Air pollution monitoring with a Q-switched C0 2 -laserlidar using heterodyne detection

Stefan Lundqvist, Carl-Olof Falt, Ulf Persson, Bo Marthinsson, and Sverre T. Eng

A differential absorption lidar (DIAL) using a Q-switched CO2 laser and a heterodyne receiver has been de-veloped. The DIAL system is highly automated with computer-controlled laser-line selection and signalprocessing. The transmitter operates at a pulse-repetition frequency of 20 kHz and has an average outputpower of 1.8 W. A wideband HgCdTe detector is used together with a high speed adding buffer to detectthe return signals. The system has been used in a field experiment to monitor ethylene emission from a pet-rochemical factory.

I. Introduction

The growing concern over the effects of the releaseof pollutant gases in the ambient air and the cost ofproduction losses due to undiscovered leakages has in-creased the need for operational versatile air pollutionmeasuring systems. The use of a CO2 differential ab-sorption lidar (DIAL) operating against a topographicaltarget permits monitoring of several interesting pollu-tants, such as ethylene, ozone, ammonia, and vinylchloride, without the need to position remotely coop-erative retroreflectors.

Several groups have reported measurements of airpollutants, using high power CO2 TEA lasers as theradiation source. 1 4 In many monitoring situationsthere is a demand for eye-safe operation, which hasmade it interesting to investigate the use of low powertransmitters together with more sensitive receivingsystems. Killinger and Menyuk5 have reported mea-surements of ethylene using a high pulse-repetitionfrequency mini-TEA CO2 laser together with high speedaveraging of the directly detected return signal.5 Co-herent heterodyne detection of pulsed laser radiationat 10.6 Aim has been shown to provide an SNR severalorders of magnitude higher than incoherent detection.6 7

The increased sensitivity of this detection system allowsthe use of lower transmitter output power and providesan increased range capability of the DIAL system.

In 1974 it was suggested by Inaba and Kobayashi8that a high PRF transmitting laser together with a

The authors are with Chalmers University of Technology, De-partment of Electrical Measurements, S-412 96 G6teborg, Sweden.

Received 2 January 1981.0003-6935/81/142534-05$00.50/0.© 1981 Optical Society of America.

heterodyne receiver and high speed averaging shouldbe used in a DIAL configuration to monitor releases ofair pollutants. However, until recently the necessaryequipment to operate an electrooptically Q-switchedCO2 laser at repetition frequencies above 10 kHz has notbeen readily available.

This paper describes a differential absorption lidarusing a Q-switched CO2 laser and a heterodyne receivertogether with a high speed adding buffer. The DIALsystem is highly automated with computer-controlledlaser-line selection and signal processing. The systemhas been used to measure ethylene emission from asteamcracker plant.

II. Measurement PrincipleIn the following we describe open long-path single-

ended absorption measurements using topographicaltargets. This measurement method provides highsensitivity at low required transmitted power withoutthe use of a cooperative reflector.

A laser beam propagating through the atmosphere isattenuated exponentially according to

PR()) = exp[-k(X)L], (1)PoG\)

where PO(X) is the transmitted power, PR (X) is the re-ceived power, L is the measured distance to the target,and k (X) is the total average extinction coefficient at thewavelength X, which is composed of a sum of absorptionand scattering contributions.

If we have N different gases where gas number n hasthe average concentration Cn and absorption coefficientan, we can write

P () = TR(G) exp -L[[aM() + E an(X)Cn], (2)PO(X) n1

where R (X) is the reflectivity of the topographical target,

2534 APPLIED OPTICS / Vol. 20, No. 14 / 15 July 1981

T is a measure of the constant attenuation in the ex-perimental setup, and aM(X) is the Mie extinctioncoefficient. To eliminate the system losses T, we haveto measure at two different wavelengths. We thenobtain the concentration of one of the gases, providedthat interference from other gases and the wavelengthdependence in the Mie extinction coefficient can beneglected. We also assume that the differential re-flectivity of the topographical target is sufficiently smallto give negligible error in the calculations of the differ-ential absorption. The influence of the turbulence onthe measurement result can be reduced by using fastwavelength switching, keeping X1/X2 1 or by long timeaveraging. 8

The optical layout of the DIAL system is shown inFig. 1. A Q-switched grating-tuned CO2 laser is usedas the transmitted radiation source. The heterodynereceiver requires a continuous CO2-laser source toprovide the local oscillator signal for the detector. Thislocal oscillator signal comes from a second grating-tunedCO2 laser, which is frequency locked to the transmitterlaser. To close the frequency control loop, the beamsfrom the two lasers are mixed in a separate referencedetector. The heterodyne receiver is operated near thequantum limit, giving good range capability with lowtransmitter output power.

The signal from the receiver detector can, after en-velope detection, be written as

VRi = (PLiPR0)1", (3)

where PRi is the received power and PLi is the local os-cillator power at the wavelength Xi. The envelope de-tected reference detector signal is

Voi = g(PLiPoi)'/1,

Fig. 1. Optical block diagram of the coherent lidar system.

Fig. 2. Block diagram of the electronics for the coherent lidar system,including the computer, laser control, and the signal electronics.

(4)

where Poi is the transmitter output power at Xi and gis a constant depending on the optical configuration.

Measuring at two wavelengths we obtain the gasconcentration from Eq. (2):

L[a(X2) - a(Xl) PR2PO1)

where L is the path length calculated by the systemfrom the measured time of flight of the laser pulse.

Using Eqs. (3), (4), and (5), we can write

L[C (X2 ) -a(X)l] VR2n ivV- (6)

Thus by measuring the signals from the reference andreceiver detectors, the system can calculate the averagegas concentration over the measurement path.

Ill. Experimental ApparatusA block diagram of the DIAL system electronics is

shown in Fig. 2. The CO2 lasers, designed and built inour laboratory, have stepper-motor controlled gratingadjustments and piezoelectrically controlled cavitytuning and are passively temperature stabilized withInvar rods. Since both the stepper motors and thepiezoelectrical translators are controlled by the com-puter, it is possible to select one line out of fifty-four

available ones and to trim the cavity of the transmitterlaser under computer control. The local oscillator laseris operated sealed off. To achieve high gain at thepulse-repetition frequency used, the Q-switchedtransmitter laser is operated with a flowing gas system,giving an average output power of 1.8 W at a pulse-repetition frequency of 20 kHz. A CdTe electroopticalmodulator together with a high-voltage pulse generatoris used for the Q -switching. This technique provideshigh pulse repetition rates and easy adjustment of thepulse shape. The FWHM value of the pulse is 125 nsec.This pulse width is determined by the laser itself. Thelength of the pulse tail can be set by adjusting the drivepulse to the electrooptic modulator.

The beam from the Q-switched CO2 laser is trans-mitted along the optic axis of a 30-cm diam Newtonianreceiving telescope and has a diameter of -7 cm. Thisconfiguration was chosen to give isolation of the trans-mitter and the receiver. Since the received signal isreturned from a rough surface, we use a receiver that hasa large aperture with a narrow field of view, which justcovers the illuminated spot.9 Due to the distortion ofthe received wave front by atmospheric turbulence, avery small improvement in the SNR will result frommaking the aperture diameter larger than a certain ef-ficiency saturation diameter r. An expression for ro

15 July 1981 / Vol. 20, No. 14 / APPLIED OPTICS 2535

has been obtained by Fried.10 Assuming strong tur-bulence (C = 10-'4 m-213), a total path length of 5 km,and a wavelength of 10 gum, we find that ro becomes 30cm.

Degnam and Klein" have calculated the minimumheterodyne detection loss for various obscuration ratios(the ratio between the diameters of the primary andsecondary mirror) and different local oscillator distri-butions. Their results as applied to our system, whichhas an obscuration ratio of 0.25, indicate that the ob-scuration loss is <1.5 dB.

A portion of the transmitter-laser beam is fedthrough a small hole in the transmitter mirror andmixed with the local oscillator beam in a HgCdTe ref-erence detector. The signal from this detector is am-plified and fed to the IF controller, which is operatedsynchronously with the Q-switched laser via the systemclock. The IF control maintains a constant heterodynebeat frequency of 15 MHz. The pulse tail of thetransmitter laser is adjusted to give good frequencycontrol loop performance.

A HgCdTe detector is also used as the first mixer inthe heterodyne receiver to detect the backscatteredlaser radiation. With a local oscillator power of 1 mWand a bias voltage of 100 mV, the noise equivalent powerwas 5 X 10-20 WHz-1.

The IF signal from the heterodyne receiver is am-plified in a 5-100-MHz bandwidth, envelope detected,and then digitized in an 8-bit analog to digital converter(ADC) that has a maximum sample rate of 20 MHz.The signal averager is a fast-adding buffer memory. A16-bit adder performs 5 million adding operations persecond and stores the result in a 256-word 16-bit ECLmemory. The numbers of integrated buffers can be setunder computer control and depend on the dynamicrange of the received signal. A normalization signal forgas concentration measurements is obtained by digi-tizing the envelope-detected signal from the referencedetector.

The program structure of the software controlling theoperation of the DIAL system is shown in Fig. 3. Theprogram is interactive and asks the operator to give acommand that invokes a subroutine, which carries outthe needed operations. A measurement starts with thecomputer setting the lasers to operate on the selectedline. The transmitter output power is maximized bythe computer using the digitized value from thepower-monitoring pyroelectric detector. The IF con-troller sets the LO laser to give the selected intermediatefrequency. Measurements are carried out in such a waythat successive data points are added together andstored. The number of samples in each measurementcan be selected by the operator, permitting fast or slowaverages. The number of laser shots summed in ameasurement depends upon the reflectivity of the to-pographic target.

Computation of the gas concentration correspondingto the measured differential absorption is performedafter the measurements have been collected and stored.Data can be displayed numerically on a printer orgraphically on a plotter, on an oscilloscope, or on a video

Fig. 3. Program structure for the software controlling the DIALsystem. There are five groups of commands: laser control; mea-surements; computations; display; and file handling. There are threedata buffers: reference data (REF); absorption line data (ABS); andcomputed results (RES). Measurements are controlled by thePARAM and MEASURE commands. Computation of the gas con-centration corresponding to the differential absorption is performed

by the COMPUT command.

D

a

-L

0 3000DISTANCE Em]

Fig. 4. Return signal when the lidar beam was aimed through themist of a cooling tower plume. The echo at 2 km is from the hillside

along the line of sight beyond the cooling tower.

terminal. The system has an autoplot facility, whichcontinuously produces an average in real time on anoscilloscope. This ability is very useful when aimingthe lidar, since the signal level produced from a singleshot is very low.

To be able to save data for later computations ordisplay, one can move data to data files on a flexibledisk. Data in a file can be subtracted from the data ina buffer, permitting the removal of unwanted zero sig-nals from the measured data buffer.

IV. Results and Discussion

The DIAL system was evaluated in a three-week fieldexperiment at Stenungsund, Sweden, in June 1980. Inthis experiment the optical system and the computerwere mounted in a van, and the laser beam was trans-mitted through the open rear door. The measurementsite was just outside a steamcracker plant producingethylene and propylene.

One most prominent feature of this system is the fastaveraging capability. Figure 4 shows the integrated

2536 APPLIED OPTICS / Vol. 20, No. 14 / 15 July 1981

- - - - - - -

LaL

4-'

a

-)

Mf

0

CO

L

_

L

-

LC-

4-'

In

0

DISTANCE ml

DISTANCE ml

3000

3000

Fig. 5. Normalized return signals from a hillside at two CO2 laserwavelengths, P(14) and P(20), of the 10.4-,um band. The ratio of thetwo signals was used to calculate the average amount of ethylene along

the optical path.

L

aLC

-

C,

0DISTANCE ml

Fig. 6. Return signal from an aerosol plume above the steamcrackerplant. The data shown are the results of 4 X 106 measurementsperformed in a 7-min time period. The dotted line is the averaged

signal level.

return of 1.5 million laser shots, requiring 2.5 min toperform. During this measurement the lasers wererunning on the P(20) line in the 10.4-Am band of CO2.The large echo at 350 m is from the mist above a coolingtower in the plant. Also seen in this figure is a returnsignal from the hillside, 2 km behind the crackerplant.

Ethylene measurements were made using a 1650-mmeasurement path through the steanicracker plant.

We used the P(14) and P(20) laser lines in the 10.4-Mmband. This choice gives a very small interference fromwater vapor. The absorption coefficients used in thesemeasurements have been measured in our laboratory.12The P(14) and P(20) lines have absorption coefficientsof 35.8 and 2.18 cm-1 ati-1, respectively. Using a brickwall as a target, we integrated 1 min on each line andobtained the normalized return signal shown in Fig. 5.This corresponds to a path average ethylene concen-tration of 113 ppb. The SNRs using this integratingtime were 21 and 10 dB for the P(20) and P(14) line,respectively.

To test the DIAL system's ability to detect aerosolbackscatter, we performed measurements over a slantpath high above the steamcracker plant. Since thevisibility during the time of these measurements wasvery high, we wanted to utilize the aerosol plumeemitted from the plant. The return signal shown in Fig.6 was obtained by integrating over 4 million pulses,which takes 7 min.

A 700-m wide aerosol plume can be seen over thefactory area. The clear peak in the beginning wascaused by cooling-tower mist drifting into the mea-surement path. Due to the low signal level in thismeasurement, we obtained large digitizing errors. Thedotted line indicates the averaged signal level. By in-creasing the video amplification and thus making better

15 July 1981 / Vol. 20, No. 14 / APPLIED OPTICS 2537

P(14)

-- - - - -

.

, , ,

use of the dynamic range of the ADC, one could makethis error very small. However, if this system were tobe used for range-resolved air-pollution measurementsusing aerosol backscatter, the transmitter laser outputpower should probably be increased by at least a factorof 10 to permit shorter averaging times.

V. Conclusion

A Q-switched C02-laser lidar with heterodyne de-tection has been built and used to monitor ethylene bylooking at the backscattered radiation from topogra-phical targets. The system uses a computer for real-time signal processing and control. The field experi-ment showed that the DIAL can successfully be used toperform single-ended monitoring of diffused air pollu-tants using only 1.8-W average output power. Resultsfrom experiments where the backscattered radiationfrom aerosols was measured indicate that range-re-solved air-pollution measurements could be performedusing this technique.

The authors would like to acknowledge the technicalsupport of H. van Ginhoven. They also thank M.Shumate for his many helpful suggestions. This workwas supported by the Swedish Defense Research In-stitute (FOA).

This is an amended version of a paper presented atthe Topical Meeting on Coherent Laser Radar for At-mospheric Sensing, Aspen, Colo., 1980.

References1. E. R. Murray and J. E. van der Laan, Appl. Opt. 17, 814

(1978).2. K. Asai, T. Itabe, and T. Igarashi, Appl. Phys. Lett. 35, 60

(1979).3. W. Baumer, K. W. Rothe, and H. Walther, "Range-Resolved

Measurements of Atmospheric Pollutants," in Proceedings,Ninth International Laser Radar Conference, Munich, 2-5 July1979 (DFVLR, Munich, 1979), pp. 200-202.

4. P. L. Kelley, D. K. Killinger, N. Menyuk, A. Mooradian, P. F.Moulton, and W. E. De Feo, "Development and PreliminaryOperation of a 5- and 10-Am DIAL system," in Proceedings,Ninth International Laser Radar Conference, Munich, 2-5 July1979 (DFVLR, Munich, 1979), pp. 209-210.

5. D. K. Killinger and N. Menyuk, in Digest of Topical Meeting onCoherent Laser Radar for Atmospheric Sensing (Optical Societyof America, Washington, D.C., 1980), paper ThC3-1.

6. J. W. van Dik et al., Digest of Topical Meeting on Coherent LaserRadar for Atmospheric Sensing (Optical Society of America,Washington, D.C., 1980), paper ThB1-1.

7. J. M. Cruichshank, Appl. Opt. 18, 290 (1979).8. A. G. Kjelaas, P. E. Nordal, and A. Bjerkestrand, Appl. Opt. 17,

277 (1978).9. G. A. Massey, Appl. Opt. 4, 781 (1965).

10. D. L. Fried, Proc. IEEE 55, 57 (1967).11. J. J. Degnan and B. J. Klein, Appl. Opt. 13, 2397 (1974).12. U. Persson, B. Marthinsson, J. Johansson, and S. T. Eng, Appl.

Opt. 19, 1711 (1980).

Students Present and Spectroscopists PastAt an afterdinner talk at the Eleventh Okazaki Conference on The

Molecular Potential Function-Present Status and New Aspects, R.Norman Jones, who retired from the National Research Council ofCanada about 2 years ago and is now a guest professor in the De-partment of Chemistry at the Tokyo Institute of Technology, ad-dressed the problems of cross-cultural communication with his stu-dents and, in an unrelated topic, talked about spectroscopists: past,present, and future.

Jones noted that Japanese students, while proficient in reading andwriting English, indeed a sizable number of them submit their doc-toral theses in English, rarely have to speak it. His students are aninterdisciplinary group including solid state physicists, physical andorganic chemists, biochemists and geochemists, and therefore, hislectures are broadly based emphasizing the analytical uses of infraredand Raman spectroscopy. Lecture notes, slides, and transparenciesaid the students. However, getting the students to participate inEnglish is a problem and is influenced by long traditions based on theold Chinese teacher-student relationships. To overcome this prob-lem Jones set up a chemistry colloquium in English in which thestudents talk English to one another rather than to him.

The colloquium met weekly, and at each session two students gavea 15 min talk in English about their own research problems. Thespeaker was introduced by one of three members of the ProgramCommittee for the colloquium. Occasionally one or two studentsfrom another university in Tokyo were invited to be guest speakers.Because the Japanese students felt obliged to make sure that the guestspeaker had a good discussion afterwards, conversation was generatedand tensions from speaking in a foreign language relaxed.

Jones's other topic was Spectroscopists: Past, Present, and Future.He presented biographical sketches of two of the men who built 19thcentury infrared spectroscopy: W. W. Coblentz and William Her-schel. Jones noted the lack of biographic or autobiographic data oncontemporary scientists suggesting that their correspondence, labo-ratory records and notebooks, grant applications, and personalmemorabilia are not preserved. He noted that the Royal Society ofLondon, the American Institute of Physics, and the AmericanChemical Society were trying to do something about this. In addition,the Coblentz Society had organized through its Archives Committeea Documentation Center for the History of Molecular Spectroscopy,now located at the Library of Bowdoin College in Brunswick,Maine.

2538 APPLIED OPTICS / Vol. 20, No. 14 / 15 July 1981