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We introduce a novel laser-scanning optical microscopy technique that employs optical-frequency-comb (OFC) lasers. This method facilitates multimodal spectroscopic imaging by analyzing interferograms produced via a dual-comb spectroscopic approach. Such interferograms capture comprehensive light information, including amplitude, phase, polarization, frequency, and time of flight information, enabling multimodal imaging from a single measurement. We demonstrate the potential of this technique across several spectroscopic imaging applications., Comment: 11 pages, 8 figures

We developed an optical phased array using an optical frequency comb and demonstrated its proof-of-principle. Optical phased arrays have been actively developed in recent years as a technology that can control the wavefront of light without any mechanical devices like phased array radar. Conventional optical phased arrays have been implemented using optical integrated circuits, but it has been difficult to achieve broadband operation with simple control. This is because control and calibration of a large number of phase modulators are required for each wavelength, and the dispersion of the waveguide makes whole bandwidth phase control of ultrashort pulses difficult. In contrast, we have developed a novel optical phased array that realizes wavefront control of ultrashort pulses generated by mode-locked laser by phase control of the comb, using high controllability of the comb and an optical array antenna with free-space optics. This is achieved by simply controlling the ratio of the two radio frequencies of the comb to realize a broadband optical phased array while suppressing environmental fluctuations. Experiments demonstrated broadband optical dot scanning at an optical frequency by forming an optical dot pattern and suppressing the environmental fluctuation by controlling the comb frequency. This innovative optical technology enables direct control of wavefronts by optical frequencies, i.e. controlling transverse modes by longitudinal modes.

We present quasi-real-time dual-comb spectroscopy (DCS) using two Yb:fiber combs with ~750 MHz repetition rates. A computational coherent averaging technique is employed to correct timing and phase fluctuations of the measured dual-comb interferogram (IGM). Quasi-real-time phase correction of 1-ms long acquisitions occurs every 1.5 seconds and is assisted by coarse radio frequency (RF) phase-locking of an isolated RF comb mode. After resampling and global offset phase correction, the RF comb linewidth is reduced from 200 kHz to ~1 kHz, while the line-to-floor ratio increases 13 dB in power in 1 ms. Using simultaneous offset frequency correction in opposite phases, we correct the aliased RF spectrum spanning three Nyquist zones, which yields an optical coverage of ~180 GHz around 1.035 $\mu$m probed on a sub-microsecond timescale. The absorption profile of gaseous acetylene is observed to validate the presented technique., Comment: 11 pages, 7 figures

We demonstrate balanced-detection dual-comb spectroscopy (DCS) using two free-running mechanical sharing dual-comb fiber lasers assisted by an all-computational digital phase correction algorithm. The mutual coherence between the combs allows us perform mode-resolved spectroscopy of gaseous hydrogen cyanide by digitally compensating residual timing and offset frequency fluctuations of the dual-comb signal. Setting the repetition rate difference between the combs to 500 Hz (1.5 kHz) yields more than 2000 resolved radio frequency comb lines after phase correction in a 3-dB bandwidth centered at 1560 nm of wavelength. Through coadding the corrected interferograms (IGMs), we obtain a single time-domain trace with a SNR of 6378 (13960) and 12.64 (13.77) bits of dynamic range in 1 second of averaging. The spectral SNR of the coadded trace reaches 529 (585), corresponding to a figure of merit of SNR of 1.3$\times$10$^6$ (1.4$\times$10$^6$). The measured absorption spectrum of hydrogen cyanide agrees well with the HITRAN database., Comment: 12 pages, 5 figures

Optically generated terahertz (THz) oscillators have garnered considerable attention in recent years due to their potential for wide tunability and low phase noise. Here, for the first time, a dissipative Kerr microresonator soliton comb (DKS), which is inherently in a low noise state, is utilized to produce a stepped-frequency THz signal ($\approx$ 280 GHz). The frequency of one comb mode from a DKS is scanned through an optical-recirculating frequency-shifting loop (ORFSL) which induces a predetermined frequency step onto the carrier frequency. The scanned signal is subsequently heterodyned with an adjacent comb mode, generating a THz signal in a frequency range that is determined by the repetition frequency of the DKS. The proposed method is proved by proof-of-concept experiments with MHz level electronics, showing a bandwidth of 4.15 GHz with a frequency step of 83 MHz and a period of 16 $\mu$s.

The Multi-Armed Bandit (MAB) problem, foundational to reinforcement learning-based decision-making, addresses the challenge of maximizing rewards amidst multiple uncertain choices. While algorithmic solutions are effective, their computational efficiency diminishes with increasing problem complexity. Photonic accelerators, leveraging temporal and spatial-temporal chaos, have emerged as promising alternatives. However, despite these advancements, current approaches either compromise computation speed or amplify system complexity. In this paper, we introduce a chaotic microresonator frequency comb (chaos comb) to tackle the MAB problem, where each comb mode is assigned to a slot machine. Through a proof-of-concept experiment, we employ 44 comb modes to address an MAB with 44 slot machines, demonstrating performance competitive with both conventional software algorithms and other photonic methods. Further, the scalability of decision making is explored with up to 512 slot machines using experimentally obtained temporal chaos in different time slots. Power-law scalability is achieved with an exponent of 0.96, outperforming conventional software-based algorithms. Moreover, we find that a numerically calculated chaos comb accurately reproduces experimental results, paving the way for discussions on strategies to increase the number of slot machines.

Spectroscopic polarimetry (SP) is a powerful tool for evaluation of thin film, optical materials, and biological samples because it can provide both polarimetric and spectroscopic characteristics of objects. However, its performance is often hampered by the mechanical instability and the limited data acquisition speed arising from the mechanical polarization modulation. Dual-comb spectroscopic polarimetry (DCSP) based on a combination of SP with dual-comb spectroscopy can acquire optical spectra of amplitude ratio and phase difference in p- and s-polarization components of the output light from simultaneous measurement of optical spectra of optical amplitude and phase in p- and s-polarization components without the need for mechanical polarization modulation. In this article, we combine the DCSP with polarization control pulse sequences (PCPS) with different polarizations and time delays for a more detailed analysis of the sample's polarization response based on Jones matrix. We obtain Jones matrix of a sample as a function of wavelength by measuring those optical spectra while multiplexing the incident light into multiple polarizations instead of a single polarization. Such Jones matrix DCSP (JM-DCSP) is applied for analysis of optical elements with known polarization property and its experimental result is in good agreement with theoretical values, indicating the validity of the proposed method. JM-DCSP will further expand the application scope of SP., Comment: 24 pages, 8 figures

THz oscillators generated via frequency-multiplication of microwaves are facing difficulty in achieving low phase noise. Photonics-based techniques, in which optical two tones are translated to a THz wave through opto-electronic conversion, are promising if the relative phase noise between the two tones is well suppressed. Here, a THz ($\approx$ 560 GHz) wave with a low phase noise is provided by a frequency-stabilized, dissipative Kerr microresonator soliton comb. The repetition frequency of the comb is stabilized to a long fiber in a two-wavelength delayed self-heterodyne interferometer, significantly reducing the phase noise of the THz wave. A measurement technique to characterize the phase noise of the THz wave beyond the limit of a frequency-multiplied microwave is also demonstrated, showing the superior phase noise of the THz wave to any other photonic THz oscillators (> 300 GHz).

We report the thermal control of a dissipative Kerr microresonator soliton comb via an optical sideband generated from an electro-optic modulator. Same as the previous reports using an independent auxiliary laser, our sideband-based (S-B) auxiliary light also enables to access a stable soliton comb and to reduce the phase noise of the soliton comb, greatly simplifying the setup with an auxiliary laser. More importantly, because of the intrinsically high frequency/phase correlation between the pump and S-B auxiliary light, the detuning between the pump and resonance frequency is automatically almost fixed, allowing the 18 times larger "effective"soliton existence range than the conventional method using an independent auxiliary laser, as well as the scanning of the soliton comb of more than 10 GHz without using microheaters.

Optical injection locking is implemented to faithfully transfer the phase noise of a dissipative Kerr microresonator soliton comb in addition to the amplification of the Kerr comb. Unlike Er-doped fiber and semiconductor optical amplifiers, the optical injection locking amplifies the comb mode without degrading the optical signal-to-noise ratio. In addition, we show that the residual phase noise of the optical injection locking is sufficiently small to transfer the relative phase noise of comb modes (equivalent to the repetition frequency) of low phase noise Kerr combs, concluding that the optical injection locking of a Kerr comb can be an effective way to generate low phase noise THz waves with a high signal-to-noise ratio through an optical-to-electronic conversion of the Kerr comb.

Rapid and large scanning of a dissipative Kerr-microresonator soliton comb with the characterization of all comb modes along with the separation of the comb modes is imperative for the emerging applications of the frequency-scanned soliton combs. However, the scan speed is limited by the gain of feedback systems and the measurement of the frequency shift of all comb modes has not been demonstrated. To overcome the limitation of the feedback, we incorporate the feedback with the feedforward. With the additional gain of > 40 dB by a feedforward signal, a dissipative Kerr-microresonator soliton comb is scanned by 70 GHz in 500 $\mu$s, 50 GHz in 125 $\mu$s, and 25 GHz in 50 $\mu$s (= 500 THz/s). Furthermore, we propose and demonstrate a method to measure the frequency shift of all comb modes, in which an imbalanced Mach-Zehnder interferometer with two outputs with different wavelengths is used. Because of the two degrees of freedom of optical frequency combs, the measurement at the two different wavelengths enables the estimation of the frequency shift of all comb modes.

Dual-comb microscopy (DCM), an interesting imaging modality based on the optical-frequency-comb (OFC) mode and image pixel one-to-one correspondence, benefits from scan-less full-field imaging and simultaneous confocal amplitude and phase imaging. However, the two fully frequency-stabilized OFC sources requirement hampers DCM practicality due to the complexity and costs. Here, a bidirectional single-cavity dual-comb fiber laser (SCDCFL) is adopted as a DCM low-complexity OFC source. Computational image correction reduces the image blur caused by the SCDCFL residual timing jitter. Nanometer-order step surface profilometry with a 14.0 nm uncertainty highlights the image-corrected DCM effectiveness. The proposed method enhances the DCM versality and practicality., Comment: 25 pages, 7 figures

This paper demonstrates that the precise phase controllability of an optical frequency comb enables all-optical signal processing for the first time. A novel all-optical Hilbert transform is presented with precise control of relative carrier-phase and envelope of optical pulse train based on frequency control utilizing an optical frequency comb. With the proposed all-optical signal processing method, fully non-scanning one-shot three-dimensional (3D) imaging can be realized with high image resolution. The technique can be applied to coherent phase imaging simultaneously. A precise pair of 90$^\circ$-phase-shifted optical pulses over the entire spectral bandwidth can be generated based on the precise optical phase controllability of an optical frequency comb, thereby facilitating a real-time and precise all-optical Hilbert transform to obtain amplitude and phase of optical signal in a single shot of ultrashort pulses. In our experiments, we realized single-shot 3D imaging with an uncertainty of 5 $\rm{\mu}$m and obtained a surface profile with a resolution of 200 $\times$ 200 pixels., Comment: 7 pages

In this study, we demonstrate the concept of combining optical combs with optical vortices for the first time. By combining the advantages of the both light sources, we realize an optical vortex comb technology for arbitrary spatiotemporal phase manipulation. This idea corresponds to simultaneous control of the longitudinal and transverse modes of light based on the high controllability of the optical comb. As a proof-of-concept experiment, we applied an optical vortex comb to the generation and rotational control of a ring-shaped optical lattice. Furthermore, to demonstrate the utility of this technique, a rotational optical manipulation of microspheres was demonstrated using the optical ring lattice as an optical tweezer light. Here, we present a new applicability of the optical comb by utilizing its characteristics far beyond the conventional range.

Conventional optical synthesis, the manipulation of the phase and amplitude of spectral components to produce an optical pulse in different temporal modes, is revolutionizing ultrafast optical science and metrology. These technologies rely on the Fourier transform of light fields between time and frequency domains in one-dimensional space. However, within this treatment it is impossible to incorporate the quantum correlation among photons. Here we expand the Fourier synthesis into high dimensional space to deal with the quantum correlation, and carry out an experimental demonstration by manipulating the two-photon probability distribution of a biphoton in two-dimensional time and frequency space. As a potential application, we show manipulation of a heralded single-photon wave packet, which is never explained by the conventional one-dimensional Fourier optics. Our approach opens up a new pathway to tailor the temporal characteristics of a biphoton wave packet with high dimensional quantum-mechanical treatment. We anticipate such high dimensional treatment of light in time and frequency domains could bridge the research fields between quantum optics and ultrafast optical measurements., Comment: 12 pages, 7 figures

Dissipative Kerr-microresonator soliton combs (hereafter called soliton combs) has been rapidly progressing as compact frequency combs. Comb mode scanning of the soliton combs with a large range and fast speed is of paramount importance for applications such as LiDAR and spectroscopy, requiring large and rapid frequency scanning of a pump continuous-wave (CW) laser as well as resonance frequency of a microresonator. Here, we demonstrate the generation of a soliton comb by a distributed feedback (DFB) laser toward the comb mode scanning with a large range and fast speed. Compared with conventional pump CW lasers (i.e. external cavity diode lasers: ECDLs), DFB lasers can be frequency-scanned more largely and rapidly without mode-hopping. In addition, because of the fast scan speed of the DFB laser, a single soliton comb is generated simply by controlling the injection current of the DFB laser, greatly simplifying the system without having any additional optical modulators such as a carrier-suppressed single-sideband modulator (CS-SSB modulator), acousto-optic modulator (AOM), and auxiliary CW laser.

Dual-comb spectroscopy (DCS) is useful for gas spectroscopy due to high potential of optical frequency comb (OFC). However, fast Fourier transform (FFT) calculation of a huge amount of temporal data spends significantly longer time than the acquisition time of an interferogram. In this article, we demonstrate frequency-domain DCS by a combination of DCS with lock-in detection, namely LID-DCS. LID-DCS directly extracts an arbitrary OFC mode from a vast number of OFC modes without the need for FFT calculation. Usefulness of LID-DCS is demonstrated in rapid monitoring of transient signal change and spectroscopy of hydrogen cyanide gas., Comment: 28 Pages, 4 figures

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 figures

We demonstrate two fully and tightly phase locked 750 MHz ytterbium (Yb) fiber frequency combs that are independently stabilized to a continuous wave (CW) laser with <1 rad RMS phase error. A bulk EOM and a single stack PZT are separately utilized as the fast actuators for cavity length stabilization. The carrier envelop frequencies are phase locked by single loop feedback to laser diode current, showing 1.6 MHz servo bumps. The in-loop fractional frequency instabilities are ~1.5e-18 at 1s for both combs. To the best of our knowledge, this is the highest repetition rate in fiber based low phase noise combs tightly locked to optical frequency reference., Comment: 4 pages, 5 figures, 1 table

Optical frequency combs have attracted attention as optical frequency rulers due to their tooth-like discrete spectra together with their inherent mode-locking nature and phase-locking control to a frequency standard. Based on this concept, their applications until now have been demonstrated in the fields of optical frequency metrology and optical distance metrology. However, if the utility of optical combs can be further expanded beyond their optical-frequency-ruler-based application by exploiting new aspects of optical combs, this will lead to new developments in optical metrology and instrumentation. Here, we report a fibre sensing application of optical combs based on a coherent frequency link between the optical and radio-frequency regions, enabling high-precision refractive index measurement of a liquid sample based on frequency measurement in radio-frequency region. Our technique encodes a refractive index change of a liquid sample into a radio-frequency comb by a combination of an intracavity multi-mode-interference fibre sensor and wavelength dispersion of a cavity fibre. Then, the change in refractive index is read out by measuring the repetition frequency of the radio-frequency comb with a frequency counter and a frequency standard. Use of an optical comb as a photonic radio-frequency converter will lead to the development of new applications in high-precision fibre sensing with the help of functional fibre sensors and precise radio-frequency measurement., Comment: 20 pages, 6 figures

We propose a novel measurement technique that exploits the high coherent controllability of multi-comb systems, which corresponds to a generalization of the frequency control between the comb sources. In this paper, we particularly focus on the arbitrary relative carrier envelope phase (CEP) control through the relative offset frequency of two combs. We successfully demonstrate polarization-modulated comb generation and its coherent detection in the developed system. The proof-of-principle experiment indicates the potential of multi-comb systems, enabling a rapid, precise, and arbitrary coherent modulation method utilized for a wide variety of metrological applications., Comment: 13 pages, 4 figures

Confocal imaging and phase imaging are powerful tools in life science research and industrial inspection. To coherently link the two techniques with different depth resolutions, we introduce an optical frequency comb (OFC) to microscopy. Two-dimensional (2D) image pixels of a sample were encoded onto OFC modes via 2D spectral encoding, in which OFC acted as an optical carrier with a vast number of discrete frequency channels. Then, a scan-less full-field confocal image with a depth resolution of 62.4 um was decoded from a mode-resolved OFC amplitude spectrum obtained by dual-comb spectroscopy. Furthermore, a phase image with a depth resolution of 13.7 nm was decoded from a mode-resolved OFC phase spectrum under the above confocality. The phase wrapping ambiguity can be removed by the match between the confocal depth resolution and the phase wrapping period. The proposed hybrid microscopy approach will be a powerful tool for a variety of applications., Comment: 32 page, 6 figures

We have developed a hyperspectral imaging scheme that involves a combination of dual-comb spectroscopy and Hadamard-transform-based single-pixel imaging. The scheme enables us to obtain 12,000 hyperspectral images of amplitude and phase at a spatial resolution of 46 um without mechanical scanning. The spectral resolution is 20 MHz, as determined by the linewidth of a single comb mode, and the spectral interval is 100 MHz over a spectral range of 1.2 THz centred at 191.5 THz. As an initial demonstration of our scheme, we obtained spectroscopic images of a standard test chart through an etalon plate. The thickness of an absorptive chromium-coated layer on a float-glass substrate was determined to be 70 nm from the hyperspectral phase images in the near-infrared wavelength region., Comment: 19 pages, 6figures

Real-time measurement of the absolute frequency of continuous-wave terahertz (CW-THz) waves is required for characterization and frequency calibration of practical CW-THz sources. We proposed a method for real-time monitoring of the absolute frequency of CW-THz waves involving temporally parallel, i.e., simultaneous, measurement of two pairs of beat frequencies and laser repetition frequencies based on dual THz combs of photocarriers (PC-THz combs) with different frequency spacings. To demonstrate the method, THz-comb-referenced spectrum analyzers were constructed with a dual configuration based on dual femtosecond lasers. Regardless of the presence or absence of frequency control in the PC-THz combs, a frequency precision of 10-11 was achieved at a measurement rate of 100 Hz. Furthermore, large fluctuation of the CW-THz frequencies, crossing several modes of the PC-THz combs, was correctly monitored in real time. The proposed method will be a powerful tool for the research and development of practical CW-THz sources, and other applications., Comment: 27 pages

Dual terahertz (THz) comb spectroscopy is a promising methods for high accuracy, high resolution, and broadband THz spectroscopy because the mode-resolved THz comb spectrum possesses both characteristics of broadband THz radiation and narrow-linewidth continuous-wave THz radiation and all frequency mode of THz comb can be phase-locked to a microwave frequency standard. However, requirement of stabilized dual femtosecond lasers has often hindered wide use of this method. In this article, we demonstrated the adaptive sampling, dual THz comb spectroscopy, enabling use of free-running dual femtosecond lasers. To correct the non-linearity of time and frequency scale caused by the laser timing jitter, an adaptive sampling clock is generated by dual THz-comb-referenced spectrum analysers and is used for a timing signal in a data acquisition board. The demonstrated results did not only indicate the implementation of dual THz comb spectroscopy with free-running dual lasers but also implied the superiority of its spectroscopic performance over the dual THz comb spectroscopy with stabilized dual lasers., Comment: 28 pages, 6 figures

We demonstrated combination of gapless terahertz (THz) comb with dual-comb spectroscopy, namely gapless dual-THz-comb spectroscopy, to achieve the spectral resolution equal to width of the THz comb tooth. The gapless THz comb was realized by interpolating frequency gaps between the comb teeth with sweeping of a laser mode-locked frequency. The demonstration of low-pressure gas spectroscopy with gapless dual-THz-comb spectroscopy clearly indicated that the spectral resolution was decreased down to 2.5-MHz width of the comb tooth and the spectral accuracy was enhanced to 10-6 within the spectral range of 1THz. The proposed method will be a powerful tool to simultaneously achieve high resolution, high accuracy, and broad spectral coverage in THz spectroscopy., Comment: 12 pages, 4 figures