Published in Applied Optics 40, 840-845 (2001) 

1

Detection and Measurement of Middle Distillate Fuel Vapors Using Tunable 

Diode Lasers

Kevin L. McNesbyΨ, Richard T. Wainner, Robert G. Daniel, R. Reed Skaggs, 

J.B. Morris, Andrzej W. Miziolek, 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 

24 pages/12 pt dbl sp 

3958 words 

5 figures 

15 references

Abstract 

A sensor for rapid (10 msec response time) measurement of vapors from the 

hydrocarbon-based fuels JP-8, DF-2, and gasoline is described. The sensor is 

based upon a previously reported laser mixing technique that uses two tunable 

diode lasers emitting in the near-infrared spectral region [1] to measure 

concentrations of gases having unstructured absorption spectra. The fiber-mixed 
 Published in Applied Optics 40, 840-845 (2001) 

2

laser beam consists of two wavelengths, one of which is absorbed by the fuel 

vapor, and one of which is not absorbed. By sinusoidally modulating the power 

of the two lasers at the same frequency but 180 degrees out of phase, a 

sinusoidal signal is generated at the detector (when the target gas is present in 

the line of sight). The signal amplitude, measured using standard phase 

sensitive detection techniques, is proportional to fuel vapor concentration. Limits 

of detection, at room temperature, are reported for vapors of the three fuels 

studied. Improvements to be incorporated in the next generation of the sensor 

are discussed. 

Introduction 

This paper describes initial results of quantitative measurements of room 

temperature vapor from the hydrocarbon-based fuels JP-8, DF-2, and gasoline 

using a sensor based upon near-infrared tunable diode laser absorption 

spectroscopy. These fuels typically are mixtures of several hundred 

hydrocarbons, with an average molecular weight near 110 grams/mole (vapor), 

150 grams/mole (liquid), and boiling points near 440K [2]. The fuels are liquids 

near room temperature and have flash points (often depending on age, storage 

conditions, etc.) in the vicinity of 315K. Recent work [2] on JP-8 found that no 

single component was present in amounts greater than 1% by volume. This 

same study identified the most prevalent hydrocarbon in JP-8 as 

methylcyclohexane. Because these fuels change composition with age, 
 Published in Applied Optics 40, 840-845 (2001) 

3

formulation location, and formulation time, the physical constants above should 

be used only as estimates. 

The development of the sensor described here was driven by the U.S. Army 

need for rapid, in-situ, quantitative measurements of JP-8 fuel vapors for 

detection of fire hazards in crew compartments on board military vehicles. When 

it was found that DF-2 (also called diesel fuel #2) and gasoline vapors could be 

measured with no modification to the sensor, these fuels were included in this 

preliminary study. When a fire occurs on board a military vehicle (in worst case 

scenario a ballistically initiated fuel/air "mist-fireball explosion"), sensors detect 

and extinguish the fire (often using halocarbons) in as little as 250 ms [3]. In 

order to properly evaluate fire suppression equipment, it is necessary to make 

several fuel vapor concentration measurements during the suppression event. A 

constraint on the sensing device is that it must ultimately be disposable, since insitu measurements of fuel/air explosions may destroy/damage the test 

equipment. Hence the need for a sensor capable of making rapid, in-situ fuel 

vapor measurements, while keeping (expensive) electronics and optics at remote 

locations. 

The method of detection and sensor type used here has been previously 

reported by us for detection of vapor from pyridine [1], but is readily applicable to 

any gas or vapor that has unstructured absorption spectra. Briefly, we employ a 

two laser mixing technique that relies upon absorption of radiation of one of the 
 Published in Applied Optics 40, 840-845 (2001) 

4

lasers by the first overtone of the C-H stretching vibration of hydrocarbon vapor 

(near a wavelength of 1.71 μmeters). The near-infrared spectral region is used 

because of the commercial availability of room temperature lasers and detectors 

and because laser radiation in this spectral region may be transported at low loss 

using inexpensive optical fibers. 

Background 

Fieldable detectors for hydrocarbon vapor measurements typically rely upon 

infrared absorption (DOAS (differential optical absorption spectrometry), FT-IR 

(Fourier transform infrared) emission/absorption) [4,5] or electrochemical / 

electrothermal techniques [6]. These methods give good results, but have found 

limited use for measurements of fire scenarios just described. FT-IR and DOAS 

spectrometers are usually bulky, expensive, measurements require 

placement/alignment of interferometer/source optics and detector or retroreflector, and are generally limited to a single line of sight. Placement of the 

instrument within the test environment (crew compartment) is not feasible 

(potential damage), looking across the entire measurement space does not 

isolate the region of interest, and although extractive sampling has been shown 

to be useful [3], it does not provide necessary temporal information. Additionally, 

the interferometer temporal resolution is limited by the mirror scan speed 

(although some new designs are approaching scan rates near 1 KHz) [7]. 

Nevertheless, use of FTIR and DOAS for measurement of the fundamental C-H 

stretch in hydrocarbon fuels near 3.4 um can yield sensitivities in the ppb range, 
 Published in Applied Optics 40, 840-845 (2001) 

5

and because the full spectrum may be measured (when using FT-IR), it is often 

possible to determine species of hydrocarbons present in a simple mixture. For 

stable, relatively safe sampling sites with an accessible line of sight, incoherent 

infrared absorption techniques (FT-IR, DOAS) are often the techniques of choice. 

Electrothermal/electrochemical detectors are generally inexpensive (2-5 K$), are 

often one-man portable, and have sensitivities (~100's of ppm) well below lower 

explosion limits for most hydrocarbon vapors [2]. Time sensitivity is typically from 

1 to 15 seconds. Some units may have accuracy problems when used in the 

presence of methane gas, and may be less sensitive to heavier hydrocarbon 

vapors [6]. For measurement sites requiring full portable instrumentation with 

time resolution from one to tens of seconds, these instruments can also provide 

excellent results. 

 

Gas detection methods using near-infrared tunable diode lasers can use optical 

fibers and relatively inexpensive (room temperature operation) semiconductor 

detectors to separate electronics and laser sources from hazardous testing 

locations. Using modulation techniques and phase sensitive detection methods, 

temporal resolution routinely exceeds 1 ms per measurement. Detection 

methods employed (typically wavelength modulation spectroscopy (WMS) and 

balanced ratiometric detection (BRD) techniques) [8,9] give good results for gas 

phase concentration measurements of small molecules with well resolved rovibrational absorption lines, such as hydrogen fluoride, methane, carbon 

monoxide, water, etc [10]. However, large and/or heavy molecules may not 
 Published in Applied Optics 40, 840-845 (2001) 

6

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 [11]. 

For mixtures of heavy molecules, e.g., the middle distillate fuels, the spectra may 

be even less structured. Figure 1 shows the near-infrared gas phase absorption 

spectrum of dry air saturated at 294K by the hydrocarbon fuels JP-8, DF-2, and 

gasoline (0.75 meter optical path), measured using a Fourier transform 

spectrometer. As suggested by the spectra shown in Figure 1, and by separate 

measurement in our laboratory, the vapors from each fuel fit criteria [1] to be 

classified as a gas having unstructured absorption spectra ( often called a "broad 

band absorber") near the 1.71 μmeter wavelength spectral region. Both WMS 

and BRD, as usually employed, are not suitable for gases that have broad, 

unstructured absorption spectra [8,9,10]. For both techniques, the main limitation 

for fuel sensing is the narrow tuning range of the diode laser. Because the 

tuning range is usually several orders of magnitude smaller than the width of the 

spectral feature in broad band absorbers (0.4 cm-1 vs. >100 cm-1), it is not 

possible to tune from the region of maximum absorption to a region of zero 

absorption. The purpose of this work is to overcome this limitation for 

measurement of such molecules, while approaching the sensitivity and time 

response of WMS and BRD methods. 
 Published in Applied Optics 40, 840-845 (2001) 

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Measuring Broad Band Absorbers with Narrow Band Lasers 

The method used here takes advantage of the residual amplitude modulation 

(RAM) which occurs when a DFB diode laser is driven by a time varying injection 

current. This RAM occurs when the wavelength of a semiconductor tunable 

diode laser is varied by modulating the laser injection current. Typically (for 

WMS and BRD), RAM is a nuisance, and what is important is the wavelength 

modulation. For the experiments described here, the RAM is what enables the 

technique to work, and the (relatively) small variation of wavelength during the 

modulation is largely ignored (but will be commented upon later). For the 

experiments described here, two different DFB lasers are driven using a 

sinusoidally varying injection current. Each laser injection current is modulated at 

the same frequency but out of phase by 180 degrees [12]. These two lasers are 

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

When the power output amplitudes of each sinusoidally modulated 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 whose magnitude is proportional to 

the concentration of absorbing gas. 
 Published in Applied Optics 40, 840-845 (2001) 

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The simple mathematical description begins by considering Beer's law [13] for 

narrow band laser radiation exiting an absorbing sample gas of concentration c 

molecules per cm3

, for a path length of l cm. The absorption coefficient 

(cm2/molecule) at the most intense region of absorption (the resonant 

wavelength, λR) , is α(λR). I0(λR) is the intensity of the incident radiation at the 

resonant wavelength. I(λR) is the intensity of radiation at the resonant 

wavelength after exiting the sample gas: 

I(λR)=I0(λR) exp(-α(λR)cl) (1) 

If the incident radiation at the resonant wavelength is now power modulated 

sinusoidally at frequency a, and time t, with depth of modulation DR , the intensity 

of the radiation at the resonant wavelength after exiting the gas becomes: 

I(λR)=I0(λR)(1+DR sin(at)) exp(-α(λR)cl) (2) 

To get a measurement of transmission off resonance (important to generate a 

differential signal and to discriminate against scattering by particles such as 

smoke), we add a second laser source at a wavelength not absorbed by the gas, 

λNR . This non-resonant wavelength is mixed with the resonant wavelength, 

power modulated at the same frequency but 180 degrees out of phase with the 

resonant wavelength modulation, and has a depth of modulation DNR . If the 

radiation exiting the gas is now focussed on an (ideally) wavelength-insensitive 
 Published in Applied Optics 40, 840-845 (2001) 

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detector (see Discussion), the power at this detector, I(λR +λNR), may be given 

by: 

I(λR +λNR)=[I0(λR)(1+DRsin(at)) exp(-α(λR)cl) +I0(λNR)(1+DNR) sin(at+π)] (3) 

To process this signal, we multiply by a reference sine wave of amplitude p in 

phase with and at the same frequency, a, as the power modulation of the two 

lasers. We then integrate over C periods of power modulation: 

C 0

π

∫ {[I0(λR) (1+DR) exp(-α(λR)cl) -I0(λNR) (1+DNR)] sin(at) psin(at)}dt 

=(C/2)pπ [I0(λR) (1+DR) exp(-α(λR)cl) -I0(λNR) (1+DNR)] (4) 

Rearranging, assuming exp(-α(λR)cl) <<1 and using A (Absorbance) is equal to 

α(λR)cl : 

2C 0

π

∫ {[I0(λR) (1+DR) exp(-α(λR)cl) -I0(λNR) (1+DNR)] sin(at) psin(at)}dt 

=Cpπ [I0(λR) (1+DR) -I0(λNR) (1+DNR) - I0(λR) (1+DR)A] (5) 

For the special case where the power and depth of modulation of the two lasers 

are equal, Equation 5 becomes: 

2C 0

π

∫ {[I0(λR) (1+DR) exp(-α(λR)cl) -I0(λNR) (1+DNR)] sin(at) psin(at)}dt 

=-Cpπ [I0(λR) (1+DR)A] (6) 
 Published in Applied Optics 40, 840-845 (2001) 

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Equation 6 is just the output of a lock-in amplifier operating on the first harmonic 

for measurement of the amplitude of the sine wave created by the differential 

absorption (for small absorptions) of the mixed laser beam by the gas at 

concentration c and path length l. This output is proportional to the number of 

periods, C, over which the integration is carried, the amplitude, p, of the 

reference sine wave, the incident power, I0(λR), of the laser at the resonant 

wavelength, and the Absorbance, A (and hence gas concentration when 

pressure broadening of the spectral feature is not important). It is worth noting 

that Equation 6 also indicates that lock-in output is increased by increasing depth 

of modulation (DR). As will also be discussed later, Equation 5 shows that it is 

important that the two laser power amplitudes and depths of modulation are as 

equal as possible, especially for small values of Absorbance. 

Experimental Apparatus 

A diagram of the experimental apparatus is shown in Figure 2. The two DFB 

laser diodes (Sensors Unlimited) used to produce the mixed wavelength probe 

beam have nominal wavelengths of 1.307 μmeters and 1.71 μmeters. The laser 

diodes are driven using an ILX LDC-3900 modular laser diode controller. The 

variable phase sinusoidal injection currents are supplied by two SRS Model 

DS345 function generators operating on a common timebase, Sinusoidally 

modulated (100 KHz, modulation depth adjusted to near 100%) laser diode 

radiation from each diode laser is mixed onto a common fiber using a fiber optic 

coupler (Gould Fiberoptics) and launched into free space using a GRIN-lens 
 Published in Applied Optics 40, 840-845 (2001) 

11

tipped single mode fiber (Sentech Corp.). The output beam is directed into a 

variable pathlength gas cell (Laser Photonics L5210) set at the minimum setting 

of 4 meters. Radiation exiting the 4 meter path length cell is detected using a 

New Focus Model 2034 InGaAs large element photodetector (maximum 

responsivity near 1600 nm). The detector signal is demodulated at the laser 

injection current modulation frequency using a lock-in amplifier (SRS 830), 

displayed on an oscilloscope (LeCroy 7200), and recorded on a laptop computer 

(Dell Latitude) running LabView software (National Instruments). JP-8 and DF-2 

were milspec grade and obtained from the fuel depot at the Aberdeen Proving 

Ground. The gasoline was regular grade obtained during the summer months at 

a commercial vendor in Aberdeen, MD. All were used as supplied. Air was 

saturated with fuel vapor using a bubbler apparatus based upon a nitrogen 

sparger. Air flow through the stainless steel frit immersed in the fuel was typically 

100-250 cm3

/minute. Near infrared spectra shown in Figures 1 and 3 were 

recorded using a Bomem DA-8 (using a liquid nitrogen-cooled InSb detector) 

interfaced to a Wilks 0.75 meter path length cell. 

Results 

Figure 3 shows the vapor phase absorption spectrum of air saturated by vapor at 

294K from JP-8, DF-2, and gasoline between wavelengths of 1.3 and 1.75 

micrometers superimposed upon the emission from the optical fiber carrying the 

mixed wavelength laser beam. Both sets of spectra were measured using a FTIR spectrometer (Bomem DA-8). The near infrared tunable diode laser radiation 
 Published in Applied Optics 40, 840-845 (2001) 

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was transported using optical fibers to the external source port on the FT-IR 

spectrometer. This figure shows that the emission of the laser at 1.71 um falls 

near the most intense region of the C-H stretch (first overtone) absorption for the 

three fuels studied, and that the 1.3 um laser falls in a region where there is no 

measurable absorption of the laser radiation by the fuels studied. Because of the 

narrow emission linewidth of the lasers, the reported lineshape in this figure is 

just the instrument lineshape (in this case, sinc) of the FT-IR spectrometer [14]. 

Figure 4 shows the gasoline vapor concentration (measured using the mixed 

laser sensor) as dry air saturated by gasoline vapor slowly displaces (several 

hundred cm3

/minute) the dry air in a 13.7 liter volume optical cell (4 meter path 

length). Also shown in this figure is the change in oxygen concentration that 

occurs during the displacement (oxygen sensor courtesy of Oxigraf, Inc., 

Mountain View, CA). This figure shows the sensor response as the vapor 

concentration approaches and exceeds the lower explosive limit (LEL) for most 

hydrocarbon vapors (e.g., for JP-8 the LEL is approximately 9500 ppm [2]). Each 

data point is the average of the lock-in amplifier output measured over 10 

milliseconds. All measurements were made at atmospheric pressure. 

 Figure 5 shows the measured fuel vapor concentration as the dry air in the13.7 

liter volume optical cell is displaced by air saturated (at 294K) by JP-8 vapor. 

Run 1 is for air saturated by vapor using a fresh fuel sample in the bubbler. Run 

5 is the fifth re-run of the test without changing the fuel sample in the bubbler. 
 Published in Applied Optics 40, 840-845 (2001) 

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The difference in peak hydrocarbon levels between runs shown in Figure 5 is 

caused by the gradual removal of lighter hydrocarbons from the JP-8. For 

brevity, results on DF2 are not presented, since the results for DF-2 were similar 

to those for JP-8. The partial pressures at atmospheric pressure (measured 

using a capacitance manometer) for gasoline, JP-8, and DF-2 fuels were found 

to be approximately 13, 3.5, and 1.75 torr (±20%), respectively. We measured 

the C-H stretch first overtone absorption coefficient for the fuel vapors to be 

approximately 2.7 (±.4) X 10 -21 cm2/molecule, which is in reasonable agreement 

to that for the first overtone of the C-H stretch for methane (8.7 X 10-22

cm2/molecule) [15]. We estimate the current limit of detection for each of these 

fuel vapors to be approximately 100 ppm-meter. 

Discussion 

An implication of the results presented here is that the device may be used to 

measure hydrocarbon vapor concentration for any middle distillate fuel. 

However, careful examination of the C-H overtone bands for each of the fuels 

shows that the contours of the absorption features, when normalised to each 

other, are not truly superimposable. So, for best accuracy the sensor should be 

calibrated for each fuel vapor to be measured. 

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

mixed laser technique for concentration measurements of vapors from the middle 
 Published in Applied Optics 40, 840-845 (2001) 

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distillate fuels. However, the technique is applicable to measurement of any 

vapor that exhibits unstructured absorption spectra. All that is required is 

knowledge of the absorption spectrum of the target gas and proper selection of 

on and off resonance lasers. It should be mentioned that we have made no 

attempt to characterize the sensor for vapors at different temperatures. This is 

important since the shape of the C-H overtone absorption band may vary with 

temperature. Additionally, we have made no attempt to account for absorption 

by finely dispersed fuel droplets. We are currently setting up the experimental 

apparatus to address these shortcomings. 

As mentioned previously, predicted by Equation 5, and verified experimentally, 

the most important element for maximizing sensitivity is the matching of the 

modulation amplitudes of the two lasers. The sensitivity reported by us for 

hydrocarbon vapor (100 ppm) in the near IR is similar to that achievable with FT 

spectrometers but approximately two orders of magnitude worse than that for 

methane vapor measured using WMS or BRD (approx. 1 ppm for field 

measurements). Differences in detector response at different wavelengths may 

be compensated for by changing the power of each laser, and is not an important 

factor affecting sensitivity. The underlying cause of our poor sensitivity (relative 

to methane detection using WMS or BRD) is the mismatch in light - injection 

current (L-I) curves for the two lasers, and that the L-I curve (as implied by the 

use of curve) for each laser is not linear. A Fourier transform of the power output 

of each laser, when driven by the sinusoidal injection current, showed significant 
 Published in Applied Optics 40, 840-845 (2001) 

15

contributions to the RAM at frequencies other than those used for the laser 

modulation (typically 100 KHz, depth of modulation close to 100%). So, when 

the two laser outputs were mixed onto a common fiber, the measured output of 

the mixed laser beam, in the absence of any absorbing gas, always showed an 

AC component. This AC component shows up in the demodulated signal. We 

are currently investigating the use of dynamic modulation control, which will 

custom tailor one of the injection current drive waveforms, to actively null the raw 

signal every few milliseconds. An alternative approach, also being investigated, 

is to switch to square wave modulation and to sample discretely twice during 

each modulation period. Finally, perhaps the best choice (our original choice, but 

precluded by cost considerations) to minimize unwanted AC components in the 

mixed signal is to use downstream electro-optic modulation, eliminating the effect 

of non-linearity in the laser L-I characteristic. Rapid changes in technology for 

the telecommunications industry have made this approach near cost-effective. 

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. 
 Published in Applied Optics 40, 840-845 (2001) 

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References 

[1] K. L. McNesby, R.T. Wainner, A. W. Miziolek, W.M. Jackson, and I.A. 

McLaren, "High Sensitivity Laser Absorption Measurements of Broad Band 

Absorbers in the Near-Infrared Spectral Region", to be published in Appl. Optics. 

[2] D.W. Naegeli and K.H. Childress, "Lower Explosion Limits and Compositions 

of Middle Distillate Fuel Vapors", SAE paper # 982485, (1998). 

[3] S.H. Modiano, K.L. McNesby, P.E. Marsh, W. Bolt and C. Herud, "Quantitative 

measurements by Fourier transform infrared spectroscopy of toxic gas 

production during inhibition of JP-8 fires by CF3Br and C3F7H", Appl. Optics, Vol. 

35, pp. 4004-4008 (1996). 

[4] Infrared Methods for Gaseous Measurements - Theory and Practice, J. 

Wormhoudt, Ed., Marcel-Dekker, New York, 1985; 

[5] Guenther P.L.; Stedman D.H.; Bishop G.A.; Beaton S.P.; Bean J.H.; Quine 

R.W.," A hydrocarbon detector for the remote sensing of vehicle exhaust 

emissions", Review of Scientific Instruments, Vol. 66, pp. 3024-3029 (1995) 

[6] http://www.brandtinst.com/biosystems/appnotes/equiv3.htm 

[7] P.R. Griffiths, B.L. Hirsche, and C.J. Manning, "Ultra-rapid-scanning Fourier 

transform infrared spectrometry", Vib. Spectroscopy. Vol. 19 p.165 (1999). 

[8] D.S. Bomse, A.C. Stanton, and J.A. Silver,"Frequency modulation and 

wavelength modulation spectroscopies: comparison of experimental methods 

using a lead-salt diode laser", Appl. Optics, Vol. 31, 718-731 (1992). 

[9] P.C.D. Hobbs, "Ultrasensitive laser measurements without tears", Appl. 

Optics, Vol. 36, pp. 903-21, (1997). 
 Published in Applied Optics 40, 840-845 (2001) 

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[10] P. Werle, "A review of recent advances in semiconductor laser based gas 

monitors", Spectrochimica Acta Part A, Vol. 54, pp.197-236 (1998); 

[11] G. Hertzberg, Infrared and Raman Spectra, Van Nostrand Rheinhold Co., 

New York, 1945. 

[12] personal communication, Mark Allen, Physical Sciences, Inc., 20 New 

England Business Center, Andover, MA 01810. 

[13] 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). 

[14] P.R. Griffiths and J.A. de Haseth, Fourier Transform Infrared Spectrometry, 

Wiley-Interscience, New York, 1986. 

[15] Linnerud, P. Kaspersen, and T. Jaeger, “Gas monitoring in the process 

industry using diode laser spectroscopy”, Appl. Physics B, Vol. 67, pp. 297-305, 

(1998). 
 Published in Applied Optics 40, 840-845 (2001) 

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 Figure 1 

0

20

40

60

80

100

120

140

160

1.2 1.7 2.2 2.7 3.2 3.7

Unleaded

Gasoline

(regular)

JP-8

DF2

Wavelength (micrometers)

%

T

r

a

n

s

mis

sio

n
 Published in Applied Optics 40, 840-845 (2001) 

19

 Figure 2 

Laptop

Computer Oscilloscope Lock-inAmplifier

GRIN Lens 1

InGaAs Detector

4 m optical cell 

Laser 1

1.31

microns

Function

Generator 1

Laser

Driver 1

Laser 2

1.71

microns

Function

Generator 2

Laser

Driver 2

Fiber

optic 

coupler
 Published in Applied Optics 40, 840-845 (2001) 

20

 Figure 3 

-0.1

0

0.1

0.2

0.3

0.4

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

-0.25

0.25

0.75

1.25

Wavelength (micrometers)

Absorbance

Laser Emission (arb. units)

Laser 1 Laser 2

Gasoline vapor

JP-8 vapor

DF-2 vapor
 Published in Applied Optics 40, 840-845 (2001) 

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 Figure 4 

0

2000

4000

6000

8000

10000

12000

0 100 200 300 400 500

0

0.05

0.1

0.15

0.2

0.25

0.3

Time (seconds)

ppm Gasoline Vapor

Fraction Oxygen

approximate LEL for

hydrocarbon vapors
 Published in Applied Optics 40, 840-845 (2001) 

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 Figure 5 

0

500

1000

1500

2000

2500

3000

0 1000 2000 3000 4000

ppm JP-8 vapor

Time (seconds)

Fresh sample

Sample after 5 runs
 Published in Applied Optics 40, 840-845 (2001) 

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Figure Captions 

Figure 1: The infrared transmission spectrum of room temperature vapor from 

JP-8, DF2, and gasoline. The fundamental C-H absorption in each of these fuels 

is near a wavelength of 3.3 μmeters. The first overtone of the C-H stretch is near 

a wavelength of 1.71 μmeters. Spectra are offset for clarity 

Figure 2: The experimental apparatus used in these experiments. 

Figure 3: The C-H first overtone region for the fuels used in these experiements. 

Also shown is the spectrum of the mixed laser beam (lineshapes of the mixed 

laser beam at 1.3μm and 1.7μm are due to the instrument lineshape of the 

interferometer (sinc)). 

Figure 4: The approach to the lower explosion limit for hydrocarbon vapor, 

measured with the mixed laser sensor, as the dry air in a 13 liter optical cell is 

slowly displaced by air saturated with gasoline vapor at 1 atmosphere and 294K. 

Also shown is the change in oxygen concentration during the displacement 

(oxygen sensor courtesy of Oxigraf, Inc.). 

Figure 5: The increase in JP-8 vapor within the 13 liter optical cell as the dry air 

in the cell is displaced with air saturated by JP-8 vapor at 1 atmosphere and 
 Published in Applied Optics 40, 840-845 (2001) 

24

294K. The difference between Run1 and 5 is the loss of volatile hydrocarbons 

from the fuel sample.