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摘要:
为了解决光纤多普勒测振仪远距离非接触测量物体振动时，信号偏弱、过零点波形凌乱、难于解调的问题，本文提出了一种新型光纤准直系统。本系统主要采用在小型C-lens光纤准直器后端增加整形系统并借助ZEMAX软件对高斯光束进行准直并优化，通过对光纤准直系统成品进行耦合测试实验，使用C-lens准直器与光纤准直系统进行信号耦合效率的测试对比。实验结果表明：改进后的准直系统能够满足2 m的工作距离，空间返回光耦合效率最大能达到6.3%，极大地提升了多普勒信号的对比度，提高了远距离光纤多普勒测振仪对振动的测量精度。
Abstract:In order to solve the problem of weak signal, messy waveform at zero-point and difficult demodulation, when the optical Doppler vibrometer in the long-distance non-contact measurement, in this paper, a new type of optical fiber collimation system is proposed. The system mainly uses an augmented beam shaping system at the end of a small C-lens optical fiber collimator and the Gaussian beam is collimated and optimized by ZEMAX software. Through the coupling test of finished product of optical fiber collimation system, and compared with the signal coupling efficiency of C-lens collimator. The experimental results show that the improved collimation system can meet the working distance of 2 meters, and the coupling efficiency of space return optical up to 6.3%, which greatly enhances the Doppler signal contrast and improves the long-distance optical fiber Doppler vibration measurement accuracy.
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Key words:
- fiber collimator /
- optical fiber Doppler measurement system /
- coupling efficiency /
- EDFA
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Overview: When an optical Doppler vibrometer with the C-lens or G-lens collimator is used in the long-distance non-contact measurement, there exists problems like weak signal, messy waveform at zero-point and difficulty of demodulation. The rear end of G-lens fiber collimator is plane. The refractive index of the lens along the gradient changes, and the focal length can be achieved by changing the length of the lens. Generally speaking, 0.23 cycle length is used to achieve the collimation effect. The C-lens collimator for our own research and development, with good long-distance performance and low cost, uses a constant refractive index of the thick lens. The rear end of the spherical surface of the front bevel 8°. In order to solve these problems, a new configuration type of collimation system is obtained by adding a beam shaping system at the end of a small C-lens fiber collimator and optimizing the Gaussian beam with ZEMAX software. The 1550 nm invisible laser beam emitted by the fiber collimator is collimated and optimized through the beam shaping system, and irradiate on the surface of the vibrating object at a distance of 2 m. The beam reflected by the vibrating object is coupled back to the C-lens collimator through the beam shaping system to achieve a return light signal with high coupling efficiency.
The experimental setup is built for the designed collimation system as shown in the figure. At first, the beam emitted by the DFB laser is amplified by the EDFA. The use of the optical circulator effectively enables the return light to be output from the 3-port, which introduces a small amount of insertion loss compared to the conventional 3 dB coupler, thereby significant improvement of the utilization of the light power is emitted by the light source. Then passing through the C-lens collimator and the beam shaping system and incident perpendicularly onto the surface of the objectThe beam is finally reflected by the surface of the object and returned to the system and outputted through the circulator port 3. Lastly, the optical power of the output light can be measured by optical power meter.
The experimental results provide a reference for improving the contrast of the Doppler interference signals obtained by long-distance non-contact measurement of vibrating objects. The results show that the system can meet the working distance of 2 m and the space coupling efficiency of the return optical transmission reaches 6.3%, it greatly reduce the subsequent data processing work of vibration measurement experiment.
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图 10 C-lens准直器与新型光纤准直仪的耦合测试曲线图
Figure 10. C-lens collimator and new fiber collimator coupling test curve
表 1 光学设计指标
Table 1. Optical design index
Parameter Value Work wavelength/nm 1550±30 Zoom ratio 9× Length range/m 2 Total length/mm < 90 
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表 2 EDFA主要参数
Table 2. EDFA main parameters
Parameter Value Work wavelength/nm 1530~1568 Input power/mW 0.25 ~2 Adjustable gain/mW 200 Noise index/mW 3.55 Power supply DC+5 V
(Working current: 350 mA)Pigtail FC/APC
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表 3 C-lens准直系统耦合效率测试数据
Table 3. C-lens collimation system coupling efficiency test data
Wave-
length/nmP1/mW P2/mW P3/μW Coupling
efficiency
(P3/P2)/%0 0 0 0 2.1 1.670 0.2 0.01 4.1 3.205 1 0.03 6.1 4.767 3 0.06 8.1 6.313 13 0.02 1550 10.1 7.816 16 0.02 12.0 9.397 28 0.03 14.0 10.86 33 0.03 16.1 12.49 37 0.03 18.1 13.96 52 0.037 20.1 16.74 104 0.06
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表 4 改进后准直系统耦合效率测试数据
Table 4. Improved collimation system coupling efficiency test data
Wave-
length/nmP1/mW P2/mW P3/mW Coupling
efficiency
(Р3/Р2)/%0 0 0 0 2.1 1.670 0.042 2.51 4.1 3.205 0.096 3 6.1 4.767 0.192 4.1 8.1 6.313 0.308 4.88 1550 10.1 7.816 0.406 5.2 12.0 9.397 0.523 5.55 14.0 10.86 0.614 5.65 16.1 12.49 0.727 5.8 18.1 13.96 0.838 6 20.1 16.74 1.05 6.3
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