C. Loth

Two-micrometer heterodyne differential absorption lidar measurements of the atmospheric CO2 mixing ratio in the boundary layer.

A 2 microm heterodyne differential absorption lidar (HDIAL) has been operated at the Instïtut Pierre Simon Laplace, Laboratoire de Météorologie Dynamique (Paris) to monitor the CO(2) mixing ratio in absolute value at high accuracy in the atmospheric boundary layer. Horizontal measurements at increasing range are made to retrieve the optical depth. The experimental setup takes advantage of a heterodyne lidar developed for wind velocity measurements. A control unit based on a photoacoustic cell filled with CO(2) is tested to correct afterward for ON-line frequency drift. The HDIAL results are validated using in situ routine measurements. The Doppler capability is used to follow the change in wind direction in the Paris suburbs.

Wind Infrared Doppler Lidar Instrument

The purpose of the project WIND (wind infrared Doppler lidar) is the development of an airborne conical scanning CO2 Doppler lidar in French-German cooperation by CNRS/CNES and DLR. The instrument and its peculiarities are described. Measurements of the Doppler shift from a moving platform with an accuracy of 1 m/s require instantaneous access to the position data of the platform. Therefore this part of the instrument is described in detail. Ground-based tests, airborne tests and a validation flight were performed. The instrument can be used for me- soscalic meteorology to test models and contribute to a spaceborne Doppler lidar in the future.

Double-pulse dual-wavelength alexandrite laser for atmospheric water vapor measurement

We describe a new alexandrite laser source arrangement designed to measure atmospheric water vapor using the differential absorption lidar technique. This laser is capable of emitting two pulses at two appropriately selected wavelengths within a single flash lamp discharge. A narrow spectral linewidth of Deltalambda < 1 pm is obtained for each pulse by intracavity filtering with a birefringent filter and two Fabry-Perot interferometers. Wavelength commutation between the two pulses is performed by electro-optically tuning the birefringent filter. The temporal separation between the two pulses can be chosen between 50 and 70 micros and each pulse duration is <250-ns (full width at half-maximum). Typical output energies of 50 mJ/pulse at each wavelength are obtained with this laser system at a 10-Hz repetition rate for a 1.3-kW input electrical power.

Correcting winds measured with a Rayleigh Doppler lidar from pressure and temperature effects

The molecular channel of the space-based Doppler lidar ADM-Aeolus relies on a double Fabry—Perot (FP) interferometer. The difference in photon numbers transmitted by the two FPs divided by their sum- the so-called Rayleigh response—is a function of the central frequency of the spectrum of the laser light backscattered by the atmosphere, so that a proper inversion enables the measurement of Doppler shifts and line-of-sight wind velocities. In this paper, it is shown that the relation-ship between the Rayleigh response and the Doppler shift does not depend on the sole characteristics of the instrument, but also on the atmospheric pressure and temperature (through the Rayleigh—Brillouin effect), and the likely presence of a narrow-band radiation due to particle scattering. The impact of these on the precision of inverted Doppler shifts (or line-of-sight winds) is assessed showing that a correction is needed. As they are lacking the appropriate precision, climatology profiles of pressure, temperature or aerosols cannot be used as an input. It is proposed to use data predicted by a numerical weather prediction system instead. A possible correction scheme is proposed. Its implication on the quality of retrieved Rayleigh winds is discussed.

Study of a 1-Watt Repetitive Dye Laser

A repetitive flashtube pumped dye laser (pulse rate up to 30 Hz) is described. The untuned laser output average power with rhodamine 6G is 1 W. When tuned by a Perot-Fabry etalon, 0.36 W is obtained in a bandwidth of 3 A. Output power is given for several dyes emitting between 440 nm and 680 nm. The behavior of flashtubes and dye solutions after long use is discussed.

Narrow bandwidth Q-switch alexandrite laser for atmospheric applications

Abstract An acousto-optics Q-switch alexandrite laser with a 0.5 pm linewidth is described. The output energy is 20 mJ with a pulse duration of 100 ns (fwhm). A new cavity arrangement is used in which a main non dispersive resonator acting as a forced oscillator is continuously self-injected by a narrow line from an auxiliary cavity. In a separate experiment we report on a (broadband) pulse emission within 100 μs with an equal energy of 50 mJ in each pulse.

WIND an Airborne Doppler Lidar for Atmospheric Applications Developed in French-German Cooperation

WIND is a joint project between France (CNRS-CNES-Meteo France) and Germany (DLR) to develop an airborne wind Doppler lidar for meteorological applications. The instrument specifications are derived from the measurement objectives as well as the state-of-the-art in technology. Presently an operational airborne wind lidar can be designed around the CO2 laser technology, heterodyne detection, and a conical scanning of the lidar line-of-sight to sample the atmospheric wind field. The 10-micron spectral domain is suitable for long range measurements for it corresponds to an atmospheric window and an adequate backscatter coefficient in the troposphere. The first flights are scheduled early 1995 on board the Falcon 20 operated by DLR.

Improvement of Heterodyne Detection with Optical Amplifier and Pulsed Coherent Doppler Lidar

Abstract This paper reports on some new experimental results obtained by the integration of an optical amplifier into a Doppler lidar system. Measurements have been performed at a hard target and in the atmosphere. The result was an improvement in the signal to noise ratio and a significantly higher range for the lidar with the optical amplifier.

WIND: the joint French-German airborne Doppler lidar

An airborne coherent Doppler Lidar to retrieve mesoscale wind fields has been developed in the frame of the Franco- German WIND project. The instrument is based on a pulsed CO2 laser transmitter, heterodyne detection and wedge scanner. The performance of the instrument operating on the ground and in the aircraft is reported.

SNR-Loss Budget of the Airborne Doppler Lidar WIND

A simulation of the airborne Doppler lidar WIND performance shows that the scientific requirement can be met with an instrumental SNR-loss up to 15 dB. A budget between the SNR degradation sources is established and leads to detailed specifications for the instrument components.