Laser Heterodyne Spectrometer Research Articles

Mid-infrared laser heterodyne spectrometer byhollow optical fiber and its newly designedcoupler.

We demonstrate a newly designed, to the best of our knowledge, hollow optical fiber coupler for a mid-infrared (IR) laser heterodyne spectrometer that mixes a targeted light source with local oscillator (LO) light. The hollow fiber achieves a high transmission efficiency ∼80-90%/m, not only for a coherent laser source but also for an incoherent blackbody source. The branching characteristics of the hollow optical fiber coupler are found to be strongly dependent on the curvature and length of the input port fiber, indicating that the branching ratio could be designed independently for each input port. Our laboratory measurements demonstrate that the branching ratio and transmittance of the coupler can be varied by coupling a flexible fiber to the input side owing to the excitation of higher-order modes. Using the hollow optical fiber coupler, a high-resolution emission spectrum of the quantum cascade laser at 10.3µm for our C O 2 laser-based heterodyne spectrometer is successfully achieved. Using a C O 2 laser with a hollow fiber and a blackbody as a direct input signal in free space, we obtain the sensitivity performance of IR laser heterodyne spectrometer as 2000-3000K of the system noise temperature. This suggests that the transmission of a coherent LO laser through a hollow optical fiber has almost the same sensitivity for the IR heterodyne detection as that without a fiber.

Simulation of wind field detection by laser heterodyne spectrometer

The middle- and upper- atmosphere wind field are important parameters that characterize the middle- and upper-atmosphere environment, respectively. The detection of the middle- and the upper-atmosphere wind field are of great significance in the civil field and military field. Laser heterodyne spectroscopy technology is a passive remote sensing detection technology with high spectral resolution and sensitivity, and has developed rapidly in recent years. The laser heterodyne spectrometer that takes laser heterodyne spectroscopy technology as its core, is developed due to its small size, light weight and stable structure. The verification of the ground-based wind field detection performance of the laser heterodyne spectrometer is a key part of its application to satellites. In this paper, a wind speed simulation device is built in a laboratory environment to achieve a wind speed change from 0 m/s to 25 m/s in a wind field. A laser heterodyne spectrometer with a spectral resolution of 0.003 cm<sup>–1</sup> is used to measure the CH<sub>4</sub> absorption spectrum without and with a wind field for different wind speeds, the resolution of measuring wind speed is 3 m/s. For relative and absolute calibration of the distributed feedback laser (DL) frequency, an interference fiber with a free dispersion range <i>D</i><sup>*</sup> = 0.01167 cm<sup>–1</sup>, a wavemeter and a reference cell is used. The experimental results effectively verify the wind measurement performance of the laser heterodyne spectrometer and prove the possibility of using the laser heterodyne spectrometer to measure the atmospheric wind field.

Measurement and Concentration Inversion of Ozone in Golmud by Laser Heterodyne Spectrometer

Measurement and Concentration Inversion of Ozone in Golmud by Laser Heterodyne Spectrometer

Atmospheric CO<sub>2</sub> column concentration retrieval based on high resolution laser heterodyne spectra and evaluation method of system measuring error

In this paper, a near-infrared laser heterodyne spectrometer developed by the laboratory is used to investigate the inversion of greenhouse gas column concentration and approximately evaluate the system measurement errors based on the optimal estimation algorithm. Firstly, the spectral database and the calculation results from the reference forward model are compared with the ground-based FTIR results, thereby selecting the detection window, the corresponding laser and detector. Secondly, the optimal estimation concentration inversion algorithm based on the reference forward model is established, and the Levenberg-Marquardt (LM) iterative method is adopted to realize the inversion of the concentration and vertical distribution profile of atmospheric CO<sub>2</sub> column in the whole layer, and the long-term observation comparative experiment is carried out to verify the feasibility of this algorithm. Finally, by simulating the selected detection window spectrum in different white noise, the approximate corresponding relationship between the system signal-noise-ratio (SNR) and CO<sub>2</sub> column concentration measuring error is eventually obtained. This research is an indispensable theoretical calculation part of the detection system and will conduce to improving the application of laser heterodyne technology in atmospheric observations.

Instrument line shape function of laser heterodyne spectrometer

Laser heterodyne is a kind of technique based on coherent detection with high sensitivity and spectral resolution for spectrum measurements. For these reasons, it has been widely used in many research fields, such as trace gases’ detection of earth’s or terrestrial planets’ atmosphere. However, when the laser heterodyne spectrometer is used for measuring the spectrum, the instrument line shape (ILS) function usually smooth the spectrum, which affects the inversion results of the gas column density. In previous researches, the radio frequency (RF) filter response function was usually used as the ILS, but recent studies indicated that the ILS without consideration of the influence of lock-in amplifier was not precise enough. In order to obtain the ILS function of the laser heterodyne spectrometer, the main factors which influence the ILS are analyzed, including the RF filter bandwidth, integral time and low-pass filter of lock-in amplifier, and the process is based on the principle of laser heterodyne technology and the flow of heterodyne signal processing. The presented ILS is the convolution of RF filter, wavelength variation in the integral time and the low-pass filter. In addition, for testing the effectiveness of the ILS in this paper, the laser heterodyne spectrometer which was built in our laboratory is used for the multiple measurement of the absorption of water vapor and methane in the band of 3.53 <inline-formula><tex-math id="M2">\begin{document}${\text{μm}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181620_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181620_M2.png"/></alternatives></inline-formula> and the column densities are retrieved with different ILS. The experimental results show that the actual resolutions of the laser heterodyne spectrometer are about 0.005 cm<sup>–1</sup> and 0.025 cm<sup>–1</sup> when the integral times are set to be 10 ms and 100 ms respectively. Furthermore, the RF filter response function and the ILS function presented in the paper are respectively used in the procedure of water vapor and methane inversion. The results show that when the ILS function used for the retrieval, the sum of squared residual reduces about 16% and the residuals at the peak of methane absorption reduces almost 100% compared with the scenario when using the RF filter function. Above all, the comprehensive analysis of the laser heterodyne spectrometer in this paper indicates that the ILS function is more precise than pioneering studies and this work will be helpful for retrieving the precise profiles of trace gases.

Measurements of O 3 and N 2O in Alaska with a tunable diode laser heterodyne spectrometer

Total column amounts and vertical profiles of O 3 and N 2O have been observed with a tunable diode laser heterodyne spectrometer at Poker Flat, Alaska (65°N, 147°W) from February 17 to March 10, 1997 and from April 1 to May 8, 1997. Poker Flat was usually located outside the polar vortex during these early to late spring periods, but sometimes located inside the vortex as in the cases of March 10 and April 17. The correlation plots between O 3 and N 2O show that O 3 amount in the lower stratosphere was chemically depleted inside the polar vortex.

Sub-Doppler Heterodyne Frequency Measurements on Isotopic Species of Carbon Monoxide and Improved Frequencies for Secondary Standards near 60 THz

We report a series of sub-Doppler frequency measurements on four isotopic species of carbon monoxide (CO). The observed rovibrational transitions in the electronic ground stateX1Σ+of CO cover the infrared spectral region between 58.5 and 63.3 THz (1951–2112 cm−1). The CO laser heterodyne spectrometer with optional microwave sideband generation utilizes heterodyne techniques to measure the frequency of a CO laser, which is locked to saturation signals of CO transitions at low pressure, against combination frequencies of two saturation-stabilized CO2lasers. Frequencies for ground state transitions of three isotopic species of CO are determined with very high accuracy, including the first heterodyne frequency measurements in the fundamental band of13C18O. A systematic correction (between −4 and −13 kHz) of our earlier published CO sub-Doppler frequencies became possible with the 1994 improvement of the CO2laser standard frequency values. The updated frequencies together with the new measurements exhibit absolute frequency uncertainties around 20 kHz (Δν/ν ≈ 3 × 10−10). They are the most accurate frequencies of CO ground state transitions suitable for secondary frequency standards and calibration of infrared spectrometers in the 60-THz spectral region.

Absolute intensity measurement of aυ 3 ozone line at saturated vapor pressure with a laser heterodyne spectrometer

A laser heterodyne spectrometer had been developed ten years ago at the Reims GSMA laboratory (France). This high-resolution spectrometer is used for the first time to carry out absolute intensity measurements. This type of spectrometer exhibits a well-known and very narrow apparatus function (0.0002cm−1). Spectra of ozone were recorded at saturated vapor pressures of ozone at liquid argon temperature and the low pressures were checked by ultraviolet absorption. Results concerning theυ3 (10, 5, 6) → (9, 5, 5) ozone line are presented.

Heterodyne Spectroscopy of the 63 μm O I Line in M42

We have used a laser heterodyne spectrometer to resolve the emission-line profile of the 63 μm3P1-3P2 fine-structure transition of O I at two locations in M42. Comparison of the peak antenna temperature with that of the 158 μm C II fine-structure line shows that the gas kinetic temperature in the photodissociation region (PDR) near θ1C is 175-220 K, the density is greater than 2 × 105 cm-3, and the hydrogen column density is about 1.5 × 1022 cm-2. A somewhat lower temperature and column density are found in the IRc2 region, most likely reflecting the smaller UV flux. The observed width of the O I line is 6.8 km s-1 (FWHM) at θ1C, which is slightly broadened over the intrinsic line width by optical depth effects. No other significant differences between the O I and C II line profiles are seen, which shows that the narrow emission from both neutral atomic oxygen and ionized carbon comes from the PDR. The O I data do not rule out the possibility of weak broad-velocity emission from shock-excited gas at IRc2, but the C II data show no such effect, as expected from nonionizing shock models.

Precise and absolute intensity measurement of the ν 3 (10, 5, 6)-(9, 5, 5) Ozone line with high resolution spectrometers

A new Tunable Diode Laser Spectrometer (T.D.L.S.) has been developed at the Reims G.S.M.A. Laboratory (France). In order to check the preliminary results obtained by this apparatus, a comparison between spectra obtained with T.D.L.S. and a Laser Heterodyne Spectrometer (L.H.S.) has been made. The study of a spectral ozone line by these two high resolution techniques is presented and the results are compared and discussed.

Ground‐based remote sensing of methane height profiles with a tunable diode laser heterodyne spectrometer

Height distributions of methane in the troposphere and stratosphere were derived from high resolution absorption spectra observed with a ground‐based tunable diode laser heterodyne spectrometer. The center wavenumber of the measured methane absorption line is 1223.1561 cm−1. In the retrieval of methane height profiles, a volume mixing ratio of methane was assumed to have a constant value in the troposphere and to decrease with a constant rate in the stratosphere. The tropospheric mixing ratio and the decreasing rate in stratosphere were derived to be 1.7±0.1 ppmv and −0.06 ppmv/km, respectively, for measurements at Tsukuba (36.0°N, 140.1°E) on December 17 and 20, 1991.

Column measurements of stratospheric trace species over Åre, Sweden in the winter of 1991–1992

Total vertical column amounts of stratospheric HCl, HF, ClONO2 and HNO3 are reported from high resolution infrared solar absorption spectra taken during the European Arctic Stratospheric Ozone Experiment in the winter of 1991–1992. These ground based measurements were made near Åre in North Sweden (63.4°N, 13.1°E) at an altitude of 800 m using a Fourier transform spectrometer and tunable diode laser heterodyne spectrometer. On 9th January 1992 the HCl vertical column dropped to 1.2 × 1015 molecules cm−2 from a November average of 3.6 × 1015 molecules cm−2. The HCl drop occurred at a time when Åre was below the polar vortex and the lower stratospheric ClO column above Åre was elevated to ∼2 × 1015 molecules cm−2 as measured by the Microwave Limb Sounder experiment on the Upper Atmosphere Research Satellite. These measurements indicate conversion of lower stratospheric chlorine from reservoir to chemically active forms.

Laboratory and atmospheric ozone measurements by infrared-laser heterodyne detection

A laser-heterodyne spectrometer with a resolution of 5 MHz has been used to record laboratory and atmospheric spectra of ozone in the 10 μm region. The necessary parameters to process the atmospheric spectra are defined through the laboratory spectra. The atmospheric ozone concentration versus altitude and the atmospheric CO2 volume mixing ratio have been obtained.

Comparison of ozone profiles from ground‐based laser heterodyne spectrometer and ozonesonde measurements

To evaluate the accuracy of the vertical profiles and total column densities obtained by a ground‐based laser heterodyne spectrometer, four ozonesonde measurements were performed at Sendai, Japan in November 28–December 2, 1989. The altitudes, widths and maximum values of the peaks in the ozone vertical distributions obtained by the laser heterodyne observations agreed well with those obtained by the ozonesondes, though the fine structures in the ozone distributions can not be resolved by this remote sensing method. The total column densities and their temporal variation obtained by the laser heterodyne observations were also consistent with the ozonesonde data and the Dobson spectrophotometer data at Sapporo and Tateno.

Laser heterodyne spectrometer using a liquid nitrogen cooled tunable diode laser for remote measurements of atmospheric O_3 and N_2O

A laser heterodyne spectrometer with a tunable diode laser as a local oscillator has been developed for ultrahigh resolution IR spectroscopy of the absorption spectra of trace constituents in the earth's atmosphere. The spectral resolution and SNR of the developed system are 0.0013 cm(-1) and ~500, respectively. This performance is sufficient for retrieving the altitude profile of atmospheric O(3) through an inversion method. The altitude profile of atmospheric N(2)O has been also obtained for the first time with this laser heterodyne technique. The high resolution and high SNR have been accomplished by adopting a tunable diode laser which operates at liquid nitrogen temperature and by developing an effective technique to eliminate the optical feedback of the local oscillator laser beam.

波長可変半導体レーザーヘテロダイン分光計による大気オゾン及び一酸化二窒素高度分布の遠隔計測

波長可変半導体レーザーヘテロダイン分光計による大気オゾン及び一酸化二窒素高度分布の遠隔計測

Stratospheric ozone measurements with a tunable diode laser heterodyne spectrometer

The vertical mixing ratio profiles of stratospheric ozone have been obtained through an inversion of the ozone absorption spectra in the 9µm band measured by a tunable diode laser heterodyne spectrometer with a spectral resolution of 80 MHz (0.0027 cm−1). The observations were made on June 22, 1988, and on August 22, 1988, at Sendai (38°15′N, 140°50′E), Japan. The values of the signal‐to‐noise ratio of the observed spectra are 190 for the data of June 22, 1988, and 300 for the data of August 22, 1988. The accuracy of the mixing ratio and the vertical resolution is examined. The results are compared with data obtained by Dobson spectrophotometers and an ozonesonde.

Infrared studies of hydrocarbons on Jupiter

Measurements of ethane and ethylene emission lines on Jupiter were made using a dual CO 2 laser heterodyne spectrometer. Retrieved abundances of these products of methane photolysis in the Jovian stratosphere were used to test existing photochemical models and to investigate a localized “hot spot” in the northern auroral region.

Heterodyne spectroscopy of the J = 17-16 CO line in Orion.

We have obtained high-resolution spectra of the 153 micrometers J = 17-16 CO line in the BN-KL region of Orion using a laser heterodyne spectrometer. The line shows broad wings (30 km s-1 FWHM at BN) characteristic of the plateau emission as well as a narrower component probably associated with the quiescent gas in the molecular ridge. From an analysis of the plateau emission together with that observed in lower J CO transitions, we derive an excitation temperature of 180 +/- 50 K and minimum column density of 1 x 10(18) cm-2 for CO in this component, which constitutes 80% of the total integrated intensity of the J = 17-16 line near BN. The peak intensity of the narrower component observed at 0.8 km s-1 resolution increases relative to that of the plateau component toward theta 1C and away from BN, while the width decreases from 10 to 4 km s-1 (FWHM).

Laser heterodyne spectrometer for monitoring atmospheric minor constituents.

An infrared laser heterodyne spectrometer has been developed to measure solar absorption lines of atmospheric minor constituents by using a PbSnTe diode laser and a CO2 laser as local oscillators. The system is capable of operating aboard an airplane in order to detect stratospheric molecules in very low concentrations by observing the sun near sunset or sunrise at the horizontal direction. The test operation on the ground shows that the sensitivity of the photomixer attains the quantum noise limit when a diode laser as well as a CO2 laser is used and the number of absorption lines of stratospheric ozone are successfully measured.