Sub-aperture stiching and CGH mixed compensation for the testing of large convex asphere

In order to achieve high-precision testing of large convex asphere, a testing method combining sub aperture stitching and computer generated hologram compensation is proposed.

 

1. Introduction

Large convex aspheres play a crucial role in modern optical systems, such as large - aperture telescopes and high - power laser systems. However, the accurate testing of large convex aspheres remains a challenging task. Traditional testing methods often encounter difficulties due to the large size and complex surface shape of the aspheres.

2. Principle of Sub - aperture Stitching

Sub - aperture stitching is a technique that divides the large - aperture surface into several smaller sub - apertures. Each sub - aperture can be measured independently using high - precision interferometers. The key idea is to overlap the adjacent sub - apertures partially and then use algorithms to stitch the measured data together. By doing so, the measurement range can be extended to cover the entire large - aperture surface. This method can effectively reduce the requirements for the aperture size of the measurement equipment.

 

3. Principle of CGH Compensation

Computational Holography (CGH) is a powerful tool for compensating the aberrations in the testing of aspheres. A CGH element is designed to generate a specific wavefront that can cancel out the wavefront errors introduced by the asphere under test. The CGH is usually fabricated on a transparent substrate, such as a quartz plate. When a reference wavefront passes through the CGH, it is modulated into a wavefront that matches the conjugate of the wavefront of the asphere, enabling accurate interferometric measurement.

4. The Advantages of the Mixed Compensation Method

The combination of sub - aperture stitching and CGH compensation offers several advantages. Firstly, it can overcome the limitations of both single methods. Sub - aperture stitching can handle the large - size problem, while CGH compensation can address the complex surface - shape - related wavefront errors. Secondly, this mixed method improves the measurement accuracy significantly. By compensating the wavefront errors in each sub - aperture and then stitching the compensated data, the overall error of the measurement can be minimized. Thirdly, it increases the flexibility of the testing system. Different CGH elements can be designed for different types of aspheres, and the sub - aperture stitching can be adjusted according to the specific requirements of the asphere.

5. Application Steps of the Mixed Compensation Method

5.1 Sub - aperture Division

The first step is to divide the large convex asphere surface into appropriate sub - apertures. The size and number of sub - apertures need to be determined based on the size of the asphere, the accuracy requirements of the measurement, and the aperture size of the interferometer. A balance should be struck between the number of sub - apertures (more sub - apertures may lead to higher accuracy but also more complex data processing) and the measurement efficiency.

5.2 CGH Design for Each Sub - aperture

For each sub - aperture, a CGH element is designed. The design process involves calculating the phase distribution of the CGH to compensate for the wavefront errors of the corresponding sub - aperture. This calculation is based on the optical design parameters of the asphere and the specific position and orientation of the sub - aperture.

5.3 Measurement of Each Sub - aperture

After the CGH elements are fabricated, each sub - aperture is measured using an interferometer with the CGH in place. The interferometer measures the wavefront of the sub - aperture, and the CGH compensates for the wavefront errors, resulting in a more accurate measurement of the sub - aperture surface.

5.4 Data Stitching

The measured data of each sub - aperture are then stitched together. Specialized algorithms are used to align the overlapping regions of the sub - apertures and merge the data. These algorithms take into account the measurement errors and uncertainties in each sub - aperture to ensure a seamless and accurate stitching of the data.

6. Conclusion and Future Outlook

The sub - aperture stitching and CGH mixed compensation method provides an effective solution for the testing of large convex aspheres. It has shown great potential in improving the measurement accuracy and efficiency. In the future, with the continuous development of optical manufacturing and measurement technologies, this method is expected to be further optimized. New materials and fabrication techniques for CGH elements may be developed, and more advanced algorithms for sub - aperture stitching may be proposed, leading to even more accurate and efficient testing of large convex aspheres.