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.

Atmospheric effects on C02 differential absorption lidar sensitivity

Roger R. Petrin, Douglas H. Nelson, Mark J. Schmitt, Charles R. Quick, Joe J. Tie , and Mike Whitehead Los Alamos National Laboratory

MS E543 Los Alamos, NM 87545

ABSTRACT

The ambient atmosphere between the laser transmitter and the target can affect C02 differential absorption lidar (DIAL) measurement sensitivity through a number of different processes. In this work, we will address two of the sources of atmospheric interference with C02 DIAL measurements: effects due to beam propagation through atmospheric turbulence ad extinction due to absorption by atmospheric gases. Measurements of atmospheric extinction mdex different atmospheric conditions are presented and compared to a standard atmospheric transmission model (FASCODE). We have also investigated the effects of atmospberic turbulence on system performance. Measurements of the effective beam size after propagation m ampared to model predictions using simultaneous measurements of atmospheric turbulence as input to the model. These results are also discussed in the context of the overall effect of beam propagation through atmospheric turbulence on the sensitivity of DIAL measurements.

1. INTRODUCTION

CO, differential absorption lidar is an important remote sensing technique for a variety of applications ranging from effluent identification and characterization to geophysical structure identification to monitoring of meteorological phenomena. N m m u s ground based, airborne and satellite based systems have been deployed and utilized for various studies.14 A number of important factors contribute to the popularity of CO, laser based LIDAR systems. One of the major advantages to using CO, laser based lidar is the relatively mahm laser technology available from commercial CO, systems. Systems with a variety of output formats (CW-loOWz, kW average power, 100's kW peak powers) are available. Another advantage is in the CO, spectral region the atmosphere is relatively transparent allowing long range operation with modest laser energy. This reduces power and size requirements as compared to other laser technologies. A third important factcx is the broad tunability (9-11 pm) available in a spectral region where many materials exhibit characteristic "fingerprint" spectral signatures. This becomes especially important in applications where identification of numerous components is important.

As part of the design and chamcterization of a CO, LIDAR system for any application a number of issues must be considered. In this work, we not only address issues associated with atmospheric effects that influence any CO, DIAL, application, but also consider those important to applications involving multi-spectral DIAL from hard targets. In this approach, multiple wavelengths are used in the measurement rather than just two as in the usual DIAL systems. Since the transmitted energy contains a broad spectral content, the spectral fingerprint of the material of interest, either the target itself in geophysical measurements or effluents in a pollution monitoring system, will be present in the signal return's spectral characteristics. The use of a large number of wavelengths is ne4xsa-y for multiple component identification, but inaoduces additional concerns in system design. Before such a system can be utilized effectively, it is necessary to identify and investigate atmospheric effects that could adversely effect performance of this type of CO, DIAL system.

In a broad sense, atmospheric effects on CO, DIAL systems can be divided into two categories: e f f m that change the spectral characteristics of the transmitted energy and effects that change the spatial characte&&s of the transmitted energy. Absorption atmospheric gases present under ambient conditions is the primary reason for the f m e r effects in the CO, spectral region. Although known as an "optical window," the 9-11 pm spectral region contains absorption features which could impact performance of a CO, DIAL system operating over a broad spectral range. Although many atmospheric gases absorb in this region, the primary gases of concern ate water vapor, carbon dioxide, and ozone. The atmospheric background absorption caused by these gases must be corrected for and understood before the spectml characteristics of the returning signal can be used to identify the spectral fingerprints of the materials of interest. Atmospheric turbulence is the major cause for changes in the spatial characteristics of the transmitted energy. Small index of refraction variations along the beam path alter the amplitude and phase characteristics of the propagating beam. Theoretical models describing laser beam propagation through atmospheric turbulence have been developed to describe the transmitted beam's spatial characteristics. Using these models, it is possible to predict the effective beam size on target in turbulent conditions, an important parameter

in determining system performance. In this work we compare the results of field experiments investigating these two types of effects with the predictions of the relevant parts of a comprehensive model for CO, DIAL.

2. MODEL

The CO, DIAL model used here is desgibed in detail in Ref. 9 so only a brief overview is given. The complete model contains the entire DIAL system: transmitter, ambient atmosphere, hard target, and receiver. The transmitter portion of the model contains information about the spatial, spectral, and temporal characteristics of the laser and optical system used while the receiver contains similar information about the detector and receiving optics. The transmitter and receiver hardware sections of the model are connected by a model of the field conditions which includes the effects of atmospheric turbulence, molecular and aerosol absorption and scattering, target spectral response, and speckle on the signal return. In practice, the output of the model is reported in a signal-to-noise ratio (SNR) for each wavelength used. Since we are concerned mainly with the fnst two parts of the field conditions model, we describe them in more detail here.

Small spatial variations in the index of refraction caused by turbulent temperam fluctuations in the atmosphere alter the amplitude and phase characteristics of a propagating beam. These spatial variations introduced by the turbulence lead to intensity modulation (also hown as scintillation), beam spreading, and beam wander. The first two of these phenomena effect the beam size on a shot-to-shot basis while the last affects only the average or effective beam size over a period of time. While intensity modulation can cause a reduction in the illuminated a m of the target, beam spreading and beam wander increase the effective beam size.

The problem of laser propagation through turbulence and the expressions used to describe beam spreading and beam wander are described in detail in Ref. 10-1 1. An effective beam size is calculated by adding the effects of atmospheric turbulence, leading to beam steering and beam wander, to diffraction limited propagation. The statistical beam diameter at the target can be expressed as

where pd is the free space diffraction term, p, is the scattering term and pw is the long-term contribution to the beam size due to centroid wander. These terms have the form

4R2 (1-.62bC / DX)’l3 >””, ( k P J 2

P3 =

2.97R2 p: = 2 513 113 ’ k P c DX

(3)

(4)

where k=2dh, 0, is the transmitted beam diameter, R is the range, fi is minus the distance to the focus of a diffraction limited beam, and pc is the transverse coherence length.

The transverse coherence length is the transverse distance across the beam above which the phase of a propagating beam becomes uncorrelated and can be evaluated in various limits. For a plane wave we have, pE = ps, where

(5)

For a spherical wave, the appropriate expression is pc = po,

For a Gaussian beam the expression is more complicated: pc = pOc I 2 -

The geometry of the specific system under consideration determines which of these limits is appropriate.

In each of the above expressions, the index of rekaction structure parameter, C,”, is the critical parameter far describing optical mbulence. It is based on Kolmogorov analysis of atmospheric turbulence and is the mean-square statistical average of the difference in the index of refraction between two points which are separated by the distance rl2 given by l 2

where the angled brackets stand for an ensemble avemge and values are given in m-u3. Since temperature fluctuations are mainly responsible for the variations in index of refraction found in C,” in the COz laser spectral region, it is appropriate to consider the temperature structure parameter ,C:, as given by l3

r:’3 -

c: = (9)

where 11‘ and 1; are the temperatures measured at two points separated by a distance r12. C,‘ is related to C,’ for a wavelength of 10 pm through the following relation l3

with T as the ambient air temperature in Kelvin and P the barometric pressure in millibars.

By utilizing values of the index of refraction structure pamneter, calculated from measurements of the temperature structm parameter using Eq. (lo), as input to Eqs. (1-7), the effective beam size can be calculated for each of the different methods of calculating the transverse coherence length. These results can then be compared to experimental results to determine which regime the system operates in.

Although an expression describing the statistical effects of intensity modulation can be written, unlike for beam wander and beam spnxding it is difficult to write an expression describing the overall spatial aspects of this phenomenon. The spatial scale for which the intensity modulation occurs is considered to be rc =2.1 pc. In extremely turbulent conditions, 100% intensity modulations occur on this spatial scale. However, this is not the scale of interest for an effective beam size.

What is known is that within the beam size deterrmned . by Eqs. (1-7) there will be regions with no illumination because of intensity modulation. Thus, the illuminated area will be smaller than expected. The extent of the reduction cannot at present be determined analytically.

The effects of molecular and aerosol absorption and scattering on atmospheric transmission have been extensively Standard codes exist for calculating atmospheric transmission under different atmospheric conditions. For our

calculations, we have utilized FASCODE model and the HITRAN database since we require high resolution transmission data for the specific CO, laser lines used. Detailed descriptions of this model can be found in Ref. 16-18. In addition to using measured spectral parameters for gases present in the ambient atmosphere, FASCODE also includes a phenomenological model for determining tbe wafer continuum absorption and calculates effects due to aerosol scattering. Parameters describing the ambient atmosphere (temperam, relative humidity, barometric pressure, concentrations of relevant atmospheric gases, wind speed, inversion layers, etc.) can be specified for test conditions of interest. For high resolution measurements, however, some discrepancies are known to exist, especially for regions of strong water abs~rption.'~

3. MEASUREMENTS

The DIAL system used in the field experiments was housed in a mcdi fd trailer and was vibration isolated by three compressed air filled support legs extending through the floor of the trailer. The transmitter system consisred of two pulsed CO, lasers, each delivering approximately 20 mJ / pulse out of the trailer, and operating between 10-100 Hz. The pulses from each these lasers consisted of an initial 250 ns pulse containing forty percent of the energy and a 2 ps tail containing the remaining energy. Pulse energy for each laser was monitored on a shot-to-shot basis using pyroelectric energy meters. The lasers were separately tunable on a shot-to-shot basis using galvanometer controlled gratings. The output energy was directed through a variable beam expander to allow for divergence control (0.16 - 2.3 m d ) before exiting the trailer via the final turning mirror. The beam diameter at the output was approximately 15 cm. The final turning mirror was also used to collect the signal return. After this mirrar, the receiver consisted of a 40 cm telescope coupled to a liquid nitrogen cooled HgCdTe detector. The signal from the detector was amplified as required and integrated with boxcar averages to produce the mmkd signals. Triggering of the boxcars and digitizers was controlled via optical triggers from each laser. The entire transmitter/receiver system was computer controlled using a window-based interface running on a UNiX-based workstation.

The typical operating conditions for the ammospheric transmission measurements involved tuning consecutive lasea shots over a 44 line se~uence and then averaging over 50-300 sequences. For the beam propagation experiments the wavelength was fixed at the lop20 line. Data was obtained over a flat, desert terrain at round-trip distances of -7 and -13 km, using returns from flame-sprayed aluminum (FSA) targets for the atmospheric transmission measurements. The terrain consisted of short desert scrub over the majority of the beam path and a section of dry lake for the last 1 km to the target. Time averaged beam profile measurements were obtained by scanning a metal pole, -30 cm diameter, located -3 km from the trailer, at a height of approximately 10 m. Initial beam divergences or approx. 0.19-2.0 mr were used. The ambient atmospheric temperature varied from -3OC to -4OC and the water vapor partial pressure from -0.3 to -0.9 kpa..

Measurements of C: were obtained with thermocouple probes during system operations. These probes were set at beam height on tripods at two locations along a line oblique to the beam line. Each C: probe consisted of two Omega, Type T, 0.001 inch copperconstantan thermocouples separated by approximately 0.5 meters. The attached electronics provided an output voltage every second which is averaged over multiple readings during that second. Readings from the C: probes, along with temperam and barometric pressure data from a nearby meteorological station, were used to calculate C,' using m. (10 ) above. The one second readings from the C: probes were averaged over 10 minute periods.

Another measurement of C," was acquired using a Locwleed saturation resistant scintillometer. It consisted of an LED transmitter and receiver located approximately 0.8 km apart. The scintillometer procluced path aveaaged values of C,' based on turbulence induced irradiance Variations of the LED transmission using the relation

C,' = 0.689 < (1, - 1d2 '07/3L-3 7'

where 1 A were the intensities detected by the photodiodes, I was the average intensity, D was the diameter of the transmitter and receiver and L the propagation distance. The transmitter and receiver were both at a 2 meter height, slightly lower titan the 3 meter beam height. These measurements were avedaged over one minute. The scintillometer was useful in that it gives a path averaged value over a distance of 850 meters. This propagation path included sections of both the dry lake and the short desert shrub.

and I

4. RESUL TS AND DISCUSS ION

4.1 A ~ o s D ~ ~ ric turbu lence effec&

Figure 1 shows the typical results of the C,' measurements for both the thermocouple probes and the scintillometer over a 24 hour period. The gaps in the thermocouple p b e data reflect those periods when the probes were not operational. When operating, the thermocouple probe and scintillometer measurements track each other reasonably well. The different turbulence level measured with the scintillometer is due to the difference in height between it and the probes. The typical diurnal cycle for C,' is clearly reflected in the results, along with the dramatic drop in turbulence expected just before sunset. The feature at approximately 13:OO is due to cloud cover.

Typical beam profile measurements for a 7 km path length are shown on Figure 2. Although the profiles shown are for an initially small beam divergence, approx. 0.190 mrad, the beams from both lasers typically overlapped and displayed the same general spatial characteristic, essentially Gaussian, regardless of the turbulence level or initial beam divergence. 'Ihe lines through the data represent a best fit Gaussian for each laser.

The effects of atmospheric turbulence on the beam propagation are detaikd in Figure 3 which shows the relative increase in beam diameter with turbulence for a variety of initial beam divergences. As prediaed, smaller initial beam divergences are affected much more tban larger initial beam divergences by higher atmospheric turbulence. The larger divergence beam shows little variation in relative size with turbulence. Concentrating on the smallest initial beam divergence used, 0.190 mrad, we can examine how well the measurements correspond to model predictions. The best comparison between the model and experiments results when using the more complicated Gaussian beam form for the transverse cohmnce length. This is not surprising considering the geometry of our system and the general Gaussian shape of our beam. The Rayleigh range for our system k approx. 1.5 km. At the ranges used here, 3.39 km to the target, we are at a range of approximately twice the Rayleigh m g e therefore not yet in a region where the spherical wave approximation will hold. However, this beyond the range of plane wave behavior. At longer ranges or higher turbulence levels, the results of the Gaussian and spherical wave forms of the tmnsverse coherence length converge while for shorter ranges or lower turbulence levels, the Gaussian and plane wave forms converge. At this range and turbulence level we are in the region where the Gaussian beam approximation gives best agreement with the data.

An important caveat to these results, however, is the possible e m in C,' measurements. The measurements of turbulence used are either point probes located near the trailer or the Lockheed scintillometer, located 2 km away. The point probes accurately measure C,' (+/- 25%) but give only point measurements. Spatial fluctuations of C: along the beam are not monitored. The L o c W scintillometer only samples a portion of the beam path. Considering the possible error in these measurements, it is more difficult to rule out the plane wave and spherical wave approximations.

It is informative to consider the effects that beam size variations due to atmospheric turbulence will have on the signal-to-noise ratio (SNR) of the system. In many situations in DIAL, S N R is limited by speckle noise since relatively small beam sizes are required to obtain good beam overlap with the target of interest. For a large initial beam divergence and beam size on target, speckle noise will exhibit a higher SNR limit than for a smaller illuminated spot. Since the beam size for large initial beam divergences is generally independent of turbulence, however, the S N R should remain essentially constant as turbulence increases. For smaller initial beam divergences, however, this is not true. Now as the turbulence level increases, the effective beam divergence increases. Whether this increases or decxeases the SNR, however, depends on a number of issues. If beam spreading is the dominant cause of the increase in divergence, then one could expect to see an increase in the S N R (a larger spot on target leads to higher SNR). If, however, beam wander is the cause for the increased effective beam diameter, then on any single shot the beam size is roughly the same size as in low turbulence and it is only the average size of the beam which indicates the increased size. Now different portions of the target would be sampled on eacfi shot, and, even if the reflectivity were the same, the speckle patterns produced would not be and the SNR would decrease.

In practice, the results thus far are ambiguous. For smaller initial beam divergences, no significant vend seems to be apparent. Generally, for larger initial beam divergences, there is a tmd showing inaeased noise (lower SNR) for higher turbulence levels.(Figure 4) This behavior is not consistent with the effects expected from beam spreadiig and beam wandez Additionally these effects rae small for 1.2 mrad beam divergence conside& here (Fig. 3), It is possible, however, that intensity modulation of the transmitted energy is causing the observed decrease in the SNR. This can be undeastood qualibtively as follows. At higher turbulence levels, deep intensity modulations cause portions of the target not to be illuminated. Thus the size of the illuminated area on the target is actually reduceQ causing a deaease in the speckle SNR limit. More importantly, however, the intensity pattern on the target will fluctuate since the spatial pattern of the atmospheric turbulence changes. This will contribute to a lower SNR.

4.2 Aano~~beric msm -ssion

Figure 5 shows results of one of the measurements of the relative absorption coefficient vs. wavelength to that predicted by FASCODE using HITRAN for a 13.8 km path length. The data has been normalized such that the results are relative to the absorption coefficient of the 10P28 line and do not represent absolute values of the absorption coefficient Good agreement is found the 10 pm region with Iess than a 0.01 km-I difference between the predicted and measured values (slightly larger errors, up to 0.04 km-', appear in some measurements). The same is genedly true in the 9R branch between 9.24-9.34 pm In the 9P region, around 9.5 pm, however, agreement is typically worse. The experimentally measured relative absorption coefficients in this spectral region are systematically higher than those predicted. Ignoring the 9P region, the 10R20, and the 9R12 lines, the standard deviation of the difference between measured and predicted values across the spectrum is 0.0033 km-'. This is a measuce of the sensitivity of a multi-wavelength DIAL system using just the model predictions to account for atmospheric transmission. Only variations in absorption coefficient larger than this can possibly be attributed to other than atmospheric absorption. Note that in practice, a separate background scan of the atmospheric transmission would be obtained and used to as a correction so the sensitivity would then depend on how constant the atmospheric transmission remained over the time between the background and actual measurements.

The large differences between the measured and predicted relative absorption coefficients in the 9P branch are most likely associated with ozone absorption which is significant in this spectral region. Nine of eleven CO, lines used in this spectral region have significant ozone absorption. In our measurements, these nine lines all show higher than pnxkted absorption. The model uses a concentration of 26 ppb ozone, the value for the standard 1976 atmosphere. Although no other measurement of the ozone concentration was obtained during our measurements, the larger measured values for the absorption coefficient indicates the ozone concentration was higher than this value.

The two Significantly larger differences between the measured and predicted values not in the 9P branch, one in the l O u m region and one in 9R branch, appear on lines where water vapor has significant absorption. These are the 9R12 snd 1OR20 lines. Considering all the measurements obtained, the results for the 10R20 line vary significantly. At times higher absorption coefficients are measured than predicted, at times the reverse. This can be expected since this line is also nearly compietely absorbed so typically has a low SNR. For the 9R12 line, the measured value is always smaller than the prediaed value. Although this is expected from previous results comparing the HITRAN database to CO, transmission measurements, other lines significantly absorbed by water vapor which have in the past shown large deviations do not in this work.Ig Therefore we draw no definite conclusion about the cause for this deviation.

Figure 6 shows the comparison of the results of eleven measurements of our atmospheric transmission measurements with model predictions. The average difference between the measured and prediczed absorption coefficients, relative to 10P28, determined from a set of six separate measurements at 3.28 km and a set of five measurements at 6.94 km are shown. Each measurement consists of an average of 50-300 shots at each wavelength and was obtained over 2-10 minute time intervals. Environmental conditions vary from measurement-to-measurement but are essentially constant for the duration of a measurement.

These results show two trends, one of which is related to the normalization process. First, the 6 km data bas an offset compared to the 3 km data, with the 6 km measurements consistently indicating lower absorption than predicted by the model (except in the 9P region where ozone interference is present). No meaning can be aaached to this, however, since it is simply a function of the line chosen for nomaliztion. An important trend evident in the data is a slope indicating higher

I measured absorption as wavelength increases. The difference between the measured and predicted values tends to be more positive as wavelength increases. This may be related to relative responses of the detectors in the system. The eneagy measurements are obtained using pyroelectric energy meters and the signal return is measured using a HgCdTe deteaor. The differing spectral responses of these devices could be the cause of the variation. Alternatively, it could indicate an error in the water continuum absorption calculation in the model, a known problem for some temperaturehelative humidity conditions.*'

$. CONCLUSIONS

I

The results presented here indicate reasonable agreement between our comprehensive CO, DIAL model and experimental results for atmospheric effects. For beam propagation through atmospheric turbulence, the model is acarately simulating measurements for both small and large initial beam divergences. For small initial beam divergences there arc indications that the Gaussian beam approximation for the transverse coherence length produces more accurate results. Further work is required to verify this. The atmospheric transmission results also show reasonable agreement with the model. For individual measurements, the relative absorption coefficients for atmospheric transmission can be simulated quite accurately (to better than 0.01 km-I) over a broad spectral range. Errors OCCUT primarily in the region where O:, absorption is strong. Some small systematic variations in the diffemm between measured and predicted values are indicated by the 6.94 km results. Further investigation of these is underway to determine if they ace model or measurement related.

5. ACKNOWLEDGMENTS

The authors would like to acknowledge useful discussions on speckle and noise issues with George E. Busch ad Edward P. MacKerrow, useful discussions on atmospheric transmission with Robert K. Sander, and technical assistance fmm William M. Porch, John J. Jolin and Charles Fite. This work was suported by the US. Department of Energy.

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. Figure 1. Atmospheric turbulence as measured by a Lockheed scintillometer and two thermocouple probes. Note the typical d i d variation and the sharp drop near 17:OO just before sunset. The difference in turbulence level between the probes ad the scintillometer is due to the height difference (3 m vs. 2 m).

Figure 2. Experimentally measured beam profiles and Gaussian fits for both lasers. The dotted lines are the experimental results and the solid lines are best fit Gaussian profdes.

Figure 3. Relative increase in beam divergence vs. atmospheric turbulence level for two different initial beam divergences. Note that the smaller initial beam divergence is affected more severely than the larger initial beam divergence. The three lines are model predictions for three different methods of calculating the transverse coherence length.

Figure 4. Measured noise vs. wavelength for two turbulence levels. The beam divergence was 1.2 nuad and the range was 3.39 km.

Figure 5. Difference between measured and predicted values for the absorption coefficient relative to that of 10P28. The large differences in the 9.5 fun region are due to ozone absorption. The range was 6.94 km.

Figure 6. Average results of measurements of differences between measured and predicted values for the absorption coefficient relative to that of 10P28 for 3.39 km and 6.94 km ranges. The 3.39 km result is the average of 6 measurements and the 6.94 km result is the average of 5 measurements.

IO-'*

I 0-13

I 0-14

10-15

1 0-l6 0

Probe 1 V Probe 2 Lockheed Scintillometer

........... Y ----

4 8 12 16

Time (hours) 20 24

Laser 0

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Angle (mrad)

I I I I I

; :: .. . . I. . I ... . . .. .

I I

Laser 1

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Angle (mrad)

a, 0 I= a,

a, > E?

n E

m E

.-

(u Q)

3 3 0

9 a, 0 S Q) 0)

i! b- €

m (u a,

4 , 0 Laser0 Low Divergence: 0.194 mr 0 Laser 1 /

Plane Wave / -- / S p h erica I Wave

Gaussian Beam -- . 3

2

1

0 10-15 I 0-14 I 0-13 10-12

2 4 High Divergence: 1.2 mr a, E?

.- B n E 3 m €

Plane Wave -.-. Spherical Wave

Gaussian Beam

----

(u a,

0 = 2

9 8 E ? I 2 5 E 8 0 m 10-15 I 0-14 I 0-13 10-

S a,

0.10 n s v .- 2 0 Z

0.05

0.00

- 0

. "J'. . -. UUIV. .ww Low Turbulence

0 . c).

9.0 9.5 10.0 10.5 11.0

Wavelength (prn)

0.06

0.04 n v

'E E. 0.02

-0 a, 0 -0 Q)

c .-

*: 0.00 d

I v)

-0.02 E 8 d

-0.04

-0.06

I I I 6.94 km FSA Target

00 0

0:

o 0

9.0 9.5 10.0 10.5 11.0

Wavelength (pm)

0.06

0.04

0.02

0.00

-0.02

-0.04

-0.06

I I I 3.39 km FSA Target Average of 6 measurements

9.0

0.06

0.04

0.02

0.00

-0.02

-0.04

-0.06

9.5 10.0

Wavelength (pm)

10.5 11.0

I I 6.94 km ~ ~ A T a r g e t Average of 5 measurement:

0 .t

9.0 9.5 10.0

Wavelength (pm)

10.5 17.0