As a significant optical precision measuring technology, optical interferometry is known for its wavelength-level measurement accuracy. As the industry calls for higher demands on measurement accuracy, the requirements on the sensing range of measurement methods are also increasingly wide. Additionally, due to the influence of human traffic, construction, and other factors, some measurement methods insensitive to environmental changes should be urgently proposed. These demands on the industry drive the development of optical interferometry. A lot of interference testing techniques have emerged in the development of optical interferometry. One of these techniques is lateral shearing interferometry which is common optical path interferometry and features a wide detection range and low need for environmental stability or coherence of the light source. As a result, this kind of interferometry has a variety of applications.

The lateral shearing interferometry originated from the Ronchi test in 1923 with a history of one hundred years, it can be divided into double-beam lateral shearing interferometry and multi-beam lateral shearing interferometry. The double-beam lateral shearing interferometry needs at least two interferograms with different shear directions to recover the full wavefront phase information. This is because the interferogram of double-beam lateral shearing interferometry only contains the wavefront phase information about the shear direction, while this problem is resolved by multi-beam lateral shearing interferometry. Through this technology, the wavefront can be copied into multiple wavefronts to different emission angles at the same time, and then a multi-beam lateral shear interferogram is formed over the overlapping region of the observation surface. Therefore, each interferogram contains information about the phase differences between the wavefronts in multiple shear directions. Thus, only one interferogram is necessary to recover the wavefront to be measured through technical steps such as phase extraction, phase unwrapping, and wavefront reconstruction, which greatly improves the measurement efficiency and makes real-time detection of instantaneous wavefronts possible.

In multi-beam lateral shearing interferometry, it is necessary to copy incident light waves of multiple light waves simultaneously, which requires obtaining the mutually interfering working wavefront and removing unwanted advanced diffracted light, and the key is in the design of light-splitting elements. The initial development of multi-beam lateral shearing interferometry employs prisms with high work surface processing requirements as the light-splitting elements, and then gratings are adopted to reduce the difficulty in device processing and improve the accuracy of the interference wavefront vector direction. Compared with other traditional interference methods, this technique greatly simplifies the whole measurement system. Quadri-wave lateral shearing interferometry (QWLSI) is characterized by high accuracy, large dynamic range, high resolution, and strong anti-interference ability. For example, due to the system simplicity, the QWLSI based on randomly coded hybrid grating theoretically requires only a two-dimensional grating and CCD. Thus it is the research focus of many research institutions in China and abroad. In recent years, QWLSI has been applied to the aberration detection of lithography objective lens, the surface shape detection of aspheric elements, and the wavefront sensing of large aperture splicing telescope. However, in pursuit of higher measurement accuracy today, a series of improvements still need to be conducted. Therefore, summarizing the completed work is a necessity, which is beneficial to better guide future work.

The development of our research team in several QWLSI areas is outlined. First, we describe the beam splitter design in QWLSI. Based on the randomly coded grating designed by Zhejiang University, our research group designs a global random coded hybrid grating that can better suppress the high-order secondary diffracted light and a phase-only grating based on global random coding constraint with low processing difficulty and high light transmittance. Second, we present our efforts to process interferograms. In the interferogram preprocessing field, we study the extension technique appropriate for two-dimensional QWLSI interferogram to reduce the measurement error caused by the boundary effect. The virtual grating phase-shifted Moiré fringe approach has been researched in terms of phase extraction. The present Fourier transform method's issues with substantial edge errors and weak anti-noise ability are resolved, and the algorithm's phase extraction accuracy and spectrum leaking are thoroughly investigated. In phase unwrapping, our team has studied the parallel phase unwrapping technique, which can speed up interferogram processing and significantly improve efficacy. Additionally, we research the algorithm of employing the wavefront differential phase of QWLSI in multiple directions to reconstruct the wavefront for improving the accuracy and anti-noise ability of wavefront reconstruction. Third, we investigate the QWLSI errors, including the processing error and installation and adjusting errors of the grating. Technical support for QWLSI installation and adjustment is provided by the quantitative results of the machining error tolerance of the grating and the influence of installation and adjustment error on measurement accuracy. The built fundamental QWLSI device based on the research on important technologies is then introduced, and the measurement accuracy in absolute terms is provided.

The QWLSI with gratings as beam splitters have been the recent research subject in pertinent academic institutions domestically and internationally. Our research team has created the fundamental QWLSI device based on important studies. As expanding the application of QWLSI in related sectors, we will conduct research on the essential technologies to enhance the amplitude and spatial frequency of the detectable wavefront distortion in QWLSI.