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high-resolution positioning system for linear encoder calibration was developed. The optical sources were stabilized to a femtosecond optical comb for high accuracy and direct traceability to the optical frequency comb. This system showed a control resolution of 17 pm by using an optical zooming interferometer. The stability of the stage was 4.5 nm at 200 seconds, which can be improved by easy modification of optical setup and lasers. [ABSTRACT FROM AUTHOR]

A fiber-based frequency comb has been developed to measure the frequency of a 633-nm iodine-stabilized laser at the National Metrology Institute of Japan (NMIJ). The measured frequency was consistent with the previous results measured with a Ti:sapphire-based comb. The NMLJ comb was shipped to Australia for the validation of the measurement capability of the National Measurement Institute, Australia (NMIA) and NMIJ combs using a common microwave reference and a common optical frequency. Consequently, the frequency consistency of the two combs was approximately 8 × 10-17. Furthermore, we demonstrate that an absolute mode number of the comb can easily and clearly be determined by using the two combs. [ABSTRACT FROM AUTHOR]

We have demonstrated optical frequency synthesis using a fiber-based frequency comb system, referenced to an ultrastable microwave oscillator. The oscillator is based on a high Q-factor cryogenic sapphire resonator cooled with liquid helium. The 100-MHz signal synthesized from the 10.8-GHz oscillation frequency of the cryogenic sapphire oscillator (CSO) was used to stabilize the repetition frequency of the mode-locked fiber laser. When the synthesized optical frequency was compared with a rubidium two-photon stabilized laser at 778 nm, the measured fractional frequency stability was ∼6 × 10-14 τ-1/2 for the averaging times τ between 1 and 100 s, and the best frequency stability was 3.0 × 10-15 for an averaging time of 1280 s. [ABSTRACT FROM AUTHOR]

A continuous-wave optical parametric oscillator (OPO) was phase locked to an optical frequency comb in the 830-nm region. The optical frequency of the OPO was controlled by changing the cavity length of the pump laser. The residual phase noise under phase locking was 220 mradrms. and the energy concentration to the carrier was 95 %. Furthermore, the optical frequency fluctuations of a free-running OPO were measured by using an optical frequency comb that was phase locked to an atomic clock. The measured fluctuations were around 10 MHz in an hour. [ABSTRACT FROM AUTHOR]

Proposes a method of long-term stablization of the repetition rate and the spectral envelope in a femtosecond mode-locked (fs-ML) laser pulse by a simple control method without modifying the laser cavity configuration. Stabilization of repetition rate; Cross-correlation measurement between temporally different pulses in fs-ML pulse train.

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 (≈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). Low phase-noise THz wave generation is in high demand for applications such as wireless communications and radars. Here, the authors generate low-noise THz waves via photodetection of the carrier of a Kerr frequency comb, which is repetition-rate stabilized using a two-wavelength delayed self-heterodyne interferometer. [ABSTRACT FROM AUTHOR]

We demonstrate balanced-detection dual-comb spectroscopy 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, we obtain a single time-domain trace with a signal-to-noise ratio (SNR) of 6378 (13 960) and 12.64 (13.77) bits of dynamic range in 1 s of averaging. The spectral SNR of the coadded trace reaches 529 (585), corresponding to a figure of merit of SNR of 1.3 × 106 (1.4 × 106). The measured absorption spectrum of hydrogen cyanide agrees well with the HITRAN database. [ABSTRACT FROM AUTHOR]

Nonlinear optical processes are intrinsically dominated by the phase relationships among the relevant electromagnetic fields, including the phase of nonlinear polarization produced in them. If one can arbitrarily manipulate these phase relationships at a variety of desired interaction lengths, direct and highly designable manipulations for the nonlinear optical phenomenon could be achieved. Here, we report a proof-of-principle experiment in which a high-order Raman-resonant four-wave-mixing process is used as a representative nonlinear optical process and is tailored to a variety of targets by implementing such arbitrary manipulations of the phase relationships in the nonlinear optical process. We show that the output energy is accumulated to a specific, intentionally selected Raman mode on demand; and at the opposite extreme, we can also distribute the output energy equally over broad high-order Raman modes in the form of a frequency comb. This concept in nonlinear optical processes enables an attractive optical technology: a single-frequency tunable laser broadly covering the vacuum ultraviolet region, which will pave the way to frontiers in atomic-molecular-optical physics in the vacuum ultraviolet region. Adaptive optics permits control of linear and nonlinear optical phenomena in order to achieve the desired output signal. Here, arbitrary manipulation of phase relations are used to engineer nonlinear interactions in a higher-order Raman-resonant four-wave-mixing platform. [ABSTRACT FROM AUTHOR]

A continuously tunable, phase-locked, single-frequency, continuous-wave (cw) terahertz generator has been demonstrated around 120 GHz, corresponding to the spectral bandwidth of the F-band unitraveling-carrier photodiode (UTC-PD) used as a photomixer in this study. This cw-terahertz generator is based on photomixing of an accurately tunable cw laser and a tightly fixed cw laser in the optical frequency region, phase locked to two independent optical combs. The continuous tuning range of the presented method was three orders of magnitude around 0.1 THz and two orders around 1 THz broader than that of a previous photomixing method in which two cw lasers are phase locked to a single optical comb, and fully covered the available spectral bandwidth of the F-band UTC-PD. The spectral behavior of the tight locking and continuous tuning of 120 GHz cw-terahertz radiation was confirmed in real time by use of a terahertz-comb-referenced spectrum analyzer. This cw-terahertz generator shows promise as a terahertz clock and synthesizer for terahertz frequency metrology. [ABSTRACT FROM AUTHOR]

Abstract: We report a method for optimizing the amplification of femtosecond optical pulses by using dispersion management. The amount of dispersion provided to the seed optical pulse of an erbium-doped fiber (EBF) has an optimal region that enhances the output power of an amplifier. The power enhancement is accompanied by spectral broadening, which originates from adiabatic narrowing in the erbium-doped fiber. The amplified optical pulses can be used to generate an octave-spanning optical frequency comb (OFC) by employing a highly nonlinear fiber (HNLF). [Copyright &y& Elsevier]

Optical frequency combs (OFCs) have become essential tools in a wide range of metrological and scientific research fields. However, in the reported literature, OFCs that cover the visible spectral range have a limited bandwidth and pulse energy. These drawbacks limit their potential applications, such as high-signal-to-noise ratio spectroscopic measurements. In this work, we demonstrate a broadband, high-power optical frequency comb covering the visible to near-infrared range (550 nm to 900 nm) with a high average power of approximately 300 mW. This is accomplished by the power scaling of optical pulses from a fully stabilized Er:fiber comb, coherent spectral broadening and finally the utilization of a PPLN's χ (2) nonlinearity. The broadband, high-power, fully stabilized visible OFCs showcased in this work offer reliable laser sources for high-precision spectroscopic measurements, imaging, and comparisons of optical clocks.

By combining dual-comb spectroscopy and optical vortices, which have the orbital angular momentum (OAM) of light, we propose a novel measurement concept, "dual-vortex-comb spectroscopy (DVCS)." We extend the dual-comb spectroscopy to angle dimensions by utilizing the characteristic helical phase structure of optical vortices. We demonstrate a proof-of-principle experiment on DVCS that realizes in-plane azimuth-angle measurements with an accuracy of ∼0.1 mrad after cyclic error correction, whose origin is verified by a simulation. We also demonstrate that the measurable angle range is tailored by the topological number of the optical vortices. This is the first demonstration of dimensional conversion between the in-plane angle and dual-comb interferometric phase. This successful result can expand the applicability of optical frequency comb metrology to new dimensions.

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 µm probed on a sub-microsecond timescale. The absorption profile of gaseous acetylene is observed to validate the presented technique.

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 access to a stable soliton comb and reduces the phase noise of the soliton comb, greatly simplifying the set-up 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, which allows an 18 times larger "effective" soliton existence range than the conventional method using an independent auxiliary laser, as well as a 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 terahertz (THz) waves with a high signal-to-noise ratio through an optical-to-electronic conversion of the Kerr comb.

We present a mechanical sharing, dual-comb fiber laser based on an all-polarization-maintaining cavity configuration and a nonlinear amplifying loop mirror mode-locking mechanism. This simple setup yields dual-optical frequency combs with a high level of mutual coherence without active servo control. We realized a high relative stability with a standard deviation of 0.27 Hz and a relative beat note between the dual-frequency combs with a full-width at half-maximum of ∼50 H z . Dual-frequency combs were found to have high relative stability and mutual coherence owing to passive common-mode noise suppression using a mechanical sharing laser cavity. This laser configuration can significantly simplify dual-comb spectroscopy.

Rapid and large scanning of a dissipative Kerr-microresonator soliton comb with characterization of all comb modes along with the separation of the comb modes is imperative for the emerging applications of frequency-scanned soliton combs. However, the scan speed is limited by the gain of feedback systems, and measurement of the frequency shift of all comb modes has not been demonstrated. To overcome the limitation of the feedback, we incorporate feedback with feedforward. With an additional gain of >40 d B by a feedforward signal, a dissipative Kerr-microresonator soliton comb is scanned by 70 GHz in 500µ s , 50 GHz in 125µ s , and 25 GHz in 50µ 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 two different wavelengths enables 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. Although the residual timing jitter in the SCDCFL blurs the image of a static object acquired by DCM, computational image correction significantly suppresses the image blur. Nanometer-order step surface profilometry with a 14.0 nm uncertainty highlights the computationally image-corrected DCM effectiveness. We further discuss a possibility to expand the computational image correction to a dynamic object and demonstrate its preliminary experiment. The proposed method enhances the DCM generality and practicality due to low-complexity OFC source.

Optical frequency combs generated from microresonators (especially microresonator soliton combs) have been attracting significant attentions because of the potential to be fully chip-scale. Among various promising applications of soliton combs, coherent optical communications and mm/THz wireless communications require low phase noise of the comb modes and low relative phase noise between the comb modes, respectively. Here, we measure the phase noise of a soliton comb, investigating how the thermorefractive noise of a microresonator influences on the phase noise. We observe the quadratic increase of the phase noise of the comb modes, as the comb mode number, counted from the wavelength of a pump cw laser, increases. In addition, we measure the relative phase noise between the comb modes, showing less influence of the phase noise of pump cw lasers by comparing soliton combs generated from pump cw lasers with low and large phase noise.