Interferometry and its Applications in Surface Metrology

Interferometry has been a time-honored technique for surface topography measurement. Interferometric measurements of surface shape are relative measurement techniques in which the shape of a known surface is compared with that of an unknown surface, and the difference is displayed as a series of interference fringes. Noise attached in the interference fringes can have catastrophic effects on the phase-unwrapping process, so denoising is essential before reconstruction. Some noise may be generated due to vibrations when multiple images over a finite time period are captured for reconstruction by phase-shifting technique. This harmful noise is drastically reduced when fast phase shifting–based single-shot parallel four-step combined with Fizeau interferometer is applied. Measuring the shape of strongly curved surfaces using two-beam interferometry is very complicated due to the higher fringe density. This problem may be solved by multiple-beam interferometry, thanks to the very sharp interference fringes. The experimental results show the feasibility and high precision of multiple-beam interferometry

Surface Characterization by the Use of Digital Holography

Digital holography (DH) is an attractive measuring optical technique in the fields of engineering and science due to its remarkable accuracy and efficiency. The holograms are recorded by an interferometer and reconstructed by numerical methods such as Fresnel transform, convolution approach, and angular spectrum. Because harmful coherent noise often arises when long coherent lengths are used, bright femtosecond pulse light with ultrashort coherent length may be the solution to reduce both spurious and speckle noises. Since the usual DH uses a visible light, it is difficult to visualize 3D internal structure of visibly opaque objects due to their limited penetration depth. The terahertz (THz) radiation has a good penetration capability; thus, 3D visualization of both surface shape and internal structure in visibly opaque object can be achieved via THz-DH technique

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

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 optical-comb-referenced frequency synthesizer, is effectively used for multiple synthetic wavelengths within the range of 32 um to 1.20 m. A multiple cascade link of the phase images among an optical wavelength (= 1.520 um) and 5 different synthetic wavelengths (= 32.39 um, 99.98 um, 400.0 um, 1003 um, and 4021 um) 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.Comment: 19 pages, 7 figure

Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy

Terahertz (THz) spectroscopy is a promising method for analysing polar gas molecules mixed with unwanted aerosols due to its ability to obtain spectral fingerprints of rotational transition and immunity to aerosol scattering. In this article, dynamic THz spectroscopy of acetonitrile (CH3CN) gas was performed in the presence of smoke under the atmospheric pressure using a fibre-based, asynchronous-optical-sampling THz time-domain spectrometer. To match THz spectral signatures of gas molecules at atmospheric pressure, the spectral resolution was optimized to 1 GHz with a measurement rate of 1 Hz. The spectral overlapping of closely packed absorption lines significantly boosted the detection limit to 200 ppm when considering all the spectral contributions of the numerous absorption lines from 0.2 THz to 1 THz. Temporal changes of the CH3CN gas concentration were monitored under the smoky condition at the atmospheric pressure during volatilization of CH3CN droplets and the following diffusion of the volatilized CH3CN gas without the influence of scattering or absorption by the smoke. This system will be a powerful tool for real-time monitoring of target gases in practical applications of gas analysis in the atmospheric pressure, such as combustion processes or fire accident

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

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 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 axial range ( = 2.0 mm) to the axial resolution ( = 34 nm), achieves 5.9 × 105, 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