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In a laser Doppler vibrometer (LDV), the laser Doppler effect is used to real-time acquire target displacement, velocity, and acceleration. Fiber optic laser vibrometers have received widespread attention in recent years due to their strong environmental adaptability and high integration advantages. With the expansion of detection target distance, higher requirements have been put forward for the measurement resolution of laser vibrometers. The LDV system typically employs a transceiver integrated telescope structure for laser emission and target return light reception. The aperture and focal length of the transceiver telescope determine its basic structure, directly affecting the emission and reception efficiency of laser energy. Additionally, the speckle effect generated by the scattering of rough targets affects the coupling of light energy entering the fiber optic for interference, thereby influencing the LDV measurement resolution. Based on relevant theories such as Gaussian beam waist transmission, rough target generation, Fresnel diffraction integration, and fiber optic coupling, a transceiver integrated fiber optic laser vibrometer optical field transmission model is established. Numerical simulation and analysis of the emission transmission process of ideal Gaussian laser and the coupling process of surface target echo reception are conducted. Based on the assumption of laser vibrometer speckle noise limitation, an evaluation scheme for the instrument’s noise baseline under rough target return light conditions is proposed. Numerical simulation experiments are conducted for a typical fiber LDV application scenario with an alignment distance of 1 km, a single-mode fiber mode field radius of 5 μm, and a laser wavelength of 1550 nm. The results indicate that the focal length and aperture of the transceiver telescope determine the distribution of system energy utilization and further affect the instrument’s noise baseline. Simulation results show that when the F-number of the transceiver lens reaches 3.3, LDV achieves the highest system energy utilization at this focal length, verifying the correctness of the simulation model. The simulation results can serve as a basis for the design of transceiver lenses for fiber optic laser vibrometers, laser anemometers, and other devices. -
Keywords:
- laser Doppler effect /
- wave propagation /
- speckle /
- fiber coupling
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图 1 典型激光外差振动测量系统原理图
Figure 1. Schematic diagram of a typical laser heterodyne vibration measurement system.
图 2 焦距600 mm, 口径200 mm情况下光强和相位分布 (a) 光纤内部光强; (b) 光纤内部相位; (c) 发射过程D面光强; (d) 发射过程D面相位; (e) 接收过程D面光强; (f) 接收过程D面相位; (g) 接收过程A面光强; (h) 接收过程A面相位
Figure 2. Intensity and phase distribution under the condition of a lens focal length of 600 mm and a diameter of 200 mm: (a) Intensity inside fiber; (b) phase inside fiber; (c) intensity on D plane during the emission process; (d) phase on D plane during the emission process; (e) intensity on D plane during the receiving process; (f) phase on D plane during the receiving process; (g) intensity on A plane during the receiving process; (f) phase on A plane during the receiving process.
图 3 透镜焦距600 mm, 口径200 mm情况下系统能量利用率概率分布图
Figure 3. Probability distribution diagram of system energy efficiency under the condition of a lens focal length of 600 mm and a diameter of 200 mm.
图 4 粗糙目标不同透镜参数下评价因子$\sigma $分布曲线
Figure 4. Distribution curve of evaluation factor $\sigma $ for rough target under different lens parameters.
图 5 镜面目标不同透镜参数下评价因子$\sigma $分布曲线
Figure 5. Distribution curve of evaluation factor $\sigma $ for specular target under different lens parameters.
表 1 激光发射传输过程说明表
Table 1. Explanation table of laser emission transmission process.
状态/过程 光场分布表示 理论基础 近似条件 A $ {U_{{\text{A, TX}}}}(x, y) $ 单模光纤光场分布理论 光纤高斯光场分布假设 A→B $ {U_{{\text{B, TX}}}}(x, y) $ 高斯光束自由空间传输理论 傍轴近似 B→C $ {U_{{\text{C, TX}}}}(x, y) $ 透镜相位调制理论 理想薄透镜假设
理想圆孔硬边光阑假设C→D $ {U_{{\text{D, TX}}}}(\xi , \eta ) $ 菲涅耳衍射积分理论 傍轴近似 
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表 2 激光回波接收耦合过程说明表
Table 2. Explanation table of laser echo reception coupling process.
状态/过程 光场分布表示 理论基础 近似条件 D $ {U_{{\text{D, RX}}}}(x, y) $ Johnson转化系统理论 粗糙表面平稳随机过程假设 D→C $ {U_{{\text{C, RX}}}}(x, y) $ 菲涅耳衍射积分理论 傍轴近似 C→B $ {U_{{\text{B, RX}}}}(x, y) $ 透镜相位调制理论 理想薄透镜假设
理想圆孔硬边光阑假设B→A $ {U_{{\text{A, RX}}}}(x, y) $ 菲涅耳衍射积分理论 傍轴近似 光纤耦合 — 光纤耦合理论 —
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表 3 不同目标不同焦距对应口径和F数拐点比较
Table 3. Transition points of aperture and F-number corresponding to different focal lengths for various targets.
收发透镜焦距 镜面目标 粗糙目标 口径/mm F数 口径/mm F数 400 mm 105 3.81 120 3.33 500 mm 130 3.84 155 3.22 600 mm 155 3.87 185 3.24 700 mm 180 3.89 210 3.33
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