La Reunion
OPAR (Reunion Island Atmospheric Physics Observatory) is a scientific structure involving four partners: Reunion Island Council, Reunion Island University, Pierre Simon Laplace Institute and CNRS. The site is located at the university of St Denis de la Reunion, 55.5°E, 21°S in the Southern-western part of Indian Ocean, 80 m ASL. This structure has been officially created in February 2003, but the instrumental setup at Reunion Island started to be installed at the beginning of the nineties, with stratospheric Rayleigh Lidar measurements. Due to the increasing interest in atmospheric measurements at this latitude, some other lidar systems (tropospheric and stratospheric ozone lidars, Doppler Lidar, tropospheric aerosols) have been implemented later. 

Till now, all the instruments have been installed at Saint Denis de la Reunion, on a campus a few meters above sea level and within the atmospheric boundary layer. In order to increase the number, quality and diversity of observations of atmospheric dynamical and chemical parameters started in the nineties, a new facility should be built at the top of the Maïdo mountain, on the western part of Reunion Island. It will become the main instrumented facility of OPAR. By the year 2011, most optical instruments installed on the Reunion University Campus at Saint Denis de la Reunion will move to this new facility.

Rayleigh Mie Raman (RMR) Lidar
It is the first LIDAR that has been installed at Reunion Island since 1994, for stratospheric temperature measurements, using at first Rayleigh scattering. Progressively, its capabilities have been increased in order to enable measurementof some other physical parameters, such as aerosols (Mie scattering + polarization), tropospheric temperature (vibrational Raman scattering), tropospheric ozone (Raman differential absorption) and water vapour (Raman scattering).
The laser is a solid-state Neodyme-YAG laser (Spectra Physics Quanta Ray 290 Pro) with second and fourth Harmonic Generators,generating either 532 nm (temperature, aerosols, water vapour) or 266 nm (tropospheric ozone) pulses with a 30 Hz Pulse Repetition Frequency. Wavelength selection is operated through KDP Harmonic generator crystal insertion in the optical beam at the output of the laser head. Dichroic mirrors seperates the different wavelengths at the output of the laser, enabling the use of two optical paths for ozone measurements and temperature-aerosol measurements.
Both wavelengths used for the differential ozone measurement are generated by the following principle. 266 nm pulses cross a Raman cell filled with a gas mixture (Deuterium + Helium). The Raman effect generates two UV signals at 289 nm and 316 nm.

The lidar optical receiver consists of four 500 mm diameter parabolic mirrors at the focal point of each is a set of optical fibers. Received signals are transmitted through the fibers are are converted in eletrical signals by photomultipliers (Hamamatsu R1477S type). Electronics and signal processing are similar to those installed at Observatoire de Haute Provence. Rayleigh channels enable temperature measurements in the stratosphere and the lower mesophere. Since 1999, a Raman channel has been in operation, enabling tropospheric temperature measurement. Polarization channels have been in operation since 1997. Troposphercic ozone measurements have been conducted since 1998.
The originality of the receiving system consists in the concept of double optical fibers that are mechanically tied (Muliple fibers collector, Baray et al., Applied Optics, 1999). Indeed, because of the differences of wavelengths (UV versus visible) and of altitude ranges, it was not possible to use the Rayleigh fibers for the ozone DIAL system. Tropospheric ozone measurements require a larger field of view in order to enable an optimum optical covering in all the schedulded altitude range (middle and high tropical troposphere) and a bigger fiber diameter (1,5 mm). Both fibers are mechanically tied at the focal point, the distance between them corresponding to the angular distance between the beams at both wavelength ranges. Phase matching of both fields of view is realized through the whole move of the fibers near the focal points. This technique enables the use of the UV optical system, realizing optical alignments in visible wavelengths.
Temperature measurement in lower atmospheric layers (12-25 km) runs with a Raman channel (N₂) coupled with two Rayleigh channels, which enables temperature and density measurements between 12 and 90 km,when the sky is clear.

Stratospheric ozone lidar
This LIDAR is devoted to stratospheric ozone measurements (17-45 km) using the Differential Absortion technology. Both wavelengths are generated with two different lasers:
* A Neodyme YAG laser generates the OFF non absorbed wavelength 355 nm. The fundamental frequency is 1064 nm. The 355 nm wavelength corresponds to the third harmonic, obtained when the laser beam crosses two KDP crystals at the output of the laser head. The laser pulse frequency is 30 Hz and the output power is nearly 5 Watts.
* An exciplexe laser generates the ON absorbed wavelength. This laser is a high pressure pulsed laser. The amplifying medium consists of a mixture of rare gazes (Neon and Xenon) and halogen gaz (Hydrogen Chlorure). An electrical discharge in the gas generates some chemical reactions which produces a complex halogen molecule (Xenon Chlorure). The radiative deactivation generates stimulated emission at 308 nm.

Receiving optics consist of four 500 mm diameter, at the focal points of which ar our optical fibers. Backscattered signals are focused on the cores of these fibers, then transmitted by the fibers to a spectrometer. Each beam is then divided in two parts (8% and 92% of the energy) by a glass membrane. Each beam is then detected by photomultipliers and digitized by the electronics. This fast counting system (400 Mhz bandwidth), digitizes the temporal signal with a 1 µs resolution (150 m spatial resolution). A mechanical chopper enables the control of the laser firing rate and the obturation of the backscattered signal in the lower layers. The electronic obturation system has been put into service since 2002 to enable a better selection of measurements in the high altitude layers.

Contact information
Jean-Luc Baray
OPAR, Reunion Island University
15 av. R. Cassin
97715 St Denis Messag. cedex 9
Phone: 02 62 93 86 64
Email: jean-luc.baray@univ-reunion.fr

Observatory for atmospheric physics in La Réunion website