The maximally attainable accuracy of an airpollution monitoring system is investigated. The system is composed of a tunable pulsed dye laser as the light source, photodiodes as the opto-electric converter and a low noise electronic signal processor specifically designed by the authors. The extreme value of the accuracy is given in terms of the standard deviation of the attenuation. The value is 3.4×10(-4)[Nep√(pulse number)] for an averaged value for multiple laser shots. Also the wavelength reproduceability of a dye laser was examined, which resulted in that a computer should take a part in the wavelength control in order that this method should be feasible

Koga, Ryuji

Kosaka, Megumi

Sano, Hiroya

Tanada, Yoshihiro

Okayama University Scientific Achievement Repository

Memoirs of the School of Engineering,Okayama University Vo1.13, February 1979A Perfonnance Analysis For An Air-PollutionMonitoring Using A Pulsed Dye LaserHiroya SANO*, Ryuji KOGA*, Yoshihiro TANADA*and Megumi KOSAKA*(Received February 7, 1979)SynopsisThe maximally attainable accuracy of an air-pollution monitoring system is investigated. The systemis composed of a tunable pulsed dye laser as the lightsource, photodiodes as the opto-electric converter anda low noise electronic signal processor specificallydesigned by the authors. The extreme value of the ac-curacy is given in terms of the standard deviation ofthe attenuation. The value is 3.4xlO-4[Nep/pulse number]for an averaged value for multiple laser shots.Also the wavelength reproduceability of a dye laserwas examined, which resulted in that a computer shouldtake a part in the wavelength control in order that thismethod should be feasible.1. IntroductionRaman scattering methods l ) had revealed to fail in gaseous air-pollution monitoring due to its inherently small Raman cross sectionsof the pertaining gas molecules. The resonance absorption whose crosssection is several orders larger than that of Raman, realizes a systemthat may achieve a real-time and sensitive air-pollution monitoring 2).Inevitable pulsed features of available high powered tunablelasers give rise to the idea of laser radar configuration with longlight pathlength, which can detect a few parts per billion of densi-* Department of electronics195196 Hiroya SANO, Ryuji KOGA, Yoshihiro TANADA and Meg-umi KOSAKAties for chromatic gaseous air pollutants.Application of the resonance absorption to a system which measuresthe absorption over a short pathlength, l[m], for instance, has beenexplained to be, however, impossible due to the noise arising in thepertaining electric and electronic apparatuses 3) •The authors have been occupied in the development of the electro-nic apparatuses which deal with electronic pulse signal in high signalto noise ratio4 ,5). Their experience led to an attempt to examine thefeasibility of the short-path length system for a point gas analyzerwhich is compact and gives local gas densities.This report deals with the minimal noise that may arise in thelight attenuation measurement for practical values of parameters.Also experiments have been made by a specifically built apparatus in-cluding a pulsed dye laser. Difference between theoretical resultsand that by experiments is attributed to the persisting electromagnet-ic impulsive noise. Earlier results of this subject have been report-ed in Japanese6 ) , but more convincing results are added along with thediscussions on the wavelength reproduceability which implies the span-accuracy deterioration in the resulting density values based on theexperimental data.2.1 principles and apparatuses used in experimentsNitric Oxide (NO) , sulfur dioxide(S02) and ammonia(NH 3) have dis-tinct absorption lines commonly in the 220-230 nm wavelength region,whereas other atmospheric constituents have rather continuous andsmooth spectral profiles there.Let a monochromatic pulsed laser light of energy PI and waveleng-th A be transmitted in polluted atmosphere over a path length L, to beattenuated to energy PT. Lambert-Beer's law implies.. (A) == In (PI /PT) = L ac.L B. (A) ,iE-I t. t.( 1)where .. is hereafter nominated as "attenuation" and c i ' B i are thedensity in parts per million and the absorption cross section of thei-th species, respectively. The coefficient a appears by replacingthe molecular number density Ni with the density c i .A Performance Analysis of Air Pollution MonitovingEmployment of adjoint spectra?) leads to197a.J~ f S1(A)·T(A)·dll (A)aL J ....,A(2)which stands for a numerical filter for the measured absorption spec-trum T. The notion of the adjoint spectra was introduced by the au-thors extending the well-known method of least squares.A measured absorption spectrum is inevitably associated withrandom noise n, asL + n. (3 )If the noise has no correlation with the wavelength nor with timeinstance, the variance of the obtained density c. becomesJVar(c.) Var(n) J {S1(A)}2dlJ.(A). (4)J (aL) 2 A JThis expression suggests that the variance depends on the specificadjoint spectrum employed. The adjoint spectrum is not unique. Thevariance of n is the value characteristic to a specific instrumentand is governed by a principle and the functioning elements employedtherein. A pourpose of this paper is to give the Var(n) value.2.2 Description of the examined apparatusOptical schematic diagram is shown in Fig.I.Fig. 1 Schema of the short pathabsorption method for the pointair-pollution monitoring.Beam~ splitter.0'"<:'" ....... ,'d ~ Po., ....OJ.... .,'" >,"''0PhotodiodeThe beam from a pulsed dye laser is splitted into two. One is moni-tored by a photo-diode to yield a charge Qr which stands for the ener-gy of the incident light pulse. The other goes through pollutedatmosphere of path L and is led to another photo-diode to give chargeQT' viz.,198 Hiroya SANO, Ryuji KOGA, Yoshihiro TANADA and Megumi KOSAKAexp(-t) exp(-E) ·exp(-.), (5)where the factor exp(-E) stands for an unbalance between the compositoptical and electrical efficiency of the two signal path.It is, however, improper to normalize QT by Qr directly when t issmall, which requires a large number of bits in the analog to digitalconversion prior to the division carried out by an digital computerand also requires the electronic circuits for impossibly large dynamicrange.In order to circumvent this difficulty, a particular circuit con-figuration was devised by the authors 5 ). The system named as the ins-tantaneous difference compensation (IDe) system, deals with the diff-erence signal, Qr-QT and the normalizing signal, Qr' instead of QTand QT' The t value is given as( 6 )When t is much smaller than unity, this approximation becomes correctand the range that Qr-QT spans small, or rather small dynamic range isrequired for the amplifier for the difference signal. A blockdiagramis shown as Fig.2.Pre-amplifiersFig.2 A Blockdiagram of the electronicsignal processor based on the rDesystem.""'tlo....'tl.s.8'"\\\\ ,,,,\ ,AccumulatorsThe gate functions as a time window of one microsecond duration, whichsuppresses the stationary shot noise induced by the background lightand the dark current of the photodiode.A Performance Analysis of Air Pollution Monitoving 199A part of the electronic circuits is mutually shared by the dif-ferencial and the normal signals in order to reduce the number ofelements and to retain some consistency between the two signals.Actually the gating function was performed by a hybrid Walsh waveformanalyzer which had been developed by a part of the authors 4 ).3. Schemes for noise suppression and its effects3.1 Mechanical vibration of the optical systemAngle perturbation of optical elements results in the fluctuationof E in Eq. (5). When an optical surface is in near right angle withthe incident light, this effect is small. The vibration of the beam-splitting surface is, therefore, the leading element for this effect.If the incident light beam is linearly polarized, the reflectance Ris determined by the incidence angle &, the polarization angle ~ andthe index of refraction n of the semi-transparent layer. For a parti-cular values of the parameters, an equationdd& R(&,~,n) o ( 7)holds, which means that a proper choise of the parameters gives riseto an immunity against mechanical vibration. The solutions of Eq. (7)are given by Fig.3, where the values of R are associated as welL.g. deg •<: 80 \ 10 6=60·........'"N.... 70""'"'" oi.-<0 0a. <:4-l '"60 0.5 ....0 0Fig.3 Optimal angle ~ between the"".-<.-< 4-lincident and the polarization planes 0'"<: .,;w.r.t. the index of refraction n, '" 50incidence angles &. .-<for some ~Resulting reflectance, R, are as- .... ~~....sociated. a. 40 001 3Index of refraction, n200 Hiroya SANO, Ryuji KOGA, Yoshihiro TANADA and Megumi KOSAKA---0-- VIBRATION APPLIED9060WITHOUT VIBRATION3010-6 L..........L_....L.._.l.-......L_...l.---::o............L_.l.-....JoFig.4 Experimental result for theeffect of the polarization angleof the incident light.POLARIZATION ANGLE [deg.]Figure 4 is the results of an experiment to exemplify this ef-fect. The incident light is linearly polarlized by a polarized filt-er whose extinction ratio is below 10- 3 . A beamsplitting cube wasused, whose transmission ratios were more than 45% for both transpar-ent and reflected beam. The cube was put on a cloth cushion whichoffers an intentional,unstable suspension, and the optical bench wasvibrated by an electric motor. Distinct rejection for vibration-induced fluctuation is seen at ~;60o. The angle setting was very cri-tical to find the optimal angle.3.2 Laser power fluctuation and the IDC systemThe laser power fluctuates from each pulse to another and withlong time duration. As the tunable light source, a dye laser whichis pumped by a pulsed nitrogen laser is used. Rating energy is 0.7[mJ] per pulse at 10 [pps] when dye, 7D4MC, is used as active medium.Figure 5 shows energies of each pulse of the laser output and Fig.G,long time transition.Fig.5 A record of laser outputenergy of every pulse.Q)::l.....Q) I1lIII >.....::l Q)a.>~"j .....0'>11l........Q) Q)l': ...Q)~...Q)III coj c:: L-~,---,~--,_ ......._-,-_-,,-_....._...J[32 pulses/div.]A Performance Analysis of Air Pollution Monitoving 201Fig.6 A long time record ofthe laser output.Qi;:l.....Q)lllUl>.....;:lQ)11.>...... ....>. ....I:1>lll .....~ .....Q)Q)J:;~Q)~~Q)UlIII IIIH a L---''----''-----I._........_ ........_-'-_-'-----J[10 min/div.]The laser energies are transformed to electric pulse heights bya photo-diode, and are fed to a digital waveform recorder. The fluc-tuation is decomposed into two constituents: the gaussian which hasno correlation between each pulses, and another, which has a l/f fea-ture including drift term. This is clearly seen in Fig.7 which showsthe resulting Var(t) versus m, the number of accumulation for boththe difference and the normalyzing signal prior to the normalization.The vertical axis is scaled in moVar(t). ~ihen m is small, the curveis parallel to the abscissa, which depicts the ergodic feature of thenoise. The curve, however, rises up as m increases, which is attribu-ted to the non-ergodic component in the noise.10-3,..-------------.....,Fig. 7 Dependence of mo Var ('0 onthe number of accumulation, m.10-5 L-_..L--J.....J...JL..J...L.U-L-_.L....-....L.....L..I-U~1 10 100NUMBER OF ACCUMULATIONS, mResidual noise of the IDC system due to the laser power fluctua-tion becomes(8 )where xp stands for the relative fluctuation of the laser power aroundits average value. The IDC residue, Eg. (8), is attributed to that the202 Hiroya SANO, Ryuji KOGA, Yoshihiro TANADA and Me~umi KOSAKAdifference and the normalizing signals are generated for the alterna-ting laser shots respectively in order to reduce the number of elec-tronic components. Full performance of the IDC system is, therefore,attained for ~ of unity.3.3 Noise in opto-electronic conversion and in the signal processingA photo-diode generates a stationary noise, which is of the shotnoise induced by the DC current, and of the thermal noise. Taking theinternal noise of the preamplifier into account by a notion of thenoise figure, a power spectral density p of the noise current thatflows through the load resistor becomesp ( 9 )where the variables stand forq : the charge of an electron,IB : background current,I o dark current,R load resistance,T absolute temperature,k Boltzmann's constant,F a noise figure of the pre-amplifier.Utilization of a time window with short time duration along witha pulsed laser makes it possible to reduce that stationary noise. Letx e be the ratio of the fluctuation of the output voltage of the sys-tem illustrated in Fig.8, to that of the averaged value.IntegratorTime window(S'~itch)Fig.8 Suppression of sta-tionary noise by a timewindow and an integrator.PhotodiodeSignalI s=QOo(tl.-7'StationarynoiseIT;.L ~ Output)f~GRLQoexp (-tiT f),,).A Performance Analysis of Air Pollution MonitovingThe variance of x p is calculated from a theory about the integral ofrandom signal over a definite time interva1 8 ) as203pTf x+e-x-l(11 Np Q) (l-e- x )(10)where x stands for the ratio of Tg to Tf.The data processing by a computer requires the analog to digitalconversion, which implies the quantization noise as1 ( ) 2 - 2MVar(xAD) = 12 Tmax '2 , (ll)where Tmax: the full scale of L value of the AD converter,M :number of bits of the digital output signal.The corpuscular feature of light results in the shot noise in thesignal charge, asVar (x ) = 1-11 ,q 11 Npwhere(12)the relative fluctuation in the signal chargethat is induced by the photo-conductive phoenomenonfor an incident light pulse,the quantum efficiency of the photodiode,the number of photons of the incident light pulse.3.4 Numerical and experimental resultsParameters of the apparatus built for test are listed in Table 1,and values of Var(f) calculated by these parameters, in Table 2.By the experiment, it was found to be difficult to control the unbal-ance II-EI within 10- 2 , due to technical difficulties in electroniccircuits. If the fluctuation in f is Gaussian, the variance of f redu-ces to 11m by averaging m laser shots. Taking this IDC residue intoaccount, the theoretical evaluation gives the variance of f as 3.4x10- 4 [Nep~m], whereas it should be 1.15 xIO- 7 [Nep2· m] for a hypothe-'tical function to reduce the unbalance of IDC.The experiment resulted in the variance of about 1.8xlO- 5 [Nep2.m], whereas the value was l.OxlO- 6 [ Nep2. m] for a simulation in thatthe photo-diodes were replaced by an electric pulse generator and thelaser system was stopped because it radiates intensive electro-magnet-ic impulse noise around it despite its careful shields. The differ-ence between the two values are considered to be of the IDC residue204 Hiroya SANO, Ryuji KOGA, Yoshihiro TANADA and Me/{umi KOSAKAand the electromagnetic noise.Table 1 Parameters for the calculationof the fluctuation in f.SNR in laser lutputPhoton numbers of incident lightPhoto diodesQuantum efficiencyDark currentBackground light currentLoad resistancea 2 (xp)Np2.21xlO- 31.5 xI0 9 (a)0.31 ].!A25 ].!A2xl0 3 QPre-amplifierNoise figureCorner frequencyAperture time of the time windowFl/TfTg101 MHz1].lS(b)Analog to digital converterFull scaleNumber of bitsInherent unbalance of the IDeAmbient temperatureMLl~T(c)(a) Limited by the range of linearity.(b) A presumable practical value.(c) Empirical value through the experiment.Table 2 Fluctuation in f calculatedfrom the Table 1.Eqs. Var(f) [xl Nep2]4(8) 4.42XIO- 7(12 ) 1.56xIO- 9(9) , (10) 1. 42xlO- 9(9) , (10) 1. 44xlO- 8(11) 1. 27xlO- 10TermsQuantization noisein AD conversionThermal and amplifiernoiseShot noiseby signalby background lightand dark currentIDC residueTotalA Performance Analysis of Air Pollution Monitoving4. Wavelength reproduceability of the dye laserA wavelength deviation 6Ak from a prescribed value Xk causes anerror in the final result of c. after the numerical filter, viz.,1-2056c.1-( 13)where the integration and the adjoint spectrum are replaced by the sum-mation and sik' respectively. The suffix k stands for the pertainingwavelength Ak •Taking only a species of gas absorption spectrum into considera-tion, the 6s i ( Ak ) is denoted as(14)In order for 6s i be zero for a 6Ak , the nominal wavelengths Xk , ksKshould be chosen so that an equation(15)holds, which reduce the effect from the inherent error in wavelengthsetting. After this has been performed, the second term of Eq. (14)becomes dominant.The dye laser employed in this experiment is equipped with agrating as the tuning facility, which is driven by a step-motor accord-ing to digital signals from a computer. However a manual establish-ment of the initial wavelength on the system startup is necessitated.The spectral profile of the y(O,O) band of nitric oxide is composedof discrete and separated lines each of which corresponds to a specificelectronic transitiqn and rotational-vibrational subleve19). Repeatedmeasurements of the spectrum for a line results in the wavelength ac-curacy within 0.04 cm- 1 of standard deviation with the step-motor cont-rol. This line is of Q(15) branch of the electronic A2E+-~rrl/2 tran-sition. The reproduceability of the manual wavelength setting on thesystem startup is within a few kayser which is enough small to identifyeach absorption line of nitric oxide. By comparing the line profile ofthe measured with that given by Tajime et aZ. 9 ), the line profile isrevealed to be governed by the laser power spectral profile rather thanthat of nitric oxide.206 Hiroya SANO, Ryuji KOGA, Yoshihiro TANADA and Megumi KOSAKAThe absorption spectra of ammonia and sulfur dioxide under theatmospheric pressure are taken also, both revealed to be of continu-ous spectra. This is compatible with the line profile in infraredregion taken by a ultra-high resolution diode spectrometer lO )Typical values of 8= lS~(Ak) for the three gases are listed in2 1.-Table 3, which implies that the 6c i value determined by Eq. (13) takessome dozens percents of the rated t value for a wavelength error intro-duced by mannual setting. The error from the step-motor control is,however, much less than this. An algorithmic wavelength control execu-ted by a computer is, therefore, necessary and will enables the systemto obtain a value of t(Ak ) that is free from an error induced by thewavelength missettings. The volume of the program is considered to bemoderate for the installation to a microcomputer, though are they notmade practically.Table 3 Values of 8 for characteristicpeaks of NO, 502 and NH 3 spectra.Gas species Wavelength [nm] [3 [ (pmr2 ]NO y(O,O) bamd 1. 26xlO- 1NH3 224.47 8.75xlO- 3S02 225.75 1.98xlO- 35. Concluding remarksThe attainable accuracy of the short pass absorption measurementis 3.4 x lO- 4 [Nep·;Pulse nurnEer] of the standard deviation, which isstill dominated by an error in the specific electric-circuit con-figuration, IDe system, which is developed by the authors. The valuecorresponds to 0.63 [ppm.m.;Pulse-numoer] for the density measurementfor nitrogen dioxide at wavelength of 462.8 [nm].The wavelength reproduceability of the dye-laser employed istolerable on the assymption that a computer pertains to the wavelengthcontrol.The principal direction to further reduce the fluctuation in anobtained density values is to use a photo-diode which may respondlinearly to a light pulse of higher energy. The difficulty in thewavelength reproduceability can be overcome by an employment of a refe-rence cell which contains pure nitric oxide in reduced pressure.A Performance Analys is of Air Pollution MonitovingReferences1) H. Inaba: Laser Monitoring of the Atmosphere (E.D. Hinkley, Ed.),(Springer, 1976), p.153.2) R. L. Byer and M. Garbuny: App1. Opt., 12(1973).3) T. Tsuji, H. Kimura, Y. Higuchi and K. Goto: Jap. J. App1. Phys.,15 (1976) 1743.2074) Y. Tanada, and H. Sano: Trans. IECE, J59-A(1976) 101(in Japanese).5) H. Sano, Y. Tanada, R. Koga and T. Ono: J. IlIum. Eng. Inst. Jap.,61(1977) 35 (in Japanese).6) H. Sano, R. Koga, and Y. Tanada: to be published in Trans. Soc.Inst. Contr. Eng. (in Japanese).7) R. Koga, Y. Tanada, H. Sano: J. Spectroscop. Soc. Jap., 27(1978)297(in Japanese) .8) W. B. Davenport, Jr., and W. L. Root: An Introduction to The Theoryof Random Signal And Noise, (McGraw-Hill, 1958).9) T. Tajime, T. Saheki, and K. Ito: Appl. Opt., 17(1978) 1290.10) G. A. Antcliffe and J. S. Wrobel: App1. Opt., 11(1971) 1543.

A Performance Analysis For An Air-Pollution Monitoring Using A Pulsed Dye Laser

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