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摘要
针对大平面菲索干涉图像的相干噪声问题，采用旋转毛玻璃片的方法改变光束的相干性，来降低干涉系统的噪声。通过对毛玻璃的转速与条纹对比度和系统信噪比之间的关系进行仿真，获得最佳干涉条纹状态所需要的毛玻璃控制参数。在不同控制参数下获取干涉图像，并分别对各图像的条纹对比度和系统信噪比进行分析。研究结果表明，增加的毛玻璃转速，虽然在一定程度上降低了干涉图像的对比度，但却有效地提高了信噪比，便于后续的干涉图像处理。
Abstract
Since the coherent noise affected the quality of the Fizeau's interferograms in the large aperture, the coherence of the beam was changed by rotated diffuser to reduce the noise of the interfering system. The relationships among the speed of the rotated diffuser, the contrast of the fringes and the SNR of the system were simulated. Then, the control parameters of rotated diffuser would be required in the optimum interference fringe. The interference images were obtained under different control parameters, and the fringe contrast and system SNR of each image were analyzed. The results showed that the contrast can be reduced by increasing the speed of the rotated diffuser in a certain extent, but the SNR can be improved effectively and it was convenient to process the interference image later.
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Key words:
- system noise /
- rotated diffuser /
- Fizeau interferometer /
- signal-to-noise ratio (SNR) /
- contrast
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Overview
Overview: In the interference system, the defects of the optical elements on the surface became a new light source, and coherent noise was generated under the irradiation of the laser beam. Then, a Newton ring and a bull’s-eye would be formed. They would affect the quality of the interfereogram acquisition, further influence the subsequent of interference image processing, and bring the measurement error. These coherent noises which influenced the interference results were the intrinsic noise of the system. The intrinsic noise were a difficult problem when the interferometer was designed. Therefore, experts and scholars made a lot of efforts in order to suppress the intrinsic noise. This paper mainly analyzed the noise reduction performance of rotated diffuser in Fizeau interferometer, and analyzed the relationship among the speed of diffuser and the SNR , contrast of speckle, contrast of fringe in order to improve the quality of interference images.
In order to cope with the coherent noise existing in the interferometric system, the rotated diffuser was adopted to suppress the coherent noise. The relative relationship among the contrast of fringe, speckle contrast , the SNR and rotated diffuser speed on the quality of the interfering image were obtained. Then, the control parameters of rotated diffuser would be required in the interference fringe through the verification experiment. The interference images were obtained under different control parameters. With the speed of the rotated diffuser increased, the relative contrast of the speckle, the fringe contrast and the SNR of the interference images were analyzed. And a good consistency was shown through the simulation results.
According to the measurement principle, we set up the Fizeau interference experiment system. The time delay of test light and reference light in interfering the system is τRT = 5×10-8, detector acquisition period T = 1/714 s, circular pupil diameter (entrance pupil diameter) D = 7 mm and wavelength λ = 650 nm. Assume that the angular velocity of the rotated diffuser is ω = 2πf, where f is the rotational speed of the diffuser, r = 20 mm, z = 35 mm.
The experimental results showed that it was rotated diffuser that could suppress the noise of the system in the interfering system significantly. With the speed of the rotated diffuser increased, meanwhile, the fringe contrast of the images would be decreased. However, the SNR would be improved. It is indicated that the rotated diffuser did have the effect of suppressing coherent noise. It provided the basis for the processing design of the diffuser in the interferometer, so as to realize the high-precision surface inspection.
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图 3 带旋转毛玻璃的Fizeau干涉系统。1.测试镜；2.参考镜；3.扩束镜；4.分光棱镜；5.激光器；6.毛玻璃；7.成像透镜；8. CCD；9.调速装置
Figure 3. Fizeau interference system with rotated diffuser. 1. TF; 2. RF; 3. Expansion lens; 4. BS; 5. Laser; 6. Rotated diffuser; 7. Imaging lens; 8. CCD; 9. Speed control device
图 4 毛玻璃转速对图像质量的影响。(a)条纹对比度与毛玻璃转速的相对关系；(b)散斑对比度与毛玻璃转速的关系；(c)图像信噪比与毛玻璃转速的相对关系
Figure 4. The influence of rotation frequency on image quality. (a) The influence to the fringes contrast; (b) The influence to the speckles images; (c) The influence to SNR
图 5 毛玻璃转速与各参数的相对关系。(a)条纹对比度与毛玻璃转速的相对关系；(b)散斑对比度与毛玻璃转速的相对关系；(c)图像信噪比与毛玻璃转速的相对关系
Figure 5. The relationship between rotated diffuser and the parameters; (a) The influence to the contrast of contrast; (b) The influence to the speckles images; (c) The influence to SNR of the fringes
图 6 旋转毛玻璃不同转速时的干涉图像。(a) f=10 r/s；(b) f=20 r/s；(c) f=30 r/s；(d) f=40 r/s；(e) f=50 r/s；(f) f=60 r/s；(g) f=70 r/s；(h) f=80 r/s
Figure 6. The interference images at different speeds of the rotated diffuser. (a) f=10 r/s; (b) f=20 r/s; (c) f=30 r/s; (d) f=40 r/s; (e) f=50 r/s; (f) f=60 r/s; (g) f=70 r/s; (h) f=80 r/s
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参考文献
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