNR), current spatial resolution is restricted to 15–50 m, which is far from practical requirements. Here we propose a novel opto-mechanical time-domain analysis based on coherent forward stimulated Brillouin scattering probing to greatly improve the achievable spatial resolution. The coherent transverse acoustic wave is first created by a long activation pulse and then probed by a short two-tone probe pulse. The two-tone probing process involves a coherent stimulated interaction between the probe pulse and the excited transverse acoustic wave. The interaction, which we first propose here, shows a distinct phase-sensitive characteristic. This new coherent stimulated probing process, if it is well controlled, will enhance the forward stimulated Brillouin scattering intensity and thus improve the SNR of the sensing. Moreover, higher SNR backward stimulated Brillouin scattering is used to detect the intensity evolution of the probe pulse. Owing to this new sensing scheme combined with a more robust demodulation algorithm, we demonstrated a 2 m spatial resolution opto-mechanical measurement over a 225 m long fiber in which we were able to distinguish air from alcohol. These advances greatly facilitate the practicability of forward stimulated Brillouin scattering.

We demonstrate a time-domain distributed temperature sensing based on birefringence effect on transient Brillouin grating (TBG) in a polarization-maintaining photonic crystal fiber (PM-PCF), which uses two short pump pulses (2 ns) to excite a TBG and a long probe pulse (6 ns) to map the transient Brillouin grating spectrum (TBGS) associated with the birefringence. The 2 ns pump pulses defines a spatial resolution of 20 cm and a temperature measurement range of a few hundred degrees Celsius, and the long probe pulse provides a narrow TBGS with a temperature resolution of 0.07 degrees C.

We report a high-spatial-resolution and long-range distributed temperature sensor through optimizing differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA). In DPP-BOTDA, the differential signal suffers from a signal-to-noise ratio (SNR) reduction with respect to the original signals, and for a fixed pulse-width difference the SNR reduction increases with the pulse width. Through reducing the pulse width to a transient regime (near to or less than the phonon lifetime) to decrease the SNR reduction after the differential process, the optimized 8/8.2 ns pulse pair is applied to realize a 2 cm spatial resolution, where a pulse generator with a 150 ps fall-time is used to ensure the effective resolution of DPP-BOTDA. In the experiment, a 2 cm spatial-resolution hot-spot detection with a 2 °C temperature accuracy is demonstrated over a 2 km sensing fiber.