A novel point diffraction interferometer with reflective shearing optical structure is developed. The substrate of interferometer has an angle with optical axis, and the incident converging spherical wavefront will be reflected by the front and rear surfaces of the substrate, respectively. Then, an interferogram with carrier frequency is obtained. Using Fourier transform algorithm, the wavefront can be retrieved from one single interferogram. In this article, the intensity distribution formulas of interferogram are derived, and the system error and the major parameters of interferometer are also discussed. The new design with compact structure and high resolution is contrast adjustable and suitable for dynamic wavefront measurement.
A single-shot reflective shearing point diffraction interferometer (S-SRSPDI) is designed for large-aperture dynamic wavefront measurements. The PDI is integrated on the small substrate with properly designed thin film. The wavefront under test is reflected by the front and rear surfaces of the substrate respectively to generate an interferogram with high linear-carrier frequency, which is used to reconstruct the wavefront by means of the Fourier transform algorithm. In this paper, the analytic formula of intensity distribution of the interferogram is derived. The parameters related with the carrier frequency of fringes are discussed. The method to optimize the contrast of the interferogram is proposed by analyzing the reflective polarization effects. In addition, the spurious fringes of the interferogram are removed by the proper designed blocking film. S-SRSPDI was applied to detect the dynamic wavefront with a diameter of 400 mm. The measured aberrations are in good agreement with those obtained by the shearing method, which verifies that the proposed S-SRSPDI is a powerful tool for large-aperture dynamic wavefront measurements.
Simultaneous phase-shifting interferometry based on a 2×2 retarder array with random fast-axes (RARF-SPSI) is proposed for real-time wavefront measurements. The retarder array is used as the phase-shift component, where the phase retardances are π/2, π, 3π/2, and 2π and the four fast-axes of the four retarders can be somewhat random. In this paper, the mathematical model of RARF-SPSI is built by using a Stokes vector and a Mueller matrix, the phase demodulation method through solving equations is derived, and the coefficient matrix of the equations that is associated with the azimuth of the fast-axes is calculated by Fourier analysis. Then the corresponding simulation analysis is executed. In the experiment, four simultaneous phase-shifting interferograms are captured and the phase distribution under test is demodulated through the proposed method. Compared with the four-bucket phase-shifting algorithm adopted in traditional simultaneous phase-shifting interferometry, the ripple error is suppressed well. The advantage of the proposed RARF-SPSI is that there is no need to calibrate the fast-axes of the phase-shift component before measuring; in other words, the phase demodulation error caused by the azimuth error of fast-axes is eliminated.
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