DIODE LASER-RASED SENSOR FOR 

FAST MEASUREMEN'I OF BINARY GAS MIXTURES 

Kevin L. McNeshy, R. Reed Skaggs, Jeffrey B. Morris, 

Brian Kennedy. Andrzej W. Miziolek 

US Army Rcseiirch Lahor:itory 

Aberdeen Proving Ground, MD 2 lOOS-SO66 USA 

W. M. Jackson 

Department of Chemistry 

University of California-Davis 

Davis. CA OS616 USA 

Ian A. McLaren 

McLaren Research 

Mountain View, CA 94043 USA 

ARSTRACT 

The develupnient and chnracterizntion of a gns sensor to iiieiisure hinary mixtures 11f oxygen and the vapor fr111n II 

hcrics (if volatile organic compounds. with ~i time resolution 111 10 msec, is described. The hensor is diode laserhnsed, all signal procehsing is perfomied disitully. and control of the setisor and electronics. data collection, and real 

titiic readout is by laptop computer. 'The discussion will tbcus on the methods used hr laser selccti~ii. phase 

\eiisitive detection. data storage, nnd concentration riieasurements f11r species that do not posess well-rcsi~lvcd 

riir;itional-vibrational spectra. 

INTRODUCTION 

For many automated fire suppression systems onboard military vehicles, the nominal time-tofire-out. after detection by thermal or light sensors. is 250 msec. Because of this critical requirement. detailed evaluation of new fire suppression systems installed onboard military vehicles 

requires measurements of fuels. oxidizers. fire suppressants, and combustion products with a 

time resolution at least as Past BS the fire suppression event. For the equipment and testing 

described here, the period of measurement is IO msec or faster. Because of the extreme hostility 

of the testing environment, the laser head and contrnl electronics need to be in a safe environment remote from the test site, and the sensing probe needs to be inexpensive and readily 

replaceable. Other performance characteristics desired lor the system are similar to those for an 

"ideal" industrial gas monitor [I]. These include the ahility to measure correctly without 

interference from other gases and from airborne particles such as dust and smoke: the ahility to 

compensate automatically for pressure and temperature fluctuations; and high wavelength 

stability for extended times, high reliability, being relatively insensitive to vibration, operation at 

room temperature, and being composed of components with wide availability. 

This paper describes efforts towards the development of a high measurement speed binary gas 

sensor for simultaneous quantitative measurements of fuel and oxygen concentrations. The 

method of detectinti relies upon absorption of radiation near a wavelength of 760 nm by oxygen. 

and absorption of rddiation near a wavelength of 1700 nm by hydrocarbon based fuels [2J. These 

spectral regions were selected because of the commercial availability of room temperature lasers 

and detectors operating in this spectral range and because laser radiation in this spectral range 

H:ilon Optim? Tcchnic:d Warking Conterencc 21-3 April I999 19 I 
may be easily transported at low loss using inexpensive optical fibers. The sensor development 

consists of two concurrent parts. Part one consists of development of the method of measurement for gases with and without well-resolved rO-VibrdtiOnal transitions, and the design and 

construction of the light source and fiber optics necessary for remote measurement. Part two 

consists of the design and implementation of the instrument control electronics, and the interface 

to allow instrument control and data acquisition using a laptop computer. These efforts are 

described in this paper. 

BACKGROUND 

Most gas phase concentration measurements using near infrared tunable diode laser absorption 

spectroscopy use wavelength modulation spectroscopic (WMS) techniques [3,4] or balanced 

ratiometric detection (BRD) [5,6]. Wavelength modulation spectroscopy (WMS) typically 

employs distributed feedback (DFB) tunable diode lasers. High sensitivity is achieved by phase 

sensitive detection at frequencies (KHz to MHz) where noise contributions to signal are minimized. Detection is typically made at twice the measurement frequency, and absorption features 

have a second derivative lineshape. Measurements are made using a single probe laser beam. 

Balanced ratiometric detection (BRD) uses a balanced photoreceiver to cancel laser intensity 

noise. To use this technique, a signal and reference beam are required. These are typically 

generated by splitting the output beam from a single laser source. It is necessary to balance the 

DC photocurrent from signal and reference lasers, in the absence of any absorption, prior to 

making a measurement. The noise cancellation circuit subtracts the common noise in signal and 

reference beams. Differences in photocurrent level caused by attenuation of the sigal beam 

intensity are reported as signal. This technique has the advantage of reporting the true absorption 

lineshape, and it may be employed with widely tuneable lasers such as external cavity tunable 

diode lasers, which are not suitable for use with phase sensitive detection at high modulation 

frequencies. 

Both of these techniques give good results for concentration measurements of molecules with 

narrow, well-resolved ro-vibrational absorption lines, such as hydrogen fluoride, water, or carbon 

monoxide [7]. Large and/or heavy molecules may not posses well-resolved ro-vibrational absorption bands because of overlap between fundamental, combination, and overtone vibrations, 

and because of rotational constants that may be less than typical gas phase linewidths [SI. Figure I shows the near-infrared vapor phase absorption spectrum of the hydrocarbon-based turbine 

fuel JP-8 near a wavelength of 1700 nm (measured using a Fourier transform spectrometer), and 

the calculated absorption spectrum of oxygen near a wavelength of 760 nm 121. For JP-8 vapor, 

the absorption spectrum near a wavelength of 1700 nm is essentially a continuous hump. 

Because of the lack of well-resolved rO-VibI'dtiOnal transitions in the absorbance spectrum of JP-8 

vapor (Figure I), conventional WMS is not a practical method for determination of vapor phase 

concentration of JP-8 fuel. Also, at this time, external cavity tunable lasers are not currently 

available near a wavelength of 1700 nm. 

192 Halon Options Technical Working Conference 27-2') April IYYY 
0 007 

JP-8 vapor, 10 cm path length 

355K, 4cm~' resolution, 

256 scans. 

0005 

'I 0 007 

1270 1370 1470 1570 1670 1770 1870 1970 

Wavelenglh (nanometers) 

3.00E-02 

2.50E~02 

2OOE02 

150E 02 

2 1 OOE 02 

c 

YI 

I 

5 OOE 03 , 0 00E+00 

759 759.5 760 760.5 761 761.5 762 762 

Wavelength (nanometers) 

O,, 1 m path length, 

296K, calculated using 

HITRAN database. 

Figure 1. The measured near-infrared absorbance \pectrum in the wavelength region near 

1700 nm of JP-8 vapor at 3SSK. and the calculated abwrbance spectrum in the 

wavelength region near 760 nm of 02. 

PHASE SENSITIVE DETECTION 

The proposed method for detection of JP-8 vapor, and all other detection nicthods based upon 

phnse-sensitive detection, rakes advantage of the followin_e relations: 

,,nl m(sin at)n(sin bt) dt = 0 a#b (1) 

2C ,?j ni(sin at)n(sin bt) dt = Cmnrr a=b (3 

For this discussion, a and h are laser amplitude modulation lrequencies. t is time, m and n are 

amplitudes. and C represents the number of periods over which the integration is carried out. 

If the signal of interest is sinusoidal at frequency a (in Equation I), upon multiplication of this 

periodic signal by a refcrcnce sinusoid at frequency b, the resulting integral will be nonzero only 

when a=b. If the measurement is made at a frequency at which the Fourier component of the 

broadband noise in the signal is small. the integral of the product of the signal and the rcferencc 

H:ilon Options Technical Worhinf Conlerenoc 27-29 April I999 193 
will exclude broadband signal noise, and only include the signal noise component at the reference frequency. Furthermore, if the signal amplitude is proportional to the desired quantity being 

measured, the result of the integration in (Equation 2) yields a value directly proportional to the 

quantity of interest. 

EXPERIMENTAL DESIGN 

The proposed method uses two fixed frequency amplitude modulated lasers, each modulated at 

the same frequency hut out of phase by 180 deg.* These two lasers are combined to produce the 

probe beam. Ideally, the combined output of the two lasers will have an amplitude that is 

invariant with time (in the absence of absorption of either laser output intensity). 

The key to the method is selection of one laser wavelength coincident with a wavelength absorbed by the gas of interest, and selection of the second laser wavelength in a region where no 

absorption is anticipated. When the mixed, modulated probe beam is passed through a gas which 

preferentially absorbs the laser radiation from laser one, the signal at the detector is modulated at 

the laser modulation frequency and, for small absorptions, has an amplitude proportional to the 

concentration of absorbing gas. Figure 2 shows the calculated signal at the detector for a 

moderately strong absorbing gas for square wave modulation of the probe laser beam 

components. 

MODULATION DESIGN 

A time (t) dependent square wave, S(a,n,t) at frequency a with amplitude n may be expressed as a 

sum of sine waves: 

S(a,n,t) = 0.636nsin(at) + 0.212nsin(3at) + 0.1273nsin(Sat) + .... (3) 

The signal at the detector, in the absence of any absorption, from two combined square wave 

amplitude modulated lasers differing in phase by 180 deg is: 

Sl(a,n,t)+S’(a,m,t) = (0.636nsin(at) + 0.212nsin(3at) + 0.1273nsin(Sat) + .... ) + 

(0.636msin(at+x) + 0.212msin(3at+x) + 0.1273msin(Sat+n) + .... (4) 

Using the relationship -sin(ax) = sin(ax + x), multiplying by the reference sine wave (frequency 

locked to the laser modulation) and integrating gives: 

2C OnJ (S l(a,n,t) + S2(a,m,t))psin(at) dt = Cpx(.636n-.636m) (3 

Here, p is the amplitude of the reference sine wave and C is the number of periods over which 

the integration is carried out. Equation 5 shows that for a probe beam composed of modulated 

lasers with equal amplitude, the signal at the detector is zero in the absence of any absorption. 

* Mark Allen, personal communication, Physical Sciences, Inc., 1999. 

194 Halon Options Technical Working Conference 27-29 April 1999 
6.00000E~01 - E 5.00000E~Oj m 'si 4.00000E~01 - E 3.OOOOOE-01 - 2 2.OOOOOE-01 - & 1.OOOOOE-01 - iij O.OOOOOE+OO 7 

0 0.0002 0.0004 0.0006 0,0008 0.001 

Time (sec) 

~- 

0.0012 

- 

Signal at Detector - strong absorption 

. 6OOOOOE~Ol 

I ". 

c, = 3~00000E~01 

5 200OOOE-01 ~ 

& 1.00000E-01 

O.OOOOOE+OO 

m 

.- 

0 0,0002 0.0004 0.0006 0,0008 0001 

Time (sec) 

0~0012 

Figure 2. The calculated signal at the detector for a moderately strong absorbing 

gas for square wave modulation of the probe laser beam components. 

When the amplitudes of the modulated lasers differ. then the zero absorption signal must be 

corrected for offset. When the output radiation of laser two is preferentially absorbcd by an 

analytc gas in the path of thc probe beam, Equation 5 becomes: 

2C ,?j (S I(a,n,t) + S?(a,m,t)) (exp-acl)psin(bt) dt = Cpn(.636n-.636m(exp(-acl))) (6) 

Where a is the absorption coefficient of the analytc pas (cm'/molecule), c is analyte gas concentration (molecules/cm'), and I is the probe beam path length through thc analytc gas (cm). When 

the product acl << I, exp(-acl) may be rewritten asl-A, (wherc A is absorbance = (acl)) so 

Equation 6 may be rewritten: 

3C ,?j (Sl(a.n,t) + S2(a.m.t)) (exp-acl)psin(bt) dt = Cprr(.636n-.636m + ,636mA) (7) 

For component lasers of equal amplitude, Equation 7 becomes: 

2C ,?I (S l(a,n,t) + S?(a.m.t)) (exp-acl)psin(ht) dt = ,636CpnmA (8) 

Halon Oplion\ Technical Working ('ontcrcncc 21-21) April IYYY I 95 
Equation 8 shows that the summation of the amplitudes of the two out of phase lasers is directly 

proportional to the absorbance, A, and hence to the concentration of the analyte gas. Additionally, the signal is directly proportional to the amplitude of the reference sine wave, to the base 

amplitude of the detector laser signal, and to the number of periods over which the integration is 

camed out. Using sine wave modulation of the component lasers instead of square wave modulation changes Equation 8 to: 

2C ,yj (S I(a,n,t) + S2(a,m,t)) (exp-acl)psin(bt) dt = CprrmA (9) 

Equation 9 may be considered the working equation for using this simple method of analysis. It 

is interesting to note that changing from square wave to sinusoidal amplitude modulation results 

in almost a 40% increase in signal intensity. 

EXPERIMENTAL CONSIDERATIONS 

Table 1 lists spectral parameters for several gas phase molecules. For 0.1-torr partial pressure in 

air of CO gas at room temperature, measured over a 10 cm path (13 ppm/m). the calculated 

absorbance, A, is 3.55 by 10-6. Figure 3 shows the value of the integral in Equation 9 versus 

absorbance over a range of absorbances from I by 10 ' to 1 X IO-', using a modulation frequency 

of 1 MHz, averaging over 1000 periods, using a reference amplitude of 10 V, and assuming xm 

is approximately equal to one. The error introduced by the assumption that acl << 1 is seen to 

become important only at relatively high concentrations of analyte gas. 

TABLE 1. ABSORPTION COEFFICIENTS FOR SELECTED ATMOSPHERIC GASES. 

Molecule h (absorption region, pm) Absorption Coefficient (cm'/mol) 

HF 1.33 1.3 by 10~'" 

Hz0 1.393 2.1 by 

co 1.561 2.3 by 

CH4 1.65 1 8.7 b Y 

Instrument Control 

To accommodate both WMS and BRD detection methods, the laser diodes need to be driven by 

complex waveforms that switch rapidly between the two modulation methods, suggesting the 

need for an arbitrary waveform generator. In addition, the complex demodulation requirements 

of the different detection methods involve a flexibility beyond the scope of traditional analog 

lock-in amplifiers. These considerations, together with the eventual need for high component 

integration and the ability to operate under extreme environmental conditions without signal 

drift, led to the selection of a Digital Signal Processing (DSP) approach to waveform generation 

and demodulation. 

196 Halon Options Technical Working Conl'erence 27-29 April l9Y9 
9.00E-01 

8.00E-01 

7.00E-01 

6.0flE-01 

m 5.00E-01 

4.00E-01 

3.00E-01 

2.00E-01 

1.00E-01 

0.00Et00 

- 

c 

0.00E+00 2.00E-04 4.00E-04 6.00E~04 

Absorbance (ecl) 

7.0flE+02 

6.00E+O2 ~ 

5.00E+02 i 

X 4.00E+02 

E 3.00E+O2 ~ 

2.00E+O2 ! 

l.OflE+flZ ~ 

0.00E+00 

c 

0) 

0 .00 E + 00 2.00E-01 4.00E-01 6.0flE-01 

Absorbance (ecl) 

Figure 3. Signal versus absorbance for concentration measurement of 

broad band absorbers using amplitude modulation spectroscopy. 

For initial laboratory development of the DSP system. it was decided to usc ii Pentium-based 

laptop computer running Windows 95, allowing rapid program development using readily 

available programming languages. A custom peripheral unit. the McLaren Research DLM-2A 

Diode Laser Modulator / DSP Lock-In, was developed to provide the required Digital-to-Analog 

(for diode laser waveform generation) and Analog-to-Digital (for acquisition of the detected signal) conversions, together with high-speed DSP functionality. Thc ensemble is controlled from a 

custom data acquisition and control program (called DLM) runnins on thc laptop computer. 

The DLM-2A communicates with the laptop computer via the latter's parallel port. using the 

Enhanced Parallel Port (EPP) mode for transfer speeds up to I MB/sec. A Field Programmable 

Gate Array (FPGA) in the DLM-?A contains the required EPP interface, the waveform generation circuitry, and dam and control logic for the Analog-Digital and Digital-Analog Convcrtcrs. 

The requircd DSP functions~Synchronous demodulation and signal averaging-arc also implcmented in the FPGA. This allows modulation and detection frequencies higher than would be 

the case if the laptop's CPU were performing all of the signal processing lasks. 

H;ilnn Oprions Tcchnio;d Workins Conlcrcncc 27-29 April IYJY 197 
Before an experiment, the diode laser waveform generation parameters (Le., modulation frequencies and amplitudes) are selected via software and sent to the DLM-2A, which then starts 

generating the repetitive complex waveforms for the laser diodes, with a cycle time of IO msec or 

less. The detector waveform is digitized, then demodulated and integrated within the FPGA, and 

streamed across to the laptop where it is analyzed in real time. displayed on a "virtual oscilloscope" screen. and stored on disk as required. The analysis algorithms used are typically (a) the 

peak height of the demodulated 2f component (for WMS modulation), and (b) the integrated 

difference between the two detected laser intensities (for BRD modulation). The eventual goal is 

to determine a single-quantity result, within IO msec of the introduction of a combustible gas 

mixture, that can determine whether or not fire-suppression action is needed. 

The use of a data acquisition and control system developed specifically for the task at hand, with 

dedicated (but programmable) hardware to offload signal-processing tasks from the host CPU 

and optimized assembly-language algorithms for signal analysis, allows a considerably faster 

response time than would be possible using a commercial package such as LabView. In addition, 

the highly-integrated DSP approach allows for a smooth transition from the laboratory-based 

development system to a field-deployable unit, at which point the laptop computer can be 

replaced by an environmentally-rugged embedded processor. 

~ 

+PGA 

-Waveform Generator 

-AID, DIA 

*Mixer 

EXPERIMENTAL APPARATUS 

- 

Laser Driver 

GRMLeni? 

Laser 3 

1HI "rn Mohlatai 

198 

Figure 4. Component diagram of the binary gas sensor. 

Halon Options Technical Working Conference 21-29 April 1999 
Laser 1 

1720 nm 

1 EAnt 1 

Modulator 

- Polarizer 1 

E-0 

Amplitude 

Modulator 1 .) = - ~. - 

Figure 5. Optical layout of the binary ga.; sensor 

Fiber 

Combiner 

Modulator 

Driver 1 

H:ilon Options Technkil Warking Coriterencc 27-29 April I'J94 19') 

- 

Laser 2 

1600 nm 

- Polarizer 2 

E- 0 

Amplitude 

Modulator 2 
ACKNOWLEDGMENT 

This work is supported by the Next Generation Fire Suppression Technology Program. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

REFERENCES 

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200 Halon Options Technical Working Conference 27-29 April 19Y9 

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