Flow vector measurement has important research significance and application value in fields of biomedicine, such as blood flow monitoring, quantitative drug delivery, and biochemical analysis. At present, laser feedback interferometry technology has been widely used in the field of velocity measurement by virtue of the merits of auto-alignment, and cavity gain, which can acquire the flow velocity information by monitoring the interference signal between the backscattered light and the original optical field in the cavity. However, the uncertainty of the feedback angle of incident light and scattered light causes a challenge in obtaining real-time velocity vector information. To solve these problems, Professor Liang Lu's team of Anhui University proposed a two-dimensional flow vector measurement system based on all-fiber laser feedback frequency-shifted multiplexing technology. Through innovative optical design, the two-dimensional vector velocity information can be decomposed into one-dimensional velocity information in the frequency domain. The distribution of the two-dimensional vector flow velocity in the pipeline is accurately identified from the rate distribution characteristics, and the incident angle of the laser beam on the fluid is also obtained. This work provides a highly efficient non-contact solution for 2D vector velocity sensing in various complex scenes. Relevant research results were recently published in Photonics Research, Volume 12, Issue 7, 2024.[ Lei Zhang, Jialiang Lv, Yunkun Zhao, Jie Li, Keyan Liu, Qi Yu, Hongtao Li, Benli Yu, Liang Lu, "Two-dimensional flow vector measurement based on all-fiber laser feedback frequency-shifted multiplexing technology," Photonics Res. 12, 1371 (2024)]

The setup of the two-dimensional laser frequency-shifted feedback flow velocity measurement system is depicted in Figure. 1. Due to the laminar distribution of fluid in the pipeline, the scattered particles in different locations both provide feedback light, which causes the superposition of the information carried by the flow velocity. The multi-channel acoustic-optic frequency-shift multiplexing technology has been adopted to identify the flow velocity information of different dimensions, which makes the flow velocity signals of different dimensions undergo different interference optical paths and frequency-shift channels. Among them, the velocity signal in the X-dimension is mapped to the fundamental frequency channel in the frequency domain, while the velocity signal in the Y-dimension is shifted twice to the frequency doubling channel, finally, the velocity vector information is extracted by the shape and position distribution of the frequency spectrum signal of laser feedback interference. Furthermore, the angle between incident light and fluid motion can be obtained by the orthogonal double beam measurement structure, and the reconstruction of flow velocity distribution in the pipeline also be realized.

Figure. 1 Setup of two-dimensional laser feedback frequency-shifted flow vector measurement.

The corresponding author of this study, Professor Liang Lu, said: "In many practical applications, the accurate measurement and control of vector information of fluid velocity is crucial. The traditional one-dimensional velocity measurement method is difficult to obtain the complete vector information of the fluid motion. The two-dimensional flow vector measurement method based on the laser feedback frequency-shift multiplexing can realize the decomposition and recognition of signal, and the ability of this system that obtains vector flow velocity and accurately measure velocity distribution under unknown incident angle is verified by theoretical simulation and experimental verification. This method can be extended to a higher dimensional flow velocity measurement system to meet the requirements of more extensive flow vector velocity measurement." In follow-up studies, our team will optimize the structure of the laser source and the sensing system, such as using a random laser with modeless characteristics as the light source to explore reconstruction and measurement of the spatial flow velocity distribution with high sensitivity and high precision. Meanwhile, the team also will study its applications in biomedical monitoring, microfluidic sensing, and complex turbulence.