Optical EngineeringPhase-shifting interferometer with pulse modulation based on a downsampling technique
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A phase-shifting interferometer that uses pulse modulation and downsampling is proposed and demonstrated. Pulse modulation is a direct modulating technique that realizes a rapid, large change in the wavelength of a laser diode with a constant optical power. For instance, an interference signal of 10 kHz is observed after the injection of pulse current. However, an expensive high-speed charge coupled device (CCD) camera is required to acquire a high-speed interference signal. If we consider a periodical signal, the same waveform repeats at every period. Therefore, the signal varying with high frequency can be captured by a low-frequency sampling pulse. This signal processing technique, the so-called downsampling, enables us to acquire rapid interference signals with a standard CCD camera. The experimental results using a flat mirror and a concave mirror showed measurement errors of 10 and 28 nm in root mean square (RMS), respectively. The repeatability of the experimental result between the first and the second is 13 nm in RMS.