Point diffraction interferometer (PDI) has become the most promising measurement tool since it is firstly proposed by Linnik and its key component used to generate the reference wave can be called as the reference wave source (RWS). A new RWS based on the silicon nitride (SiN) waveguide is now proposed, aimed at providing a spherical reference wave with high NA and high accuracy. Simulation results show that the PV and the RMS of the reference wave generated by the waveguide RWS are 2.86×10-4λ (λ=532nm) and 4.83×10-5λ respectively, and the maximum light transmittance of this RWS could reach 24%. In addition, the NA of the reference wave reaches up to 0.58.
A novel aspherical surface measurement system based on randomly encoded hybrid grating (REHG) wavefront sensor is proposed. The system utilizes a non-null testing compensation lens to partially compensate the normal aberration of aspheric surface under test, which is more general than the null testing system. Different from the conventional interferometers, the REHG wavefront sensor exhibits strong anti-vibration ability and larger dynamic range due to its common path and self-reference characteristics, and is especially suitable for the measurement of aspherical surface. In this paper, a simulation model combing quadriwave lateral shearing interferometry based on the REHG and non-null aspheric testing technology is established. The generation mechanism of retrace error is analyzed. The iterative reverse optimization (IRO) retrace error correction method based on system modeling is discussed. Computer modeling is performed based on the actual experiment configuration, and an optimization function is set with polynomial coefficients of aspherics under test as variables. Through the reverse optimization with iterative ray tracing, the retrieved wavefront of the REHG sensor in the actual experiment system and the simulated wavefront in the system model will meet the constraint condition, and the figure error of aspheric surface can be then reconstructed by the optimal solution of polynomial coefficients in the model. The feasibility and accuracy of the aspherical surface non-null measurement system based on the REHG wavefront sensor are demonstrated by the simulation experiment result with different asphericities.
In this paper, a novel randomly encoded hybrid grating (REHG) interferometric wavefront sensor with the features of high-precision, high-resolution, high-dynamic-range and anti-vibration is proposed. The REHG consists of a randomly encoded binary amplitude grating and a phase chessboard. The far filed Fraunhofer diffractions only contain ±1 orders in two orthogonal directions. Different from the cross grating lateral shearing interferometer (CGLSI), there is no need of order selection mask for quadriwave lateral shearing interference. Without the influence of periodical Talbot effect, a continuously variable shear ratio can be obtained with the REHG, which makes it possible to control the dynamic range and measurement sensitivity of the wavefront sensor. A high-precision calibration method for shear ratio based on the shearing wavefront feature extraction and the generalized wavefront retrieval algorithm are employed to ensure the accuracy of the wavefront measurement results. The REHG wavefront sensor can work in collimated beam and convergent beam modes. Due to self-referenced and common-path characteristics, the REHG wavefront sensor can applied to different application fields in situ. Compared to the ZYGO interferometer, the results of the optical aberration and spherical surface measured by the REHG are highly precise and also show good repeatability. By applying two REHG wavefront sensors with different shear ratio, a wideband sensitivity-enhanced interferometric microscopy with real-time visualization can retrofit existing bright-field microscopes into quantitative phase microscopes.