OPO LIDAR SOUNDING OF TRACE ATMOSPHERIC GASES IN THE 3 – 4 μ M SPECTRAL RANGE

The applicability of a KTA crystal-based laser system with optical parametric oscillators (OPO) generation to lidar sounding of the atmosphere in the spectral range 3–4 μm is studied in this work. A technique developed for lidar sounding of trace atmospheric gases (TAG) is based on differential absorption lidar (DIAL) method and differential optical absorption spectroscopy (DOAS). The DIAL-DOAS technique is tested to estimate its efficiency for lidar sounding of atmospheric trace gases. The numerical simulation performed shows that a KTA-based OPO laser is a promising source of radiation for remote DIAL-DOAS sounding of the TAGs under study along surface tropospheric paths. A possibility of using a PD38-03-PR photodiode for the DIAL gas analysis of the atmosphere is shown.


INTRODUCTION
To overlap the near-and middle IR region, radiation from optical parametric oscillators based on nonlinear crystals is used [1].However, laser radiation is required to be monochromatic in the standard DIAL technique.Finally, the problem of standard DIAL measurements at only two radiation wavelengths means that the disturbing absorption is ignored.The result of this is error introduced by a priori uncertain absorption coefficient.These disadvantages do not affect differential optical absorption spectroscopy (DOAS) [2].However, DOAS capabilities for vertical profiling are restricted; only path-average measurements have been carried out until now.A technique which combines the advantages of both methods-spatial resolution of the DIAL technique and identification of gases in DOAScould be a promising approach to a solution of the problem.The aim of this work is the development of a technique for lidar sounding of trace atmospheric gases, which combines DIAL and DOAS, and its validation in a numerical experiment for estimating the capabilities of lidar sounding of the gas composition of the atmosphere in the 3-4 µm spectral range with an OPO-based laser system.

DIAL AND DOAS TECHNIQUE FOR LIDAR SOUNDING
The P R signal from a scattering layer of ∆z in thickness can be represented as where P0(λ) is the laser radiation power, AD is the instrumental function width, О(z) is the overlapping region of the laser beam and detector's field-of view, β(z,λ) is the mass coefficient of backscattering radiation, η(λ) is the efficiency of the receiving-transmitting system, Δz is the spatial resolution along the sounding path, and τ(z,λ) is the attenuation coefficient.The concentration of a gas under study is calculated by the DIAL equation where σabs is the absorption cross-section.
Disadvantages of the DIAL technique are caused by uncertainties of a priori absorption coefficients at two wavelengths.DOAS allows these disadvantages to be avoided, by monitoring the transmission in the UV, visible and IR regions in a broad band (20-100 nm).The first step in DOAS is the calculation of the ratio of the spectrum observed (IOBS) to the reference spectrum (IREF), which is found from the laser spectrum (I0) measured with the same detector.An atmospheric spectrum at a known content of absorbing gases can be used as IREF.Calculating the ratio (IOBS/IREF) and taking its natural logarithm, the optical depth can be found: Equation (3) includes the wavelength-dependent absorption (the first term in Eq. ( 3)) and the scattering Y(β), which is mainly determined by the backscattering coefficient β.The ratio is independent of the laser radiation spectrum or the spectral dependence of the receiving optics, spectrometer, and detector, which is an important advantage of DOAS.The so measured magnitude is equal to the difference in the concentration of absorbing gases in the reference case and in the case of real atmosphere.A differential spectrum is usually retrieved by several hundreds of points; the number of fitting parameters is no more than six.Thus, equation ( 2) becomes overdetermined and is solved with the least-square method.Under this approach, the fitting coefficients are varied for the fitting spectrum to match the best with the spectrum observed.In this case, if the absorption coefficients are known, the integral content of each gas can be found.
Simultaneous determination of the concentrations of several gases is an important advantage of DOAS as compared to the DIAL technique.

OPO LASER SYSTEM FOR REMOTE SOUNDING OF THE ATMOSPHERE
In this work, we consider a laser system (designed by SOLAR LS Company, Minsk) which is a part of a OPO DIAL lidar and ensures the tunable generation of nanosecond radiation pulses in the 3-4 µm range.Tables 1 and 2 present main specifications of the pumping laser and radiation converter.The DIAL-DOAS technique developed for TAG measurements was validated for estimation of the lidar signal levels, using the specifications of the above described KTA-based OPO laser system.

SIMULATION OF TAG LIDAR MEASUREMENTS
Sounding of some atmospheric gases (HCl, HBr, NO2, and N2O) along surface atmospheric paths (in the range 1-5 km) has been numerically simulated.The standard midlatitude summer model was used in the simulation.The absorption of all main atmospheric gases was considered; the HCl, HBr, and NO2 concentrations were taken equal to 1 ppm, and the N2O concentration, to 1.5 ppm.Table 3 represents the informative wavelengths used in the numerical simulation and appropriate for DIAL-DOAS sounding of the TAGs under study.Spatially and spectrally resolved lidar echo signals in the TAG wavelength range calculated for surface tropospheric paths are shown in Figs.1a and 1b for N2O and NO2, respectively, and in Figs.2a  and 2b, for HBr and HCl, respectively.

MEASUREMENT RESULTS
Energy parameters of a laser system have been measured in experiments with an OPO-based setup for estimation of optimal lidar output parameters.Two improved schemes for measuring output laser parameter are used.
In the first scheme, OPHIR PE10-C, PE25-C, and PE25-BB energy meters (EM) are used as the receiving unit.
A pumping wavelength of 1.064 nm is transformed by means of an optical parametric oscillator (OPO) based on nonlinear KTP and KTA crystals.Turning the crystal allows the radiation wavelength to be tuned in the 3-4 mm spectral range.A gold-coated end mirror allows the laser radiation to be directed to the EM detection area.The measurements result in the wavelength dependence of the radiation pulse energy.In the second scheme, a gold-coated end mirror directs the laser radiation to a topographical target, which is represented by a diffusing reflector with an albedo of 0.8.The main specifications of the setup are given in Table 4.A sounding path is 20 m long.Backscattered radiation is gathered by a receiving Cassegrain telescope (main mirror diameter is 30mm and focal length is 120 mm) and focused to the PD38-03-PR photodiode detection area.An aperture and BS-15 light filter are included in the scheme to prevent scattered radiation from saturating the photodiode.
The level of signals with the use of the above optical elements allows a suggestion about possibilities of sounding the atmosphere along a surface tropospheric path up to 1 km long.The experimental geometry is shown in Figure 3.The time dynamics of a single radiation pulse (curve 1) and the shape of a sync pulse (curve 2) are shown in Fig. 4.

CONCLUSIONS
The technique developed for lidar sounding of TAGs, which combines DIAL and DOAS, and its validation in numerical experiments confirm prospects of the use of the selected informative wavelengths.The numerical simulation performed shows that OPO laser is a promising source of radiation for remote DIAL-DOAS sounding of the TAGs under study along surface tropospheric paths.A possibility of using a PD38-03-PR photodiode for the DIAL gas analysis of the atmosphere is shown.

Figure 2 .
Figure 2. Spatially and spectrally resolved lidar echo signals of (a) HBr and (b) HCl sounding

Figure 5
Figure 5 Pulse energy as a function of the radiation wavelength when using PE10-C and PE25-C EMs as radiation detectorsFigures 5 -6 shows the effect of water vapors on the lasing lines in the 4-m spectral region without nitrogen purging of the OPO cavity, i.e., the effect of strong water vapor absorption lines is obvious.

Figure 6
Figure 6 Lidar signals detected by the PD38-03-PR photodiode: minimum detectable lidar signals (black curve), maximum detectable lidar signals (blue curve), result of averaging a statistical sample of lidar signals (red curve)

Table 1
Specifications of pumping laser

Table 2
Specifications of radiation converter

Table 3
Informative wavelengths

Table 4
Specification of the experimental setup