High NA objective lens wavefront aberration measurement using a cat-eye retroreflector and Zernike polynomial.

Wavefront aberration is one important parameter for objective lenses. When the NA (Numerical Aperture) of the objective lens becomes larger than 0.8, wavefront aberration measurement with high accuracy and low cost is difficult to realize because of the lack of a reference sphere. In this paper, a new method is proposed to measure the wavefront aberration of a high NA objective lens. A cat-eye retroreflector with a plane mirror is used to reflect the wavefront. The plane mirror is tilted in at least three different directions by certain tilt angles to collect sufficient information of the wavefront aberration under test. Specific grid-combined Zernike polynomial is built for each set of tilt angles and directions to fit the corresponding returned wavefronts. The wavefront aberration can be reconstructed from the fitting results of the returned wavefronts. The measurement accuracy is influenced by the tilt angle, tilt angle error, NA, defocus amount of the plane mirror, detector's resolution, and other random noise. The tilt angle error is the main source of the measurement error. The relative measurement error is within 5% and 1% when the relative tilt angle error is below 0.5% and 0.1% respectively. The feasibility of the proposed method is verified experimentally by measuring the wavefront aberrations of 0.14 NA, 0.65 NA, and 0.9 NA objective lenses. Wavefront aberration measurement for a high NA objective lens with high accuracy and low cost is achievable through this method.