• Chinese Journal of Lasers
  • Vol. 48, Issue 24, 2404003 (2021)
Dai Fei, Wang Kai*, Lin Wei, Li Jun, and Lei Haile
Author Affiliations
  • Research Centre of Laser Fusion, China Academy of Engineering Physics, Mianyang, Sichuan 621000, China
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    Objective Gifford-McMahon (GM) cryocooler without liquid helium has gradually become the primary method to obtain a low-temperature environment, but the typical vibration displacement is about ±14.6 μm at the second-cold stage when the compressor is operated at 50 Hz. This vibration is still too high for some applications, such as optical interference, cryogenic microwave oscillators, X-ray phase-contrast imaging, and other precision systems that require submicron-level vibration control on the sample holder. We have developed a vibration-free cryostat using GM cryocooler years ago and measured the vibrations of the sample holder using the high-speed microscopic vision method. However, a more accurate method is needed to analyze its vibration to provide reference data for further reducing the vibration of the sample holder. Moreover, the direct contact measurement method will destroy the eigenfrequency of the system, resulting in the deviation of results from the real value. Therefore, the non-contact vibration measurement methods, such as the doppler method, triangle method, and holographic interferometry, are needed to analyze the vibration of the sample holder of the cryostat. This paper presents a novel vibration-measuring method for the measurement of the vibration amplitude of the cryocooler based on the Fraunhofer double-slit interference principle. This method not only has a high resolution, but also has the ability to measure the vibration of distant objects, and is especially suitable for cryostat vibration measurement and analysis.

    Methods The laser is divided into two beams by the beam splitter after collimation. Beam 1 and beam 2 are used as the reference and signal lights, respectively. First, beam 1 and beam 2 pass through the slit, respectively, to generate slit light. Then, they are reflected by mirror 1, which is fixed, and mirror 2, which is mounted on the vibration object to be measuring and pass through the slit again. Finally, the two beams focus on the focal plane by focusing the lens on generating interference fringes, acquired by the plane-array charge-coupled device (CCD). The light intensity distribution of the fringes at the focal plane is similar to that of Fraunhofer diffraction (Fig.1) when assuming that the two-slit beams are parallel to each other and there is a specific interval between the two-slit beams after passing through the spectroscope. Because the position of the interference fringe is related to the phase of the two-slit beams, we can obtain vibration information about the sample holder by acquiring the position of maximum intensity of the interference fringe. Furthermore, the factors influencing measurement results are investigated to avoid a significant amount of experimental work.

    Results and Discussions The resolution of displacement measurement is affected by the parameters of the optical system under the same displacement speed and the same CCD sampling rate through Eq. (6) and Eq. (9). Increasing the focal lengths of the focusing lens, decreasing the slit width and the slit spacing can improve the resolution of the measurement. However, the intensity of light will decrease(Fig.4 and Fig. 5). We conclude that the appropriate slit spacing, slit width and focal length according to the actual situation of CCD are chosen. In this paper, the focal length of the focusing lens is 150 mm, and the size of a single-pixel of the CCD is 13.5 μm. By adjusting the parameters of the optical system, the number of pixels in a cycle is set at around 40, the maximum sampling rate of CCD is 3000 frame/s and the laser wavelength is 532 nm. Hence, the resolution of this device is about 6.65 nm theoretically. The results show that this method can accurately obtain the submicron-level vibration of vibrating objects, and the resolution can reach 10 nm (Table 1). Finally, the vibration curve of the vibration-free cryostat is measured using the optical system (Fig. 9).

    Conclusions The current study proposes a novel vibration measuring method for measuring the vibration amplitude of the cryocooler based on the Fraunhofer double-slit interference principle. The factors influencing measurement results are investigated, and the performance of the system is evaluated using a commercial PZT. The results show that the precision of the device can reach 10 nm in the 50--1000 nm vibration range. Furthermore, the vibration of the sample holder of the GM cryocooler after vibration reduction is measured and analyzed with this vibration measuring device. Measured results show that the vibration amplitude of the sample holder is reduced from ±15 to ±0.3 μm. However, the vibration spectrum of the cryostat shows a frequency of 1 Hz, indicating that, whereas the vibration reduction method effectively inhibits vibration transfer from cold head to sample holder, there is still a large optimization space. As a result, this work provides considerable support for further optimization of the vibration level of the cryocooler.

    1 引言

    随着低温工程技术的发展,无液氦的吉福德-麦克马洪(G-M)制冷机逐渐成为模拟极端环境的主要设备之一,因其冷头的振动位移在±15 μm左右[1],所以无法适用于对振动要求较高的科学实验[2],如光学干涉、微波成像以及X射线相衬成像等精密系统都要求将样品座的振动位移控制在亚微米量级[3]。根据自适应减振原理,本实验室已经研制了一套基于G-M制冷机的低振动制冷装置,并使用高速摄像法对其振动水平进行了初步表征[4-5],但是还需要使用更为精确的手段对其振动位移进行进一步的测量分析,为进一步降低样品座的振动幅度提供参考数据。

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    Fei Dai, Kai Wang, Wei Lin, Jun Li, Haile Lei. Vibration Measurement of Cryostat Based on Principle of Fraunhofer Double-Slit Interference[J]. Chinese Journal of Lasers, 2021, 48(24): 2404003
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    Category: Measurement and metrology
    Received: Mar. 29, 2021
    Accepted: May. 24, 2021
    Published Online: Nov. 25, 2021
    The Author Email: Wang Kai (goodkai@163.com)