2000

OSA®

Optical Society of America
80 / SaB5-1

Rapid High Sensitivity Laser Absorption Measurements of Broad Band Absorbers in the

Near Infrared Spectral Region

Kevin L. McNesby and Andrzej W. Miziolek

U.S. Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5066

Ian A. McLaren

McLaren Research, Mountain View, CA 94043

Abstract

The development of a gas sensor to measure concentrations of binary mixtures of oxygen and

several volatile organic compounds is described. The sensor is diode laser-based and all signal

processing is performed digitally.

Introduction

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 detection relies upon absorption of radiation near a wavelength of 760 nanometers by

oxygen, and absorption of radiation near a wavelength of 1700 nanometers by hydrocarbon

based fuels [1]. 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 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 rovibrational 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 what follows.

Background

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 which may be less than typical gas phase linewidths [2]. Figure 1 shows the

near-infrared vapor phase absorption spectrum ot' the hydrocarbon-based turbine fuel JP-8 near

a wavelength of 1700 nanometers (measured using a Fourier transform spectrometer), and the

calculated absorption spectrum of oxygen near a wavelength of 760 nanometers [1]. For JP-8

vapor, the absorption spectrum near a wavelength of 1700 nanometers is essentially a

continuous hump.

Detection

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

phase sensitive detection, takes advantage of the following relations:

o "f m(sin at)n(sin bt) dt = 0 a7:b

2C o"f m(sin at)n(sin bt) dt = Cmnn a=b

(1)

(2)

1
r

I

I

I For the discussion to follow, a and b are laser amplitude modulation frequencies, t is time, m and

n are amplitudes, and C represents the number of periods over which the integration is carried

out.

SaB5-2/81

jIf the signal of interest is sinusoidal at frequency a (in equation 1), upon multiplication of this

periodic signal by a reference sinusoid at frequency b, the resulting integral will be non-zero 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 reference

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 (2) yields a value directly proportional to the

quantity of interest.

Modulation Design

Experimental 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:

(4)

S(a,n,t) = 0.636nsin(at) + 0.212nsin(3at) + 0.1273nsin(5at) + .... (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 degrees is:

S1(a,n,t)+S2(a,m,t) = (0.636nsin(at) + 0.212nsin(3at) + 0.1273nsin(5at) + .... ) +

(0.636msin(at+1t) + 0.212msin(3at+1t) + 0.1273msin(5at+1t) + ....

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.

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

the same frequency but out of phase by 180 degrees [3]. 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).

Using the relationship -sin(ax) = sin(ax + 1t), multiplying by the reference sine wave (frequency

locked to the laser modulation) and integrating gives:

2C a"I (S1 (a,n,t) + S2(a,m,t»)psin(at) dt = Cp7t(.636n-.636m) (5)

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.

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 absorbed by an

analyte gas in the path of the probe beam, equation 5 becomes:

2C a"I (S1(a,n,t) + S2(a,m,t» (exp-acl)psin(bt) dt = Cp7t(.636n-.636m(exp(-acl))) , (6)

Where a is the absorption coefficient of the analyte gas (cm2/molecule), cis analyte gas

concentration (molecules/cm\ and I is the probe beam path length through the analyte gas (em).
82 / SaB5-3

When the product acl « 1, exp(-acl) may be rewritten as1-A, (where A is absorbance = (acl)) so

equation 6 may be rewritten:

2C o"J (S1(a,n,t) + S2(a,m,t)) (exp-acl)psin(bt) dt = Cpn(.636n-.636m + .636mA)

For component lasers of equal amplitude, equation 7 becomes:

2C o"J (S1 (a,n,t) + S2(a,m,t)) (exp-acl)psin(bt) dt = .636CpnmA

(7)

(8)

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 carried out. Using sine wave modulation of the component lasers instead of square

wave modulation changes equation 8 to:

2C o"J (S1 (a,n,t) + S2(a,m,t)) (exp-acl)psin(bt) dt = CpnmA (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.

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

Bibliography

[1] L.S. Rothman, RR Gamache, RH. Tipping, C.P. Rinsland, M.A.H. Smith, D. Chris, V.

Benner, M. Devi, J.-M Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S.T. Massie, L.R Brown,

RA Toth, 'The Hitran Molecular Database: Editions of 1991 and 1992", J. Quant. Spectrosc.

Radiat. Transfer, Vol. 48, p. 469, (1992).

[2] G. Hertzberg, Infrared and Raman Spectra, Van Nostrand Rheinhold Co., New York, 1945.

[3] personal communication, Mark Allen, Physical Sciences, Inc.
SaB5-4 I 83

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Wavelength (nanometers)

JP-8 vapor, 10 cm path length

355K, 4cm-1 resolution,

256 scans.

02' 1 m path length,

296K, calculated using

HITRAN database.

Figure 1: The measured near-infrared absorbance spectrum in the wavelength region near 1700

nanometers of JP-8 vapor at 355K, and the calculated absorbance spectrum in the wavelength

region near 760 nanometers of O2,