Journal
OPTICS EXPRESS
Volume 26, Issue 20, Pages 26292-26306Publisher
OPTICAL SOC AMER
DOI: 10.1364/OE.26.026292
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Funding
- Exploratory Research for Advanced Technology (ERATO)
- Japanese Science and Technology Agency (MINOSHIMA Intelligent Optical Synthesizer Project) [JPMJER1304]
- Japan Society for the Promotion of Science (JSPS) [18K04981]
- Grants-in-Aid for Scientific Research [18K04981] Funding Source: KAKEN
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Digital holography (DH) is a promising method for non-contact surface topography because the reconstructed phase image can visualize the nanometer unevenness in a sample. However, the axial range of this method is limited to the range of the optical wavelength due to the phase wrapping ambiguity. Although the use of two different wavelengths of light and the resulting synthetic wavelength, i.e., synthetic wavelength DH. can expand the axial range up to several hundreds of millimeters, its axial precision does not reach sub-micrometer. In this article, we constructed a tunable external cavity laser diode phase-locked to an optical frequency comb, namely, an optical-comb-referenced frequency synthesizer, enabling us to generate multiple synthetic wavelengths within the range of 32 mu m to 1.20 m. A multiple cascade link of the phase images among an optical wavelength (= 1.520 mu m) and 5 different synthetic wavelengths (= 32.39 mu m, 99.98 mu m, 400.0 mu m, 1003 mu m, and 4021 mu m) enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm. The axial dynamic range, defined as the ratio of the axial range (= 2.0 mm) to the axial resolution (= 34 nm), achieves 5.9 x 10(5), which is larger than that of previous synthetic wavelength DH. Such a wide axial dynamic range capability will further expand the application field of DH for large objects with meter dimensions. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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