Spectroscopic polarimetry (SP) is a powerful tool for characterization of thin film, polarization optics, semiconductor, and so on. However, mechanical polarization modulation of broadband light hampers its application for dynamic monitoring of a sample. In this article, we demonstrate the dynamic SP with features of polarization-modulation-free polarimetry and spectrometer-free spectroscopy benefiting from dual-comb spectroscopy (DCS) using a pair of optical frequency combs (OFCs). Effectiveness of the proposed system is highlighted by visualizing the hysteresis property of dynamic response in a liquid-crystal-on-silicon spatial light modulator at a sampling rate of 105 Hz.
Dual-comb spectroscopy (DCS) is an emerging spectroscopic tool with the potential to simultaneously achieve a broad spectral coverage and ultrahigh spectral resolution with rapid data acquisition. However, the need for two independently stabilized ultrafast lasers significantly hampers the potential application of DCS. We demonstrate mode-resolved DCS in the THz region based on a free-running single-cavity dual-comb fiber laser with the adaptive sampling method. While the use of a free-running single-cavity dual-comb fiber laser eliminates the need for two mode-locked lasers and their frequency control, the adaptive sampling method strongly prevents the degradation of spectroscopic performance caused by the residual timing jitter in the free-running dual-comb laser. Doppler-limit-approaching absorption features with linewidths down to 25 MHz are investigated for low-pressure acetonitrile/air mixed gas by comb-mode-resolved THz spectroscopy. The successful demonstration clearly indicates its great potential for the realization of low-complexity, Doppler-limited THz spectroscopy instrumentation.
Laser-scanning optical microscopy is widely used for the observation of microstructures and the analysis of molecular functions of samples with tightly focused light. Spectroscopic information is also available if a broadband light source is employed. General laser-scanning optical microscopy observes optical intensity by employing a sample- or laser-scanning system for the analysis of samples via reflectance, scattering, absorbance, and laser-induced phenomena. Another visualization method is using optical phase, which can enhance the image contrast of such high transparent materials and nano-step structures. However, broadband spectroscopic phase-contrast imaging with a laser-scanning configuration is slightly tricky due to the interferometric configuration is required to retrieve phase information of each wavelength. If the simultaneous measurement of amplitude and phase spectra is enabled in laser-scanning microscopy, it is possible to realize multivariate measurement to analyze more detailed information of samples based on such as complex refractive index, polarization characteristics, and so on with tightly focused light. To overcome these limitations, in this study, we proposed an optical-frequency-comb (OFC)-based laser scanning optical microscopy. The OFC technique enables fast Fourier transform spectroscopy by using well-defined two OFC lasers without any mechanical scan in the time domain. The combination of the laser scanning optical microscopy and the OFC technique realized the simultaneous and spectroscopic observation of quantitative amplitude and phase images with tight focusing down to the diffraction limit. Furthermore, we realized the analysis of polarization by the direct observation of the amplitude and phase of the orthogonal components. We applied the proposed method to the observation of nano-step structures, phase objects and anisotropic materials to provide a proof-of-principle demonstration of the proposed method. Our proposed approach will serve as a unique and powerful tool for characterizing the materials via complete characterization of optical information such as amplitude, phase, polarization and spectrum.
Since confocal laser microscopy (CLM) can acquire a high-contrast three-dimensional image, it is widely used in the field of bio-imaging. However, CLM is based on point measurement, it is necessary to mechanically scan the focal spot while keeping the conjugate relation between confocal pinhole and focal spot on the sample. Since such a mechanical scanning is vulnerable to environmental disturbance such as vibration, it needs a stable measurement environment like the active anti-vibration table. Also, for imaging of living samples, CLM cannot visualize clear image because of motion blur due to the difference in scanning time, thus it is limited to get only fixed sample images. We here propose a dual optical comb microscope combining dual optical comb spectroscopy (DCS) and two-dimensional spectral encoding (2D-SE). This combination enables one-to-one correspondence between optical frequency comb (OFC) modes and image pixels. Image information is superimposed on the mode-resolved OFC spectrum waveform by 2D-SE. Simultaneously, the confocality of all the pixels is given in parallel by a single confocal pinhole. The frame rate is limited by the data acquisition time of the interferogram, imaging rate over 1000 frames/s is possible. In this paper, we demonstrate the fast confocal phase imaging of a living paramecium.
Digital holography (DH) is a technique to reconstruct the amplitude and phase images of a sample by calculating the wavefront propagation from the interference image. Although DH enables three-dimensional shape measurement based on the phase images, axial dynamic range of a single-optical-wavelength DH is limited to less than a full or half optical wavelength due to phase wrapping ambiguity. To extend the axial range over the optical wavelength, synthesized wavelength DH has been proposed. In this method, DH is performed at two different wavelengths, and then synthesized wavelengths between them are used. However, use of a single longer synthesized wavelength degrades the axial resolution because the axial dynamic range is limited by the phase noise. To extend the axial dynamic range, one has to increase the axial range while maintaining the axial resolution of sub-wavelength. One promising approach to do it is cascade linking between multiple synthetic wavelengths with different orders. In this paper, we present multicascadelinked synthetic wavelength DH using an optical-comb-referenced frequency synthesizer (OFS). OFS is a tunable external cavity laser diode phase-locked to an optical frequency comb, and 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 and 5 different synthetic wavelengths enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm.
We demonstrate the confocal phase imaging at a video rate by a combination of dual comb spectroscopy (DSC) with 2D spectral encoding (2D-SE). After the image pixels of the sample is encoded on an optical frequency comb (OFC) by 2DSE, DSC of the image-encoded OFC passing through the confocal pinhole gives the mode-resolved amplitude and phase spectra. Based on one-to-one correspondence between the image pixels and OFC modes, the confocal amplitude and phase images are decoded from the mode-resolved amplitude and phase spectra, respectively. The phase spectrum measurement without the need for mechanical scanning enables the video-rate confocal phase imaging.
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