Published in Applied Optics 39, 5006-5011 (2000) 

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High Sensitivity Laser Absorption Measurements of Broad Band Absorbers in the NearInfrared Spectral Region

Kevin L. McNesbyΨ Richard T. Wainner, Andrzej W. Miziolek

U.S. Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5066 

W.M. JacksonΩ, and Ian A. McLaren♣

U.S. Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5066 

ΩDept. of Chemistry, University of California-Davis, Davis CA 95616 

♣ McLaren Research, Mountain View, CA 94043 

Ψ author to whom correspondence should be addressed

Abstract 

The development and characterization of a diode laser-based sensor to measure the vapor from 

broad band absorbers in the near-infrared spectral region is described. Data is presented for the 

detection and measurement of vapor from pyridine, C5H5N, using a novel technique based on 

laser mixing. The discussion focuses upon experimental determination of whether a compound 

may be classified as a "broad band absorber" and methods used for laser selection, phase 

sensitive detection, and for data reduction. 
 Published in Applied Optics 39, 5006-5011 (2000) 

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Introduction 

This paper describes efforts towards the development of a tunable diode laser-based sensor for 

quantitative measurements of vapor of broadband absorbers in the near-infrared spectral region. 

The development of such a sensor is driven by the need for rapid, quantitative measurements of 

middle distillate fuel vapors (e.g., diesel fuel, JP-8) for detection of fire hazards on board military 

vehicles. The specific application reported here is for detection of vapor from pyridine (C5H5N), 

but the method is readily applicable to measurement of vapor of any molecule that possesses a 

spectral envelope which does not show well-resolved rovibrational absorption lines and which has 

an absorption spectral width greater than the tuning range of available near-infrared tunable 

diode lasers. 

The method of detection employed here uses a laser mixing technique that relies upon absorption 

of radiation of one of the lasers (near the wavelength of 1.65 micrometers) by pyridine vapor. 

The near-infrared spectral region is used because of the commercial availability of room 

temperature lasers and detectors (the wavelength region near 1.65 micrometers is widely used 

for 2f detection of methane gas) and because laser radiation in this spectral range may be easily 

transported at low loss using inexpensive optical fibers. 

Background 

Most gas phase concentration measurements using line-of-sight near-infrared tunable diode laser 

absorption spectroscopy use wavelength modulation spectroscopy (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 made at twice the modulation frequency, and absorption features have a 

second derivative lineshape. Measurements are made using a single probe laser beam. 
 Published in Applied Optics 39, 5006-5011 (2000) 

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

noise. To use this technique, a signal and reference beam are 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. This 

technique has the advantage of reporting the true absorption lineshape, and 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. 

Broad Band Absorbers 

Both WMS and BRD techniques give good results for gas phase concentration measurements of 

small molecules with well resolved ro-vibrational absorption lines, such as hydrogen fluoride, 

methane, carbon monoxide, water, etc [7]. However, large and/or heavy molecules might not 

posses well resolved ro-vibrational absorption bands because of overlap between fundamental, 

combination, and/or overtone vibrations, and because of rotational constants which may be less 

than typical gas phase linewidths [8]. Figure 1 shows the near-infrared gas phase absorption 

spectrum of pyridine (C5H5N) and of methane (CH4) near a wavelength of 1.65 micrometers 

measured using a Fourier transform spectrometer. Shown in the inset of Figure 1 is the 

absorption spectrum of a mixture of pyridine and methane gases (total pressure 13.5 torr, Ppyridine

= 3 torr, Pmethane = 10.5 torr, path length = 400 cm-1) measured using wavelength modulation 

spectroscopy employing 2f detection. Low pressure was used to minimize absorption linewidths. 

Figure 1 shows attenuation of near infrared radiation by both pyridine and methane near a 

wavelength of 1.65 micrometers at this pressure. However, the inset to Figure 1 shows that when 

using wavelength modulation spectroscopy employing 2f detection, only the absorption by 

methane is detected, even though pyridine and methane have similar gas phase absorption 

coefficients near a wavelength of 1.65 micrometers (see Table 1). This means that there is 

minimal change in the absorption of the laser radiation by pyridine vapor during the wavelength 

scan used to detect and measure methane gas. Relative to the wavelength tuning range used for 
 Published in Applied Optics 39, 5006-5011 (2000) 

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2f methane detection, this result demonstrates the criteria that we use to determine if a molecule 

is a "broad band absorber". 

Phase Sensitive Detection 

The method used here for detection of pyridine vapor, and vapors of other molecules which may 

be classified as broad band absorbers, as well as other detection methods based upon phase 

sensitive detection, takes advantage of the following relations: 

0 

π

∫ m(sin at)n(sin bt) dt = 0 a≠b (1) 

2C 0

π

∫ m(sin at)n(sin bt) dt = Cmnπ a=b (2) 

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. 

If 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, the result of the integration in (2) yields a value directly 

proportional to the amplitude of the signal of interest. 
 Published in Applied Optics 39, 5006-5011 (2000) 

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Experimental Design 

The method used here takes advantage of the power modulation which occurrs when a DFB 

diode laser is driven by a time varying injection current. For the experiments described here, two 

different DFB lasers are driven using a sinusoidally varying injection current. Each laser is 

modulated at the same frequency but out of phase by 180 degrees [9]. These two lasers are 

combined to produce the probe beam used for the line-of-sight measurement. When the power 

output amplitudes of each laser are the same, the combined output of the two lasers will have an 

amplitude that is (ideally) invariant with time. 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 only one of the lasers, the signal at the detector is modulated at the injection current 

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

concentration of absorbing gas. 

Modulation Design 

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

as: 

S(a,n,t) = nsin(at) (3) 

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

amplitude modulated lasers differing in phase by 180 degrees is: 
 Published in Applied Optics 39, 5006-5011 (2000) 

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S1(a,n,t)+S2(a,m,t) = nsin(at) + msin(at+π) (4) 

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

locked to the laser modulation) and integrating gives: 

2C 0

π

∫ (S1(a,n,t) + S2(a,m,t))psin(at) dt = Cpπ(n-m) (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 0

π

∫ (S1(a,n,t) + S2(a,m,t)) (exp-αcl)psin(at) dt = Cpπ(n-m(exp(-αcl))) , (6) 

Where α is the absorption coefficient of the analyte gas (cm2

/molecule), c is analyte gas 

concentration (molecules/cm3

), and l is the probe beam path length through the analyte gas (cm). 

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

equation 6 may be rewritten: 

2C 0

π

∫ (S1(a,n,t) + S2(a,m,t)) (exp-αcl)psin(at) dt = Cpπ(n-m + mA) (7) 

For component lasers of equal amplitude, equation 7 becomes: 

2C 0

π

∫ (S1(a,n,t) + S2(a,m,t)) (exp-αcl)psin(at) dt = CpπmA (8) 
 Published in Applied Optics 39, 5006-5011 (2000) 

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The value of the integral in Equation 8 is directly proportional to the absorbance, A, and hence to 

the concentration of the analyte gas. Additionally, the value of the integral 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. Figure 2 shows the value 

of the integral in Equation 8 (calculated signal) for a moderately strong absorbing gas as a 

function of Absorbance, (A = αcl). The measured signal using the mixed laser method described 

here is predicted to vary linearly with pressure.

Experimental Apparatus 

A diagram of the experimental apparatus is shown in Figure 3. The two DFB laser diodes 

(Sensors Unlimited) used to produce the mixed wavelength probe beam have nominal 

wavelengths of 1.307 micrometers and 1.653 micrometers. The laser diodes are driven using an 

ILX LDC-3900 laser diode controller. The variable phase sinusoidal injection currents are 

supplied by two SRS Model DS345 function generators operating on a common timebase. The 

fiber coupled, sinusoidally modulated (100 KHz) laser diode radiation is launched into free space 

using GRIN-lens tipped single mode fibers (Sentech Corp.). The grin lens-tipped fibers are 

arranged in a quasi-co-linear, "stop-light" configuration. The near co-linear output beams are 

directed into a variable pathlength gas cell (Laser Photonics L5210). Radiation exiting the 

variable path length cell is detected using a New Focus Model 2034 InGaAs large element 

photodetector. The detector signal is demodulated at the laser injection current modulation 

frequency using a lock-in amplifier (SRS 844), displayed on an oscilloscope (LeCroy 7200), and 

recorded on a laptop computer (Dell Latitude) running LabView software (National Instruments). 

Pyridine (99%) and methane (UHP) were supplied by EM Science and Matheson, respectively, 

and were used without any further purification. Near infrared survey spectra were recorded using 

a Bomem DA-8 spectrometer using a liquid nitrogen-cooled InSb detector. 
 Published in Applied Optics 39, 5006-5011 (2000) 

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Results 

Figure 4 shows the vapor phase absorption spectrum of pyridine between wavelength values of 

1.3 and 1.75 micrometers. Also shown in Figure 4 are the spectral positions of the two lasers 

used to make the mixed wavelength probe beam. The absorption coefficient for pyridine vapor at 

a wavelength of 1.653 micrometers was measured for this work. Table 1 shows that the 

absorption coefficient for pyridine vapor at this wavelength is similar in magnitude to that for 

methane. As shown in Figure 4, the laser used for pyridine detection emits radiation that is not in 

the most intense region of the pyridine absorption band. 

Figure 5 shows the demodulated mixed laser signal for pyridine vapor versus pressure, measured 

using a 400 cm path length. Each data point represents the average of 100 measurements of 

output from the lock-in amplifier, using a time constant of 10 milliseconds, laser modulation at 100 

KHz. The measured signal is seen to be approximately linear with respect to changes in 

pressure, as predicted by Equation 8. For the sensor system described here, we estimate the 

limit of detection for pyridine vapor in air, measured at a wavelength of 1.653 micrometers, to be 

approximately 200 ppm-meter. For measurements in which the on-resonance laser wavelength 

was coincident with the center of the pyridine absorption band, the sensitivity would be increased. 

Discussion 

The main purpose of these experiments was to demonstrate the feasibility of the mixed laser 

technique for concentration measurements of vapor phase molecules which cannot be measured 

using conventional wavelength modulation spectroscopy. During the course of the design and 

implementation of the method, criteria were developed to maximize sensitivity, and several 

improvements were considered for future experiments using the technique. 
 Published in Applied Optics 39, 5006-5011 (2000) 

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For the mixed laser technique, the most important element for high sensitivity is the minimization 

of the amplitude of the unattenuated mixed laser probe beam. We discovered during the course 

of these experiments that our laser amplitudes did not vary directly with injection current, so a 

Fourier analysis of the signal at the detector for each individual laser contained components at 

frequencies other than the frequency of the variation of the injection current. These other 

frequency components made it impossible for us to get an unattenuated mixed laser power signal 

that was devoid of any AC component. Minimization of this unwanted AC component of the 

unattenuated mixed laser power signal is an important factor affecting sensitivity. 

The first improvement to the apparatus for the next set of experiments will be to fiber mix the 

lasers prior to launch into the gas cell. Because our two lasers were not truly co-linear, we 

noticed some beam walk problems when going through the long pass cell. These beam walk 

problems necessitated the use of the large element photodetector. Further experiments are 

planned using the next iteration of the technique for detection of middle distillate fuel vapors. 

 

This work was supported by the Next Generation Fire Suppression Technology Program (NGP), 

administered by the Strategic Environmental Research and Development Program (SERDP). 

The authors would like to thank Lawrence Ash of the Naval Air Systems Command for his 

assistance in development of the sensor apparatus. 

Bibliography 

[I] Linnerud, P. Kaspersen, and T. Jaeger, “Gas monitoring in the process industry using diode 

laser spectroscopy”, Applied Physics B, Vol. 67, pp. 297-305, (1998). 

[2] L.S. Rothman, R.R. Gamache, R.H. 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, 

R.A. Toth, “The Hitran Molecular Database: Editions of 1991 and 1992”, J. Quant. Spectrosc. 

Radiat. Transfer, Vol. 48, p. 469, (1992). 
 Published in Applied Optics 39, 5006-5011 (2000) 

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[3] J. Reid and D. Labrie, “Second-Harmonic Detection with Tunable Diode Lasers - Comparison 

of Experiment and Theory”, Appl. Physics, B 26, pp. 203-210, (1981). 

[4] J.A. Silver, Appl. Opt., Vol. 31, p. 707, (1992). 

[5] D.M. Sonnenfroh, W.J. Kessler, J.C. Magill, B.L. Upschulte, M.G. Allen, J.D.W. Barrick, “In-situ 

sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer”, Appl. 

Phys. B, Vol. 67, pp. 275-282, (1998). 

[6] P.C.D. Hobbs, Proc. SPIE 1376, Laser Noise, p. 216, (1991). 

[7] McNesby, K.L., Skaggs, R.R, Miziolek, A.W., Clay, M., Hoke, S.H, and Miser, C.S., “Diode 

Laser-Based Measurements Of Hydrogen Fluoride Gas During Chemical Suppression Of Fires”, 

Applied Physics B, Vol. 67, p443, 1998. 

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

[9] personal communication, Mark Allen, Physical Sciences, Inc. 
 Published in Applied Optics 39, 5006-5011 (2000) 

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87

92

97

102

107

1.6 1.65 1.7 1.75

54

55

56

57

58

Wavelength (micrometers)

% Transmittance (C

5

H

5N)

% Transmittance (CH

4)

C5H5N

CH4

 C5H5N + CH4

Figure 1: The % Transmittance spectrum of pyridine (C5H5N,

PT= 4 torr, 225 cm path length) is shown, together with the %

Transmittance spectrum of methane (CH4, PT= 10.5 torr, 15 cm

path length). The inset shows the wavelength modulation

spectrum of a mixture of pyridine and methane (PT=16 torr,

pathlength=400 cm) measured at 2f near a wavelength of 1.653

micrometers. The breadth of the 2f scan is approximately 0.4

cm-1. The absorption coefficient of pyridine at a wavelength of

1.653 micrometers was found to be 3.6 X 10-22 cm2/molecule .

2f
 Published in Applied Optics 39, 5006-5011 (2000) 

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0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

0.00E+00 2.00E-01 4.00E-01 6.00E-01

Absorbance

C

alc

ulate

d

S

gi n

al (a

r

b. u

nits) Figure 2: The value of the integral in Equation 8 (calculated

signal) is shown for a moderately strong absorbing gas as a

function of Absorbance, (A = αcl).
 Published in Applied Optics 39, 5006-5011 (2000) 

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Laptop

Computer Oscilloscope Lock-inAmplifier

GRIN Lens 1

GRIN Lens 2

InSb Detector

Multipass Cell

Laser 1

1.31

microns

Function

Generator 1

Laser

Driver 1

Laser 2

1.653

microns

Function

Generator 2

Laser

Driver 2

Figure 3: A schematic of the experimental apparatus is shown

above.
 Published in Applied Optics 39, 5006-5011 (2000) 

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70

75

80

85

90

95

100

105

1.3 1.4 1.5 1.6 1.7

-10000

-5000

0

5000

10000

15000

20000

25000

%

Tra

n

s

mitta

n

c

e

C( 5

H

5

N)

nI te

n

sity (a

r

b. u

nits)

Wavelength (micrometers)

Figure 4: The absorption spectrum of pyridine (C5H5N, 225

cm path length, 1 cm-1 resolution) is shown superimposed on

the spectrum of the mixed laser probe beam (1 cm-1 resolution).

The decrease in the signal to noise ratio in the %Transmittance

spectrum at lower wavelengths is due to decreasing sensitivity

of the InSb detector and to increasing scatter in our multipass

infrared gas cell.

C5H5N

Laser 1 Laser 2
 Published in Applied Optics 39, 5006-5011 (2000) 

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Figure 5: The measured signal for pyridine (C5H5N) vapor using

the mixed laser technique. The path length used was 400 cm. The

inset shows the signal response at the lowest pressures used in

these experiments.

0

5

10

0123

-1

0

1

2

3

0 0.2 0.4 0.6 0.8

Pressure (torr)

Signal (volts)
 Published in Applied Optics 39, 5006-5011 (2000) 

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Molecule λ (absorption region, micrometers) Absorption Coefficient (cm2

/mol) 

HF 1.33 1.3 X 10-20 

H2O 1.393 2.1 X 10-20

CO 1.567 2.3 X 10-23

CH4 1.651 8.7 X 10-22 

C5H5N 1.653 3.6 (±2) X 10-22 (this work) 

Table 1: Absorption coefficients for pyridine and for selected atmospheric gases.