Research Projects

Wavefront Sensing Technology for Surface Measurement

The objective of this collaborative project is to develop Shack-Hartmann wavefront sensing technology to measure the topography of large flat surfaces and the profile of aspherical surfaces. 



The Shack-Hartmann wavefront sensor (SHWS) is selected as the core technology, because of its high accuracy, environmental reliability and cost efficiency. The group will perform the theoretical research and experimental investigation of the core SHWS technology and image processing capability. Research directions include the improvement of lateral resolution and range, and extension of the longitudinal measurement capabilities. Research milestones include the exploration of spatial light modulators, optimisation of lens array design, digital and physical scanning, algorithms and feasibility demonstrations of novel techniques. 

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Contact PersonLi Xiang(xli@SIMTech.a-star.edu.sg)
Solution

The measurement principle of Shack-Hartmann wavefront sensor is: a lenslet array dissects the incoming wavefront into a large number of sub-apertures, then a camera measures the wavefront slope across each sub-aperture, after that the software analyses the 2-D data and reconstructs the wavefront, finally the image processing system generates the topography and derive other optical properties. 

Shack-Hartmann wavefront sensing technology can achieve nanoscale resolution in the Z direction, but its lateral resolution is lower because of the limitation of the number of lenslets in lens array. To improve the lateral resolution is one of the main research directions. The group employs the spatial light modulator (SLM) to realise the digital scanning, and the lateral resolution can be achieved in pixel scale. 

To measure aspheric surface, the group needs to generate a standard wavefront as reference. Measuring the difference between the two surfaces will result in aspheric profile. The reference surface will be generated with SLM. The measurement is a non-null measurement. Non-null measurement requires sufficient longitudinal capability, the lenslet scanning technique can provide this capability. 

A wavefront simulation platform is developed to simulate the propagation of wavefront when it is reflected by aspherical surface or flat work piece. This simulation software will provide the guidance for the design of measurement system and reconstruction algorithum. 

Both zonal and model reconstruction algorithm are used for the measurement of various wavefront, for example, the Southwell algorithm, the Zernike polynomials etc. 

Aiming for in-line measurement, the group will also develop the in-line implementation technique, mainly the alignment, reliability and error tolerance.

BenefitsOptical Wavefront Sensor
PCT/SG2005/000417
13 December 2005
Li Xiang, Zhao Liping, Fang Zhong Ping, Anand Krishna Asundi, Rinov Herawan

Patents / Awards / Achievements / DifferentiationOptical Wavefront Sensor
PCT/SG2005/000417
13 December 2005
Li Xiang, Zhao Liping, Fang Zhong Ping, Anand Krishna Asundi, Rinov Herawan

Applications

The application area of SHWS covers semiconductor, precision engineering and optical engineering.

  • Topography measurement for large flat surface
  • Profile measurement of aspherical surface
  • Performance measurement of optical system
Problems Addressed

Based on the group's survey of industrial requirements in the near future, the objective of this collaborative project is to develop Shack-Hartmann wavefront sensing technology to measure the topography of large flat surfaces and the profile of aspherical surfaces. 

With the breakthrough of manufacturing technology, work pieces with large flat surfaces or aspheric profiles can be fabricated with high throughput. However, measurement technology still cannot meet the industrial requirements. The difficulties of measurement include the large measurement range in both longitudinal and lateral directions, and higher accuracy and lateral resolution requirements. Another challenge is the capability of performing these measurements in the industrial environment.