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The Pound-Drever-Hall (PDH) technique is a powerful method for laser frequency stabilization, and a PDH loop can also act as a single-channel sensor with high resolution and fast response speed. In this work, we expand the PDH technique into a time-domain distributed form, accomplishing a fast Brillouin optical time domain analysis (BOTDA) scheme which measures the distribution of Brillouin frequency shifts along optical fiber without frequency scanning. Approaches of phase modulated probe light, synchronous demodulation for PDH error, arbitrarily accessing in single interrogation, and distributed form feedback control are employed in the system. The scheme achieves fast measurement speed and large dynamic range at the same time, as well as good response linearity and high tolerance to optical power fluctuation. In the demonstration experiment, sensing resolution of 0.4 MHz and measurement range of 300 MHz are realized, under the condition of 25 km single mode communication fiber, 5 m spatial resolution and 0.24 s measurement time (400 times of averaging).

The utilization of distributed fiber-optic sensing systems has become prevalent in monitoring both static parameters such as temperature or strain, and dynamic parameters like vibration. In order to measure multi-parameters simultaneously, we combine Rayleigh scattering and Brillouin scattering in a single sensing system. The time-gated digital optical frequency domain reflectometry is employed for vibration sensing and the Rayleigh backscattering of linear frequency modulation pulse is used as the probe of BOTDA for temperature sensing. The multi-frequency probe enables a non-step-scanning single-end BOTDA system. To simplify the hybrid system, we employ an intensity modulator for frequency modulation and pulse generation, replacing acousto-optic modulators. Both the positive and negative sidebands are received and modulated for demarcating static and dynamic parameters. This simplified system achieves sub-meter spatial resolution for vibration within a 12-km sensing range and offers a temperature measuring range of approximately 500 $^\circ \text{C}$, which is also non-step-scanning and immune to coherent fading.

Distributed acoustic sensing (DAS) is a transformative tool for applications involving dense dynamic strain measurements. Coherent detection, and matched filtering, are two effective approaches promoting the evolution of DAS technology. In the current stage, laser with linewidth of sub-kilohertz is considered to be mandatory in phase-sensitive optical time-domain reflectometry ($\Phi$-OTDR) with coherent detection and matched filtering, which restricts its further popularization in some degree. In this work, an effective phase noise compensation method for $\Phi$-OTDR with coherent detection and matched filtering is proposed, which significantly relaxes the laser linewidth limitation in DAS. The phase noise of light source is obtained by an auxiliary interferometer, and is then used to separately compensate the coherent detection signal of Rayleigh backscattering light, and the kernel function for matched filtering demodulation. In the demonstrational experiment, such signal-and-kernel phase noise compensation method is introduced into linear-frequency-modulation-pulsed $\Phi$-OTDR with 100 kHz linewidth integrable tunable laser assembly (ITLA), achieving 50 km measurement range, 68 $\mathbf {p} \bm {\varepsilon } /\sqrt{\mathbf{Hz}}$ strain resolution, and 4.2 m spatial resolution at the same time.

Distributed fiber-optic sensing system has been employed for temperature and strain measurement in health monitoring of various structures, and current systems are mainly designed for static-only or dynamic-only parameters. To combine these functions, we propose and experimentally demonstrate a single-end multi-parameter distributed sensing system which can simultaneously measure static parameters and dynamic parameters by integrating Brillouin optical time domain analyzer (BOTDA) for static parameters and phase-sensitive optical time domain reflectometry ($\varphi$-OTDR) for dynamic parameters. The probe pulse of $\varphi$-OTDR and the pump pulse of BOTDA are successively injected into the sensing fiber. The first pulse is used as the probe pulse of a regular $\varphi$-OTDR for vibration measurement, and its Rayleigh backscattering (RBS) lightwave is used as the probe of BOTDA. The second pulse is used to amplify the RBS lightwave for the measurement of static-parameters. The interaction of the pulse pair is analyzed in detail, and the hybrid system is experimentally demonstrated. The experimental results show that 3 kHz vibration sensing with 10 m spatial resolution at 14 km standard single mode fiber can be realized, and the temperature of the sensing fiber can be measured simultaneously with 5 m spatial resolution and 1.29 MHz measurement accuracy of Brillouin frequency shift.