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.

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.

Optical chirp chain Brillouin optical time-domain analysis (OCC-BOTDA) has the capabilities of fast measurement, high Brillouin threshold, and freedom from the nonlocal effect; at the same time, however, it also has problems introduced by transient stimulated Brillouin scattering. The influence of the transient interaction is reflected as the broadened asymmetric Brillouin spectrum, the ghost peak, and the frequency shift of the main peak. This introduces difficulty in computing the fiber Brillouin frequency shift with good measurement accuracy. Besides, the OCC modulation causes additional noise due to the uneven amplitude response for different frequency components. In this work, we propose a high-performance OCC-BOTDA using the principal component analysis (PCA) based pattern recognition algorithm and differential pulse-width pair (DPP) technique. After building the Brillouin spectrum database (i.e., all patterns), the fiber intrinsic Brillouin frequency shift can be recognized by the PCA algorithm from a nonstandard Brillouin spectrum profile, resulting in good measurement accuracy. Meanwhile, the DPP technique, subtracting between two Brillouin signals generated by two wide-width pump pulses, is utilized to reduce the OCC modulation noise and avoid the pulse self-phase modulation effect in long-range BOTDA sensing. In the experiment, a temperature measurement with 1.3 MHz measurement precision, 4 m spatial resolution, and 5 s measurement time is achieved over a 100 km single-mode fiber.

Brillouin-based optical fiber sensing has been regarded as a good distributed measurement tool for the modern large geometrical structure and the industrial facilities because it can demodulate the distributed environment information (e.g., temperature and strain) along the sensing fiber. Brillouin optical time domain analysis (BOTDA), which is an excellent and attractive scheme, has been widely developed thanks to its high performance in a signal-to-noise ratio, a spatial resolution, and sensing distance. However, the sampling rate of the classical BOTDA is severely limited by several factors (especially the serially frequency-sweeping process) so that it cannot be suitable for the quickly distributed measurement. In this work, we summarize some promising breakthroughs about the fast BOTDA, which can be named as an optical frequency comb technique, an optical frequency-agile technique, a slope-assisted technique, and an optical chirp chain technique.

Brillouin optical time-domain analysis requires a pulsed pump to obtain a distributed Brillouin gain spectrum (BGS) containing environmental information, whose width corresponds to spatial resolution (SR). We propose a rising edge demodulation (RED) algorithm acting on Brillouin information generated by a transient pump pulse ( < p h o n o n lifetime) via a nonlinear weight matrix to enhance SR. The distributed BGS generated by using an 8-ns transient pump pulse is processed by the RED algorithm, and its SR is enhanced from 0.8 m to 1 mm (i.e., 800-fold improvement) in the simulation and 8 mm (i.e., 100-fold improvement) in the experiment.