Erbium-doped-fiber-based broad visible range frequency comb with a 30 GHz mode spacing for astronomical applications.

We have realized a comb system with a 30 GHz mode spacing, 62 % available wavelength coverage in the visible region, and nearly 40 dB spectral contrast by combining a robust erbium-doped-fiber-based femtosecond laser, mode filtering with newly designed optical cavities, and broadband-visible-range comb generation using a chirped periodically-poled LiNbO3 ridge waveguide. Furthermore, it is suggested that this system produces a spectrum with little change over 29 months. These features of our comb will contribute to fields requiring broad-mode-spacing combs, including astronomical observations, such as exoplanet exploration and the verification of the cosmic accelerating expansion.

Ultra-precise determination of thicknesses and refractive indices of optically thick dispersive materials by dual-comb spectroscopy.

Precise measurements of the geometrical thickness of a sample and its refractive index are important for materials science, engineering, and medical diagnosis. Among the possible non-contact evaluation methods, optical interferometric techniques possess the potential of providing superior resolution. However, in the optical frequency region, the ambiguity in the absolute phase-shift makes it difficult to measure these parameters of optically thick dispersive materials with sufficient resolution. Here, we demonstrate that dual frequency-comb spectroscopy can be used to precisely determine the absolute sample-induced phase-shift by analyzing the data smoothness. This method enables simultaneous determination of the geometrical thickness and the refractive index of a planar sample with a precision of five and a half digits. The thickness and the refractive index at 193.414 THz (λ = 1550 nm) of a silicon wafer determined by this method are 0.5204737(19) mm and 3.475625(58), respectively, without any prior knowledge of the refractive index.

Cosine similarity for quantitatively evaluating the degree of change in an optical frequency comb spectra.

We introduce and experimentally apply "cosine similarity" as an index for quantitatively evaluating the degree of change in the spectra of optical frequency combs. The cosine similarity with the original spectrum increased or decreased as the amount of control applied to the combs increased or decreased; this is considered to be an appropriate indication of spectral similarity. Therefore, we apply this approach to an evaluation of the temporal spectral changes in polarization-maintaining (PM) and non-PM combs. The results suggest that there is no significant difference between the spectral stabilities of PM and non-PM combs, and reveal that the spectral sensitivity to the amount of control is a more effective factor.

40 GHz continuous, precise, and low power-loss laser frequency sweep using an electro-optic modulator.

We propose and demonstrate a novel technique for precisely and widely sweeping the frequency of a continuous-wave laser. One of the modulation sidebands of a slave laser generated with an electro-optic modulator is phase-locked to a master laser; in this situation, the slave carrier component can be swept by sweeping the modulation frequency. It does not require beat signal detection at varying and/or high frequency, thus providing a robust and reliable laser frequency sweep. Also, it requires neither a frequency comb for the sweep nor a large power loss. We successfully swept an 852 nm laser over 20 GHz; we confirmed that a second harmonic 426 nm laser could be continuously swept over 40 GHz.

Iodine-stabilized laser at telecom wavelength using dual-pitch periodically poled lithium niobate waveguide.

We demonstrate the third harmonic generation of a 1542-nm laser using a dual-pitch periodically poled lithium niobate waveguide with a conversion efficiency of 66%/W2. The generated 514-nm light is used for saturation spectroscopy of molecular iodine and laser frequency stabilization. The achieved laser frequency stability is 1.1×10-12 at an average time of 1 s, which is approximately one order of magnitude better than the acetylene-stabilized laser at 1542 nm. Uncertainty evaluation and absolute frequency measurement are also performed. The developed frequency-stabilized laser can be used as a reliable frequency reference at the telecom wavelength for various applications including optical frequency combs and precision interferometric measurement.

Polarization-sensitive dual-comb spectroscopy with an electro-optic modulator for determination of anisotropic optical responses of materials.

We propose and demonstrate a polarization-sensitive dual-comb spectroscopy (DCS) technique that employs an electro-optic modulator for determining the anisotropic optical responses of materials. This straightforward extension of the typical DCS setup directly provides amplitudes and phases in two mutually orthogonal directions of the electric field of light. Using this method, we determined the optic axis direction and the anisotropy in the complex refractive index of a sample whose optical parameter is well defined. We estimate a birefringence of the sample to be 5.49(55)×10-5 at a comb tooth in the 780 nm region.

X-ray pumping of the 229Th nuclear clock isomer.

The metastable first excited state of thorium-229, 229mTh, is just a few electronvolts above the nuclear ground state1-4 and is accessible by vacuum ultraviolet lasers. The ability to manipulate the 229Th nuclear states with the precision of atomic laser spectroscopy5 opens up several prospects6, from studies of fundamental interactions in physics7,8 to applications such as a compact and robust nuclear clock5,9,10. However, direct optical excitation of the isomer and its radiative decay to the ground state have not yet been observed, and several key nuclear structure parameters-such as the exact energies and half-lives of the low-lying nuclear levels of 229Th-remain unknown11. Here we present active optical pumping into 229mTh, achieved using narrow-band 29-kiloelectronvolt synchrotron radiation to resonantly excite the second excited state of 229Th, which then decays predominantly into the isomer. We determine the resonance energy with an accuracy of 0.07 electronvolts, measure a half-life of 82.2 picoseconds and an excitation linewidth of 1.70 nanoelectronvolts, and extract the branching ratio of the second excited state into the ground and isomeric state. These measurements allow us to constrain the 229mTh isomer energy by combining them with γ-spectroscopy data collected over the past 40 years.

Development of 8-branch Er:fiber frequency comb for Sr and Yb optical lattice clocks.

We demonstrate an 8-branch Er:fiber frequency comb with seven application ports, which can be individually optimized for applications with different wavelengths. The beat between the comb and a cw laser has a signal-to-noise ratio exceeding 30 dB at a resolution bandwidth of 300 kHz. The 8-branch frequency comb is used to perform frequency locking for four repumping and lattice lasers, and the frequency measurement of two clock lasers of strontium and ytterbium optical lattice clocks. We have achieved reliable optical lattice clock operation, thanks to the stable frequency locking and measurement obtained by using the 8-branch frequency comb. The developed frequency comb is a powerful experimental tool for various applications, including not only optical lattice clocks, but also research on quantum optics that use many frequency-stabilized lasers.

Multi-branch fiber comb with relative frequency uncertainty at 10-20 using fiber noise difference cancellation.

In multi-branch combs, the comb outputs from the branches suffer from different fiber noises, which often limit the uncertainty of the combs referring a highly-stable optical frequency. To overcome this limitation, we introduced fiber noise difference cancellation to multi-branch fiber combs. We detected and phase-locked the beat notes between the branch outputs and a common 1542 nm continuous wave laser. A piezo-electric transducer-based fiber stretcher was installed in each branch except for the branch used as the cancellation reference. We fabricated two quasi-identical combs with this mechanism and confirmed the relative frequency uncertainty by comparing them. The cancellation improved the frequency uncertainty to a low level of 10-20 at a 100000-s averaging time.

Dual-comb spectroscopic ellipsometry.

Spectroscopic ellipsometry is a means of investigating optical and dielectric material responses. Conventional spectroscopic ellipsometry is subject to trade-offs between spectral accuracy, resolution, and measurement time. Polarization modulation has afforded poor performance because of its sensitivity to mechanical vibrational noise, thermal instability, and polarization-wavelength dependency. We combine spectroscopic ellipsometry with dual-comb spectroscopy, namely, dual-comb spectroscopic ellipsometry. Dual-comb spectroscopic ellipsometry (DCSE). DCSE directly and simultaneously obtains the ellipsometric parameters of the amplitude ratio and phase difference between s-polarized and p-polarized light signals with ultra-high spectral resolution and no polarization modulation, beyond the conventional limit. Ellipsometric evaluation without polarization modulation also enhances the stability and robustness of the system. In this study, we construct a polarization-modulation-free DCSE system with a spectral resolution of up to 1.2 × 10-5 nm throughout the spectral range of 1514-1595 nm and achieved an accuracy of 38.4 nm and a precision of 3.3 nm in the measurement of thin-film samples.Spectroscopic ellipsometry is an established technique to characterize the optical properties of a material. Here, Minamikawa et al. combine the method with dual-comb spectroscopy, which allows them to obtain ellipsometric parameters including the phase difference between s-polarized and p-polarized light.

Ortho-Para-Dependent Pressure Effects Observed in the Near Infrared Band of Acetylene by Dual-Comb Spectroscopy.

We demonstrate that dual-comb spectroscopy, which allows one to record broadband spectra with high frequency accuracy in a relatively short time, provides a real advantage for the observation of pressure-broadening and pressure-shift effects. We illustrate this with the ν_{1}+ν_{3} vibration band of ^{12}C_{2}H_{2}. We observe transitions from P(26) to R(29), which extend over a 3.8 THz frequency range, at six pressures ranging up to 2654 Pa. Each observed absorption line profile is fitted to a Voigt function yielding pressure-broadening and pressure-shift coefficients for each rotation-vibration transition. The effectiveness of this technique is such that we are able to discern a clear dependence of the pressure-broadening coefficients on the nuclear spin state, i.e., on the ortho or para modification. This information, combined with the pressure-shift coefficients, can facilitate a detailed understanding of the mechanism of molecular collisions.

Generation of a frequency comb spanning more than 3.6 octaves from ultraviolet to mid infrared.

We have observed an ultra-broadband frequency comb with a wavelength range of at least 0.35 to 4.4 µm in a ridge-waveguide-type periodically poled lithium niobate device. The PPLN waveguide is pumped by a 1.0-2.4 µm wide frequency comb with an average power of 120 mW generated using an erbium-based mode-locked fiber laser and a following highly nonlinear fiber. The coherence of the extended comb is confirmed in both the visible (around 633 nm) and the mid-infrared regions.

Sub-Doppler resolution mid-infrared spectroscopy using a difference-frequency-generation source spectrally narrowed by laser linewidth transfer.

The spectral linewidth of a 3.28 µm difference-frequency-generation source has been reduced to 3.5 kHz using a laser linewidth transfer technique [Opt. Express21, 7891 (2013)]. We use an optical frequency comb with a broad servo bandwidth to transfer a narrow linewidth of a pump laser, a 1.06 µm Nd:YAG laser, to a signal laser, a 1.57 µm external-cavity laser diode. This source enables us to record the Lamb dip of the ν3 band R(2) E transition of methane with a molecular spectral linewidth of 21 kHz while the frequency axis is absolutely calibrated.

Compact iodine-stabilized laser operating at 531 nm with stability at the 10(-12) level and using a coin-sized laser module.

We demonstrate a compact iodine-stabilized laser operating at 531 nm using a coin-sized light source consisting of a 1062-nm distributed-feedback diode laser and a frequency-doubling element. A hyperfine transition of molecular iodine is observed using the light source with saturated absorption spectroscopy. The light source is frequency stabilized to the observed iodine transition and achieves frequency stability at the 10(-12) level. The absolute frequency of the compact laser stabilized to the a(1) hyperfine component of the R(36)32 - 0 transition is determined as 564074632419(8) kHz with a relative uncertainty of 1.4×10(-11). The iodine-stabilized laser can be used for various applications including interferometric measurements.

Near-infrared broadband dual-frequency-comb spectroscopy with a resolution beyond the Fourier limit determined by the observation time window.

We performed broadband dual-frequency-comb spectroscopy in the near-infrared region with a much higher resolution than the Fourier limit by using discrete Fourier transforms and spectral interleaving. We observed the resonant spectrum of a Fabry-Perot cavity over a spectral range of 187 to 218 THz using this technique, and measured its free spectral ranges and finesses. The recorded spectrum includes cavity resonance lines with widths of about 2 MHz, which is much narrower than the resolution of 48 MHz determined by the observation time window.

Design of cavity-enhanced absorption cell for reducing transit-time broadening.

To reduce the linewidth of Lamb dips, we introduce a cavity-enhanced absorption cell (CEAC) coupled with a Gaussian beam with a 1.9-mm 1/e(2) radius at the beam waist for the reduction of transit-time broadening. We state that transit-time broadening depends only on the beam radius at the beam waist. This fact is useful for the design of the CEAC, and a pair of concave and convex mirrors is thereby employed. We have carried out sub-Doppler resolution spectroscopy of the ν(3) band of CH(4) and the ν(1) band of CH(3)D using a difference-frequency-generation source and the CEAC, and the recorded Lamb dips narrow to 80 kHz (HWHM).

A novel frequency control scheme for comb-referenced sensitive difference-frequency-generation spectroscopy.

We present a novel scheme of frequency scan and wavelength modulation of a difference-frequency-generation source for comb-referenced sensitive spectroscopy. While the pump and signal frequencies are phase-locked to an optical frequency comb (OFC), the offset frequency between the signal wave and the nearest comb tooth is modulated to apply a wavelength-modulation technique, and the idler wave frequency is repeatedly swept for signal accumulation by changing the repetition frequency of the OFC. The spectrometer is applied to absolute frequency measurement of weak hyperfine-resolved rovibration transitions of the ν(1) band of CH(3)I, and the uncertainty in frequency determination is reduced by one order of magnitude in compared with that of the previous work published in Optics Express 20, 9178-9186 (2012).

Spectroscopy of 171Yb in an optical lattice based on laser linewidth transfer using a narrow linewidth frequency comb.

We propose a novel, high-performance, and practical laser source system for optical clocks. The laser linewidth of a fiber-based frequency comb is reduced by phase locking a comb mode to an ultrastable master laser at 1064 nm with a broad servo bandwidth. A slave laser at 578 nm is successively phase locked to a comb mode at 578 nm with a broad servo bandwidth without any pre-stabilization. Laser frequency characteristics such as spectral linewidth and frequency stability are transferred to the 578-nm slave laser from the 1064-nm master laser. Using the slave laser, we have succeeded in observing the clock transition of (171)Yb atoms confined in an optical lattice with a 20-Hz spectral linewidth.

Absolute frequency list of the ν3-band transitions of methane at a relative uncertainty level of 10(-11).

We determine the absolute frequencies of 56 rotation-vibration transitions of the ν(3) band of CH(4) from 88.2 to 90.5 THz with a typical uncertainty of 2 kHz corresponding to a relative uncertainty of 2.2 × 10(-11) over an average time of a few hundred seconds. Saturated absorption lines are observed using a difference-frequency-generation source and a cavity-enhanced absorption cell, and the transition frequencies are measured with a fiber-laser-based optical frequency comb referenced to a rubidium atomic clock linked to the international atomic time. The determined value of the P(7) F(2)((2)) line is consistent with the International Committee for Weights and Measures recommendation within the uncertainty.