Chongfan Technology
News
04
2025
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03
Integrated fiber optic transceiver for rotating Doppler speed measurement
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In recent years, with the exploration of the physical dimensions of light fields, it has been found that utilizing the orbital angular momentum of light fields can produce changes in light frequency related to the rotational speed of objects, known as the rotational Doppler effect. The rotational Doppler measurement technology developed based on this theory has opened up new ideas for angular velocity measurement research. In order to overcome the limitations of existing technologies, such as high costs, large size, and poor flexibility,the team of Professor Wang Jian from Huazhong University of Science and Technology's Multidimensional Photonics Laboratory (MDPL)has proposed and developed the first integrated all-fiber rotational Doppler velocimetry technology. This technology features high integration and ultra-wideband operation, providing the possibility for the realization of miniaturized new optical measurement tools, with broad application prospects in motion monitoring across multiple scenarios.
The Doppler effect is a classic physical phenomenon that describes the frequency shift of waves caused by relative motion between the wave source and the observer. Generally, the Doppler effect is considered a response to linear momentum or linear phase gradient, where the object being analyzed is in translational motion. In recent years, with the deepening of research into new physical dimensions of light fields, it has been found that the Doppler effect can also be associated with angular momentum. Thus, the Doppler effect has been further developed into its transverse form, known as the rotational Doppler effect. From the perspective of light field mode analysis, the rotational Doppler effect is described as follows: when the light field interacts with a rotating object, the orbital angular momentum (OAM) in the mode is modulated, resulting in a frequency shift proportional to the change in OAM.
Among the various methods of optical measurement, laser Doppler velocimetry developed based on the Doppler effect is a widely used technology that offers advantages such as high measurement accuracy, wide measurement range, fast response speed, and non-contact measurement, overcoming some limitations of traditional measurement technologies. In recent years, with the improvement of theoretical research on the rotational Doppler effect, the rotational Doppler measurement technology developed based on it has been applied to the measurement of rotational motion of scattering bodies, microparticles, and vortices. As an efficient new angular velocity measurement technology, rotational Doppler velocimetry shows great potential. However, the currently established rotational Doppler measurement systems are usually composed of discrete optical components configured in free space, facing significant challenges in practical applications due to high costs, large size, and poor flexibility.
This study proposes for the first time an integrated all-fiber rotational Doppler velocimetry technology (AF-RDV). The core device of this technology is an ultra-wideband mode-selective coupler, which has low insertion loss performance and can achieve mode conversion, filtering, and transmission. The mode-selective coupler is designed using single-mode and multi-mode fibers through mode coupling theory, and its manufacturing process includes: pre-tapering of single-mode fiber, twisting single-mode and multi-mode fibers, and melting tapering. By matching the effective refractive indices of the modes, conversion between the Gaussian fundamental mode in single-mode fiber and the OAM superposition mode in multi-mode fiber can be achieved. Figure 1 illustrates the conceptual principle of the all-fiber rotational Doppler velocimetry technology, where, depending on the selection of input and output ports, a reciprocal working mode can be achieved, namely forward operation (a) and reverse operation (b).
In the forward operation shown in Figure 1 (a), the Gaussian fundamental mode is input from port 1 of the single-mode fiber and is converted into a first-order linearly polarized mode, i.e., ±1 order OAM superposition mode, via the mode-selective coupler. After being illuminated by the probe light on the rotating object to be measured, it is modulated by the scattering body into a speckle with multiple OAM mode components, where different OAM modes carry rotational Doppler frequency shifts related to the mode change. The scattered light is received through port 3, and after undergoing mode filtering in the tapering region of the mode-selective coupler, the Doppler beat frequency signal is collected at port 2, where the collected signal mainly consists of the Gaussian fundamental mode in the scattered light. In the reverse operation shown in Figure 1 (b), the Gaussian fundamental mode is input from port 2 of the multi-mode fiber and is directly transmitted to port 3 to illuminate the rotating scattering body with the Gaussian fundamental mode. The scattered light is received at port 3, where the ±1 order OAM superposition mode will be converted back to the Gaussian fundamental mode and returned to port 1, while other mode components are filtered out. Thus, in both operations, by exchanging the input and output ports, the input and output modes of the system remain unchanged, indicating that the rotational Doppler effect has mode-related characteristics, and its testing system is reciprocal. The detection illumination and signal acquisition in the system are achieved through the same port, thus featuring an integrated transceiver characteristic, which can effectively enhance the system's integration.
By performing Fourier analysis on the signals collected from the output port, the Doppler beat frequency can be extracted from the spectrum, and the rotational speed information can be obtained through numerical calculations. Theoretical analysis shows that the rotational Doppler effect is wavelength-independent, which has been verified experimentally in this study, as shown in Figure 2. The mode-selective coupler designed in this study has ultra-wideband chromatic aberration performance. By using a tunable laser to perform spectral scanning measurements from 1500 nm to 1620 nm at intervals of 10 nm, it was observed that the modes converted by the mode-selective coupler maintain high purity. Therefore, the all-fiber rotational Doppler velocimetry system developed in this study has ultra-wideband working performance, and the measurement accuracy is not affected by factors such as laser linewidth and temperature drift, which can significantly reduce the costs of system setup and maintenance.
This study has achieved a significant breakthrough in motion measurement technology, developing the first all-fiber rotational Doppler velocimetry system. This system features integrated transceiver and ultra-wideband operation, with advantages of high integration and low cost, promising to provide a miniaturized new optical measurement tool with broad application prospects in physics, life sciences, and industrial production.
In future research, the author team will optimize the all-fiber rotational Doppler velocimetry technology and conduct broader application validation. For example, high-order mode-selective couplers will be developed to enhance the measurement accuracy of the system. Additionally, the packaging of the system and external field testing will also be key focuses to ensure its stability and reliability in practical applications. The goal is to maximize the potential of AF-RDV technology in the field of optical measurement, providing more efficient solutions for motion monitoring in multiple scenarios.
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