Development of pinhole filter in high precision interferometer

An enhanced chromatic dispersion probe for simultaneous measurement of dual-axis absolute and relative displacement with nanometric resolutions

“…It can also be observed that in figures 6(c) and (d), the side lobes of the measured spectra within the measurement ranges of axis 1 and axis 2, respectively, are effectively suppressed after enhancement of the chromatic dispersion probe by employing an optical 4f system in which a smaller pinhole is employed and placed on the focal planes of the two achromatic lenses, which filters out more of the short wavelengths outside of the focal point on the focal plane caused by a focal length shift of the achromatic lens, resulting in suppressed side lobes in the output spectra and further expansion of the measurement range of the probe. Moreover, a smaller pinhole acts as a low-pass filter in the spatial frequency domain that eliminates spatial high-frequency noise from the scattered light caused by the dust in the air or on the optical components, resulting in a smoother spatial energy distribution and an optimized signalto-noise-ratio [48] compared with the previous work [43]. Finally, larger absolute measurement ranges of 225 µm and 220 µm for axis 1 and axis 2, respectively, were realized and shown in figures 6(a) and (b) compared with those of the absolute measurement ranges of 165 µm and 180 µm for axis 1 and axis 2, respectively, in our previous work [43].…”