Three-dimensional optical coherence digital-null deformography of multi-refractive-surface optics with nanometer sensitivity

Abstract

Knowledge of the lens deformation during the reliability test is critical for lens design and fabrication. Refractive surface distorts the optical path of probing light, and poses a great challenge to measuring the test-induced nanoscale changes of all refractive lens surfaces simultaneously. In this work, we present an optical coherence digital-null deformography (ODD). A digital null, i.e., the interference signals (including intensity and phase) of the backscattered probing light from each lens surface, was recorded prior to the test with a phase-sensitive optical coherence tomography (OCT). Then the post-test lens was physically aligned to the digital null by actuating a hexapod iteratively with a digital null alignment (DNA) method, so that the refractive distortion was matched. Finally, the changes between the aligned lens and its digital null were measured with an intensity centroid shift (ICS) at micron scale and a joint wavenumber (k)-depth (z) domain phase shift (kz-PhS) at nanoscale. We demonstrate that the proposed kz-PhS has a sensitivity of 4.15 nm and a range of 5 µm without phase wrapping; and the sensitivities of DNA are z translation 0.04 µm, x/y translation 0.24 µm, tilt 0.0003°, and rotation 0.03°. A lens drop test was performed with ODD. Circumventing refractive distortion by the null measurement, ODD can visualize the test-induced changes of all refractive surfaces non-destructively and simultaneously, and it will greatly facilitate lens design and fabrication.