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We present the first broadly tunable KNbO3 OPO in tracking-free configuration (TFC), pumped by a flashlamp- pumped Ti:Sapphire laser. Tuning the pump laser from 733 to 841 nm yielded to an OPO tuning range from 908 nm to 1402 nm for the signal, and 2103 to 3803 nm for the idler. This range was limited by the mirror coatings, and continuous tuning should be achievable up to and beyond 400 nm. Threshold was as low as 15 MW/cm2 and efficiencies up to 10% have been observed without AR-coatings on the crystal.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present the results of a detailed Lidar study on urban aerosols performed in summer 1996 in Lyon. Mie calculations havebeen performed to determine the optical backscattering and extinction coefficients of the real size distribution obtained bysampling, and then used for Lidar measurements. The solid particles have been sampled using cut-off filters. Size distributionand composition, determined by scanning electronic microscopy (SEM) and X-Ray microanalysis, reveal two main modes at0. 1 and 0.9 .tm, the composition of which is soot for the first one and 60 % soot - 40 % silica for the second. The combinedSEM and lidar techniques allowed to obtain the first quantitative Lidar profiles of urban aerosols. Potential and limitations ofthe method are critically discussed.Keywords : lidar, aerosols, microscopy, urban 1. INTRODUCTION Atmospheric aerosols are today a key issue in air pollution. Their impact on public health and activity in heterogeneouschemistry are well recognised although few comprehensive quantitative studies have been reported so far. This is especiallytrue for urban aerosols, which variety in size and composition make a complete characterisation difficult. For instance, due tothe lack of analysing techniques, today's legislation is limited to specify a certain mass of particles per cubic centimetre',whatever the size and the composition. This is particularly unsatisfactory since obviously particles smaller than 1 im aremuch more relevant for public health than bigger ones .

We developed a new concept for lidar measurements: multispectral differential absorption. Instead of comparing the backscattered light at one absorbed and one reference wavelengths (DIAL), a broadband laser is sent into the atmosphere and the returned signal is spectrally resolved on the receiver side (MULTIDIAL). Potential advantages are thoroughly reviewed. First, experiments combining lidar with a Fourier transform spectrometer are presented. Broadband nontracking OPOs are being built for this application in the infrared to detect VOCs. This idea was also applied to the UV range. A tunable (280-300 nm) broadband Ce:LiSAF laser has been developed for this purpose. The detection is performed by a gated amplified CCD-OMA.