Absolute measurement of optical flat surface shape based on the conjugate differential method

Area of Science:

• Optical metrology
• Interferometry
• Surface metrology

Background:

• Accurate measurement of absolute surface shape is crucial for optical components.
• Traditional differential methods face limitations in precision and reliability.

Purpose of the Study:

• To propose and validate the conjugate differential method for absolute surface shape measurement of flat mirrors.
• To enhance the precision and reliability of interferometric measurements.

Main Methods:

• The conjugate differential method utilizes slight transverse shifts of the optic at orthogonally bilateral symmetry positions.
• Measurement involves a four-step test to generate phase difference maps.
• Applied differential wavefront reconstruction methods include Fourier transform, Zernike polynomial fitting, and Hudgin model.

Main Results:

• The conjugate differential method demonstrated improved precision in slope approximation by reducing multi-step test couplings.
• Experimental results showed better agreement with the classical three-flat test compared to the traditional differential method.
• The deviation in RMS value was less than 0.3 nm.

Conclusions:

• The conjugate differential method offers a more reliable and precise approach for absolute surface shape measurement of flat mirrors.
• This method enhances interferometric metrology for optical testing.