Binzhi Zhang

Design and fabrication of imaging optical systems with freeform surfaces

Freeform surfaces provide more degrees of freedom for design of optical systems, and enhance the ability of compensation and correction aberrations. Freeform surfaces are of advantage to balance the unsymmetrical aberrations, especially for the wide-field off-axis optical systems. This paper focus on an off-axis reflective optical system, which focal length is 550mm, F# is 6.5 and field of view (FOV) is 76°. The system adopts some freeform surfaces. We discuss the problems we noticed in processes of design, manufacture, measurement and alignment, and the solutions. At last, the periodical research result and the expected performance are given.

Design and fabrication of CGH for aspheric surface testing and its experimental comparison with null lens

Computer-generated hologram (CGH) is an effective way to compensate wavefront in null test of aspheric surfaces and freeform surfaces. Our strategies of CGH design and fabrication for optical testing are presented, and an experiment demonstrating the compensation results of CGH and null lens is also reported. In order to design complex CGH, software was developed, with which we can design a CGH including three sections: main section for compensating wavefront in null test, alignment section for adjusting the relative position between CGH and interferometer, and fiducial section for projecting fiducial marks around the optics under test. The design result is represented in GDS II format file which could drive a laser-direct-writer-machine to fabricate a photomask. Then, a 1:1 replication process is applied to duplicate the patterns from photomask to a parallel optical substrate whose surface is error better than λ/60 rms. Finally, an off-axis aspheric surface was tested with CGH and null lens respectively. The test result with CGH (0.019λ;rms) is almost the same as the result with null lens (0.020λ rms). This experiment also demonstrated that fiducial marks projected by CGH can be used to guide the alignment of the optics and measurement of its off-axis distance.

Manufacturing and testing of a cubic SiC surface

The free surface and unrotational-symmetric surface optical elements have been applied more and more widely along with the development of optical design technology, although they are still difficult for manufacturing. In this letter, a SiC unrotational-symmetric aspheric surface whose surface equation is z=3\lambda(x3+y3)(\lambda=0.6328 \mum) has been introduced. The tilt abstraction is adopted to minimize the material removal. The surface figures are peak-to-valley (PV) value of 0.327\lambda and root-mean-square (RMS) value of 0.023\lambda. A non-null testing method based on digital mask is proposed to test this surface. The accuracy of the method is testified by the experiment of standard sphere testing.

Fabrication and testing of a 600mm diameter brazing SiC mirror

SiC is a preferential material for space large aperture mirror at present. Aperture of 1 meter sic mirror can meet most requirements of space optical systems. For larger aperture sic mirror, it is not economical and safe to make the monolithic body directly. The SiC mirror brazed assembly with several segments can solve these problems. In this paper, one Φ600 mm sic plane mirror as demonstration welded with 3 radial segments is roughly grinded to the required figure at first, then the mirror is finely grinded and polished by CCOS technology, the RMS of the figure error is less than 1/20λ (λ = 632.8nm) finally. The problems of the preferential removal near braze joint and material removing rate influenced by deformation of mirror surface are solved. Methods of reducing the residual stress of mirror are discussed. At last, some measurements and experiments are carried on, and the results conclude that the brazed SiC mirror can meet the requirement of large aperture optical systems.

Test of an off-axis asphere by subaperture stitching interferometry

A new method combined subaperture stitching with interferometry(SSI) is introduced. It can test large and off-axis aspheric surfaces without the aid of other null optics. In this paper the basic principle and theory of the technique are analyzed. The synthetical optimization stitching model and effective stitching algorithm are established based on homogeneous coordinates transformation and simultaneous least-squares method. The software of SSI is devised and the prototype for testing of large aspheres by SSI is designed and developed. An off-axis asphere with the aperture of 376mm×188mm is tested by this method. For comparison and validation, the asphere is also tested by null compensation.The synthesized surface profile is consistent to that ofthe entire surface from null test; and the difference of PV and RMS error between them is 0.047 λ and 0.006 λ, respectively. So it provides another quantitative measurement for testing large aspheric surfaces and off-axis aspheres besides null-compensation.

The manufacturing and testing of an unrotational-symmetric SiC mirror

In previous work, wavefront coding technology has been applied on an off-axis three mirror anastigmatic optical system. The secondary mirror is selected as the wavefront coded element. After redesigned the surface of secondary mirror becomes an unusual unrotational-symmetric surface with cubic term, which can not be tested by traditional null testing with compensator. For preparing for manufacturing and testing this kind of elements, a simple cubic surface whose equation is z=3λ(x3 + y3) (where x, y is normalized coordinate, λ=0.6328 μm) is polished. The final surface figure is 0.327λ(PV) and 0.023λ(RMS). The manufacture of this surface is introduced in this paper. The tilt component is subtracted to minimize the material removal. Also a non-null method is described for testing the experimental element. The deviation from a reference plane of the cubic surface is regarded as system error. In another words, the ideal cubic surface is set as the reference artificially. A special system error file for interferometer can be created so that the cubic term can be extracted during the testing process automatically. The residual error is just the departure from the ideal figure of the surface under machining by this way. The error and effective range is also presented. But the method may not be practical for the secondary mirror as wavefront coded element because the surface of that kind is convex asphere added cubic term. An improved non-null method is discussed for testing this kind of surface.