Optical windows are important for a wind tunnel to enable observation or imaging of the internal flow-field. In order to reduce the interference to the internal flow and not to modulate the input observation ray field as much as possible, the interior and exterior surfaces of the optical window often adopt complex free-form surface design and must be used in pairs. This requires high accuracy of both the surface form and their relative position. In this paper, Trace-Pro optical software is used for the ray tracing analysis of the wind tunnel observation window defined by discrete points on the inner and outer surfaces. The conclusion is that the collimated beam is also the collimated beam after passing through the single observation window, which means the transmitted wave-front can be well resolved by a standard interferometer. The ray data in Trace-Pro is imported into MATLAB to obtain the modulated wave-front error which is contributed by the difference between the inner and outer surfaces of the monolithic optical window. It hence can be used to guide the corrective machining of the window surface. In addition, the influence of different misalignment on the interferometric test of the window is analyzed. Finally, the method is experimentally demonstrated on an optical free-form window. The surface positioning error is reduced with corrective machining based on the measured transmitted wave-front.
Aspheric surface acts an important role in modern optical systems. The null test utilizing a computergenerated hologram (CGH) is fundamental for precision test of aspheres. Its measurement accuracy mainly depends on the fabrication precision of CGH. However, it is not easy to exactly characterize the fabrication error of the CGH as well as its contribution to measurement uncertainty. In this paper, a new CGH wavefront error evaluation method is presented. The CGH fabrication errors such as duty-cycle error, etching depth inhomogeneity, pattern distortion, etc., are related to the fringe spacing based on elaborate measurement of the CGH microstructures. A scanning white-light interferometer and a high-precision two-axis stage are employed to sample the microstructure at a series of designed locations on the CGH. When the fabrication error is modeled through experiments, it can then help to realize rapid measurement of any other CGHs with significantly reduced number of sampling. The second step is then modeling the contribution of CGH fabrication error to measurement uncertainty according to the scalar diffraction theory. Meanwhile, the wavefront error induced by CGH fabrication error can also be characterized through ray-tracing in lens design software. The fabrication error is incorporated into the discrete phase data of CGH surface and its contribution to the final measurement uncertainty is evaluated through simulations.
Cylindrical mirrors are key optics of high-end equipment of national defense and scientific research such as high energy laser weapons, synchrotron radiation system, etc. However, its surface error test technology develops slowly. As a result, its optical processing quality can not meet the requirements, and the developing of the associated equipment is hindered. Computer Generated-Hologram (CGH) is commonly utilized as null for testing cylindrical optics. However, since the fabrication process of CGH with large aperture is not sophisticated yet, the null test of cylindrical optics with large aperture is limited by the aperture of the CGH. Hence CGH null test combined with sub-aperture stitching method is proposed to break the limit of the aperture of CGH for testing cylindrical optics, and the design of CGH for testing cylindrical surfaces is analyzed. Besides, the misalignment aberration of cylindrical surfaces is different from that of the rotational symmetric surfaces since the special shape of cylindrical surfaces, and the existing stitching algorithm of rotational symmetric surfaces can not meet the requirements of stitching cylindrical surfaces. We therefore analyze the misalignment aberrations of cylindrical surfaces, and study the stitching algorithm for measuring cylindrical optics with large aperture. Finally we test a cylindrical mirror with large aperture to verify the validity of the proposed method.