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Calcium monohydroxide radical (CaOH) is receiving an increasing amount of attention from the astrophysics community as it is expected to be present in the atmospheres of hot rocky super-Earth exoplanets as well as interstellar and circumstellar environments. Here, we report the high-resolution laboratory absorption spectroscopy on low-J transitions in A ˜ 2 Π ( 0 , 0 , 0 ) − X ˜ 2 Σ + ( 0 , 0 , 0 ) band of buffer-gas-cooled CaOH. In total, 40 transitions out of the low-J states were assigned, including 27 transitions that have not been reported in previous literature. The determined rotational constants for both ground and excited states are in excellent agreement with previous literature, and the measurement uncertainty for the absolute transition frequencies was improved by more than a factor of 3. This will aid future interstellar, circumstellar, and atmospheric identifications of CaOH. The buffer-gas-cooling method employed here is a particularly powerful method to probe low-J transitions and is easily applicable to other astrophysical molecules.

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.

Direct frequency comb spectroscopy (DFCS) in the mid-infrared region, especially in the long mid-infrared region (>5 μm), is a powerful technique for atmospheric remote sensing [1] and fundamental spectroscopy such as studies of C 60 [2]. DFCS has been demonstrated in Fourier transform spectroscopy, dual-comb spectroscopy, and techniques involving a dispersive spectrometer. Although a dispersive spectrometer with a virtually-imaged phase array (VIPA) can probe transient molecules by recording spectra within 10 μs [3,4], it has the disadvantages of low throughput and limited-bandwidth, which present challenges for sensitive, broadband detection at high time resolution.

We have developed a dispersive spectrometer using a compact immersion grating for direct frequency comb spectroscopy in the long-wave infrared region of 8-10 {\mu}m. A frequency resolution of 463 MHz is achieved, which is the highest reported in this wavelength region with a dispersive direct frequency comb spectrometer. We also demonstrate individual mode-resolved imaging of the frequency comb spectrum by Vernier cavity filtering and apply this to obtain both simple and complex molecular spectra. These results indicate that the immersion grating spectrometer offers the next advancement for sensitive, high-resolution spectroscopy of transient and large/complex molecules when combined with cavity enhancement and cooling techniques.

Long-wavelength mid-infrared frequency combs with high power and flexible tunability are highly desired for molecular spectroscopy, including investigation of large molecules such as C60. We present a high power, phase-stabilized frequency comb near 10 {\mu}m, generated by a synchronously-pumped, singly-resonant optical parametric oscillator (OPO) based on AgGaSe2. The OPO can be continuously tuned from 8.4 to 9.5 {\mu}m, with a maximum average idler power of 100 mW at the center wavelength of 8.5 {\mu}m. Both the repetition rate (frep) and the carrier-envelop offset frequency (fceo) of the idler wave are phase-locked to microwave signals referenced to a Cs clock. We describe the detailed design and construction of the frequency comb, and discuss potential applications for precise and sensitive direct frequency comb spectroscopy., Comment: 15 pages, 4 figures

Sub-Doppler resolution spectroscopy of the fundamental vibration bands of H 35 Cl and H 37 Cl has been carried out from 87.1 to 89.9 THz. We have determined the absolute transition frequencies of the hyperfine-resolved R (0) to R (4) transitions with a typical uncertainty of 10 kHz. We have also yielded six molecular constants for each isotopomer in the vibrational excited state, which reproduce the determined frequencies with a standard deviation of about 10 kHz.

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.

Real-time absorption and dispersion dual-comb spectroscopy is achieved in the 3-μm region without mode-locked lasers or active stabilization. The resolved lines of flat-top frequency combs with adjustable line-spacing and tunable center wavelength enable precise spectroscopy.

A set-up of mid-infrared dual-comb spectroscopy with electro-optic modulators records broadband molecular spectra with high signal-to-noise ratio and resolved comb lines within milliseconds. A tunability from 85.6 to 96.71 THz is demonstrated.

Dual-comb spectroscopy with electro-optic modulators and nonlinear fibers records broadband spectra with high signal-to-noise ratio and resolved comb-lines within milliseconds. The spectrometer is tunable from 185.0 to 195.9 THz and from 85.6 to 96.7 THz.

We demonstrate frequency stabilization of a modulation-free laser to a saturated absorption spectral line of atoms in a transversal magnetic field. This stabilization scheme has been proposed for wide capture range in comparison with the dichroic atomic vapor laser lock (DAVLL) scheme and demonstrated for a Doppler-broadened spectral line in J. Opt. Soc. Am. B, 26, 1216 (2009). In this paper, a 1083-nm external-cavity laser diode is frequency-stabilized to the sub-Doppler spectral line of helium transition ( 2 3 S 1 , m J = 0 ↔ 2 3 P 0 ). Even though the error signal shape strongly depends on the pump beam polarization, the stabilized frequency is expected to be insensitive to the pump beam polarization.

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.

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).

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.

Absorption spectroscopy of fundamental ro-vibrational transitions in the mid-infrared region provides a powerful tool for studying the structure and dynamics of molecules in the gas phase and for sensitive and quantitative gas sensing. Laser frequency combs permit novel approaches to perform broadband molecular spectroscopy. Multiplex dual-comb spectroscopy without moving parts can achieve particularly high speed, sensitivity and resolution. However, achieving Doppler-limited resolution in the mid-infrared still requires overcoming instrumental challenges. Here we demonstrate a new approach based on difference-frequency generation of frequency-agile near-infrared frequency combs that are produced using electro-optic modulators. The combs have a remarkably flat intensity distribution, and their positions and line spacings can be freely selected by simply dialing a knob. Using the proposed technique, we record, in the 3-μm region, Doppler-limited absorption spectra with resolved comb lines within milliseconds, and precise molecular line parameters are retrieved. Our technique holds promise for fast and sensitive time-resolved studies of, for example, trace gases.

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.

We discuss the design and performance of an all polarization-maintaining self-referenced frequency comb that can maintain optical coherence with an optical reference laser under strong vibrations from a shaker table and a moving vehicle.

We demonstrate a self-referenced fiber frequency comb that can operate outside the well-controlled optical laboratory. The frequency comb has residual optical linewidths of < 1 Hz, sub-radian residual optical phase noise, and residual pulse-to-pulse timing jitter of 2.4 - 5 fs, when locked to an optical reference. This fully phase-locked frequency comb has been successfully operated in a moving vehicle with 0.5 g peak accelerations and on a shaker table with a sustained 0.5 g rms integrated acceleration, while retaining its optical coherence and 5-fs-level timing jitter. This frequency comb should enable metrological measurements outside the laboratory with the precision and accuracy that are the hallmarks of comb-based systems. Work of the U.S. government, not subject to copyright

We have being developing optical frequency combs, especially fiber-based frequency combs with narrow linewidth, for laser frequency measurement and control. We are also working on Yb and Sr optical lattice clocks for precision frequency metrology.

We have determined 183 sub-Doppler resolution spectral frequencies of 12CH4 using a difference-frequency-generation source, an enhanced-cavity absorption cell, and an optical frequency comb from 86.7 to 93.1 THz with a typical uncertainty of 3 kHz.

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.

We demonstrate a completely polarization-maintaining fiber frequency comb operating at a 200 MHz repetition rate and show stability and noise performance consistent with precision measurement applications. This design is compatible with a robust, fieldable comb.

We report as regards an optical-optical frequency transfer with relative frequency stability at 10−21-level by canceling fiber noise between plural optical amplifier branches.

We report on the development of an ytterbium (171Yb) and a strontium (87Sr) optical lattice clock at the National Metrology Institute of Japan (NMIJ). The result of absolute frequency measurement of the 171Yb lattice clock is shown. We also report on narrow-linewidth fiber-based frequency combs that are used not only to measure the absolute frequency of the optical lattice clocks but also to servo control the frequency of the lasers for cooling and trapping Yb and Sr atoms.

We describe continuous-wave laser frequency synthesis using optical frequency combs for the optical lattice clock system developped at NMIJ.

We have been developing a new clock laser system for Yb and Sr optical lattice clocks. The system is based on an infrared laser with a narrow linewidth, and a fiber-based frequency comb transferring the linewidth to required plural wavelengths. We demonstrate that fiber-based frequency combs with a multi-branch configuration can transfer laser linewidth to another wavelength at the millihertz level. An intra-cavity electro-optic modulator is employed to obtain a broad servo bandwidth for repetition rate control.

We have been developing a new clock laser system for an Yb optical lattice clock. The system does not include a high finesse cavity at 578 nm, but a fibre-based frequency comb, which is tightly phase-locked to a narrow linewidth Nd:YAG laser at 1064 nm. The Nd:YAG laser is phase locked to a high finesse cavity at 1064 nm made from ultra low expansion glass.

We have carried out dual-comb spectroscopy and observed in a simultaneous acquisition a 140-THz-wide spectrum from 1.0 to 1.9 {\mu}m using two fiber-based frequency combs phase-locked to each other. This ultra-broad wavelength bandwidth is realized by setting the difference between the repetition rates of the two combs to 7.6 Hz using the sub-Hz-linewidth fiber combs. The recorded spectrum contains five vibration-rotation bands of C${_2}$H${_2}$, CH${_4}$, and H${_2}$O at different wavelengths across the whole spectrum. The determined transition frequencies of C${_2}$H${_2}$ agree with those from the previous sub-Doppler resolution measurement of individual lines using CW lasers within 2 MHz., Comment: 13 pages, 4 figures, 1 table

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).

We have carried out sub-Doppler resolution spectroscopy of the ν3 band of CH412 using a difference-frequency-generation source, an enhanced cavity absorption cell, and an optical frequency comb, and we have added 150 lines to the transition frequency list published earlier [Opt. Express19, 23878 (2011)]. It now covers 204 lines from the P(12) to R(8) transitions, involving 21 weak forbidden transitions, with a typical uncertainty of 3 kHz for allowed transitions and 12 kHz for forbidden transitions. The combination differences between the allowed and forbidden transitions enable us to determine a scalar rotational constant and three scalar distortion constants of the ground vibrational state with greatly reduced uncertainties.

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.

We develop an all-fiber-based frequency comb with an intra-cavity waveguide electro-optic modulator. A comb mode is successfully phase-locked to an optical reference. The servo bandwidth of repetition rate is approximately 400 kHz.