ATMOSPHERIC REMOTE-SENSING REFERENCE DATA FROM GOME: PART 1. TEMPERATURE-DEPENDENT ABSORPTION CROSS-SECTIONS OF NO2 IN THE 231–794 nm RANGE

Absorption cross-sections of NO₂ between 231–794 nm have been measured in the 221–293 K temperature range, using the global ozone monitoring experiment (GOME) flight-model (FM) satellite spectrometer. The spectra have a resolution of about 0.2 nm below 400 nm and of about 0.3 nm above 400 nm. These are the first reference spectra of NO₂ covering at the same time the entire UV–visible–NIR spectral range and a broad range of relevant atmospheric temperatures. The new absorption cross-sections are important as accurate reference data for atmospheric remote-sensing of NO₂ and other minor trace gases.     

Investigation on infrared laser absorption spectroscopy measurement of acetylene trace quantities

A laser-based infrared spectrometer was developed for use in high-resolution spectroscopic analysis of trace gases in the atmosphere. Continuous wave, broadly tunable coherent infrared radiation was generated from 8 to 19 μm in a gallium selenide crystal by laser difference-frequency mixing. Measurements of acetylene trace concentrations using laser absorption spectroscopy are reported. The measurements have been performed by using the P(21), Q(11), and R(9) lines of the ν₅ band, respectively, in order to investigate optimal detection conditions: a trade-off choice between higher line absorption strength for sensitive detection and better spectral discrimination from lines overlapping for open path trace-gas monitoring applications. Minimum detectable concentrations are compared under different experimental conditions. The ν₅ R(9) line seems to be close to optimum in terms of absorption strength and freedom from spectral interference for spectroscopic detection of acetylene trace concentration at atmospheric pressure.     

Efficient ultrafast ultraviolet generation based on frequency doubling in short-period periodically-poled KTiOPO4 crystal

We have efficiently generated ultrafast coherent ultraviolet (UV) pulses based on second-harmonic generation in a periodically-poled KTiOPO₄ crystal with the period as short as 2.55 μm. The highest output power is measured to be 20.1 mW at 377.1 nm with the conversion efficiency of 15.6%. Such an output wavelength is probably among the shortest ones generated by forward second-harmonic generation in a periodically-poled KTiOPO₄ crystal.     

Polarization Performance of a New Spectrometer Based on a Multilayer-Coated Laminar Grating in the 150-190-eV Region

We constructed a polarization spectrometer based on a laminar grating applied with a Mo/B₄C multilayer coating. The multilayer-coated grating had been previously evaluated to have high polarizance at around 6.7 nm (184 eV) due to the Brewster reflection. In this study the polarization spectrometer was found to be usable in a range from 150 to 190 eV. Using it, σ and π emission spectra in the B 1s emission from a CrB₂ single crystal were separately recorded for the first time in the soft x-ray region. The spectral feature of the emission was reasonably well reproduced by the band calculation.     

Generation of narrow-band photon pairs for quantum memory

We demonstrate a narrow-band photon pair source on a rubidium (Rb) D₂ line (780 nm) using a periodical poled KTiOPO₄ crystal via a degenerate optical parametric oscillator far below threshold. The single longitudinal mode is selected by a spectral filtering cavity. The measured cross correlation between two photons shows that the generated photon has a linewidth of 21 MHz, which is comparable to the typical linewidth of an atomic-based quantum memory. The detected photon pair rate is about 2.7/s/MHz/mW. This system could be utilized for many applications in quantum information field, such as the storage and retrieval of a photon in Rb atomic ensemble and so on.     

Measurements of N2- and O2-broadening and shifting parameters of methane spectral lines in the 5550–6236 cm−1 region

The absorption spectra of mixtures of methane (CH₄) with N₂ and O₂ at different partial pressures of both CH₄ and buffer gases for three temperatures 240, 267, and 296 K have been recorded using the Bruker IFS 125 HR FTIR spectrometer in the 5550–6236 cm^?1 region. The multispectrum fitting procedure has been applied to these spectra to recover the spectral line parameters. The main goal of this procedure was the determination of the N₂- and O₂-broadening and shifting coefficients and the exponents of their temperature dependences. These parameters have been derived for 452 assigned lines with good values of the signal to noise ratio. The rotational dependence of the mean values of these parameters is discussed. The temperature dependence exponents were observed for both N₂ and O₂ buffer gases.     

Specific heat of KTiOPO4 within the 80–300 K range

The specific heat of the KTiOPO₄ crystal has been measured with a vacuum adiabatic calorimeter within the 80–300 K range. A peak-shaped anomaly in the specific heat indicating a phase transition has been revealed in the C _p (T) curve at T≅279 K. Numerical integration of smoothened experimental C _p (T) curves yielded the thermodynamic functions of KTiOPO₄, namely, the entropy, enthalpy, and reduced Gibbs energy. The entropy and enthalpy of the observed transition have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 497–498 (March 1999)     

Efficient Nd:YAG pumped mid-IR laser based on cascaded KTP and ZGP optical parametric oscillators and a ZGP parametric amplifier

Efficient conversion into the mid-IR of a low pulse-energy (2.5 mJ) Nd:YAG laser is achieved by cascaded KTiOPO₄ (KTP) and ZnGeP₂ (ZGP) optical parametric oscillators followed by a ZGP optical parametric amplifier. The first stage 2.13 μm degenerate KTP OPO uses four KTP crystals in a walk-off compensated geometry and an elliptical pump beam focal geometry to produce up to 2.2 W from 6.3 W incident. The 2.13 μm e-ray pumps a Type-I ZGP OPO, which produces 0.5 W of light in the 3.8-4.8 μm spectral region that in turn is amplified by a 2.13 μm o-ray pumped optical parametric amplifier generating 0.84 W with an M² of <2.     

A Raman system for multi-gas-species analysis in power transformer

We have built a complete Raman detection system for multi-trace-gas diagnosis, which is suitable for analyzing the dissolved gases in electric power system. In the system, a high-sensitivity CCD device connected to a spectrometer is used as the detection unit of the Raman system. A near-confocal cavity is used for improving the detection sensitivity of the system. In the effective spectral range of about 570–710 nm, Raman spectra of eight typical gases are achieved by using this Raman system. The detection limits for different gases have been obtained: 126 ppm for CO₂, 21 ppm for CH₄, 63 ppm for C₂H₄, 42 ppm for C₂H₂, 96.6 ppm for H₂. The detectability of the system satisfies the requirements of gas diagnosis in power transformer.     

Two-photon-excited fluorescence in KTiOPO4 polycrystals

The spectra of two-photon-excited fluorescence in KTiOPO₄ were obtained at room temperature. A coppervapor laser was used as a source of excitation light and provided two emission lines (λ=510.6 and 578.2 nm). The laser operated at a high pulse-repetition rate (～ 10⁴ Hz) and featured a peak power of about 10⁴ W, average power of 1 W, and pulse duration of 20 ns. The fluorescence spectra of crystalline KTiOPO₄ are compared with the resonance fluorescence spectra of KTiOPO₄ at 4.2 K. The measured decay time of fluorescence was found to be less than 16 ns. The efficiency of conversion of the laser radiation to fluorescence was about 10^?10 under saturation conditions.     

Measurement of the thermal expansion coefficients of ferroelectric crystals by a moiré interferometer

A moiré interferometer is used to measure the thermal expansion of two ferroelectric crystals, LiNbO₃ and KTiOPO₄. The crystal samples are patterned with a chromium reflective grating and used as a diffractive component in a reflective grating interferometer. The thermal expansion of all the three axes of congruent LiNbO₃ and of x and y axes of the flux-grown KTiOPO₄ were measured from room temperature to 200 °C. For this temperature range the thermal expansion coefficient has been modeled by a second-order polynomial and its coefficients have been estimated by accurate analysis of the resulting moiré fringe pattern.     

Intracavity laser spectroscopy of CH<Subscript>4</Subscript> and NH<Subscript>3</Subscript> gases by using a pulse-periodic Cr<Superscript>2+</Superscript>:ZnSe laser

The results of studying the characteristics of an intracavity laser spectrometer based on a polycrystalline Cr²⁺:ZnSe laser operating in a pulse-periodic regime with the pulse-repetition rate of 3 kHz and pulse duration of ∼50 ns are presented. Intracavity spectra of absorption of NH₃ and CH₄ gases in the vicinity of 2.35 μm are measured. The estimate of the spectrometer sensitivity is provided.     

Ground-based solar absorption studies for the Carbon Cycle science by Fourier Transform Spectroscopy (CC-FTS) mission

Carbon cycle science by Fourier transform spectroscopy (CC-FTS) is an advanced study for a future satellite mission. The goal of the mission is to obtain a better understanding of the carbon cycle in the Earth's atmosphere by monitoring total and partial columns of CO₂, CH₄, N₂O, and CO in the near infrared. CO₂, CH₄, and N₂O are important greenhouse gases, and CO is produced by incomplete combustion. The molecular O₂ column is also needed to obtain the effective optical path of the reflected sunlight and is used to normalize the column densities of the other gases. As part of this advanced study, ground-based Fourier transform spectra are used to evaluate the spectral region and resolution needed. Spectra in the 3950–7140 cm^?1 region with a spectral resolution of 0.0042 cm^?1 recorded at Kiruna (67.84°N, 20.41°E, and 419 m above sea level), Sweden, on 1 April 1998, were degraded to the resolutions of 0.01, 0.1, and 0.3 cm^?1. The effect of spectral resolution on the retrievals has been investigated with these four Kiruna spectra. To obtain further information on the spectral resolution, optical components and spectroscopic parameters required by the future mission, high-resolution solar absorption spectra between 2000 and 15000 cm^?1 were recorded using Fourier transform spectrometers at Kitt Peak (31.9°N, 111.6°W, and 2.1 km above sea level), Arizona, on 25 July 2005 and Waterloo (43.5°N, 80.6°W, and 0.3 km above sea level), Ontario, on 22 November 2006 with spectral resolutions of 0.01 and 0.1 cm^?1, respectively. Dry air volume mixing ratios (VMRs) of CO₂ and CH₄ were retrieved from these ground-based observations. The HITRAN 2004 spectroscopic parameters are used with the SFIT2 package for the spectral analysis. The measurement precisions for CO₂ and CH₄ total columns are better than 1.07% and 1.13%, respectively, for our observations. Based on these results, a Fourier transform spectrometer (maximum spectral resolution of 0.1 cm^?1 or 5 cm maximum optical path difference (MOPD)) operating between 2000 and 15000 cm^?1 is suggested as the primary instrument for the mission. Further progress in improving the atmospheric retrievals for CO₂, CH₄, and O₂ requires new laboratory measurements of the spectroscopic line parameters.     

Ultrahigh-resolution saturation spectroscopy using slow molecules in an external cell

We present the narrowest molecular lines so far recorded in the 10 µm spectral region. A linewidth of 80 Hz (HWHM) has been obtained for theP(39)A ₁ ³ (–) line of OsO₄, by selecting slow molecules (T _eff=0.6 K) in saturation spectroscopy at low laser fields (30 nW) and low pressures (2 × 10^–6 Torr). In these conditions, the contribution of the fast molecules is greatly reduced because of the finite size of the beam. This method, applied previously to methane at 3.39 µm, is used for the first time in an external cell and improves by a factor 8 the best resolution of our spectrometer. Heterodyne detection and double frequency modulation have been necessary to extract a signal at a contrast of only 10^–6. The physical ideas concerning this regime are described and a detailed analysis of the line shape is given.     

Spectral estimation of NMR relaxation

In this paper, spectral estimation of NMR relaxation is constructed as an extension of Fourier Transform (FT) theory as it is practiced in NMR or MRI, where multidimensional FT theory is used. nD NMR strives to separate overlapping resonances, so the treatment given here deals primarily with monoexponential decay. In the domain of real error, it is shown how optimal estimation based on prior knowledge can be derived. Assuming small Gaussian error, the estimation variance and bias are derived. Minimum bias and minimum variance are shown to be contradictory experimental design objectives. The analytical continuation of spectral estimation is constructed in an optimal manner. An important property of spectral estimation is that it is phase invariant. Hence, hypercomplex data storage is unnecessary. It is shown that, under reasonable assumptions, spectral estimation is unbiased in the context of complex error and its variance is reduced because the modulus of the whole signal is used. Because of phase invariance, the labor of phasing and any error due to imperfect phase can be avoided. A comparison of spectral estimation with nonlinear least squares (NLS) estimation is made analytically and with numerical examples. Compared to conventional sampling for NLS estimation, spectral estimation would typically provide estimation values of comparable precision in one-quarter to one-tenth of the spectrometer time when S/N is high. When S/N is low, the time saved can be used for signal averaging at the sampled points to give better precision. NLS typically provides one estimate at a time, whereas spectral estimation is inherently parallel. The frequency dimensions of conventional nD FT NMR may be denoted D₁, D₂, etc. As an extension of nD FT NMR, one can view spectral estimation of NMR relaxation as an extension into the zeroth dimension. In nD NMR, the information content of a spectrum can be extracted as a set of n-tuples (ω₁, … ω_n), corresponding to the peak maxima. Spectral estimation of NMR relaxation allows this information content to be extended to a set of (n + 1)-tuples (λ, ω₁, … ω_n), where λ is the relaxation rate.     

Mid-infrared ZGP OPO pumped by near-degenerate narrowband type-I PPKTP parametric oscillator

The total pulse energy of the signal and idler in a near-degenerate type-I periodically poled KTiOPO₄ (PPKTP) optical parametrical oscillator (OPO) was spectrally confined within a 2 nm spectral bandwidth at 2.13 μm. This was achieved by using a volume Bragg grating as the output coupler. Both the signal and the idler from the PPKTP OPO were then simultaneously used to pump a mid-infrared ZnGeP₂ (ZGP) OPO. The 2 nm bandwidth was narrower than the ZGP crystal acceptance bandwidth and, thus, made efficient conversion in the second OPO possible. A total slope efficiency of 10% from 1.06 μm to the 3.5–5 μm region was demonstrated, generating 250 μJ in the mid-IR with only 3.6 mJ of 1.06 μm pump energy. This corresponds to a Nd:YAG pump to mid-IR conversion efficiency of 7%. PACS 42.65.Yj; 42.72.Ai; 42.40.Eq     

Fourier transform measurements of SO2 absorption cross sections: II.: Temperature dependence in the 29 000–44 000 cm (227–345 nm) region

This paper is the second of a series that reports results on the measurements of the absorption cross section of SO₂ in the UV/visible region at high resolution and that investigates high temperatures in support to planetary applications. Absorption cross sections of SO₂ have been obtained in the 29 000–44 000 cm⁻¹ spectral range (227–345 nm) with a Fourier transform spectrometer at a resolution of 2 cm⁻¹ (0.4500 cm MOPD and boxcar apodisation). Pure SO₂ samples were used and measurements were performed at room temperature (298 K) as well as at 318, 338 and 358 K. Temperature effects in this spectral region are investigated and are favorably compared to existing studies in the literature. Comparison of the absorption cross section at room temperature shows good agreement in intensity with most of the literature data, but shows that most of the latter suffer from inaccurate wavelength scale definition. Moreover, literature data are often given only on restricted spectral intervals. Combined with the data described in the first part of this series of papers on SO₂, this new data set offers the considerable advantage of covering the large spectral interval extending from 24 000 to 44 000 cm⁻¹ (227–420 nm), at the four temperatures investigated.     

Radiation of the Plasma of a Transverse Discharge in a Helium–Krypton–Elegas Mixture

We carried out a spectroscopic investigation of the degradation of the active medium of a pulsed-periodic KrF emitter based on a He/Kr/SF₆ mixture (P = 10–150 kPa) with pumping by a transverse volumetric discharge. The plasma radiation spectra in the range 200–620 nm at different stages of degradation of the working mixture and the dynamics of the radiation of inert gases as well as of the products of decomposition of SF₆ molecules in the plasma are studied. It is shown that since the number of discharge pulses is 10⁴, rather effective formation of excited sulfur molecules is observed which decompose with emission in the spectral range 260–550 nm. This can be employed for developing a wideband lamp based on the system of KrF(B–X; D–X), S₂(B–X), and S₂(f–a) bands.     

基于差频中红外激光的痕量气体高分辨光谱检测研究

研究了基于差频光源的高分辨中红外激光光谱检测系统,差频中红外光源使用两台近红外半导体激光器作为种子光源,采用PPLN晶体作为非线性混频器件,结合准相位匹配技术实现了3.2~3.7 μm中红外相干光源输出,最大差频输出功率约为1 μW.以CH₄为例检验了系统的高分辨红外光谱检测特性,选择CH₄分子3 028.751 cm^-1 v₃基频吸收线作为分析谱线,10 cm光程的检测限为0.8 ppm.光谱数据分析表明,系统检测限主要受到标准具光学噪音的限制.     

Studies of vibrational relaxation in CDF3 following intense laser excitation

The V-T/R relaxation time of CDF₃ was measured studying the laser-induced infrared fluorescence emitted by vibrationally excited CDF₃. Following excitation by the 10R(12) line of a TEA CO₂ laser infrared fluorescence has been detected without spectral resolution in the 1100–700 cm^–1 range. A decay rate of 28.8 ms^–1 Torr^–1 was obtained for pure CDF₃ when it is excited with a fluence of 390 mJ/cm². Measurements have also been made in the presence of different bath gases (He, Ne, Ar, Xe, and CHF₃).