Research On Key Techniques And Applications Of The Near-Infrared Large Aperture Wavelength-Tuning Interferometer

The planar optical elements with large aperture have found wide applications in the fields of astronomy, aerospace, high power laser and others. One example is the Shenguang III high power solid-state laser facility in which a large number of optical elements with large aperture and high quality are employed. An interferometer that has capability of measuring the surface shapes and optical homogeneity of the elements with large aperture during the process of manufacturing and applications is then required. To this end, a wavelength tuning-based phase-shifting interferometer with a diameter of600mm has been developed, which is illuminated by a source in the near-infrared wavelength range. Although wavelength tuning-based method has the advantage of measuring elements with large aperture over hardware shifting methods, the required value of wavelength tuning is a function of both the changes of the wavelength and the length of the cavity between the transmission flat and the reflective flat. And the wavelength resolution increases with the increasing of the length of the cavity. Second, the measurements of large optical elements have to be performed at the Brewster angle or a small angle in some applications in which the cavity length is very large, which makes it difficult to accurately calibrate the phase shifts. It is therefore necessary to find methods of calibrating the phase shifts at small cavity lengths and calculating the phase at large cavity lengths. In addition, the interferometer developed is illuminated by the near-infrared wavelength range with clear aperture size of600mm, enough transmission in mid-spatial frequency and capability of calculating the phases of discontinuous regions and performing measurements at a large cavity length, which lead to more strict requirements to phase calculation, optical design, and system alignment. In this thesis, these key techniques and their applications are investigated.The precision of calculated phase is directly determined by the precision of phase shifts. In this thesis, two methods are proposed to calibrate the phase shifts for measurements at different cavity lengths. The first method is based on the Lissajous figures in which phase shifts are obtained by fitting of Lissajous figure with high precision. In this method, no equal phase shifts are required and it can work at a small cavity length (about0.01m). However, it has the limitations of requiring interferograms with high contrast and the phase difference between two points should not be multiples of π. These limitations are overcomed by the second method in which phase shifts are found by Fourier transform of the intensities captured at one point at different time. This method requires equal phase shifts and can be used in the wavelength tuning-based phase-shifting interferometer. Two groups of measurement experiments were carried out with both the wavelength tuning interferometer and the Zygo GPI interferometer. The results show the difference of the PV values is less than λ/50, which demonstrated that the phase shifts can be calibrated by both methods at high precision with the latter method of higher stability.Compared with measurements at small cavity lengths, the phase shifts will deviate from π/2due to the limitation of the phase shift step resolution when measuring at large cavity lengths, which results in error in final phase calculation. In order to solve the problem, the random phase-shifting algorithm with wavelength tuning is analyzed, in which the phase shifts are assumed to be unknowns and the phases are calculated through iterative spatial and serial least-squares fitting. The difference between the PV values of the wavefronts obtained at a large cavity length and a small cavity length is less than λ/70. An adaptive phase selecting method is then proposed, in which several interferograms with a phase shift π/2are chosen from a large number of interferograms. It is found that the difference between the PV values of the wavefronts obtained at a large cavity length and a small cavity length is less than λ/300. The results show both methods can be employed in measurements at large cavity lengths with the latter method of higher precision.A seed point unwrapping algorithm based on Discrete Cosine Transform (DCT) algorithm is proposed to unwrap the wrapped phases in discontinuous regions, in which the phases in discontinuous regions are unwrapped with the seed point algorithm respectively and they are then unified with the interferometric orders obtained by DCT algorithm. The phases of discontinuous regions can rapidly and correctly unwrapped by the method which demonstrated the validity and the precision of the method.To meet the requirements of the interferometer developed, a single aspherical lens was used for collimation, and double telecentric configuration for imaging and a rotating diffuser is removed in the image system. Based on the resolution of the output wavelength and the controller of the laser, a voltage driver with high precision is developed, in which the wavelength can be tuned by the driver with high precision. Due to the invisibility of the near-infrared light, the adjusting method with visible light is proposed. The optical quality and the measurement repeatability of the interferometer were evaluated and it was found that measurement uncertainty of the interferometer was better than λ/15.Finally, the potential applications of the interferometer are discussed. The parallel plates were first measured in only two steps:first with and second without the parallel plate in the cavity between transmission flat and reflective flat. The Fourier transform algorithm was used to separate several groups of the fringes. Both the numerical simulation and the measurement experiments demonstrate the validity of the presented method. The method is characterized by its high precision and simplicity and can be used for measuring the parallel plates with large aperture. The interferometer was then employed to measure the optical elements with large aperture and at a large cavity length with data of high precision.