A two-dimensional (2-D) shearing interferometer based on an amplitude chessboard grating was designed to measure the wavefront aberration of a high numerical-aperture (NA) objective. Chessboard gratings offer better diffraction efficiencies and fewer disturbing diffraction orders than traditional cross gratings. The wavefront aberration of the tested objective was retrieved from the shearing interferogram using the Fourier transform and differential Zernike polynomial-fitting methods. Grating manufacturing errors, including the duty-cycle and pattern-deviation errors, were analyzed with the Fourier transform method. Then, according to the relation between the spherical pupil and planar detector coordinates, the influence of the distortion of the pupil coordinates was simulated. Finally, the systematic error attributable to grating alignment errors was deduced through the geometrical ray-tracing method. Experimental results indicate that the measuring repeatability (3σ) of the wavefront aberration of an objective with NA 0.4 was 3.4 mλ. The systematic-error results were consistent with previous analyses. Thus, the correct wavefront aberration can be obtained after calibration.
Lateral shearing interferometry is an attractive technique to measure the wavefront aberration of high numerical aperture optical systems, of which using two-dimensional grating can divide and shear the wavefront in two-dimension simultaneously. A two-dimension lateral shearing interferometer based on chessboard grating was designed, which can work in dual-mode: the phase shifting mode and the Fourier transform mode. In the phase shifting mode, the phase shifting was realized by moving chessboard grating along the shearing direction in the image plane. In the Fourier transform mode, the spatial carrier frequency was realized by positioning the grating at the Talbot distance of the objective image plane. An experimental setup was designed to measure a 10×, NA0.25 microscope objective at 632.8nm wavelength. The objective was measured by the experimental setup in dual-mode, the results showed that the wavefront of the objective was 0.172λ RMS; in the phase shifting mode, the repeatability (3σ) of RMS was 1.1mλ; in the Fourier transform mode, the repeatability (3σ) of RMS was 2.7mλ; after correcting the coordinates of the wavefront, the differences of Z5 to Z36 between phase shifting mode and the Fourier transform mode were better than 8mλ.
Lateral shearing interferometry was an attractive technique to measure the wavefront aberration of high numerical
aperture microscope objective lens. A two-dimension lateral shearing interferometer based on chessboard grating was
designed for microscope objective wavefront metrology. By positioning the chessboard grating at the Talbot distance of
the objective focal plane, the wavefront was divided and sheared in two-dimension. By applying two-dimensional
Fourier transform method and differential Zernike polynomial fitting, Zernike coefficients of the wavefront were
obtained. A 10x, NA0.25 microscope objective was measured at 632.8nm wavelength, the results showed that the
wavefront of the objective was 0.755λ PV, 0.172λ RMS, the repeatability(3σ) of RMS at random grating position was
2.3mλ, the repeatability(3σ) of Z5 to Z36 at random grating position were better than 17mλ.
In Cartesian coordinate system, a flat can be expressed as the sum of even-odd, odd-even, even-even and oddodd
functions. In the traditional three-flat even and odd function method, odd-odd function is difficult to obtain. In
our paper the odd-odd function can be solved by use the Dove prism which can rotate the optical axis. The odd-odd
function can calculate exactly. The even-odd, odd-even, even-even can be solved by rotating the flat 180°like the
traditional method. Only five configurations are used to test the flats. The theoretical derivation and analysis are
In order to get three-dimensional distribution of the optical material which has very high index homogeneity, measuring index homogeneity of the main direction of a sample, then opening a rectangle window in the sample to measure the index homogeneity of the window direction. Zernike circle polynomials are in widespread use for wavefront analysis because of their orthogonality over a circular pupil and their representation of balanced classical aberrations, for the main direction is circular, we use the Zernike circle polynomials to fit the main direction interferometric data. From result of fitting the main direction interferometric data, we find the measuring error which must be taken into account. So it is an important question how to choose the orthonormal polynomial for fitting the window direction interferometric data. Using the Zernike circle polynomials as the basis functions, the orthonormal polynomials of the rectangular pupil be obtained from the circle polynomials by the Gram–Schmidt orthogonalization process, using the first fifteen items of the orthonormal polynomials of the rectangular pupil fit the interferometric data of the window direction, we get a good fitting precision, find the measuring error of some samples considerable at the same.