Compensated differential Zernike fitting method for wavefront aberration metrology based on grating lateral shearing.

Area of Science:

• Optical Engineering
• Metrology
• Wavefront Sensing

Background:

• Lateral shearing interferometry is a classical method for measuring wavefront aberrations in optical systems like lithographic lenses.
• Spherical wavefronts distort on planar detector surfaces, leading to coordinate distortion.
• Increasing numerical aperture (NA) causes significant variations in shear ratios, introducing errors in traditional methods.

Purpose of the Study:

• To address the coordinate distortion and shear ratio variation issues in lateral shearing wavefront aberration measurements.
• To develop a method that accurately reconstructs wavefronts, particularly for high NA optical systems.

Main Methods:

• Ray-tracing was employed to calculate the shearing ratio distribution within the shearing region.
• A compensated differential Zernike fitting method was proposed to correct for coordinate distortion and shear ratio variations.

Main Results:

• The uncompensated method shows increasing relative error with NA (1% at 0.1 NA, 10% at 0.3 NA, >100% above 0.7 NA).
• Compensation is crucial for NA > 0.3, significantly reducing reconstruction errors.
• The proposed method demonstrated accuracy through simulations and experimental validation.

Conclusions:

• The compensated differential Zernike fitting method effectively solves the coordinate distortion and shear ratio variation problems.
• Accurate wavefront aberration measurement requires compensation for high NA optical systems.
• This method enhances the reliability of wavefront sensing in advanced optical metrology.