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Laser Doppler velocimetry (LDV) based on a differential laser Doppler system has been widely used in fluid mechanics to measure particle velocity. However, the two outgoing lights must intersect strictly at the measurement position. In cross-interface applications, due to interface effects, two beams of light become easily disjointed. To address the issue, we present a laser velocimeter in a coaxial arrangement consisting of the following components: a single-frequency laser (wavelength λ = 532 nm) and a Twyman–Green interferometer. In contrast to previous LDV systems, a laser velocimeter based on the Twyman–Green interferometer has the advantage of realizing cross-interface measurement. At the same time, the sensitive direction of the instrument can be changed according to the direction of the measured speed. We have developed a 4000 m level laser hydrothermal flow velocity measurement prototype suitable for deep-sea in situ measurement. The system underwent a withstand voltage test at the Qingdao Deep Sea Base, and the signal obtained was normal under a high pressure of 40 MPa. The velocity contrast measurement was carried out at the China Institute of Water Resources and Hydropower Research. The maximum relative error of the measurement was 8.82% when compared with the acoustic Doppler velocimeter at the low-speed range of 0.1–1 m/s. The maximum relative error of the measurement was 1.98% when compared with the nozzle standard velocity system at the high-speed range of 1–7 m/s. Finally, the prototype system was successfully evaluated in the shallow sea in Lingshui, Hainan, with it demonstrating great potential for the in situ measurement of fluid velocity at marine hydrothermal vents. [ABSTRACT FROM AUTHOR]

A novel technique to perform dynamic strain measurement with a Brillouin optical time‐domain reflectometry (BOTDR) sensing system based on short‐time Fourier transform (STFT) is proposed. Unlike traditional frequency scanning methods, the proposed system samples the Brillouin backscattering signal directly after it has been downconverted to the intermediated frequency (IF) region. The Brillouin frequency shift (BFS) of the sensing fibre was reconstructed from the recorded time‐domain data continuously without dead‐time, which provided a much higher refreshing rate of the strain measurement. Therefore, dynamic strain variation over an optical fibre can be measured with a single‐end structure that is more flexible and reliable. A 16.7 Hz strain variation on a 12 m section was successfully detected at the end of a 270 m sensing fibre with 4 m spatial resolution and 45 με uncertainty. [ABSTRACT FROM AUTHOR]

• A new method is proposed to measure C and α -factor based on the self-mixing effect. • Parameter C and α are obtained by tracking LETA area of the jumping points. • This scheme has the optimum theoretical change sensitivity compared to other methods. • The measured C in in the range of 1.0-4.5, and α -factor is evaluated as 2.41±0.146. In this study, a practical and straightforward method is presented for measuring the optical feedback coefficient C and the linewidth enhancement factor α by means of the mode-hopping phenomena in the self-mixing interferometric system. A theoretical analytical model, in combination with the laser energy transition area of the phase jumping points and the C -parameter, is described deeply. In the experiment, a laser self-mixing vibration sensing system is fabricated to further verify this proposed theoretical model and simultaneously obtain the geometric area S R , F under different C values. From the actual measurements, the C -parameter is exactly measured by tracking the change of the geometric area S R , F , over the range of 1.0-4.5. Then, the α -factor of this self-mixing sensor is obtained as 2.41±0.15 with an accuracy of 6%. In addition, the absolute and relative error of the experimental results are less than 0.14 and 7.43%, respectively. In a word, the measured results agree well with the simulated values, of which validates the good applicability and practicability of this measurement method. [ABSTRACT FROM AUTHOR]