C. Thiebeaux

Atmospheric laser heterodyne detection

Abstract We describe in this paper what has been done for ten years at Reims in the area of atmospheric heterodyne laser spectrometry. The recent technical progress allows now to use this technique in two different ways. The first is devoted to the radiometry with low spectral resolution [0.1 cm −1 ] of “cold” gases (300 K) in the troposphere. The second is devoted to the transmission spectrometry of the atmosphere (high resolution 0.002 cm −1 ) with the sun as a blackbody source. In the particular case of ozone, the gas absorption lines are in the neighborhood of the CO 2 laser lines. The earlier ozone concentration measurements were precision limited because of the atmospheric CO 2 absorption and the total integration time (>30 min) of the recorded spectrum. An alternative improved method is presented where the atmospheric CO 2 absorption is eliminated with the use of a 18 O 12 O 18 O laser. An acousto-optic spectrometer (AOS) gives the multiplex advantage and reduces the total duration of a spectrum to only one minute.

Heterodyne spectrometer for the 10 micron region

We first consider the interest of infrared heterodyne spectroscopy to study the vertical distribution of atmospheric constituents such O3. Then an heterodyne spectrometer is described. It operates in the 10 micron region with a CO2 laser and gives a resolution of 5 MHz. A laboratory spectrum of C2H4 is presented as a preliminary result.

Frequency and amplitude stabilization device for a cw CO2 laser

Abstract The active stabilization of a cw CO2 laser is described. The servo control system uses frequency stabilization at the maximum of the power profile. Description of the high voltage current regulated supply is given together with details and diagrams of the electronics. The results obtained by this servo-control system are presented.

Tunable heterodyne spectrometer in the 9 μm range with selected lead–salt diodes

Abstract Heterodyne spectrometry is a potentially powerful technique for astronomical or geophysical investigations. A heterodyne receiver works by mixing the radiation under study with the laser radiation from a local oscillator (LO) on a fast photodetector. The described apparatus uses Pb–salt diodes as the LO and operates between 1050 and 1150 cm −1 . The effects of the characteristics of the diode on the performances of the heterodyne receiver are detailed. Ground-based heterodyne atmospheric spectra of the ν 1 band of ozone using the sun as a source are presented.

An instrument for atmospheric detection of NH3 by laser heterodyne radiometry

Abstract A laser heterodyne radiometer has been used to detect and to identify a pollutant in the atmosphere. In this paper we describe the technique based on the gas thermal emission measurement at 10 urn wavelength. A quantitative study of the NH3 detection is detailed.

Infrared laser heterodyne ozone spectrum

An infrared laser heterodyne spectrometer (IRLHS) has been developped for both laboratory and atmospheric studies. A long path and temperature adjustable cell has been designed for laboratory spectra. An ozone spectrum with 5MHz (0.00016 cm−1) resolution is presented.

Linewidth narrowing of 10 μm diode lasers by external feedback

Abstract The spectral emission of a mid-infrared diode laser at 10 μm is investigated by two different techniques. The fringes of a high finesse confocal etalon and the beats with a stabilized carbon dioxide laser are recorded. The conjugate effects of electronic frequency stabilization by using low-pressure gas molecule absorption and the controlled optical feedback in an external cavity are shown. The linewidth is reduced from 65 to 20 MHz.

Total ozone determination by infrared heterodyne spectrophotometry

Abstract A CO 2 Laser Heterodyne Spectrophotometer (L.H.S.) has been used to record very high resolution (5 MHz) solar spectra. Observations of atmospheric ozone were made at Reims (France) (located 49°14′ North, 4°4′ East), during May 1990. The spectral data have been inverted by a non-linear least squares method written in the laboratory, to yield a total column amount of zone. The results are compared with data obtained by Dobson Spectrophotometer at the Observatory of Haute Provence (O.H.P.), France (located 43°56′ North, 5°4′ East).

Heterodyne spectra analysis

A complete explanation of heterodyne spectra is given here. The study of the folding effect has been carried out and the properties of heterodyne spectra are detailed. A fitting method using synthetic spectra is developed. Ozone heterodyne spectrum is taken as an example.

Thermal emission detection by laser heterodyne radiometry

Abstract A selective black body is considered as a self-emitting source and can be characterized by its remote thermal emission. This emission is detected at 10 μm wavelength by a laser heterodyne radiometer. Temperature, spectral, and spatial resolution conditions are given. The experimental set-up is fully described and signal-to-noise ratio measurements are presented.