Multicascade-linked synthetic wavelength digital holography using an optical-comb-referenced frequency synthesizer

Abstract

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 a few tens of microns, this method is still insufficient for practical applications. In this article, a tunable external cavity laser diode phase-locked to an optical frequency comb, namely, an opticalcomb-referenced frequency synthesizer, is effectively used for multiple synthetic wavelengths within the range of 32 µm to 1.20 m. A multiple cascade link of the phase images among an optical wavelength (= 1.520 µm) and 5 different synthetic wavelengths (= 32.39 µm, 99.98 µm, 400.0 µm, 1003 µm, and 4021 µm) 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 maximum axial range (= 0.60 m) to the axial resolution (= 34 nm), achieves 1.7×10 8 , which is much 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.

Binocular phase retrieval based on the transport of intensity equation under multiwavelength illumination

“…Yamagiwa et al 19 . constructed a tunable external cavity laser diode phase-locked to an optical frequency comb, multiple synthetic wavelengths can be generated in the range of (32 μm∼1.20 m), the application field of digital holography in large objects of instrument size is expanded.…”

Full C-band wavelength-tunable, 250 MHz repetition rate mode-locked polarization-maintaining fiber laser

“…High-repetition-rate mode-locked lasers, given their unique characteristics of an ultra-short pulse, high peak power and a broad spectrum, have played key roles in numerous applications, including frequency comb generation 1 , remote timing transfer and synchronization 2 , broadband spectroscopy 3 , microwave generation 4 , length metrology 5 – 8 , surface metrology 9 and observations of ultrafast phenomena 10 . In particular, fiber-based mode-locked lasers have been widely used as a practical tool given their reliability, compactness and low cost 11 .…”

Full C-band wavelength-tunable, 250 MHz repetition rate mode-locked polarization maintaining fiber laser

“…microwave generation [4], length metrology [5][6][7][8], surface metrology [9] and observations of ultrafast phenomena [10]. In particular, ber-based mode-locked lasers have been widely used as a practical source given their reliability, compactness and low cost [11].…”