Sub-Doppler Heterodyne Frequency Measurements near 5 μm with a CO-Laser Sideband System: Improved Calibration Tables for Carbonyl Sulfide Transitions

We present a series of sub-Doppler frequency measurements of carbonyl sulfide (OCS) rovibrational transitions covering the spectral region around 2050 cm⁻¹(61 THz). The absolute experimental uncertainties are between 21 and 84 kHz (Δν/ν = 3–14 × 10⁻¹⁰). In our spectrometer, tunable microwave sidebands are added to CO-laser lines and are used to saturate OCS transitions at low absorber pressure. The CO laser, plus a fixed-frequency sideband, is stabilized to the center of the OCS Lamb-dip signal and the laser frequency is measured against combination frequencies of two saturation-stabilized CO₂lasers. The determined OCS transition frequencies are combined with other data in a least-squares fit. These measurements improve the accuracy of several carbonyl sulfide bands for calibration purposes by one to two orders of magnitude. New molecular constants and detailed calibration tables between 860 and 3100 cm⁻¹are given.     

The CO fundamental-band laser as secondary frequency standard at 5 µm

We present a very high-resolution heterodyne spectrometer based on a CO laser which operates down to fundamental-band transitions of the molecule. This allows us to detect saturated absorption signals on these transitions at very low pressure (0.4 Pa) and laser intensity (< 1 mW/cm²), yielding a linewidth of about 250 kHz. With the CO fundamental-band laser stabilized to these saturation signals we have measured the transition frequencies of the fundamental bands of three isotopic species to an accuracy of typically 20 kHz (v/v 3 × 10^–10), referenced to the CO₂ frequency standard. Together with additional frequency measurements of the first hot bands, these provide the first heterodyne frequency data of sub-Doppler accuracy for transitions in low lying bands of CO. They now represent the most accurate secondary frequency standard in the spectral region around 5 µm (60 THz).     

Atomic frequency offset locking in a Λ type three-level Doppler broadened Cs system

We here present a comparative study of frequency stabilities of pump and probe lasers coupled at a frequency offset generated by coherent photon-atom interaction. Pump-probe spectroscopy of the Λ configuration in D₂ transition of cesium is carried out to obtain sub-natural (∼2 MHz) electromagnetically induced transparency (EIT) and sub-Doppler (∼10 MHz) Autler-Townes (AT) resonance. The pump laser is locked on the saturated absorption spectrum (SAS, ∼13 MHz) and the probe laser is successively stabilized on EIT and AT signals. Frequency stabilities of pump and probe lasers are calculated in terms of Allan variance σ(2,τ) by using the frequency noise power spectrum. It is found that the frequency stability of the probe stabilized on EIT is superior (σ∼2×10⁻¹³) to that of SAS locked pump laser (σ∼10⁻¹²), whereas the performance of the AT stabilized laser is inferior (σ∼6×10⁻¹²). This contrasting behavior is discussed in terms of the theme of conventional master-slave offset locking scheme and the mechanisms underlying the EIT and sub-Doppler AT resonances in a Doppler broadened atomic medium.     

1 W of blue 465-nm radiation generated by frequency doubling of the output of a high-power diode laser in critically phase-matched LiB3O5

Blue 465-nm radiation is generated by frequency doubling of the output of an InGaAs diode-laser oscillator-amplifier system in critically phase-matched LiB(3)O(5) (LBO). The diode-laser system emitted 4 W of single-frequency 930-nm radiation in a near-diffraction-limited beam (M(2)<1.2) . The laser power is enhanced to values of up to 150 W in a resonant external ring cavity. The LBO crystal is placed at a resonator internal focus. The frequency doubling in the LBO crystal generates blue radiation at 465 nm with a power of 1 W. The spectral width of the blue radiation is less than 3 MHz, and the value of the M(2) beam parameter is less than 1.2.     

Frequency comb-referenced narrow line width diode laser system for coherent molecular spectroscopy

We analyze in detail the frequency noise properties of a grating enhanced external cavity diode laser (GEECDL). This system merges two diode laser concepts, the grating stabilized diode laser and the diode laser with resonant optical feedback, thus combining a large tuning range with an excellent short-term frequency stability. We compare the frequency noise spectrum of a GEECDL to that of a grating stabilized diode laser and demonstrate a 10-fold reduction of the frequency noise linear spectral density. The GEECDL is phase locked to a similar laser and to a fs-frequency comb with a servo loop providing an open-loop unity-gain frequency of only 237 kHz, which is a tenth of the bandwidth typically required for grating stabilized diode lasers. We achieve a residual rms phase error as small as 72 mrad (≈ 200 mrad) for stabilization to a similar laser (to the fs-frequency comb). We demonstrate that the novel diode laser can phase-coherently track a stable optical reference with an instability of 1.8×10^-16 at 1 s. This laser system is well suited for applications that require phase locking to a low-power optical reference under noisy conditions. It may also be considered for the implementation of optical clock lasers. PACS 42.55.Px; 42.60.Jf; 42.50.Gy     

CO-laser side-band spectrometer: Sub-Doppler heterodyne frequency measurements around 5 µm

We report the realization of a tunable sub-Doppler heterodyne spectrometer with high absolute accuracy, employing side-band generation with a CO laser. The fixed-frequency CO-gas laser, working from 4.7 to 8.4µm, is made partially tunable by the use of microwave side-band generation in a CdTe Electro-Optical Modulator (EOM). This leads to tunable radiation of high spectral purity. We describe the design of the microwave EOM, adapted to the CO laser, its performance and its first application to highly accurate frequency measurements. The side-band radiation is used for sub-Doppler stabilization of the CO laser, while the carrier frequency is mixed with the frequencies of two CO₂ reference lasers. As a first result, we present measurements of OCS transitions in the 4.9µm (61 THz) region, reaching an absolute accuracy of 30 kHz (/ = 5×1O^–10). Further application of our spectrometer to calibration gases will establish a variety of InfraRed (IR) calibration standards with a new quality of accuracy. On visit from: Fachbereich Physik, Humboldt-Universität zu Berlin     

Tunable frequency-controlled laser source in the near ultraviolet based on doubling of a semiconductor diode laser

Continuously tunable ultraviolet laser radiation at 397 nm was generated by doubling the output of a semiconductor diode laser. The fundamental radiation was provided by a 150 mW AlGaAs laser diode injected by a low-power AlGaAs laser diode which was frequency stabilized by optical feedback using a new scheme of a miniature external cavity. Second-harmonic generation was produced in a lithium-triborate crystal placed in a compact enhancement cavity. The fundamental radiation was used for sub-Doppler spectroscopy of the Ar I 4s ³ P ⁰ ₀–4p ¹ P ₁ transition at 795 nm; the second-harmonic radiation was used for spectroscopy of the Ca II 4² S _1/2–4² P _1/2 transition at 397 nm.     

Very high resolution CO laser spectrometer and first sub-Doppler line-shape studies near 60 THz (5 μm)

2 laser standards. Using this technique, we can tune the CO laser frequency with absolute frequency control within the gain profile of each laser transition. The frequency uncertainty is smaller than 15 kHz, corresponding to Δν/ν=2.5×10^-10. Moreover, we obtain a reduction of the CO laser linewidth by a factor of 2 down to 65 kHz, corresponding to a spectral resolution of δν/ν=1×10^-9. With this outstanding accuracy and resolution we studied the shape of saturation dips in rovibrational lines of CO and carbonyl sulfide (OCS) at low pressure (<5 Pa). The self-pressure-broadening rate of CO was found to be γ_c=+83(7) kHz/Pa in this pressure region. This value is about four times higher than values resulting from previous measurements at much higher pressures. To our knowledge the measurements described here are the first line-shape studies with sub-Doppler resolution in the 5 μm spectral region. Received: 4 November 1996     

Absolute frequency measurements of wavelength standards 532 nm, 543 nm, 633 nm and 1540 nm

This paper describes the results of absolute frequency measurements of primary wavelength standards 633 nm, 543 nm, 532 nm, (iodine stabilized) and 1540 nm (acetylene stabilized) in CMI. The values obtained with Menlo Systems femtosecond frequency comb in CMI are compared with previous measurements of the same standards in BIPM, BEV and MPQ. Measured sub-Doppler linewidths and relative intensities of several hyperfine spectral components of iodine molecule are also presented.     

High-resolution spectroscopy of velocity-selected atoms in a thin cell

We propose a novel technique of sub-Doppler spectroscopy using a thin vapor cell. Optical pumping in a thin cell transfers atoms with small velocity components to a specific quantum state. The resultant velocity distribution appears as a sub-Doppler structure in the absorption spectrum of a probe light. A single laser beam from a laser diode is split into two paths: one beam optically pumps Cs atoms on the D₂ line, and the other probes the absorption on the same line from a perpendicular direction. Observed hyperfine-resolved spectra and their parameter dependence are analyzed on the basis of rate equations. Received: 16 January 2002 / Revised version: 11 February 2002 / Published online: 24 April 2002     

基于噪声免疫腔增强光外差分子光谱技术实现光纤激光器到1530.58 nmNH_3亚多普勒饱和光谱的频率锁定

噪声免疫腔增强光外差分子光谱技术(NICE-OHMS)由于结合了频率调制光谱与腔增强光谱两种技术,不仅可以将激光耦合到高精细度谐振腔大幅提高腔内功率,还可以实现低气压样品气体的高灵敏测量,因此基于该技术可以实现分子吸收线的饱和,获得亚多普勒光谱,从而能作为激光频率锁定的参考.本文基于光纤激光器的NICE-OHMS技术,将光纤激光器频率锁定到NH3的亚多普勒吸收线上.首先分析了基于Pound-Drever-Hall和DeVoe-Brewer技术实现激光到腔模和调制频率到腔自由光谱区频率锁定的性能,之后在腔内气压为70 mTorr条件下,测量了半高全宽为2.05 MHz的NH3亚多普勒信号,最后将1.53μm的光纤激光器频率锁定到该亚多普勒吸收线上,相对频率偏差为16.3 kHz,阿伦方差结果显示,136 s积分时间下频率稳定度达到1.6×10~(-12).     

Blue 489-nm picosecond pulses generated by intracavity frequency doubling in a passively mode-locked optically pumped semiconductor disk laser

We report the generation of blue 489-nm picosecond laser pulses by intracavity second-harmonic generation in a mode-locked optically pumped InGaAs vertical-external-cavity surface-emitting laser. Mode locking achieved by a semiconductor saturable absorber mirror generated 5.8-ps-long sech²-shaped pulses at an emission wavelength of 978 nm and a repetition rate of 1.88 GHz. Intracavity frequency doubling in a 5-mm-long lithium triborate crystal generated blue picosecond pulses with a spectral width of 0.15 nm and an average output power of up to 6 mW.     

Subkilohertz enhanced-power diode-laser spectrometer in the visible

We report on a high-performance diode-laser spectrometer operating near 657 nm with narrow linewidth (<0.6kHz) , enhanced power (as much as 40 mW), and low drift (<10Hz/s) . The spectrometer comprised an extended-cavity diode-laser frequency stabilized to a high-finesse optical resonator and a broad-area antireflection coated laser diode as an amplifier with a single-lobe emission pattern of good spatial purity. The spectrometer was used to record time-domain optical Ramsey spectra of laser-cooled Ca atoms with a resolution of 0.6 kHz.     

High precision line profile measurements on C acetylene using a near infrared frequency comb spectrometer

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.     

Erratum to: Allan-variance measurements of diode laser frequency-stabilized with a thin vapor cell

A novel method of stabilizing laser frequency that uses a sub-Doppler spectrum of atoms in a thin vapor cell has been developed. The extended-cavity diode laser is frequency-locked to a hyperfine component of the Cs D₂ line. The linewidth and the signal-to-noise ratio of the spectrum are systematically investigated to find a cell length that gives best long-term frequency stability. In the Allan-variance measurements on the beat note between two lasers thus stabilized, a frequency stability of 6.2×10^-11 is achieved at an averaging time of 5 s. PACS 42.62.Fi, 42.60.Lh, 39.30.+w     

The applications of the stark effect to the stabilization of the CO2 laser and the submillimeter laser

Methods based on the Stark effect are described for dither-free frequency stabilization of the optically pumped submm laser. The CO₂ pump laser was stabilized using a Stark Lamb dip signal of the submm lasant in an external Stark cell. An estimated frequency stability (f/f) better than ±1.4×10^–8, for one hour recording, was obtained by this method. The frequency of the submm laser was stabilized using the d.c. and a.c. Stark effects for a metal-dielectric rectangular waveguide laser. An estimated frequency stability of ±6×10^–8 was obtained for 119 m line of CH₃OH laser for one hour recording.     

Highly excited,normally inaccessible vibrational levels by sub-doppler modulated gain spectroscopy: The Na2 A1Σ+u state

Modulated gain spectroscopy is a sensitive, widely applicable, rovibronically state selective, sub-Doppler, triple resonance method for examining excited vibronic levels which are Franck—Condon inaccessible from thermally populated levels of the electronic ground state. A cw optically pumped molecular laser (OPL) prepares a steady-state population in a selected, vibrationally highly excited, rotation-vibration level of the electronic ground state (the lower level of the OPL transition). An intensity-modulated, single frequency dye laser excites part of this intracavity OPL-prepared population to the level of interest, thereby causing an increase in the OPL population inversion density, and, in turn, its output power. As the frequency of the dye laser is scanned, resonances are selectively detected by the appearance of modulation on the OPL output power; discrimination against dye laser excitations out of levels unconnected with the OPL is nearly perfect. Sub-Doppler (≈300 MHz FWHM) transitions are observed, thereby extending knowledge of the Na₂A¹Σ⁺_u state from v=44 to 62.     

A 657-nm narrow bandwidth interference filter-stabilized diode laser

We present a 657-nm external cavity diode laser (ECDL) system,where the output frequency is stabilized by a narrow-band high transmission interference filter.This novel diode laser system emits laser with an instantaneous linewidth of 7 kHz and a broadened linewidth of 432 kHz.     

Relative line intensity measurement in absorption spectra using a tunable diode laser at 1.6 μm: application to the determination of 13CO2/12CO2 isotope ratio

The measurement of relative intensities in CO₂ combination bands spectrum is performed using wavelength modulation spectroscopy (WMS) and a DFB (distributed feedback) diode laser operating at 1.6 μm. The diode laser is stabilized with an external Fabry–Pérot interferometer and absorption spectroscopy is performed in a multipass gas cell. A spectrum containing spectral lines of both ¹³CO₂ and ¹²CO₂ isotopic species is recorded. The variation of laser power during frequency scanning and the line shape are taken into account to accurately extract line intensities from experimental data. The isotopic concentration ratio is deduced from the intensity ratio. Both ratios are measured with an accuracy of about 0.5% in pure CO₂. Received: 9 June 2000 / Published online: 8 November 2000