Citation: | Meng Shi, Chen Lei, Zhu Wenhua, et al. Instantaneous wavefront measurement of large aperture optical elements[J]. Opto-Electronic Engineering, 2018, 45(1): 170536. doi: 10.12086/oee.2018.170536 |
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Overview: In order to measure the instantaneous wavefront of large aperture optical elements, a method based on the structure of oblique incidence of reflective shearing point diffraction interferometer is proposed. In the measurement, the near infrared fiber laser, which operates at 1313 nm wavelength, works as a light source. The light beam transmits through the spatial filter to generate a standard spherical wavefront. The standard spherical wavefront passes through the beam splitter and then it reaches the off-axis parabolic mirror whose diameter is 400 mm and F number is 10. The off-axis parabolic mirror realizes the transformation between the convergent wavefront and the collimation wavefront. The collimated wavefront reaches back to the off-axis mirror after being reflected by the mirror under test. The wavefront to be measured is splitted into two parts. One part is reflected directly from the slit and form the wavefront to be measured, the other part transmits through the slit and is diffracted by the pinhole to get a standard spherical wavefront works as the reference wavefront.These two kinds of wavefront form interference fringe at the target surface of the CCD and the imaging lens images the location of the exit pupil. Because of the transverse dislocation of the two beams of light, high linear carrier frequency of the interferogram is introduced.The frequency of the linear carrier is set, by adjusting the incidence angle θ, near the Nyquist frequency as close as possible. After receiving a good contrast interferogram, wavefront phase is retrieved by Fourier transform (FT) automatically to realize the dynamic measurement of instantaneous wavefront. On the basis of scalar diffraction theory and Fourier optics theory, a theoretical model of instantaneous wavefront was established and formula of linear carrier was derived.Furthermore, to study the effect which the lateral and axial defocus of pinhole have on diffraction intensity and reference wavefront quality, a mathematical model of convergent beam diffraction by a pinhole is established based on Fresnel diffraction theory. F number of the converging beam, diameter of the pinhole and so on are also taken into account. The optical path is up to 20 m, due to the long cavity of the optical path, the air current is a significance factor to the result. Besides, because of the air current, the system itself can be seen as a instantaneous wavefront happening and measurement of large aperture optical elements. The results indicate that the root mean square value is in accord with that acquired by SID4 wavefront sensor (less than 1/50λ), so about the repeated accuracy. The testing method proposed can be applied in high resolution and accuracy measurement of instantaneous wavefront
Principle of the near infrared reflective shearing point diffraction interferometery
Optical path diagram. (a) The testing optical path; (b) The optical path in reality
(a) Interferogram collected by CCD; (b) A small piece of area after being amplified; (c) Wavefront recovered
Distributions of PV (a) and RMS (b)
(a) Wavefront recovered by SID-4; (b) D-value of SID-4 wavefront and Fig. 3(c)