Dual-fiber point diffraction interferometer to measure the wavefront aberration of an imaging system

A dual-fiber point diffraction interferometer is built to measure the wavefront aberration of an imaging system with high precision. The optical intensity of the test light and reference light are controlled independently, so that high interference contrast can be obtained. The interferometer has several advantages: high precision, a flexible structure, and a quasi-common optical path. System errors of the interferometer that influence the measurement accuracy are considered and calibrated. In the experiments, a projection lens with numerical aperture of 0.3 is measured. The measured wavefront error is 1.5 nm rms, and the measurement repeatability is 35 pm rms.