Ultrahigh-resolution spectrometer for the 5-microm wavelength region

We present an ultrahigh-resolution saturation spectrometer based on a line-tunable carbon monoxide laser near 60 THz (lambda = 5 microm). A spectral resolution of 14 kHz (Dnu/nu = 2.3 x 10(-10)) for CO fundamental-band transitions was achieved, which improves on earlier results by one order of magnitude. A frequency-locking scheme using tunable microwave sidebands provides tunability and absolute frequency control of the CO laser on the kilohertz. Transit-time broadening and pressure broadening of the observed transitions are significantly reduced by use of expanded laser beams in a 24-m absorption cell at pressures down to 0.0 1Pa. The new spectrometer is suitable for the study of saturation line shapes and the development of a new generation of frequency standards in the 60-THz region.     

Optically stabilized tunable diode-laser system for saturation spectroscopy

We present an optically stabilized lead-salt diode-laser system which is the nucleus of a very-high-resolution instrument for sub-Doppler molecular spectroscopy in the mid-infrared. By application of external optical feedback, we have narrowed the diode-laser linewidth by two orders of magnitude, yielding a spectral width of less than 200 kHz. The diode- laser frequency is stabilized and controlled via the external reflector by variable-frequency offset-locking the diode-laser to a CO laser frequency. This substantial improvement in the spectral properties enabled us to perform a Lamb-dip experiment on a carbonyl sulfide (OCS) absorption line near 1985 cm^–1. We were able to detect a saturated dispersion signal at low pressure (5 Pa) with a signal-to-noise ratio of several thousand. The present paper describes the unique features of the optically stabilized tunable diode-laser system and its use as a spectroscopic tool for sub-Doppler applications.     

10 kHz detection of CO2 at 4.5 microm by using tunable diode-laser-based difference-frequency generation

A compact, high-speed tunable, diode-laser-based mid-infrared (MIR) laser source has been developed for absorption spectroscopy of CO2 at rates up to 10 kHz. Radiation at 4.5 microm with a mode-hop-free tuning range of 80 GHz is generated by difference-frequency mixing the 860 nm output of a distributed-feedback diode laser with the 1064 nm output of a diode-pumped Nd:YAG laser in a periodically poled lithium niobate crystal. MIR absorption spectroscopy of CO2 with a detection limit of 44 ppm m at 10 kHz is demonstrated in a C2H4-air laminar diffusion flame and in the exhaust of a liquid-fueled model gas-turbine combustor.     

Narrow-line phase-locked quantum cascade laser in the 9.2 microm range

We report the operation of a 50 mW continuous-wave quantum cascade laser (QCL) in the 9.2 microm range, phase locked to a single-mode CO(2) laser with a tunable frequency offset. The wide free-running emission spectrum of the QCL (3-5 MHz) is strongly narrowed down to the kilohertz range, making it suitable for high-resolution molecular spectroscopy.     

Frequency and phase-lock control of a 3 THz quantum cascade laser

We have locked the frequency of a 3 THz quantum cascade laser (QCL) to that of a far-infrared gas laser with a tunable microwave offset frequency. The locked QCL line shape is essentially Gaussian, with linewidths of 65 and 141 kHz at the -3 and -10 dB levels, respectively. The lock condition can be maintained indefinitely, without requiring temperature or bias current regulation of the QCL other than that provided by the lock error signal. The result demonstrates that a terahertz QCL can be frequency controlled with 1-part-in-10(8) accuracy, which is a factor of 100 better than that needed for a local oscillator in a heterodyne receiver for atmospheric and astronomic spectroscopy.     

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.     

激光磁共振方法测量C0激光器的频率稳定度

对于频率不能连续调谐的CO激光器,采用一种新的激光频率稳定度的测量方法是以分子的激光磁共振谱线为基准,通过测量分子的激光磁共振谱线线宽及其变化,然后用阿仑方差进行数据处理,即得出了一系列时畴下的CO激光频率稳定度,并用计算机对取样时间从0.1秒-1000秒的频率稳定度进行幂函数拟合,获得了频畴的CO激光频率稳定度的表达式。     

激光磁共振方法测量CO激光器的频率稳定度

对于频率不能连续调谐的CO激光器,采用一种新的激光频率稳定度的测量方法是以分子的激光磁共振谱线为基准,通过测量分子的激光磁共振谱线线宽及其变化,然后用阿仑方差进行数据处理,即得出了一系列时畴下的CO激光频率稳定度,并用计算机对取样时间从0.1秒-1000秒的频率稳定度进行幂函数拟合,获得了频畴的CO激光频率稳定度的表达式。     

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     

Real-time detection of 13CH4 in ambient air by use of mid-infrared cavity leak-out spectroscopy

We report on spectroscopic real-time detection of (13)CH(4) in ambient air. Our measurements were carried out by means of cavity leak-out absorption spectroscopy employing a tunable cw laser in the mid-infrared spectral region near lambda = 3 mum. A CO laser in combination with tunable microwave sideband generation was used as the light source. Using a 50-cm-long ringdown cell with R = 99.98% mirrors, we achieved a detection limit of 290 parts in 10(12) (ppt) (13)CH(4) in ambient air (integration time, 100 s). The corresponding noise-equivalent absorption coefficient was 5 x 10(-9)/cm.     

A narrow-line-width external cavity quantum dot laser for high-resolution spectroscopy in the near-infrared and yellow spectral ranges

We demonstrate a diode laser system which is suitable for high-resolution spectroscopy in the 1.2 μm and yellow spectral ranges. It is based on a two-facet quantum dot chip in a Littrow-type external cavity configuration. The laser is tunable in the range 1125–1280 nm, with an output power of more than 200 mW, and exhibits a free-running line width of 200 kHz. Amplitude and frequency noise were characterized, including the dependence of the frequency noise on the cavity length. Frequency stabilization to a high-finesse reference cavity is demonstrated, whereby the line width was reduced to approx. 30 kHz. Using a femtosecond frequency comb, the residual frequency instability was determined and found to be below 300 Hz on the time scales 1–300 s. Yellow light (>3 mW) at 578 nm was generated by frequency doubling in an enhancement cavity containing a PPLN crystal. The source has potential application for precision spectroscopy of ultra-cold Yb atoms and cold molecular hydrogen ions.     

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).     

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 far-infrared spectroscopy extended to 9.1THz

We synthesized tunable far-infrared radiation at frequencies higher than 9 THz (300 cm (-1)) by mixing CO(2) laser, (15)NH(3) laser, and microwave radiation in a W-Co metal-insulator-metal diode. We used this farinfrared radiation to accurately measure torsion-rotation transitions of CH(3)OH in the 8-9-THz region. We also measured the frequency of the aP(7, 3) (15)NH(3) laser transition.     

Frequency noise reduction in erbium-doped fiber distributed-feedback lasers by electronic feedback

A novel technique for suppressing frequency noise in an erbium-doped fiber distributed-feedback laser incorporated into a master-oscillator-power-amplifier configuration by an electronic feedback technique is presented. The frequency noise is suppressed by locking of the laser emission to a fiber interferometer. The frequency noise spectral density of the laser is reduced by as much as 20 dB over the frequency range 1 Hz-10 kHz to 1.5 Hz/Hz(1/2) +/-25% at 1 kHz with a relative intensity noise spectral density below -120 dB/Hz over the frequency range 10 Hz-1 kHz. These lasers will have applications as sources for fiber-optic interferometry, high-resolution spectroscopy, and high-bandwidth communications.     

SG-DBR半导体激光器的光谱技术研究

可调谐半导体激光吸收光谱作为一种高灵敏度、高选择性、非侵入的痕量气体实时检测技术,已在大气监测、工业控制等方面得到广泛应用。采用一种新型宽带可调谐的SG-DBR半导体激光器(可调谐范围1 520~1 570 nm)作光源,并通过自编程序对该激光器设定了18个通道,输出波长分别对应CO,CO₂以及H₂O的吸收谱线中心位置,设计和构建了一个基于近红外可调谐半导体激光吸收光谱的多组分气体光谱测量系统,描述了相关的光学系统设置,结合波长调制(wm)的二次谐波技术测量其中14个通道(分别对应CO和CO₂的吸收谱线)的吸收光谱,系统获得的CO和CO₂峰值吸收探测极限能够达到10-5。实验结果验证了SG-DBR激光器在波长调制吸收光谱多组分气体检测领域的可行性。在实际应用过程中使用单个SG-DBR激光器可以实现多组分气体的同时测量,有效降低设备成本和系统复杂性。     

High Resolution Linear Laser Absorption Spectroscopy-Review

Throughout the optical spectral region from the visible to the far infrared, lasers have in recent years become increasingly important for high resolution spectroscopy in both laboratory measurements and practical applications. In general, laser sources can be classified into broadly tunable and discretely tunable types with the gas laser belonging to the latter. While gas lasers oscillate in narrow lines scattered throughout the optical spectral region, broadly tunable laser emissions cover a range of wide spectral region depending on the lasing media and operating characteristics. For example, the Pb-salt diode laser covers the spectral region from about 2.6 to 30 μm, the dye laser from 400 to 700 nm, the color center laser from 0.8 to 3.3 μm, the difference frequency spectrometer from 2.2 to 4.2 μm and the spin-flip Raman laser from 5.2 to 6.0 μm. The very limited tunability of the gas laser can sometimes be extended to many times the Doppler width by Zeeman tuning the gain medium or pressure broadening the gain profile or electrooptically modulating the laser output frequency with a tunable microwave source.     

Generation of stable and tunable optical frequency linked to a radio frequency by use of a high finesse cavity and its application in absorption spectroscopy

The laser frequency could be linked to an radio frequency through an external cavity by the combination of Pound-Drever-Hall and Devoe-Brewer locking techniques. A stable and tunable optical frequency at wavelength of 1.5 μm obtained by a cavity with high finesse of 96000 and a fiber laser has been demonstrated, calibrated by a commercial optical frequency comb. The locking performances have been analyzed by in-loop and out-loop noises, indicating that the absolute frequency instability could be down to 50 kHz over 1 s and keep to less than 110 kHz over 2.5 h. Then, the application of this stabilized laser to the direct absorption spectroscopy has been performed. With the help of balanced detection, the detection sensitivity, in terms of optical density, can reach to 9.4×10^-6.     

All-fiber wavelength-swept laser near 2 μm

We report, for the first time to our knowledge, an all-fiber wavelength-swept Tm-doped laser based on a fiber Fabry-Perot tunable filter in the 2 μm spectral region. The laser wavelength can be continuously tuned over 200 nm from 1840 to 2040 nm in a short period of time. The demonstrated tuning speed was 12.5 μm/s with a tuning efficiency of 17.5 nm/V. The spectral linewidth of the laser was measured to be approximately 300 MHz or 0.01 cm(-1). This kind of laser can find potential applications in both high-resolution laser spectroscopy and tunable mid-IR generation via nonlinear frequency conversion.