Smaller and lighter optical systems with better performance can be built by the use of freeform optics.
However, most optical systems were constrained to traditional surfaces for the accurate metrology of
freeform surface is a challenge so far unsolved. One high-precision approach to measure freeform
surface with less time and expense is using tilted wave interferometer. A lens array is placed in the test
path of the interferometer, which can generate light source array that locally compensate the gradient of
test surface. But each source generated by lens array is not ideal spherical wave which contains
aberrations. In addition, the sources cannot be activated individually during the measurement, so that it
is impossible to perform an irregular source array according to the gradient variation of each test
surface. Thus, a novel technique based on fiber array is proposed for generating irregular source array.
Whereas, the position deviation of each fiber and phase difference produced by the length of each fiber
affect the measurement result. In this paper, the consequences of above errors are analyzed. A
calibration method can obtain the exact spatial coordinates of each fiber is suggested to calculate the
position deviation of each fiber. Meanwhile, a method based on Mach-Zehnder interference system is
presented, which can get phase difference produced by the length of each fiber accurately. Afterwards,
the data obtained by the two calibration methods are introduced into the mathematical model of system
error for eliminating the measurement error introduced by the use of fiber array. An elliptical mirror is
measured by our tilted wave interferometer based on fiber array showing the feasibility of the proposed
methods.
Compared with conventional optical elements, free-form surface optical elements, as a kind of nonrotationally symmetrical shaped component, can provide more freedom in optical design, optimize the structure of the optical system, and improve its performance. However, the difficulties involved in the measurement of free-form elements restrict their manufacture and application. A tilted-wave-interferometer (TWI) can achieve high precision in free-form surface measurement, but it requires higher space attitude error control. We analyze the relation between the alignment error and the measurement error introduced by the misalignment in free-form surface metrology with TWI. The attitude control method in the rotation direction is proposed based on the moire fringe technique. Then, combining it with the five-dimensional space attitude control method of aspherical elements, we put forward an alignment error control process in measuring the free-form surface. An experiment of measuring a free-form surface using TWI shows the effectiveness of our method.
As is well-known, optical testing has begun to emerge as a limiting factor in the application of freeform surfaces. In all kinds of published freeform optical metrology, the tilted-wave-interferometer (TWI) is the precise and flexible method for testing a freeform surface as it can compensate the local surface’s deviation from its best fit sphere by using a set of tilted waves. In the process of measurement with TWI, accurate assessment of the test surface error from the fringes plays a key role. We present a method for evaluation and characterization of surface aberrations in TWI by combining computer-generated wave technology and a retrace errors elimination algorithm. The feasibility of the method is proved by the simulation and experimental results.
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