Tunable diode laser spectroscopy of helium clusters

Helium clusters, He_N-X, containing a probe molecule, X, are studied by infrared spectroscopy for the size range N≈1∼100. Spectra are observed using a supersonic jet expansion and a tunable diode laser source operating in a rapid-scan (sweep integration) mode. The pulsed jet uses a dilute gas mixture of the probe molecule in helium, with relatively high backing pressures (5–50 bar), and a cooled (80–295 K) nozzle. Sensitivity is enhanced by multi-passing the laser beam through the jet with a toroidal mirror system. The clusters are larger than van der Waals dimers and trimers, but smaller than those encountered in the field of helium nanodroplets (N≈10³–10⁵). Furthermore, individual cluster sizes are resolved here, but not with nanodroplets, and infrared absorption is detected directly (change in transmitted laser intensity), rather than indirectly (change in cluster fragmentation). Trends in the spectra are described for five probe molecules, X=CO, SiH₄, OCS, N₂O, and CO₂. Superfluid effects dominate for clusters larger than N≈8. Notable results include the unexpected observation of broad oscillations in the effective rotational constants as a function of cluster size. PACS 33.20.Ea; 34.30.+h; 36.40.Mr; 42.55.Px     

Tunable diode laser spectroscopy with optimum wavelength scanning

In this paper, an improved signal processing method is given, aiming at a sensitivity improvement of the tunable diode laser spectroscopy based gas concentration measurement. The gas absorption spectrum is probed with an optimum wavelength scanning function which enables a more efficient curve fit than for traditional linear wavelength scanning. The wavelength scanning function is determined to be optimum, in the sense that the variance of the estimated concentration noise is minimized. This optimum scanning function depends on the signal model used in curve fitting. Several models including interfering gas spectra and etalon fringes are examined. Compared to the gas absorption spectrum recorded by ramping the wavelength linearly, the optimum spectrum waveforms have a cascade structure, which means that the optimum scanning preferably samples important points of the spectrum. The new method theoretically enables a factor of ～2 improvement on detection sensitivity of the estimated concentration. Furthermore, direct spectroscopy is superior to second harmonic detection, because the concentration noise can be an additional factor of ～2 lower.     

Infrared diode laser spectroscopy of nitric oxide

The lineshapes of the rotational-vibrational lines $\text{R(}\text{17}\text{2}\text{)}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{, R(}\text{19}\text{2}\text{)}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, and $\text{R(}\text{19}\text{2}\text{)}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ of nitric oxide were measured using a PbS_0.61Se_0.39 diode laser. These lineshapes were measured for a NO/N₂ mixture and an NO/H₂O/N₂ mixture at atmospheric pressure. This is the first high resolution spectroscopic mearuement of the rotational-vibrational lineshape of NO broadened by H₂O. The effect of the H₂O is to broaden the lines, increasing the halfwidth by up to 30% for a 10% volume concentration of H₂O. For the case of NO broadened by N₂, the measured linestrengths for the $\text{R(}\text{19}\text{2}\text{)}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{R(}\text{17}\text{2}\text{)}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ lines are within 2% of previously published values.     

用于氢原子1S-2S光谱研究的超窄线宽243nm半导体激光

近年来,随着激光稳频技术的发展,人们可以把972nm的外腔半导体激光器产生的激光进行放大和四倍频,并将它锁定到超稳定的光学法布里-珀罗腔上,从而可以获得Hz量级超窄线宽的243nm激光,使其成为研究氢原子1S-2S双光子跃迁的有力工具.文章主要介绍了超稳腔、四倍频243nm半导体激光器的研究进展及在氢原子1S-2S双光子跃迁精密光谱研究中的应用.     

用于氢原子1S-2S光谱研究的超窄线宽243nm半导体激光

近年来,随着激光稳频技术的发展,人们可以把972nm的外腔半导体激光器产生的激光进行放大和四倍频,并将它锁定到超稳定的光学法布里-珀罗腔上,从而可以获得Hz量级超窄线宽的243nm激光,使其成为研究氢原子1S-2S双光子跃迁的有力工具.文章主要介绍了超稳腔、四倍频243nm半导体激光器的研究进展及在氢原子1S-2S双光子跃迁精密光谱研究中的应用.     

Room-temperature diode laser photoacoustic spectroscopy near 2.3 μm

We describe results obtained with a new type of GaSb-based semiconductor distributed feedback multiple quantum well lasers operating in continuous-wave mode at room temperature and above. The lasers show good tunability both by the excitation current and the temperature from -1 °C to +60 °C (4170 to 4360 cm^-1). They operate in a high-power (more than 1 mW) single-mode regime and can be tuned without hopping in the methane and ammonia absorption spectral range of up to 15 cm^-1. Application to photoacoustic detection is also described. Potential use lies in the areas of analytical chemistry of gases and in atmospheric pollution research. PACS 82.80.Kq; 42.62.Fi; 42.55.Px     

可调谐半导体激光吸收光谱法对高温甲烷的测量研究

可调谐半导体激光吸收光谱(TDLAS)是一种具有高灵敏度、高分辨率、快速检测特点的气体检测技术,已广泛用于大气中多种痕量气体的检测以及地面的痕量气体和气体泄漏的检测。研究了利用TDLAS技术测量高温下甲烷浓度的实验方法,使用可加热的静态吸收池对在1653.72nm波长附近R(3)支转动跃迁的吸收线进行了测量,并计算了吸收线强。分别在相同温度不同浓度和相同浓度不同温度的两种条件下进行了实验。结果表明,利用直接吸收的方法,在实验室可以得到370K时的最小可探测限为100×10-6,500K时的最小可探测限为245×10-6(吸收池长度为10cm),可以应用在燃烧控制及喷焰气体浓度测量等多个领域。     

A simple extended cavity diode laser for spectroscopy around 640 nm

We report on an extended cavity diode laser for operation near 640 nm. The laser is continuously tunable in 10 GHz ranges with a maximum output power of 3 mW. The laser system has been constructed using off-the-shelf optoelectronic components and easily machinable mechanical parts. The constructed system has been used to study the saturated absorption of the closed 1s₅–2p₉ neon transition in a radio-frequency discharge that can be maintained at neon pressures down to 10⁻² Pa.     

Methane diode laser overtone spectroscopy at 840 nm

Overtone absorption lines of ¹²CH₄ have been examined by using a tunable diode laser (TDL) spectrometer in the region around (840 nm) where the combination overtone band ν₁+3ν₃ lies. The spectrometer sources are commercially available heterostructure GaAlAs TDLs operating in the “free-running” mode, which allowed the detection of the line positions within . The wavelength modulation spectroscopy (WMS) and the second harmonic detection technique permitted the measurements of minimum absorbances of the order of ?5×10^-6. This allowed to observe the weakest lines of the band with absorption cross-sections of the order of ?2×10^-25 cm²/molecule or /amagat. For some of them self-, air-, He- and H₂-broadening coefficients have been obtained at room temperature.     

Simultaneous detection of CO and CO2 at elevated temperatures using tunable diode laser absorption spectroscopy near 1570 nm

Simultaneous measurements of CO and CO₂ at elevated temperatures are demonstrated using a single semiconductor distributed-feedback (DFB) laser near 1570 nm. Wavelength modulation spectroscopy with second-harmonic detection is used to improve the detection sensitivity and accuracy. A proper line pair near 6368.086 and 6368.330 cm^?1 is selected using some line-selection criterions for the target temperature range of 300–1000 K. Normalization of the 2f signal with the 1f signal magnitude is used to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling. The CO and CO₂ concentrations measurements are within 2.21% and 2.55% of the expected values over the tested temperature range of 300–1000 K. The minimum detectable concentrations of CO and CO₂ at 1000 K are 80 ppm m and 153 ppm m, respectively.     

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.     

Diode laser spectroscopy of ethylene overtones at 830 nm

A tunable diode laser spectrometer has been employed to examine the 3rd overtone absorption lines of C₂H₄ at around 12 000 cm⁻¹ (830 nm). The spectrometer sources are heterostructure AlGaAs tunable diode lasers operating “free-running”. By the aid of the wavelength modulation spectroscopy with the second harmonic detection technique and a Herriott type multipass cell 63 very weak absorption lines have been observed for the first time, with cross sections as low as 2 × 10⁻²⁶ cm²/mol, equivalent to . The self-broadening coefficient has been measured for one of the most intense lines.     

Infrared diode laser spectroscopy

Three types of lasers (double-heterostructure 66 K InAsSb/InAsSbP laser diode, room temperature, multi quantum wells with distributed feedback (MQW with DFB) (GaInAsSb/AlGaAsSb based) diode laser and vertical cavity surface emitting lasers (VCSELs) (GaSb based) have been characterized using Fourier transform emission spectroscopy and compared. The photoacoustic technique was employed to determine the detection limit of formaldehyde (less than 1 ppmV) for the strongest absorption line of the v₃ + v₅ band in the emission region of the GaInAsSb/AlGaAsSb diode laser. The detection limit (less than 10 ppbV) of formaldehyde was achieved in the 2820 cm⁻¹ spectral range in case of InAsSb/InAsSbP laser (fundamental bands of v₁, v₅). Laser sensitive detection (laser absorption together with high resolution Fourier transform infrared technique including direct laser linewidth measurement, infrared photoacoustic detection of neutral molecules (methane, form-aldehyde) is discussed.     

Tunable diode laser spectroscopy of NO2 at 6.2 μm

High-resolution spectra in the ν₃ region of ¹⁴N¹⁶O₂ were measured by using a diode laser as a source to a vacuum grating spectrometer. Spin-rotation constants for the ν₃ and ν₂ + ν₃ states were determined from the observed data. Absolute strengths and transition moments for the ν₃ and ν₂ + ν₃ ? ν₂ bands were derived from the observed line strengths at 296 K. Self-broadened and N₂-broadened half-widths were measured for several of the ν₃ band lines at room temperature. Temperature-dependent half-widths for two N₂-broadened ν₃ lines are also reported.     

Tunable diode laser measurements of the band strength and collision halfwidths of nitric oxide

The strength of the fundamental absorption band of nitric oxide at 5.3 μm and collision halfwidths of nitric oxide lines broadened by nitrogen, argon, and combustion gases were measured in absorption cell, flat flame and shock tube experiments using a tunable diode laser. Room temperature absorption measurements were made in an absorption cell filled with NO/N₂ or NO/Ar mixtures or with probe-extracted combustion gases. High temperature (to 2500 K) absorption measurements were performed for NO in N₂ and NO in Ar using a shock tube, and for NO in combustion gases using a flat flame burner.Absorption measurements were made on lines from 1860–1925 cm^?1, ($\text{Ω=}\text{1}\text{2}$ and $\text{3}\text{2}$,P($\text{5}\text{2}$-R ($\text{29}\text{2}$)) resulting in a band strength of 123±8 cm^-2 atm^?1 at 273.2 K. Collision halfwidth dependencies for each broadening species were examined as a function of rotational quantum number and temperature.     

Tunable laser diode study of the ν3 band of SiF4 near 9.7 μm

Doppler-limited tunable-diode laser spectra of the stretching fundamental ν₃ of ²⁸SiF₄ near 1031 cm^?1 were analyzed and the spectroscopic constants determined. The ν₃ vibrational dipole moment derivative was determined for several rovibrational lines.     

External cavity diode laser around 657 nm

Operating a laser diode in an external cavity, which provides frequency-selective feedback, is a very effective method to tune the laser frequency to a range far from its free running frequency. For the Ca atomic Ramsey spectroscopy experiment, we have constructed a 657-nm laser system based on the Littman-Metcalf configuration with a 660-nm commercial laser diode. Continuously 10-GHz tuning range was achieved with about 100-kHz spectral linewidth, measured with beat-note spectrum of two identical laser systems.     

Mid-infrared diode laser spectroscopy of SO

The absorption spectrum of the fundamental band of SO⁺ (X²Π) has been recorded using a mid-infrared tunable diode laser spectrometer with the velocity modulation technique in an AC glow discharge of He/SO₂. Forty-two lines of SO⁺ were identified in the spectral range of 1230-1330 cm⁻¹. The observed rovibrational transitions together with the microwave data published previously were fitted to a standard effective Hamiltonian for ²Π states. Molecular constants for the ground and υ = 1 vibrational states were derived. The band origin was determined to be 1291.5299(27) cm⁻¹.     

Xenon saturation spectroscopy by diode laser

Summary Commercial diode lasers have emission wavelengths which overlap some absorption xenon lines. We have performed saturation spectroscopy of four lines, leaving from the first excited atomic configuration 5p ⁵6s, at 823.16 nm, at 828.01 nm, at 834.68 nm and at 820.63 nm, in a weak glow discharge of natural xenon. Natural xenon is a mixture of several isotopes, and the two even isotopes, Xe(129) (26% of relative abundance) and Xe(131) (21%), have a nuclear spin (1/2 and 3/2, respectively) that produces a hyperfine structure. The complex resulting spectra have been resolved and the results are compared with the available literature data. The authors of this paper have agreed to not receive the proofs for correction.     

高功率、高效率808nm半导体激光器阵列

通过设计高效率808 nm非对称宽波导外延结构,减少P型波导层和包层的自由载流子光吸收,实现腔内光吸收损耗为0.63 cm~(-1).制备的808 nm半导体激光器阵列在室温25?C下,实现驱动电流135 A,工作电压1.76 V,连续输出功率大于150 W,斜率效率高达1.25 W/A,中心波长809.3 nm,器件最高电光转换效率为65.5%,这是目前国内报道的808 nm半导体激光器阵列的最高电光转换效率,达到国际同类器件最好水平.