Initial measurements using a 1.54-μm eyesafe Raman shifted lidar Edward M. Patterson, David W. Roberts, and Gary G. Gimmestad

All authors are with Georgia Institute of Technology, At­lanta, Georgia 30332; E. M. Patterson is in the School of Geophysical Sciences, the other authors are in the Georgia Tech Research Institute. Received 29 August 1989. 0003-6935/89/234978-04$02.00/0. © 1989 Optical Society of America.

We have demonstrated an eyesafe lidar system for cloud and aerosol studies using 45-mJ/pulse 1.54-μm radiation generated by wavelength shifting the output from a pulsed Q-switched Nd:YAG laser using a CH4 Raman cell.

Eyesafe operation is desirable when lidar systems are used in programs in which personnel can be in the path of the laser beam. This is particularly important in scanning systems or in systems that are designed for automated unattended oper­ation. In practice, to be eyesafe at short ranges from the transmitter and still maintain a reasonable pulse energy output, the source wavelength must be shorter than 400 nm or longer than 1.4 μm.1 With the exception of long wave­length CO2 systems, commonly used lidar systems are not eyesafe and require stringent precautions for safe operation. Since these CO2 systems have several disadvantages, includ­ing reduced detector/preamp performance at 10 μm and the need for cryogenically cooled detectors, there is need for a short wavelength lidar that is capable of eyesafe operation. Interest in the properties of the atmosphere at near IR wave­lengths and recent developments in detectors have made the concept of an eyesafe system in the l.4r-l.8-μm range an attractive option.

We have demonstrated a short wavelength IR eyesafe lidar system for cloud and aerosol backscatter and transmission studies that is based on Raman shifting the 1.06-μm output from a Nd:YAG laser to 1.54 μm with a methane cell. Al­though earlier systems have used Raman shifted laser sources for operation at other near IR or visible wavelengths2

or for eyesafe rangefinders,3 to our knowledge, the measure­ments discussed here are the first reported measurements of atmospheric backscatter up to 11 km with such a short wavelength eyesafe system. This Letter describes the sys­tem and shows some examples of initial measurements.

Raman scattering is an inelastic scattering process that involves the excitation of a molecule to a virtual state, which then decays to an energy level other than the original one. For Stokes scattering, the molecule is initially in the ground state, and the decay is to an excited state with the Raman-scattered radiation at a lower energy and longer wavelength. When a material that exhibits Raman scattering is pumped with an intense laser beam, stimulated Raman scattering can occur, and an appreciable fraction of the incident energy can be converted into energy at the shifted wavelength. The stimulated Raman intensity is described by the equation3

in which Is(0) is the spontaneous Raman noise intensity, Gs is the gain coefficient, I1 is the pump intensity, and Z is the interaction length. Gs depends on the Raman cross section of the media, Raman linewidth, and optical pump frequency. Gases such as methane at high pressure can produce efficient conversion with more than 30% of the incident energy shifted to the longer wavelength.4

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Fig. 1. System diagram of Raman shifted eyesafe lidar system.

A diagram of our eyesafe lidar system is shown in Fig. 1. The transmitter consists of a source laser, Raman cell, and output optics. The source is a Q-switched pulsed 1.2-J/ pulse Nd:YAG (Quantel YG 58lC20) laser which pumps a 75-cm long Raman cell containing methane at ~340 psi. The laser beam is focused at the center of the cell and recollimated with two 50-cm focal length lenses as shown in Fig. 1. Stimulated Raman scattering is produced at a wave­length of 1.54 μm, an eyesafe wavelength that is in an atmo­spheric window region. The output from the Raman cell is passed through a dichroic beam splitter to remove residual 1.06-μm radiation from the transmitted beam, which is then passed through a beam expander to increase the beam diam­eter to ~6 cm, and then to steering mirrors to direct the beam. The beam expander reduces divergence and pro­duces a transmitted beam which is eyesafe at the output aperture.

The receiver unit consists of a 40-cm ƒ/4.5 Newtonian telescope with a telecompressor lens just ahead of the focal point to reduce the effective focal length to 76 cm, a 20-nm interference fiber centered at 1.54 μm, and a 0.3-mm diam room temperature InGaAs PIN diode followed with a GaAs FET low noise preamplifier (available as a unit from Analog Modules). The receiver optics define a field of view of 0.44 mrad. The preamplifier output is digitized using a DSP 20-MHz 12-bit transient digitizer. The transient digitizer out­put is recorded using an IBM PC AT computer, which is also used for data analysis and display.

A listing of system performance parameters is shown in Table I. The values that we measured for pulse energy, beam divergence, and pulse repetition rate are of particular note. The measured output from the Raman shift cell of 45 mJ/pulse was obtained when the cell was pumped with 1.06-μm radiation of 0.6 J/pulse. Although we observed signifi­cantly higher energy of 200 mJ/pulse when the cell was pumped with a 1.2-J/pulse beam, the lower pump intensity was used to reduce the potential for component damage. Our best conversion efficiency is lower than maximum values reported earlier because our system was not optimized in terms of interaction length or focal length of focusing and recollimation lenses. Our choice of cell pressure was based on earlier studies of Singh et al2 and is probably near opti-mimi.

The divergence of the transmitted beam was determined by measuring the signal intensity from a hard target while scanning the receiver field of view through the transmitted beam. This divergence was found to be ~2 mrad; the field of view of the receiver is only 0.44 mrad, so that not all the transmitted beam is in the field of view of the receiver. As discussed by Measures,5 this has the effect of reducing the

effective aperture of the system by approximately the square of the ratio of receiver field of view to transmitted beam divergence. On the basis of these measurements, we esti­mate our effective aperture at ~10 cm.

This relatively large value of the divergence and the rela­tively low pulse repetition rate (4 Hz) were due to heating of the methane in the cell by the focused laser beam. Attempts at higher pulse repetition rates caused thermal defocusing and beam spread, resulting in unacceptable beam quality. To minimize the effects of this heating on beam quality, we operated the cell in the vertical direction. The total opera­tion time of the cell was limited by sootlike particle forma­tion in the methane due to the focused laser beam. This material would deposit on the windows and absorb the laser radiation, causing heating and window damage. This prob­lem of window degradation was minimized by periodic clean­ing of the windows.

At this time, our primary estimates of quantitative back-scatter are based on analysis from measured or estimated system performance parameters. Although a hard target or other exact calibration has not yet been accomplished, we have addressed the question of system calibration in a pre­liminary fashion to verify our system performance estimates. The roughened concrete wall of a building 1.07 km from the lidar was used as a calibration target. The lidar beam was incident on the target at an angle of ~10°. The reflectivity at 1.54 μm was determined as 0.3 from THE INFRARED HANDBOOK data on concrete,6 and the reflectance from the building was assumed to be isotropic over the backward hemisphere with a value of ρ = 0.3/2Π; possible specular or retroreflectance peaks were ignored in our analysis. We did find that our calibration values for system sensitivity agreed with that expected from an analysis of system parameters within a factor of 2, which is considered acceptable at present.

Analysis of our system design parameters indicated that the system should be sensitive to Rayleigh scattering at vertical distances of more than 3 km to backscatter from thin cirrus clouds at ranges of more than 12 km and to boundary layer aerosols at ranges of more than 10 km. Because of the

Table i. System Performance Parameters

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Fig. 2. Measured lidar return from boundary layer aerosols and multiple cloud layers.

Fig. 3. Measured lidar return from thin cirrus at 11-km altitude.

relatively low values of solar radiance at 1.54 μm as well as the low scattering function of the atmosphere, sky background was expected to be much less of a problem than at shorter wavelengths, so that operation during both day and night is possible.

To date, we have measured cirrus backscatter from 11-km altitude and aerosol return from altitudes of more than 4 km in daytime conditions. Samples of data are shown in Figs. 2 and 3; the resolution in each figure is 7.5 m. Lidar returns averaged over 100 pulses showing multiple cloud layers as well as the boundary layer aerosol between the cloud layers are shown in Fig. 2. Figure 3 shows the higher altitude portion of a similar return averaged over 1000 pulses. In this plot, a cirrus cloud layer is seen at ~11 km. Based on our preliminary calibration discussed above, we estimate that this observed cirrus backscatter is ~ l × 10-6 m-1 sr-1, a value of backscatter that is consistent with expectations from the LOWTRAN 6 models.

Summarizing; we have demonstrated a viable eyesafe Raman shifted lidar system operating at 1.54 μm. Our sys­tem analysis, which has been confirmed by our- initial mea­surements, indicates that this lidar has the sensitivity to be a useful tool for measurements of tropospheric aerosols as well

as of clouds up to the tropopause with total eye safety. Further work is planned to optimize the Raman shift cell to improve conversion efficiency and beam quality, to reduce the problem of methane breakdown and window degrada­tion, and to provide for improved calibration.

This work was supported by internal research funds of the Georgia Institute of Technology. We would like to acknowl­edge helpful discussions with J. Bradshaw, C. Van Dijk, and D. Killinger.

References 1. American National Standard for the safe use of lasers, Standard

ANSI Z136.1-19809, American National Standards Institute, New York (1980).

2. U. N. Singh, Z. Chu, R. Mahon, and T. D. Wilkerson, "Optimiza­tion of a Raman Shifted Dye Laser System for DIAL Applica­tions," presented at the Fourteenth International Laser Radar Conference, Italy (1988).

3. R. W. Nichols and W. K. Ng, "Raman Shifted Nd:YAG Class I Eyesafe Laser Development," Proc. Soc. Photo-Opt. Instrum. Eng. 610,92-98 (1986).

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C. Gunterman, V. Schulz-von der Gathen, and H. V. Dobele, "Raman Shifting of Nd:YAG Radiation in Methane: an Effi­cient Method to Generate 3-μm Radiation for Medical Uses," Appl. Opt. 28, 135-138 (1989). R. M. Measures, Laser Remote Sensing Fundamentals and Ap­plications (Wiley-Interscience, New York, 1984). W. L. Wolfe and G. J. Zissis, Eds., The Infrared Handbook (Environmental Research Institute of Michigan, Ann Arbor, 1985).

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