Phase shifting point diffraction interferometer for calibrating optical flats

Abstract

The total range of all interferometric methods for testing planeness of a test flat is most extensive. In particular, the “three flat test” method has been use for decades now to determine the flatness deviations a test flats. But even with the advancement of the analysis methods in the “three-flat test” method, little attention is paid to errors contributed by the beamsplitter and the auxiliary optics. To address this problem, a point diffraction-reference surface interferometer that can either use a pinhole or a single-mode fiber as a point diffraction source has been developed. The interferometer configuration employs a two-step flat surface measurement procedure to provide relative measurement of optical flats. In the interferometer configuration, the first reference flat is maintained in the same position for both first and second measurement while the test flat is replaced by a second reference flat in the second measurement. This mode of measurement allows subtraction of errors introduced by the first reference flat, the beamsplitter and the auxiliary optics as they are not moved from their positions during measurements. From the two-step flat surface measurement analysis, the peak-to-valley (PV) value of the random measurement errors over the entire surface profile was 65.6±1.0 nm, and the rms surface error was 16.0±0.2 nm. In addition, the precision and accuracy of optical phase-shifting technique is critical to the measurement uncertainty phase-shifting interferometers. The accuracy of optical phase shifters is limited by the inherent characteristics of the piezo-actuators (or PZT) such as nonlinearities, hysteresis, creep and thermal drift. To overcome this problem, a new phase-shifting technique based on two acousto-optic modulators (AOMs) where the inherent characteristics of the PZT do not affect the required phase-shifts is explored. The acousto-optic phase-shifting technique was successfully applied to control and measure the required optical phase shifts directly in the Mach-Zehnder interferometer. This was accomplished by designing a digitally programmable phase shift and measuring system that can precisely measure the phase delays between the two AOMs driving signals. Since the input signal frequency used in these experiments was 57.256 MHz with the incremental delays of 0.25 ns, an approximate phase resolution of 5 degrees was achieved.