Abstract：Raman Lidar can detect water vapor mixing ratio by atmospheric water vapor vibration Raman scattering echo signal associated with the water vapor concentration. However, vibration Raman scattering spectra would drown in the sun background light due to the weak Raman scattering echo signal, therefore the measured time is usually at night. All radiation below 300nm known at solar-blind ultraviolet band is absorbed by the ozone layer in the stratosphere. The shorter the wavelength is, the stronger the energy is. To realize the detection of atmospheric water vapor at daytime and night time, a Raman Lidar is developed at a solar-blind ultraviolet band. The system consists of the laser, telescope, photoelectric acquisition and signal processing part. Briefly, the forth harmonic output (ultraviolet 266 nm) of an externally triggered, 10 Hz repetition rate, Nd∶YAG laser is employed as the transmitter. The bore sight assembly uses a turning prism controlled by a New Focus actuator. With 400 mm diameter, 0.5 m rad field of view, a telescope forms the main part of the receiving optics. To obtain signals with fine separation and high efficient extraction, three dichroic mirrors separate out the detection channels by reflecting light with longer wavelengths while transmitting light with shorter wavelengths, a combination of narrow bandwidth (FWHM=1 nm) interference filters is employed to filter the backscattered signal. The rejection rate of the Mie-Rayleigh scattering signals reaches to 10-7. Before reaching the photomultiplier tube (PMT) in each channel, a plano-convex lens is employed to focus the backscattered signal on the front face of the PMT. The backscattered radiation is collected and analyzed at four wavelengths of interest, 266.0 nm for the elastic scattering, 277.6, 283.6 and 294.5 nm for the Raman scattering of O2, N2 and H2O molecules, respectively. The four PMTs output signals are then input into a multi-channel digitizer to record the backscattered signal, which is used to retrieve the water vapor profile. We use the standard atmospheric scattering models and aerosol extinction coefficients, set system of the sampling interval to 80 ns, cumulative average pulse number to 36 000, the signal-to-noise ratios of atmospheric water vapor measurement are simulated. The simulation results show that there exists influence on ozone absorbing mainly at the Solar-blind Ultraviolet Raman Lidar detection range. The signal to noise ratio simulation results show that the measurement height of the designed Solar-blind Ultraviolet Raman Lidar system can be up to 3.5 km during the daytime measurement. The optimal parameters of Lidar system are obtained based on the detailed analysis and the discussion of the SNR of echo signals. It is concluded that this new solar-blind ultraviolet band Raman Lidar system has the advantage of measuring the water vapor in the daytime without the influence of solar background radiation.
Key words：Raman lidar; Solar-blind; Atmosphere water vapor; Ozone absorption
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