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The ν 1 + ν 3 vibration band of acetylene (C2H2) in the near infrared region was recorded with a dual-comb Fourier-transform spectrometer. We observed 56 transitions from P ( 26 ) to R ( 29 ) at six different column densities. The integral line intensity was determined for each recorded absorption line by fitting the line profile to Lambert-Beer’s law with a Voigt function. Thanks to the outstanding capability of dual-comb spectroscopy to cover a broad spectrum in a relatively short time with high resolution and high frequency precision, we determined the reliable line strength for each ro-vibrational transition as well as the transition dipole moment for this band.

We apply an intensity-modulation technique to dual-comb spectroscopy to improve its detection sensitivity. The scheme is demonstrated via Doppler-free optical–optical double-resonance spectroscopy of Rb by modulating the intensity of a pump laser with frequencies set at rates 3 times lower and 50,000 times higher than the difference in the repetition rates of the two frequency combs. The signal-to-noise ratios are enhanced by 3 and 6 times for slow and fast modulations, respectively, compared to those of conventional dual-comb spectroscopy without any intensity modulation. The technique is widely applicable to pump-probe spectroscopy with dual-comb spectroscopy and provides high detection sensitivity.

We propose and demonstrate a novel method for temperature determination using dual-comb spectroscopy and a novel analysis technique, “Rotational-state Distribution Thermometry: RDT.” We obtained the spectral profile of a vibration–rotation band of 12C2H2 using dual-comb spectroscopy at room temperature. The line-center absorbance was determined for each transition by fitting the line profile to a Gaussian function. A model function that relates the rotational temperature of the molecule to the distribution of the line-center absorbance was introduced; the gas temperature was determined by fitting the distribution to the model function. The determined temperature agrees to within 0.6 K with the cell-wall temperature measured with a platinum resistance thermometer. In addition, using the spectral profile obtained in this study, we compare the present analysis with two conventional methods for determining the gas temperature; one is based on the line-center absorbance ratio, and the other on the Doppler width. The present method takes full advantage of the supreme characteristics of dual-comb spectroscopy and has the potential to offer fast and precise thermometry.

We demonstrate background-free detection schemes in optical-optical double-resonance dual-comb spectroscopy to improve measurement sensitivity. The dual-comb employing modulation spectroscopy and fluorescence detection achieved 4 and 925 times higher SNR than normal dual-comb setup, respectively.

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.

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.

We carry out sub-Doppler laser spectroscopy of hyperfine components of rovibrational transitions of molecular iodine at 578 nm using second harmonic generation of an 1156-nm external-cavity diode laser. The absolute frequencies of the hyperfine components can be determined by using an optical frequency comb to referenced to the Coordinated Universal Time of the National Metrology Institute of Japan. The hyperfine components at 578 nm can be good frequency references for the 1S o −3P o clock transition in atomic ytterbium.

Optical-optical double-resonance spectroscopy with a CW pump laser and dual-comb technique were demonstrated. We obtained the fully resolved hyperfine spectra of rubidium, and the absolute frequencies of the spectra were determined precisely.

A light source to drive the (1)S0-(3)P0 transition in Yb atoms is generated by 2 solid state lasers: a Nd:YAG laser and an Yb:YAG laser, using a sum-frequency generation (SFG) scheme. With a ridge waveguide (WG) periodically poled lithium niobate (PPLN) device, SFG power of about 150 mW is obtained at the required frequency. The zero-expansion temperature of a Fabry-Pérot etalon was determined by using a home-made fiber-based optical frequency comb running continuously for weeks. Frequency stabilization of the clock laser system was also evaluated by the optical frequency comb.

We demonstrate a self-referencing locking method for an optical frequency comb that leads to an offset-free frequency comb. It works with a standard f−2f interferometer and phase-locks the carrier–envelope offset frequency to the repetition frequency. This technique uses neither an acousto-optic modulator nor difference frequency generation. No synthesizer signals are needed aside from a reference signal to stabilize the repetition rate. We test the performance by comparing it with an independent comb system; it does not result in any performance degradation when compared with the conventional phase-locking method. The offset-free comb realized by this technique may become a powerful and universal tool for use in frequency-comb-related research.

We had reported in a previous paper the precision measurement of line profiles of a transition of 13 C acetylene in the near-infrared region, employing a tunable diode-laser spectrometer, the frequency of which is locked to an optical-comb signal [K.M.T. Yamada et al., J. Mol. Spectrosc. 249 (2008) 95–99]. In the present article we review the methodology for the experiment and the analysis in determining the Boltzmann constant from the Doppler width of a rovibrational transition on the basis of our experiences. Problems to be surmounted for further improvement in accuracy are discussed. To cite this article: K.M.T. Yamada et al., C. R. Physique 10 (2009).

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 NMIJ 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 times10-17. Furthermore, we demonstrate that an absolute mode number of the comb can easily and clearly be determined by using the two combs.

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).

Line profiles of a rovibrational transition of 13C acetylene have been measured for various pressures in the near infrared region. In order to accomplish high precision in frequency, we have employed a diode-laser, the frequency of which is locked to an optical comb. By tuning the comb frequency we have achieved a continuous frequency tuning over 2 GHz for the measurement of Doppler broadened line profiles spread over 2 GHz. In addition we have stabilized the incident power of the laser on the sample cell by adjusting the gain of a fiber amplifier via a feed-back loop. Observed profile data have been analyzed by the generalized Voigt function to determine the Gaussian width precisely: at the present we realized a precision of 10 - 3 . The zero pressure line center position was determined with a precision of 10 - 9 .

The authors 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 tau -12 for the averaging times tau between 1 and 100 s, and the best frequency stability was 3.0 10-15 for an averaging time of 1280 s

A compact iodine-stabilized laser at 531 nm is developed with a frequency stability at the 10−12 level using a coin-sized diode laser module. This laser will be used for applications including interferometric measurements.

We performed an absolute frequency measurement of the $^1S_{0}$-$^3P_{0}$ transition in $^{87}$Sr with a fractional uncertainty of $1.2 \times 10^{-15}$, which is less than one third that of our previous measurement. A caesium fountain atomic clock was used as a transfer oscillator to reduce the uncertainty of the link between a strontium optical lattice clock and the SI second. The absolute value of the transition frequency is 429 228 004 229 873.56(49) Hz., Comment: accepted for publication in Journal of the Physical Society of Japan, 7 pages, 2 figures

We succeeded in generating a coherent optical frequency comb covering more than 3 octaves from visible to mid-infrared with a waveguide-type PPLN. The generated comb is demonstrated to be useful for metrology and spectroscopic applications.

Frequency-control characteristics of a mode-locked erbium-fiber laser with an intracavity optically pumped ytterbium-fiber were investigated. The fixed frequency of the comb when the pump power to the ytterbium-fiber is changed varies with the power.

An optical frequency measurement system based on an octave-spanning optical frequency comb generated by a chirped-mirror-dispersion-controlled mode-locked Ti:Al2O3 laser and a photonic-crystal fiber is developed. All of the modes of the octave-spanning optical frequency comb are frequency-stabilized to a microwave frequency standard, where the carrier-envelope offset frequency is phase-locked with self-referencing of the comb. We investigate the methods of controlling carrier-envelope offset frequency in a chirped-mirror-dispersion-controlled mode-locked laser. The rotation of a pair of chirped mirrors is useful for setting the bias of carrier-envelope offset frequency. Although our mode-locked laser has a low pulse-repetition frequency of 150 MHz, a high signal-to-noise ratio in beats results in the direct measurement of beat frequency with a laser to be measured using a frequency counter, and enables us to phase lock carrier-envelope offset frequency merely by using a mixer analogously without the need for a prescaler, with a servo bandwidth at approximately 500 kHz. The uncertainty of our optical frequency measurement system, besides the uncertainty of microwave reference frequency, is 4×10-14, and is limited by the uncertainty of the rf synthesizer used for phase locking and by that of the beat frequency measurement. Frequency measurements of an iodine-stabilized frequency-doubled Nd:YAG laser at 532 nm, an iodine-stabilized He–Ne laser at 633 nm and a rubidium two-photon-absorption stabilized extended-cavity laser diode at 778 nm are conducted. The results contributed to the revision of the practical realization of the metre adopted by the International Conference on Weights and Measures (CIPM) in 2001.

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.

We have established an ensemble of iodine-stabilized Nd:YAG lasers to verify the frequency reproducibility of the laser. The stability, repeatability and several systematic shifts of the laser frequency are investigated by heterodyne beating of the lasers. The frequency dispersion of the ensemble of lasers is evaluated to be 0.5 kHz (corresponding to a relative frequency uncertainty of 8 × 10−13). The absolute frequency of one of the lasers (named as Y3) is measured to be 563 260 223 507 897(58) Hz using a femtosecond optical comb, when the laser was stabilized on the a10 component of the R(56)32-0 transition of 127I2 for a cold-finger temperature of −10 °C. This group of lasers, including one transportable laser, forms an ensemble of optical frequency standards, which serves for many applications such as international comparisons, optical frequency measurements, frequency calibration services and high-resolution spectroscopy.

A comparison of two independent absolute optical frequency measurements has been carried out between the National Metrology Institute of Japan/National Institute of Advanced Industrial Science and Technology (NMIJ/AIST), Tsukuba, Japan and the JILA (formerly the Joint Institute for Laboratory Astrophysics), Boulder, CO, using a portable iodine-stabilized Nd:YAG laser. The agreement between the two absolute measurements is better than the measurement uncertainty of 6.7/spl times/10/sup -13/ that can be attributed to the reproducibility limitations of the portable laser. This comparison is used to confirm the measured absolute frequency of an iodine-stabilized Nd:YAG laser at NMIJ/AIST (Y3), which is reported to the Consultative Committee for Length (CCL) for the determination of the absolute frequency value of iodine-stabilized Nd:YAG lasers.

We succeeded in observing one of the absorption lines of acetylene [P(16)] using an erbium-doped fiber-ring laser, which is the first such observation using a fiber-ring laser. The observed linewidth of the P(16) linear absorption was approximately 450 MHz. Mode competition and mode hopping were suppressed by using an ultra narrow-band fiber Bragg grating and a Fabry-Perot filter in the ring laser, respectively. Wavelength fine-tuning was carried out by changing the cavity length of the fiber-ring laser; the fine-tuning range was about 850 MHz (0.01 nm). Furthermore, wavelength tuning was achieved by controlling the temperature of the fiber Bragg grating, and the coarse timing range was about 0.2 mn.

Hyperfine structures of the R(1 2 1)35-0 and P(1 4 2)37-0 transitions of molecular iodine at the blue end of the tuning range of a frequency-doubled Nd:YAG laser are measured by using two I 2 -stabilized lasers. The absolute frequencies of the observed transitions are determined by a frequency link to an optical frequency reference (the R(5 6)32-0:a 10 hyperfine transition), using an optical frequency comb generator. The observed hyperfine transitions are good frequency references for both frequency-doubled Nd:YAG and Nd:YVO 4 lasers in the 532 nm region. High-accurate hyperfine constants are obtained for the P(1 4 2)37-0 transition by fitting the measured hyperfine splittings to a four-term Hamiltonian, which includes the electric quadrupole, spin–rotation, tensor spin–spin, and scalar spin–spin interactions.

A novel and compact Er-Yb:glass laser was developed with single-frequency output power >10 mW in a wavelength range from 1531 to 1547 nm. The laser output was efficiently frequency doubled at 770.1 nm in a single-pass periodically poled lithium niobate waveguide and a second harmonic power in excess of 15 /spl mu/W was achieved, with an internal conversion efficiency of /spl sim/220% W/sup -1/. This frequency-doubled beam was used for high-resolution saturated spectroscopy of the /sup 39/K 4S/sub 1/2 /-4P/sub 1/2 / absorption line. The best resolved crossover peak showed 16% line contrast and 10-MHz linewidth. In a second experiment, working at the fundamental frequency, saturation spectroscopy of /sup 13/C/sub 2/H/sub 2/ was achieved using a low-pressure gas cell inside a build-up cavity. The measured absorption dip has 3% line contrast and 1.2 MHz linewidth. Both the atomic and molecular saturated lines are very well suited for frequency locking in order to establish accurate frequency standards in the optical communication band.

A novel and compact diode-pumped Er–Yb:glass laser, with single-frequency output power up to 20 mW in a wavelength range from 1531 to 1547 nm, was frequency stabilized against sub-Doppler rovibrational transitions of 13C2H2. The non-linear wavelength-modulation spectroscopy technique was used to lock the laser frequency to the molecular lines. The stability of the developed optical frequency standard was mainly dominated by a white frequency-noise contribution, which sets the Allan deviation at a level of σy(τ)=2×10−11τ−1/2 for integration times in the range of 0.01 s .

We have evaluated an ultra-stable laser system, which is based on the linewidth transfer method. In this system, a Nd:YAG laser at 1064 nm stabilised to a high finesse Fabry- Perot etalon is employed as a master laser. Beat signals between the high-speed controllable optical frequency comb locked to the master laser and another ultra-stable laser operated at 1.5 μm are observed to investigate the frequency stability. The frequency stability measured by the fractional Allan deviation is about 2 × 10−15 at 1 s averaging time, which almost coincides with the estimated thermal noise limit on the optical cavity. Furthermore, using the system, the 1S0-3P0 transitions in ytterbium and strontium atoms are observed. We have demonstrated that linewidth transfer technique based on the optical frequency comb combining with the narrow linewidth laser serves as a useful ultra-stable local oscillator for optical clocks.

We extend the spectral bandwidth of dual-comb spectroscopy over a hundred THz using two optical frequency combs with relative linewidth of sub-Hz level. We record the spectrum of the entire vibration band of acetylene.

The frequency ratio of the 1S0(F=1/2)-3P0(F=1/2) clock transition in 171Yb and the 1S0(F=9/2)-3P0(F=9/2) clock transition in 87Sr is measured by an optical-optical direct frequency link between two optical lattice clocks. We determined the ratio (\nu_{Yb}/\nu_{Sr}) to be 1.207 507 039 343 340 4(18) with a fractional uncertainty of 1.5x10^{-15}. The measurement uncertainty of the frequency ratio is smaller than that obtained from absolute frequency measurements using the International Atomic Time (TAI) link. The measured ratio agrees well with that derived from the absolute frequency measurement results obtained at NIST and JILA, Boulder, CO using their Cs-fountain clock. Our measurement enables the first international comparison of the frequency ratios of optical clocks, and we obtained a good agreement between the two measured ratios with an uncertainty smaller than the TAI link. The measured frequency ratio will be reported to the International Committee for Weights and Measures for a discussion related to the redefinition of the second., Comment: accepted for publication in Opt. Express. 8 pages, 3 figures, 1 table

We have established a compact and transportable I/sub 2/-stabilized Nd:YAG laser for international comparisons of laser frequency. The root Allan variance of the portable laser has reached 3.9/spl times/10/sup -14/ when the integration time is longer than 200 s. The results of an international comparison between the National Research Laboratory of Metrology (NRLM), Tsukuba, Japan and the JILA (formerly the Joint Institute for Laboratory Astrophysics), Boulder, CO, USA show that the frequency difference of the portable laser NRLM-Y1 and the JILA laser JILA-W (f/sub NRLM-Y1/-f/sub JILA-W/) was -2.5 kHz, when the cold-finger temperatures of NRLM-Y1 and JILA-W were kept at -10/spl deg/C and -15/spl deg/C, respectively. The averaged frequency offset between two NRLM lasers (f/sub NRLM-Y1/-f/sub NRLM-Y2/) was -1.1 kHz. A frequency variation of about 1.2 kHz was found for the frequency offset between two NRLM lasers, after NRLM-Y1 was taken for a round trip to Sydney for a comparison organized by the National Measurement Laboratory, (NML), Australia.