Final Technical Report 

Laser-Based Instrumentation for Real-Time, In-Situ 

Measurements of Combustible Gases, Combustion By-Products, & Suppression 

Concentrations 

SERDP PROJECT IDENTIFIER: 1/7, 1/78, 1/789 

15 September 2002 

The views and conclusions contained in this document are those of the Army Research 

Laboratory and should not be interpreted as representing the official policies, either 

expressed or implied, of the Strategic Environmental Research and Development 

Program. 

Kevin L. McNesby, Ph.D. 

Andrzej W. Miziolek, Ph.D. 

R. Reed Skaggs, Ph.D. 

U.S. Army Research Laboratory 

Aberdeen Proving Ground, MD

Sponsored by: 

The Department of Defense 

Strategic Environmental Research and Development Program 
Table of Contents 

1. Title Page 1

2. Summary of Task Objectives 3

a. FY97-98 3 

 b. FY99-00 3 

 c. Technical Challenges 5 

 d. General Methodology 5 

 e. Technical Results 5 

 

3. Detailed Description of Project 6

a. Species of Interest: Limits of Detection (LOD) 6 

b. Laser Induced Breakdown Spectroscopy 6 

c. 2F Tunable Diode Laser Absorption Spectroscopy (TDLAS) 7 

i. Stand Alone Spectrometer System for HF Gas 7 

 Measurement 

ii. Results of HF Measurement Using Single Gas 9 

 Stand Alone Spectrometer System

 iii. Multi-position, Multi-gas Spectrometer System 10 

 iv. Results Using the Multi-Position, Multi-Gas 11 

Spectrometer System

d. Real Time Fuel/Ox Sensor 12 

 i. Background 13 

 ii. Measuring Broad Band Absorbers With Narrow 15 

 Band Lasers 

 iii. Experimental Apparatus 17 

 iv. Results 19 

 e. Second Generation Sensor Development 20 

 i. Results of Prototype Testing of Second Generation 22 

 Sensor System

 ii. Sensitivity 23 

 iii. Future Developments 24 

f. Technical Problems 25 

 g. Recommendations and Conclusions 25 

4. References 26

5. Publications 29

6. Milestones 31

 

7. Significant Project Results – Discussion and Recommendations 33

8. Conclusion 34

2
2. Summary of Task Objectives 

This final report summarizes results of a multi-year project the purpose of which 

was to implement laser-based instrumentation for real-time, in-situ measurements of 

combustible gases, combustion by-products, and suppression concentrations during 

suppression of flames and explosions on board Army combat vehicles. This effort 

directly addresses the statement of need for element 3.c of the Next Generation Fire 

Suppression Technology Program (NGP), administered by the DoD Strategic 

Environmental Research and Development Program (SERDP). The nominal project 

duration was from FY97 through FY99, although delays in accomplishing the goals set 

for the final year of the project required a no-cost extension through FY2000. 

a. FY97-FY98 

The first two years of the project (FY97-FY98) focused on adaptation of nearinfrared tunable diode laser absorption spectroscopy (TDLAS) for measurement of 

byproducts produced during chemical flame suppression (e.g., HF) over a 250 ms fire 

suppression period, such as experienced in crew compartments of military vehicles. Part 

of the testing program involved evaluating rugged, low-cost fiber-optics and couplings 

for transporting the near-infrared laser radiation from the diode laser (located at a safe 

distance from the measurement location) to the measurement location, and retrieving the 

signal. An additional part of the program for the first two years involved the adaptation 

of laser induced breakdown spectroscopy (LIBS) for measuring suppression agent 

concentrations, starting with halogenated suppressants, and then using LIBS for 

measurement of oxygen concentrations. 

Following laboratory characterization, these diagnostics would be installed in the 

crew compartment test fixture at Aberdeen Test Center for full scale fire suppressant 

testing. The ultimate goals of the first two years of the program were to understand why 

a suppressant is behaving well or poorly in full-scale suppression tests, for determining 

the repeatability of the tests, and for guiding optimization of agent distribution and 

dispersion. A simple schematic showing implementation of TDLAS and LIBS for gas 

measurement is shown in Figures 1 and 2. 

b. FY99-FY00 

The final year of the project (including the no-cost extension) concentrated on the 

development of a multi-point, field worthy, fiber-coupled, near-infrared tunable diode 

laser-based sensor for measurement and detection of combustible mixtures of oxygen and 

hydrocarbon fuels (heptane and JP-8). The goal of the project during this time was to 

build a sensor capable of quantitative measurement of fuel/oxygen concentrations before 

(minimum five data points prior to agent release), during, and after the suppression event 

3
Laptop

Computer Oscilloscope Lock-inAmplifier

Gradient Index

(GRIN) Lens

InGaAs Detector

Tunable

Diode

Laser

Function

Generator

Laser

Driver 

Figure 1: A schematic of a tunable diode laser absorption spectroscopy

(TDLAS) measurement of gases produced during fire suppression on board

Army vehicles.

Fiber Optic

Measurement Region

(Light into free space)

(Light collection)

(nominal event time equal to 250 milliseconds) with a time resolution better than 10 

milliseconds. Detection of fuel and oxygen concentration is especially important after the 

suppression event in order to predict the possibility of reignition. Minimum acceptable 

detection limits for fuel/oxygen mixtures are approximately 600 ppm fuel (0.06%) 

1 064 n m

Pulsed Laser

R adiation

1 064 n m

Pulsed Laser

R adiation

Spectrograph

G a ted Intensified

C am era

La ser induced plasm a 

w ithin me asurem ent

region

Focusing

Lens

C ollim ator / Fiber

C oupler

F igure 2: A schem atic of a laser induced breakdow n

spectroscopy (L IBS) m easurm ent for gas m easurem ent

during fire suppression on board Arm y vehicles.

4
and approximately 3000 ppm oxygen (0.3%), for a 10 centimeter pathlength at room 

temperature.

c. Technical Challenges 

The technical challenges faced during the first two years of the project were 

grouped into two broad categories. First, several challenges needed to be overcome 

relating to the ability to measure and store data continuously with a new data point 

acquired every 10 milliseconds. Secondly, it was unclear how the sensor equipment 

would perform in the harsh environment for field testing, in which the sensor assembly in 

the vehicle was subjected to shock (explosion) and heat (fire). For the third year of the 

project, the main challenge was developing a method that enabled the narrow band light 

source (the diode laser) to measure a broad band absorber (JP-8 vapor), while 

maintaining the signal to noise enhancement achievable with phase sensitive detection. 

[1,2] 

d. General Methodology 

For the first two years of the project, the measurement techniques using near 

infrared tunable diode lasers were based on well established techniques [2,3] for phase 

sensitive detection of gases that exhibit well resolved rovibrational spectra. Typically, 

the protocol was measurement and calibration of a system (e.g., HF gas measurement by 

near-infrared TDLAS near a wavelength of 1.31 micrometers, using 2f detection) in lab, 

followed by reinstallation of this system at the measurement site. During this time, LIBS 

experiments were carried out in the laboratory to determine measurement linearity and 

limits of detection for gaseous fire suppressants For the third year of the program, most 

of the effort was devoted to method development within the laboratory (measurement of 

fuel vapors using distributed feedback diode lasers), with minimum time being spent at 

full scale testing ranges.

e. Technical results 

A brief summary of technical results (excluding publications describing this work, 

which are listed separately) follows: 

1. Four-position near infrared tunable diode laser spectroscopy (NIR-TDLAS) system

installed at the Aberdeen Test Center (ATC). 

2. Tunable diode laser system for HF detection installed, calibrated, and tested in field 

and laboratory applications. 

3. Comparative testing to determine limits of detection (LOD) of HF detection using 

NIR-TDLAS, FT-IR, and using ion-selective electrode completed. 

4. Laboratory measurement of LIBS spectra of CF3Br and FM- 200 completed. 

5. Measurement of near infrared spectra of mixtures of methane and FM-200 for single 

laser detection completed. 

6. Field worthy, mobile tunable diode laser system, with fieldable calibration system, 

completed, tested for HF measurement. 

5
7. Initial TDL-based oxygen measurements during crew compartment fire suppression 

system testing completed. 

8. Time resolved measurements of HF dissipation during scavenging 

testing completed for FE-36 and FM-200. 

9. Laboratory mesaurment of pyridine vapor using distributed feedback laser. 

10. Simultaneous measurement of fue/oxygen concentrations (100 ms per point) using 

distributed feedback diode laser for gasoline, DF-2, and JP-8. 

11. Prototype Fourier transform laser spectrometer used to measure gasoline vapor 

concentrations. 

3. Detailed Description of Project 

In general the first two years of the project involved transitioning proven, 

laboratory based methods (TDLAS employing phase sensitive detection, laser induced 

breakdown spectroscopy) from the lab at the ARL to the crew compartment fire test 

facility at the Aberdeen Test Center. 

a. Species of Interest: Limits of Detection (LOD) 

Limits of detection (LOD) using TDLAS and using LIBS for several vapors are 

listed below. All LOD values listed are (to our knowledge) best achievable laboratory

values. In our experience, LOD’s in field measurement are always at least one order of

magnitude worse than lab measurements. 

Method Molecule Wavelength Approx. LOD 

TDLAS [4,5] HF 1.31 um .04 ppm-m

TDLAS [4,5] O2 0.76 um 100 ppm-m

TDLAS [4,5] CH4 1.65 um .6 ppm-m

TDLAS [4,5] CO 1.55 um 5 ppm-m

TDLAS [6] JP-8 1.71 um 100 ppm-m

LIBS [7,8] O2, CF3Br, CF3H emission 1000 ppm-v 

b. Laser Induced Breakdown Spectroscopy (LIBS) 

A schematic showing the application of LIBS to gas measurement was shown in 

Figure 2. Briefly, a pulsed laser (typically Nd:YAG, pulse energy > 50 milliJoules) is 

brought to a focus in free space such that a breakdown occurs. The breakdown is 

believed to occur as follows. Initially, in the focal volume of the laser, the high electric 

field causes an ionization of a small number of gas molecules (usually air). Free 

electrons within this plasma may then absorb laser radiation (inverse Brehmstrahlung [9]) 

resulting in further ionization accompanied by collisional heating. Temperatures within 

the laser focal volume may reach 20000 K, resulting in atomization of any molecules 

within the focal volume [7,8]. After the laser pulse, rapid deceleration of the electrons 

within the plasma leads to an initial emission of broadband radiation (Brehmstrahlung 

6
[9]) lasting tens to hundreds of nanoseconds, followed by emission of discrete 

wavelengths of light as the atomic species present in the plasma return to their ground 

states (tens to hundreds of microseconds). 

The application of laser induced breakdown spectroscopy focused along two 

areas. First, the technique was used for measurement of linearity and determination of 

limits of detection for halogenated fire suppressants [7,8]. Figure 3 shows the LIBS 

spectra of several fluorinated fire suppressants, measured in the gas phase. The limits of 

detection for each gas, using LIBS, was approximately 1000 parts per million (ppmvolume). Figure 3 shows that the intensity of F emission measured using LIBS is nearly 

directly proportional to vapor concentration. 

LIBS of Fire Suppressants

0

0 2000 4000 6000 8000 10000 12000 14000 16000

Concentration (ppm)

Signal Intensit

y (arbitrar

y units)

FM-200

Halon-1301

Halon-14

HFC134a

Figure 3: Laser induced breakdown spectra (LIBS) of several 

fluorinated fire suppressants, measured in the vapor phase. The

signal intensity is the peak F emission near 400 nm.

 

c. 2F Tunable Diode Laser Absorption Spectroscopy (TDLAS) 

 i. Stand Alone Spectrometer System for HF Gas Measurment 

The TDLAS effort was focused in two areas. The first of these was an effort 

designed to measure HF concentrations during fire suppression testing, using a stand 

alone spectrometer system assembled at the ARL. This system used a single, distributed 

feedback (DFB) tunable diode laser emitting near infrared radiation at a wavelength near

7
1.31 micrometers. This wavelength coincides with the P2 line of the first overtone 

absorption of HF gas, and is free of overlap by water lines present in most combustion 

environments [10]. Typically, the laser was modulated (wavelength tuned) at 100 Hz for 

a range of approximately 0.5 cm-1, such that the center of this tuning range coincided 

with the center of the absorption by the P2 transition in HF gas (absorption linewidth 

approximately 0.1 cm-1). Superimposed upon this “slow” modulation was a fast (50KHz 

– 100KHz) sinusoidal wavelength modulation. The wavelength range of the fast 

modulation was adjusted so that it was a small (<0.25) fraction of the absorption 

linewidth of the P2 transition (1st overtone) in HF gas. This dual modulated light beam 

was then launched onto an optical fiber, and the fiber used to transmit the beam from a 

control room (located a safe distance from the measurement site) to the measurement site. 

For field work, this usually meant the beam was transported approximately 100 meters to 

the measurement site. At the measurement site, the fiber was terminated with a gradient 

index (GRIN) lens, causing the light beam to be collimated while it passed through free 

space (the measurement region). After passing through free space for approximately 10-

15 cm, the modulated light beam impinged upon a semiconductor, room temperature 

operation light detector (typically InGaAs). The signal from the biased (12V) 

photodetector was returned to the control room via a BNC cable, demodulated using a 

lock-in amplifier, and recorded on a laptop computer running LabView software. The 

amplitude (height) of the second derivative-like signal output by the lock-in amplifier 

was proportional to the amount of light-absorbing gas (HF) present in the measurement 

region. The steps involved in laser modulation and demodulation are shown in Figure 4. 

7

8

9

10

11

12

13

14

15

16

7665.5 7665.51 7665.52 7665.53 7665.54 7665.55 7665.56

Wavenumbers (cm-1)

Signal at Detector (V)

0

0.01

0.02

0.03

0.04

0.05

0.06

Lock-in Signal Inten

sity (a.

u.)

Signal at detector without

fast modulation

Signal at detector with

fast modulation

2f signal from 

lock-in

Figure 4: The signals produced at the detector as slow and fast laser 

modulations are combined, and the 2f signal produced by the lock-in amplifier,

for a typical TDLAS measurement of HF gas. 

8
The experimental protocol, lab results, field results, and data analysis methods are 

detailed in several publications[6,10-18] from our research group. Early in the 

experimental cycle, an effort was made to use a second optical fiber to capture the light 

after it had passed through the measurement region, allowing the photodetector to remain 

in the control room. However, it was found that the alignment of the two optical fibers 

for the “pitch and catch” setup at the measurment site (typically, the crew compartment 

of a Bradley Fighting Vehicle) was extremely sensitive. So, it was decided to place the 

photodetector at the measurement site, since tests showed that small misalignments in the 

open path fiber-to-detector line of sight were less important than small misalignments in 

open path fiber-to-fiber line of sight. Additionally, the costs associated with replacing

either the fiber-to-detector or fiber-to-fiber setup within the vehicle were similar 

(hundreds of dollars). Using this system, measurement duration was 10 milliseconds 

(100 scans/second), however the time between measurements was approximately 100 

milliseconds, caused by the delay in transferring information from the lock-in amplifier, 

to an oscilloscope, and from the oscilloscope to the laptop computer running LabView 

software. 

ii. Results of HF Measurment Using Single Gas Stand Alone Spectrometer 

System 

The single gas stand alone spectrometer system for HF gas measurement was used 

for three types of testing. Results from each type of testing have been reported in the 

open literature [10,11,15,17]. Briefly, the three tests were suppression of spark and 

ballistically initiated JP-8 fires (CF3Br, FM-200) in the crew compartment of a Bradley 

Fighting Vehicle hull, suppression of JP-8 pool fires (using a handheld extinguisher 

charged with FM-200), and suppression of heptane pan fires (using a mechanically 

actuated bottle charged with a series of fluorinated propanes). For the tests conducted in 

the crew compartment of the Bradley hull, measurements of HF gas (and other 

combustion products) were also measured using extractive Fourier transform infrared 

spectroscopy. 

For these tests, the HF measurement location approximated the seating location of 

a passenger within the vehicle. Production of HF gas ranged from hundreds to thousands 

of ppm-m, depending on how quickly the fire was extinguished. For successful tests 

(fires extinguished with 250 ms of the suppressant being deployed, HF levels were 

typically in the hundreds of ppm range. For tests where the fire was not extinguished or 

suppression was not complete after 250 ms (resulting in suppressant being burned), the 

HF levels were often beyond the measurement range of both instruments (usually 5000 

ppm for TDLAS). For the JP-8 pool fire measurements, the main goal was to obtain a 

measurement of the approximate exposure to HF experienced by a firefighter while 

suppressing the fire using a hand held extinguisher. For these tests, the HF measurement 

location was approximately 5 meters from the pool fire, allowing the firefighter full 

access to the pool fire, as he was attempting to extinguish the fire as rapidly as possible.

For these tests, the HF gas concentration that was measured ranged from 0 to

approximately 1000 ppm. As only one measurement site was used, the measurement was 

9
largely a function of wind direction, although the time averaged exposure was always 

beyond acceptable levels. 

For the heptane pan fires, an attempt was made to understand the temporal

behaviour of HF gas concentration following an initial release of the commercial 

suppressant FE-36 (hexafluoropropane), with and without the addition of an acid gas 

scavenging agent that may also act as a flame suppressant (ammonium polyphosphate 

(APP)). The two mechanisms by which HF gas concentrations can be decreased during 

and following fire extinguishment by FE-36 or FM-200 are by 1) reducing the time 

required to extinguish the fire, thereby minimizing the time that the fluorine containing

suppressant is exposed to flame temperatures, or 2) releasing a scavenging agent in 

conjunction with the fire suppressant chemical to remove HF after it is produced. A 

potential chemical scavenging agent of gaseous HF is ammonium polyphosphate (APP), 

which is a commercially available chemical (Phos-chek) with fire retardant properties.

One of the drawbacks to the release of a powder agent in an occupied space is the 

possibility of reduced visibility. 

HF was produced in the full scale tests from a series of enclosed heptane/air pan

fires extinguished by either FE-36 (C3F6H2), FM-200 (C3F7H), FE-36 plus APP, or FM200 plus APP. By measuring how much the HF concentrations were reduced in fires 

extinguished with FE-36/APP or FM-200/APP mixtures compared to fires extinguished 

by FE-36 or FM-200 alone, the effectiveness of APP as a HF reduction and potential 

scavenging agent was demonstrated. In addition, spectroscopic measurements of the

amount of light attenuation (obscuration) that occurs as a hand held fire extinguisher 

containing powder fire fighting agent is released in the crew space of a M1 land combat

vehicle were performed. 

Results [17] from the full scale tests indicate that HF concentrations produced

from fires extinguished by FE-36 plus APP and FM-200 plus APP were reduced in the 

test facility and that the presence of APP accelerated the reductions. It has been 

concluded that the combination of APP in an extinguisher containing FE-36 or FM-200 

appears to reduce HF levels. Visibility reduction during extinguisher deployment was

measured inside an actual combat vehicle for extinguishers containing FE-36 plus APP 

and NaHCO3 plus N2. 

Some results of these tests are shown in Figure 5. Measurements suggest that by 

following the temporal evolution of HF gas during fire suppression, it would be possible 

to determine whether the decrease in observed HF gas levels was due to acid gas 

scavenging or the fire being extinguished more rapidly because of the presence of the 

additional suppressant. 

iii. Multi-Position, Multi-gas Spectrometer System 

The second area of investigation involved the use of a fully automated, multiposition, multi-laser instrument housed in a small trailer adjacent to the control room at

10
the measurement site. This instrument, built for the Army by Southwest Sciences, Inc. 

(Santa Fe, NM) was originally designed to measure concentrations of CO, O2, HF, and 

-200

0

200

400

600

800

1000

1200

1400

0 50 100 150 200

Time, seconds

Average H

F Concentration, pp

m-m

FE-36

FE-36 + 7 % APP

FE-36 + 15 % APP

-200

0

200

400

600

800

1000

1200

1400

0 50 100 150 200

Time, seconds

Average H

F Concentration, pp

m-m

FE-36

FE-36 + 7 % APP

FE-36 + 15 % APP

FE-36

FE-36 + 7 % APP

FE-36 + 15 % APP

Figure 5: HF gas produced during suppression of heptane pan fires by FE-36 

(hexafluoropropane) with and without the addition of APP (ammonium 

polyphosphate). 

NO2 at four different locations within the crew compartment of the Bradley Fighting 

Vehicle, achieving continuous measurement with data points every 10 milliseconds. This 

instrument used the same technique of dual modulation spectroscopy employed by the 

stand alone HF measurement system described earlier, but used a 10 KHz ramp for the 

“slow” modulation and a 1MHz sine wave for the “fast” modulation. During the course 

of development of this instrument, it was decided to focus exclusively on measuring HF 

and O2 within the Bradley hull, during suppression by FM-200 of spark and ballistically 

initiated JP-8 fires. 

iv. Results Using the Multi-position, Multi-gas Spectrometer System

Results using this system have been reported [10], but were limited to oxygen 

and HF measurements during chemical suppression of spark initiated P-8 fires. For these 

fires, a heated (~70 centigrade) spray of JP-8 liquid was begun within 

11
O xyg e n C o n c e n tr a t i o n

1 0

1 2

1 4

1 6

1 8

2 0

2 2

2 4

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0

T im e ( m illis e c o n d s )

% Oxygen

Figure 6: Oxygen concentration measured within the crew compartment of a 

Bradley Fighting Vehicle during suppression (by C3F8) of a spark initiated JP-8 

spray fire. 

the crew compartment of the Bradley hull (fuel nozzles located along one interior wall), 

and one second later this fuel spray was ignited by a series of spark plugs located along 

the wall of the interior of the vehicle. Approximately one second after ignition, the two 

onboard bottles containing the pressurized suppressant (FE-36 (C3F6H), FM-200 

(C3F7H), C3F8) were released. At the crew seating location, a sharp drop in O2

concentration occurred during the fire and suppression event. Following suppression, O2

levels returned to ambient in a few seconds. It was not clear whether the decrease in O2

levels during the fire and suppression event was due to combustion of oxygen or 

displacement of oxygen by the hot fuel vapors/expanding FM200 vapor. Results from a 

typical test measuring oxygen concentration as a function of time in the crew 

compartment of the Bradley fighting vehicle are shown in Figure 6. 

d. Real time Fuel/Ox sensor 

The last year of the funded project (FY99), plus almost a year (FY00) of 

additional work resulting from a no-cost extension, concentrated on measurement of 

12
fuel/air

ent 

Pf

pm

O2 gas was routinely achievable using the TDLAS 

chniques described previously. The main challenge for the remaining year was the 

measur

ectors for hydrocarbon vapor measurements typically rely upon 

frared absorption (DOAS (differential optical absorption spectrometry), FT-IR (Fourier 

transfo

ector

t

S for measurement of the fundamental C-H 

stretch in hydrocarbon fuels near 3.4 μm can yield sensitivities in the parts per billion 

(ppb) r

ppm)) well 

s

mixtures using “real” fuels, e.g., JP-8 turbine fuel. Specifically, the work

concentrated on development, within an initial one year period, of a multi-point, field

worthy, fiber-coupled, near-infrared tunable diode laser-based sensor for measurem

and detection of combustible mixtures of oxygen and hydrocarbon fuels (heptane and J

8). The goal of this work was to provide quantitative measurement of fuel/oxygen 

concentrations before (minimum five data points prior to agent release), during, and after 

the suppression event (nominal event time equal to 250 milliseconds) with a time 

resolution better than 10 milliseconds. Detection of fuel and oxygen concentration is 

especially important after the suppression event in order to predict the possibility o

reignition. The goal for the system was a LOD for fuel vapors of approximately 600 p

fuel (0.06%) and approximately 3000 ppm oxygen (0.3%), for a 10 centimeter path 

length at room temperature. 

The desired LOD for

te

ement of fuel vapors. 

i. Background 

Field worthy det

in

rm infrared) emission/absorption) [19,20] or electrochemical / electrothermal 

techniques [21]. These methods give good results, but have found limited use for 

measurements in fires. FT-IR and DOAS spectrometers are usually bulky, expensive,

measurements require placement/alignment of interferometer/source optics and det

or retro-reflector, 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 [22], it does no

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) [23]. 

Nevertheless, use of FTIR and DOA

ange, 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 thousand), are 

often one-man portable, and have sensitivities (~100's of parts per million (

below lower explosion limits for most hydrocarbon vapors [24]. 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 vapor

[21]. For measurement sites requiring full portable instrumentation with time resolution

from one to tens of seconds, these instruments can also provide excellent results. 

13
 

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

ers and relatively inexpensive (room temperature operation) semiconductor detectors 

to sepa

etc [25].

l

ometer. 

asoline, JP-8, DF2. Spectra offset for clarity. Structured absorptions near 1.9 

As suggested by the spectra shown in Figure 7, the vapors from each fuel fit 

riteria [27] to be classified as a gas having unstructured absorption spectra (often called 

a "broa le

fib

rate electronics and laser sources from hazardous testing locations. Using 

modulation techniques and phase sensitive detection methods, temporal resolution 

routinely exceeds 1 ms per measurement. As previously mentioned, detection methods 

employed (typically wavelength modulation spectroscopy (WMS) and balanced 

ratiometric detection (BRD) techniques) [2,3] 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,

However, large and/or heavy molecules may not posses well resolved ro-vibrationa

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 [26]. For mixtures of heavy molecules, e.g., the middle distillate fuels, the 

spectra may be even less structured. Figure 7 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 spectr

Figure 7: FT-IR spectra of dry air saturated at 294K with vapor from unleaded 

0

20

40

60

80

100

120

140

1.2 1.7 2.2 2.7 3.2 3.7

Unleaded

Gasoline

(regular)

JP-8

DF2

Wavelength (micrometers)

%

Tra

n

s

mis

s

oi n

160

g

and 2.7 micrometers are from water vapor. 

c

d band absorber"). Both WMS and BRD, as usually employed, are not suitab

for gases that exhibit broad, unstructured absorption spectra [2,3,25]. For both 

14
techniques, the main limitation for fuel sensing is the narrow tuning range of the diode 

laser. Because the tuning range of the DFB lasers is usually several orders of m

smaller than the width of the spectral feature in broad band absorbers (0.4 cm-1

vs. >100

cm-1), it is not possible using commercially available DFB lasers to tune from the region 

of maximum absorption to a region of zero absorption. A main goal of the pr

agnitude 

w Band Lasers 

sidual

mplitude modulation (RAM) that occurs as the wavelength of a semiconductor tunable 

diode l and

ly

to

nal 

ematical description begins by considering Beer's law [1] for narrow 

and laser radiation exiting an absorbing sample gas of concentration c molecules per 

velength is now power modulated sinusoidally 

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

oject was to

overcome this limitation for measurement of such molecules, while approaching the 

sensitivity and time response of WMS and BRD methods.

ii. Measuring Broad Band Absorbers with Narro

The initial method developed for the project takes advantage of the re

a

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

BRD), RAM is a nuisance, and what is important is the wavelength modulation. For this

method, 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 this method, two different DFB lasers are driven using a sinusoidal

varying injection current. Each laser injection current is modulated at the same frequency

but out of phase by 180 degrees [27]. These light output of the two lasers is combined

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 sig

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. 

The simple math

b

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 wa

a

the resonant wavelength after exiting the gas becomes: 

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

15
To get a measurement of transmission off resonance (important to generate a differential 

e

and

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

o process this signal, we multiply by a reference sine wave of amplitude p in phase with 

0

π

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

(4)

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

C 0

π

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

(5)

or the special case where the power and depth of modulation of the two lasers are equal, 

C 0

π

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

(6)

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

for mea

wer,

g

ser

e

ratio of the measurement. 

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 sam

frequency but 180 degrees out of phase with the resonant wavelength modulation,

has a depth of modulation DNR . If the radiation exiting the gas is now focussed on an 

(ideally) wavelength-insensitive detector, the power at this detector, I(λR +λNR), may be

given by: 

I(

T

and at the same frequency, a, as the power modulation of the two lasers. We then 

integrate over C periods of power modulation: 

C

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

R

2

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

F

Equation 5 becomes: 

2

=-Cpπ [I0(λR) (1+DR)A]

surement 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 po

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 increasin

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. As mentioned previously, and 

implied by Equation (6), because the measurement relies on differential absorption to

quantify gas concentrations, division of the raw signal at the detector by the average la

power should account for pseudo wavelength-independent signal intensity extinction 

from scatterers, such as smoke particles. However, it should also be mentioned that 

decreases in overall signal voltage will result in similar decreases in the signal to nois

16
iii. Experimental Apparatus

apparatus employing this method is shown in 

igure 8. The two DFB laser diodes (Sensors Unlimited) used to produce the mixed 

wavele ser

le

is

unched

nt

top

Figure 8: A diagram of the laser mixing apparatus used to measure fuel vapor 

concentrations. 

A diagram of the experimental

F

ngth probe beam have nominal wavelengths of 1.307 μm and 1.71 μm. The la

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

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

mixed onto a common fiber using a fiber optic coupler (Gould Fiberoptics) and la

into free space using a single mode fiber that is terminated by a gradient index (GRIN) 

lens (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 eleme

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 lap

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

Laptop

Computer

Oscilloscope Lock-in

Amplifier

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

Laptop

Computer

Oscilloscope Lock-in

Amplifier

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

17
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 Wilkes 0.75 meter path length cell. 

iv. Results 

Figure 9 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 μm 

superimposed upon the emission from the optical fiber carrying the mixed wavelength 

laser beam. Both sets of spectra were measured using a FT-IR spectrometer (Bomem

DA-8). The near infrared tunable diode laser radiation 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 μm 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 [28]. 

Figure 9: 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 μm 

superimposed upon the emission from the optical fiber carrying the mixed 

wavelength laser beam. 

.1

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

-0.25

Wavelength (micrometers)

a

-0

0.25

L ser Emi

s

s ts)

Laser 1 Laser 2

Gasoline vapor

0

0.1

0.2

0.3

0.75

1.25

Absorbance

ion (

arb. uni

JP-8 vapor

DF-2 vapor

0.4

18
Figure 10 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 (DFB laser- based 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 [24,29]). Each data point is the average of the lock-in 

amplifier output measured over 10 ms. All measurements were made at atmospheric 

pressure. 

ndred

13.7 liter volume optical cell. Also shown in this 

gure is the change in oxygen concentration that occurs during the displacement 

(DFB l ).

Figure 10: Gasoline vapor concentration (measured using the mixed laser 

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

cm3

/minute) the dry air in a

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 (sec)

Gasoline Vapor (ppm)

Fraction Oxygen 

gasoline

O2

fi

aser- based oxygen sensor courtesy of Oxigraf, Inc., Mountain View, CA

ter

s

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

 cm

2

/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) [4]. We estimate the limit of detection for each of these fuel vapors, using 

the mixed laser sensor, to be approximately 100 ppm-meter. 

 Figure 11 shows the measured fuel vapor concentration as the dry air in the13.7 li

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

in peak hydrocarbon levels between runs shown in Figure 11 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 pressure

at atmospheric pressure (measured using a capacitance manometer) for gasoline, JP-8, 

and DF-2

19
-0.

0.4

0.9

1.4

1.9

2.4

2.9

3.4

I

0 500 1000 1500 2000 2500 3000 3500 nstrum

ent Response (arb. units)

Fresh sample

Sample after 5 runs

1

Parts-per-million meter JP-8 vapor

 (added to 13.7 liter optical celll)

Figure 11: Ageing of JP-8 (loss of lighter hydrocarbons) caused by repeated fills 

f the optical cell used for testing the mixed laser fuel vapor sensor. 

g the

ure the full C-H absorption 

nvelope of the fuel vapor.

e. Second Generation Sensor Development 

Perhaps the best method for detection and measurement of vapor phase “broad 

band” absorbers is Fourier transform infrared (FT-IR) spectroscopy [28]. In conventional 

Fourier transform spectrometers, each “discrete” wavelength of light emitted by the 

glowbar source (most Fourier transform spectrometers use a resistively heated tungsten 

coil) is power modulated by a Michelson interferometer so that the output, as measured at 

the detector, is cosinusoidal, with a frequency in the KHz (acoustic) range. The 

amplitude and frequency of this cosine wave are a function of moving mirror 

displacement and of the wavelength of the incident light. As an example of the 

relationship between spectral resolution and mirror displacement, to resolve the line 

separation of the fundamental ro-vibrational transition in CO gas it is necessary to move 

s) caused by repeated fills 

f the optical cell used for testing the mixed laser fuel vapor sensor. 

g the

ure the full C-H absorption 

nvelope of the fuel vapor.

e. Second Generation Sensor Development 

Perhaps the best method for detection and measurement of vapor phase “broad 

band” absorbers is Fourier transform infrared (FT-IR) spectroscopy [28]. In conventional 

Fourier transform spectrometers, each “discrete” wavelength of light emitted by the 

glowbar source (most Fourier transform spectrometers use a resistively heated tungsten 

coil) is power modulated by a Michelson interferometer so that the output, as measured at 

the detector, is cosinusoidal, with a frequency in the KHz (acoustic) range. The 

amplitude and frequency of this cosine wave are a function of moving mirror 

displacement and of the wavelength of the incident light. As an example of the 

relationship between spectral resolution and mirror displacement, to resolve the line 

separation of the fundamental ro-vibrational transition in CO gas it is necessary to move 

o

Two factors led us to decide that measurement of the full absorption envelope for 

the C-H stretching vibration was necessary, and that our first generation sensor system

was inadequate for general fuel vapor sensing. First, careful examination of the C-H

overtone bands for each of the fuels tested (JP-8, diesel, and gasoline) shows that the 

contours of the absorption features, when normalized to each other, were not 

superimposable. So, being limited to sampling vapors from different fuels usin

same single wavelength would be a potential source of error. Second, changes in 

temperature, storage conditions (ageing) and changes in fuel composition between 

vendors may cause changes in the contour of the unstructured absorption envelope.

Therefore, it was necessary to develop a technique with the simplicity of the first 

generation sensor but with the capability of expansion to meas

Two factors led us to decide that measurement of the full absorption envelope for 

the C-H stretching vibration was necessary, and that our first generation sensor system

was inadequate for general fuel vapor sensing. First, careful examination of the C-H

overtone bands for each of the fuels tested (JP-8, diesel, and gasoline) shows that the 

contours of the absorption features, when normalized to each other, were not 

superimposable. So, being limited to sampling vapors from different fuels usin

same single wavelength would be a potential source of error. Second, changes in 

temperature, storage conditions (ageing) and changes in fuel composition between 

vendors may cause changes in the contour of the unstructured absorption envelope.

Therefore, it was necessary to develop a technique with the simplicity of the first 

generation sensor but with the capability of expansion to meas

e

20
one of the mirrors in the interferometer approximately 1 cm. Although highly refined, 

the net effect of this design is to limit the use of these instruments mainly to the 

laboratory, necessitate complicated electromechanical mirror transport systems, and limit 

measurement applications to repeatable or static events. Details of FT-IR spectroscopy 

are widely available in several standard texts on the subject [30]. 

The approach employed in our second generation sensor uses an array of 

addressable microphotonic semiconductor lasers, each emitting light at a different 

wavelength, and power modulated at different frequencies (typically between 100 and 

150 KHz). When the modulated light is combined onto a single path, and the combined 

beam brought to focus on a detector, the signal reported by the detector has the contour of 

an interferogram. This interferogram is analyzed by standard Fast Fourier Transform 

(FFT) techniques to yield the amplitudes of the individual component cosine waves [31]. 

These amplitudes are proportional to the average power of light at each wavelength 

reaching the detector. Figure 12 shows the interferogram generated by a two laser mixed 

beam. Laser 1 emits radiation near a wavelength of 1.31 micrometers and was power 

modulated at 100 KHz, with a depth of modulation near 95%. As with the first 

generation sensor, this laser wavelength was chosen because it is not absorbed by 

ydrocarbon vapor or by water vapor. Laser 2 emits laser radiation near a wavelength of 

sen because

is approximately at the center of the 1st overtone C-H stretching vibration of most 

middle

nd

h

1.71 micrometers and was power modulated at 110 KHz, with a depth of modulation near 

95%. Also as with the first generation sensor, this laser wavelength was cho

it

distillate fuel vapors.

3.50E-02

3.70E-02

3.90E-02

4.10E-02

4.30E-02

4.50E-02

Signal at detector

Laser 1 (100 KHz) at 1.31μ

Laser 2 (110 KHz) at 1.71 μ

y (a.u)

Figure 12: The two laser interferogram measured at the detector for the seco

generation fuel vapor sensor. 

2.50E-02

2.70E-02

2.90E-02

0.00E+00 4.00E-04 8.00E-04

Time (sec)

3.30E-02

nte

n

sit

3.10E-02 I

21
The output from each fiber coupled laser is launched onto a single, common, 

single mode optical fiber using a commercially available fiber coupler (Gould Fiber 

Optics). Each laser is power modulated, each at a different frequency, using digital

nction generators (Stanford Research Systems DS-345) operating on a common time

base. T nd the

e

ier

he

n

Figure 13: A schematic of the experimental apparatus used for Fourier 

transform laser spectroscopy (FT-LS). 

i. Results of Prototype Testing of Second Generation Sensor System 

As with the first generation sensor, the second generation sensor was tested by 

measuring gasoline vapor concentration 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 

explosive limit (LEL) for most hydrocarbon vapors 

fu

his modulated light beam is passed through a GRIN lens into free space, a

transmitted light detected using an InGaAS (New Focus model 2011) detector. Th

signal at the detector is demodulated (Agilent Infinium Oscilloscope) using a Fast Four

Transform (FFT), in a manner similar to that of commercially available Fourier transform

infrared (FT-IR) spectrometers [31]. Time for each measurement is user controlled by

varying the frequency of power modulation. Resolution is user controlled by varying t

number of periods of power modulation measured per “scan”. The experimental 

apparatus for the Fourier transform laser spectroscopy (FT-LS) system is described i

Figure 13. 

Laser 1

Analyte region

InGaAs

detector

Fiber optic

Laser 2

FFT

Laser diode controller

Function generators

GRIN

Fiber mixer lens

meter path length). Figure 14 shows the sensor response as the vapor concentration 

approaches and exceeds the lower

22
(e.g., for JP-8 and most other middle distillate fuels the LEL is approximately 10000 ppm

4,29]). For the absorbance measurement, the background for each laser was the 

demodu

nce 

tly

t

rement

cell is

vapor. For the absorbance calculation, 

e background was the demodulated signal for each laser prior to the onset of 

el vapor flow.

[2

lated signal prior to the onset of fuel vapor flow. The flow apparatus was 

characterized using our first generation sensor, and using that calibration the absorba

reported here is reasonably linear with concentration (within 5%), but was not explici

measured for these experiments. All of the factors that determine quality of

conventionally measured FT-IR spectra are of importance here (modulation depth, 

apodization, phase, etc. [28]). Most of the efforts for the prototype were focused on 

making sure the lasers were modulated in phase, using a common timebase. We did not 

investigate effects of varying the modulation depth or of varying the laser power. 

Dynamic range effects were not investigated, although we anticipate some improvemen

relative to Michelson interferometer based instruments because the Fourier frequencies 

for each laser are no longer coupled to the laser wavelength. Time for each measu

point was approximately one second. All measurements were made at atmospheric 

pressure. 

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 500 1000 1500 2000 2500

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 500 1000 1500 2000 2500

A

bsorbance

Time (sec)

LEL (Lower

Explosion Limit)

Figure 14: Absorption of laser radiation at 1.71micrometers as air in 2m

displaced by air saturated with gasoline

th

fu

ii. Sensitivity 

The absorption coefficients for most VOC’s in the near-infrared spectral region 

(~1 to 2 micrometer wavelength, two (or more) vibrational quanta per transition) are 

23
typically lower by an order of magnitude than for the mid-infrared spectral region (~2-3

micrometer wavelength, typically 1 vibrational quantum per transition) [32]. In our lab 

we have measured the near infrared absorption coefficients between wavelengths o

and 30 micrometers for several VOC’s (see Figure 7). For the military fuel JP-8 (a

mixture of several hundred hydrocarbons) we have measured [using a Bomem DA-8 FTIR spectrometer] the maximum absorption coefficient in the spectral region between a 

wavelength of 1.6 and 1.825 micrometers to be approximately 3 X 10-21

 cm

2

/m

[4]. Using the prototype FT-LS instrument described above [33] we obtained a dete

limit of approximately 200 ppm-m (parts per million per meter path) for JP-8 vapor in

or approximatrely 50 times worse than the detection limit using a commercial FT-IR 

spectrometer. It is worth noting that for the prototype sensor described here, we ob

that the power modulation of each diode laser was not purely cosinusoidal (harmonics in 

the FFT output), even though the driver current was supplied by the SRS DG535 functio

generators. We anticipate that as the number of lasers being power modulated is 

increased, the contributions to signal from frequencies other than driver frequencies

limit resolu

0

f 1.0

olecule 

ction

air,

served

n

may

tion. We have considered designing a custom driver waveform to give the 

best cosinusoidal power modulation, but have yet to actually try this. 

 Figure 15: Schematic of a 10-laser FT-LS source in the near infrared (under 

evelopment), providing reasonable coverage of the full overtone C-H absorption 

We are presently pursuing development of a 10-laser FT-LS source in the near 

frared, providing reasonable coverage of the full overtone C-H absorption envelope for 

hydrocarbon vapors. The basic idea for this sensor is illustrated in Figure 15. Analysis 

imilar to PLS techniques already employed for FT-IR spectral data could be used to 

identify mixture components [34]. Such an instrument may be readily assembled from 

0

0.02

0.04

0.06

0.08

0.1

0.12

1.625 1.675 1.725 1.775 1.825 1.875

Absorbance – air saturated by gasoline vapor1 meter path

Wavelength (micrometers)

Wavelength position of 10 diode laser

addressable source

d

envelope for hydrocarbon vapors. 

iii. Future Developments

in

s

24
comme

ide

t for

m and 

C

m) remains 

challenging. The difficulty in obtaining lasers at custom wavelengths exacerbates what 

we perceive to be the main challenge for gas sensing using near infrared DFB lasers – 

namely, sensing of molecules and mixtures of gases that do not present well resolved 

vibrational spectra. We believe we have made reasonable efforts towards solving this 

problem (see sections 3d and 3e above), but the difficulty in obtaining lasers at custom 

wavelengths makes full demonstration of a truly addressable source difficult. 

 

g. Recommendations and Conclusion 

Gas sensing using near-infrared lasers will probably continue gaining market 

share, although it seems likely that a transition will occur in which a single laser detecting 

a single gas is replaced either by an addressable source, or by a single, broadly tunable 

laser. Near infrared devices for gas sensing will likely continue to be used because of the 

ease of transmission through optical fibers, room temperature operation, high signal to 

noise ratio through phase sensitive detection techniques, and because of the large tel-com

infrastructure that continues development of electronics that may easily be adapted to gas 

sensing. However, room temperature operation mid-infrared lasers (quantum cascade 

and interband cascade designs) offer all of the advantages of phase sensitive detection 

vailable with near-infrared DFB’s, with the huge advantage of gas sensing using the 

nsors will 

ill

oratory

rcially available tel-com equipment. Here, the main impediment is finding a 

vendor for the individual laser diodes. The problem is that low demand for lasers outs

of the fiber windows near 1350 nm and 1550 nm limits availability. 

Provided that the cost of addressable light sources continues to decrease, arrays 

covering larger spectral windows may become available. While several options exis

wavelength separation, we believe that the highly developed FT-IR market may influence

the design of new solid state spectrometers, and that this approach may warrant further

consideration. 

f. Technical Problems 

The main technical difficulties associated with this project center on the 

availability of DFB lasers. With the contraction of the tel-com market at the beginning of

this century, many fabrication facilities discontinued manufacture of lasers at user 

specified wavelengths, producing lasers only within the tel-com bands near 1330 n

1550 nm. Although some of the decrease in availability of domestically manufactured 

near-infrared DFB lasers has been made up by foreign manufacturers (in particular, NE

of Japan), obtaining lasers for gas sensing (in particular, O2 near 760 n

a

fundamental vibrational transition. It seems likely that future microphotonic se

employ suites of arrays of near- and mid-infrared devices, and that these devices w

gradually displace conventional Michelson interferometer based systems in the lab

and in the field.

25
4. References 

[1] 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). 

-

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

modula

997).

rsen, and T. Jaeger, “Gas monitoring in the process industry 

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292-

opic

alysis of Fire Suppressants and Refrigerants by Laser Induced Breakdown 

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w

nt

[2

tion spectroscopies: comparison of experimental methods using a lead-salt diode

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[4] I. Linnerud, P. Kaspe

u

[5] R. Martinelli, R. Menna, D. Cooper, C. Carlisle, H. Riris, “Near-infrared InGaAs/InP 

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[6] K.L. McNesby, R.T. Wainner, R.G. Daniel, R. R. Skaggs, J. B. Morris, A.W.

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[7] Lancaster, E.D., McNesby, K.L., Daniel, R.G., Miziolek, A.W., "Spectrosc

An

Spectroscopy", Applied Optics, Vol. 38, p

[8] Williamson, C.K., McNesby, K.L., Daniel, R.G., and Miziolek, A.W., "LaserInduced Breakdown Spectroscopy for Real-Time Detection of Halon Alternative 

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[9] M.Q. Brewster, Thermal Radiative Transfer and Properties, Wiley Interscience, Ne

York (1992), p. 253. 

[10] K.L. McNesby, R.R. Skaggs, A.W. Miziolek, M. Clay, S. Hoke, and C.S.Miser, 

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Suppression Of Fires” Applied Physics B, Vol. 67, p443, (1998). 

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of Toxic Gases Produced During Firefighting Using Halons, Applied Spectroscopy , vol. 

51, pp. 678-683, 1997.

26
[12] K.L. McNesby, R.G. Daniel, A.W. Miziolek, and S.H. Modiano, "Optical 

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.L., Skaggs, R.R., Miziolek, A.W., Daniel, R.G., and Miser, C.S., 

f

.,

aser Measurements of HF Concentrations from Heptane/Air Pan 

ires Extinguished by FE-36 And FE-36 Plus Ammonium Polyphosphate,” ARL-TR, 

W.R., 

nd Horton, D., “Diode Laser Measurements Of HF Concentrations Produced from

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200

ggs, R.R., Daniel, R.G., Miziolek, A.W., McNesby, K.L., Herud, C. Bolt, W.R., 

nd Horton D., “Diode Laser Measurements of HF Concentrations From Heptane/Air Pan 

ast, in 

houdt, 

[20] Guenther P.L.; Stedman D.H.; Bishop G.A.; Beaton S.P.; Bean J.H.; Quine R.W.," A 

, Vol. 66, pp. 3024-3029 (1995) 

erud, "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). 

M

1

[13]McNesby, K

“Near-Infrared Tunable Laser Diagnostics In Laboratory Scale Flames,” Proceedings o

Laser Applications To Chemical and Environmental Analysis Topical Meeting, p. 203,

1998.

[14] McNesby, K.L., Skaggs, R.R., Miziolek, A.W., Clay, M., Hoke, S., and Miser, C.S

“Diode Laser-Based Measurements Of Hydrogen Fluoride Gas During Chemical 

Suppression Of Fires,” ARL-TR –1785, 1998. 

[15] Skaggs, R.R., Daniel, R.G., Miziolek, A.W., McNesby, K.L., Herud, C., Bolt, W.R.,

and Horton, D., “Diode L

F

submitted 1998. 

[16] Skaggs, R.R., Daniel, R.G., Miziolek, A.W., McNesby, K.L., Herud, C., Bolt,

a

Heptane/Air Pan Fires Extinguished By FE-36, FM-200, FE-36 Plus APP, or FM-200 

Plus APP,” Proceedings of the 1998 JANNAF Combustion Meeting, CPIA Publication 

685, p.161, 2000. 

[1

and Horton, D., “Diode Laser Measurements Of HF Concentrations Produced From

Heptane/Air Pan Fires Extinguished By FE-36, FM-200, FE-36 Plus APP, Or FMPlus APP,” Applied Spectroscopy, Vol. 53, p. 1143 (1999). 

[18] Ska

a

Fires Extinguished by FE-36 And FE-36 Plus Ammonium Polyphosphate,” 1998

Proceedings of the International Society for Optical Engineering: Photonics E

press, 1999. 

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

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

hydrocarbon detector for the remote sensing of vehicle exhaust emissions", Review of 

Scientific Instruments

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

[22] S.H. Modiano, K.L. McNesby, P.E. Marsh, W. Bolt and C. H

27
[23] P.R. Griffiths, B.L. Hirsche, and C.J. Manning, "Ultra-rapid-scanning Fourier

transform infrared spectrometry", Vib. Spectroscopy, Vol. 19, pp.165-176 (1999). 

el Vapors", Proceedings of the Fall Meeting of the Society of 

utomotive Engineers, SAE paper # 982485, (1998), pp. 1-7; also see Kuchta, J. M. and 

-TRTragedy: Explosions and Flight 800”, Engineering and Science, No. 2, pp. 18-29, 

semiconductor laser based gas monitors", 

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

s in

and J.A. de Haseth, Fourier Transform Infrared Spectrometry, Wileysurement of MiddleDistillate Fuel Vapors by Use of Tunable Diode Lasers” Applied Optics, Vol. 40, pp. 

., Vol. 19, p. 297 (1965).

atabase: Editions of 1991 and 

992”, J. Quant. Spectrosc. Radiat. Transfer, Vol. 48, p. 469, (1992). 

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

Middle Distillate Fu

A

R. G. Clodfelter, “Aircraft mishap fire pattern investigations. Final Repor”t APWAL

85-2057, Aero Propulsion Laboratory, (1985); M. R. Baer and R. J. Gross, “Extended 

Modeling Studies of the TWA 800 Center-Wing Fuel Tank Explosion”, Sandia National 

Laboratories Report SAND2000-0445. 44 pp., March 2000.; J. Shepherd, “Learning 

from a

1998. 

[25] P. Werle, "A review of recent advances in

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

York, 1945. 

[27] K.L. McNesby, R.T. Wainner, R.G. Daniel, A.W. Miziolek, W.M. Jackson, I.A.

McLaren “High-Sensitivity Laser Absorption Measurements of Broadband Absorber

the Near-Infrared Spectral Region” Applied Optics, Vol. 39, pp. 5006-5011, (2000). 

[28] P.R. Griffiths

Interscience, New York, 1986. 

[29] K.L. McNesby, R.T. Wainner, R.G. Daniel, R.R. Skaggs, J.B. Morris, A.W.

Miziolek, W. M. Jackson, I.A. McLaren “Detection and Mea

840-845, (2001). 

[30] J.R. Ferraro and L.J. Basile, Fourier Transform Infrared Spectroscopy : 

Applications to Chemical Systems”, Vols. 1-3, Academic Press, New York, 1978; also

see Ref. 29. 

[31] J.W. Cooley and J.W. Tukey, “An Algorithm for the Machine Calculation of 

Complex Fourier Series”, Math. Comput

[32] 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 D

1

28
[33] K.L. McNesby and A.W. Miziolek, “Fourier Transform Laser Spectroscopy”, 

accepted for publication in Applied Optics. 

[34] Xiang Zhu, Daniel T. Cassidy, “Liquid detection with InGaAsP semiconductor 

sers having multiple short external cavities”, Applied Optics-LP, Volume 35, Issue 24, 

5. Publications 

ment

la

4689-4693 (1996). 

K.L. McNesby, R.G. Daniel, A.W. Miziolek, and S.H. Modiano, "Optical Measure

of Toxic Gases Produced During Firefighting Using Halons", ARL-TR-1349, April 1997.

K.L. McNesby, R.G. Daniel, S.H. Modiano, A.W. Miziolek, Optical Measurement of 

Toxic Gases Produced During Firefighting Using Halons, Applied Spectroscopy , vol. 51,

pp. 678-683, 1997. 

R.G. Daniel, K. L. McNesby, R.R. Skaggs, P. Sagear, and A.W. Miziolek, “Spectroscopy 

of Inhibited Counterflow Diffusion Flames”, ”, 1997 JANNAF Propellant Development 

.L. McNesby, R.R. Skaggs, A.W. Miziolek, M. Clay, S. Hoke, and C.S.Miser, “Diode 

Of

d Physics B, Vol. 67, p443, (1998). 

cNesby, K.L., Skaggs, R.R., Miziolek, A.W., Daniel, R.G., and Miser, C.S., “Nearnvironmental Analysis Topical Meeting, p. 203, 1998. 

.,

el, R.G., Miziolek, A.W., McNesby, K.L., Herud, C., Bolt, W.R., and 

Air Pan

, Daniel, R.G., Miziolek, A.W., McNesby, K.L., Herud, C., Bolt, W.R., and 

Horton, D., “Diode Laser Measurements Of HF Concentrations Produced from

00

, CPIA Publication 

685, p.161, 2000. 

& Characterization Subcommittee Meeting, West Palm Beach, FL. 

K

Laser-Based Measurements Of Hydrogen Fluoride Gas During Chemical Suppression

Fires” Applie

Williamson, C.K., McNesby, K.L., Daniel, R.G., and Miziolek, A.W., "Laser-Induced

Breakdown Spectroscopy for Real-Time Detection of Halon Alternative Agents," 

Analytical Chemistry 70, 1186-1191, (1998). 

M

Infrared Tunable Laser Diagnostics In Laboratory Scale Flames,” Proceedings of Laser

Applications To Chemical and E

McNesby, K.L., Skaggs, R.R., Miziolek, A.W., Clay, M., Hoke, S., and Miser, C.S

“Diode Laser-Based Measurements Of Hydrogen Fluoride Gas During Chemical 

Suppression Of Fires,” ARL-TR –1785, 1998. 

Skaggs, R.R., Dani

Horton, D., “Diode Laser Measurements of HF Concentrations from Heptane/

Fires Extinguished by FE-36 And FE-36 Plus Ammonium Polyphosphate,” ARL-TR. 

Skaggs, R.R.

Heptane/Air Pan Fires Extinguished By FE-36, FM-200, FE-36 Plus APP, or FM-2

Plus APP,” Proceedings of the 1998 JANNAF Combustion Meeting

29
Skaggs, R.R., Daniel, R.G., Miziolek, A.W., McNesby, K.L., Herud, C., Bolt, W.R., and 

Horton, D., “Diode Laser Measurements Of HF Concentrations Produced From

Heptane/Air Pan Fires Extinguished By FE-36, FM-200, FE-36 Plus APP, Or FM-200 

Lancaster, E.D., McNesby, K.L., Daniel, R.G., Miziolek, A.W., "Spectroscopic Analysis 

Skaggs, R.R., Daniel, R.G., Miziolek, A.W., McNesby, K.L., Herud, C. Bolt, W.R., and 

orton D., “Diode Laser Measurements of HF Concentrations From Heptane/Air Pan 

d by FE-36 And FE-36 Plus Ammonium Polyphosphate,” 1998

roceedings of the International Society for Optical Engineering: Photonics East, 1999. 

orption Measurements of Broad Band Absorbers in the Nearfrared Spectral Region", Applied Optics-LP, Volume 39, Issue 27, 5006-5011 

aser Absorption 

easurements of Broad Band Absorbers in the Near Infrared Spectral Region,” 

.L. McNesby, R.T. Wainner, R.G. Daniel, R.R. Skaggs, J.B. Morris, A.W. Miziolek, 

l

.L. McNesby, A.W. Miziolek, and I.A. McLaren, "Rapid high sensitivity laser 

in

r

C),

Plus APP,” Applied Spectroscopy, Vol. 53, p. 1143 (1999). 

of Fire Suppressants and Refrigerants by Laser Induced Breakdown Spectroscopy", 

Applied Optics, Vol. 38, p1476 (1999). 

H

Fires Extinguishe

P

McNesby, K.L., Skaggs, R.R., Miziolek, A.W., and McLaren, I.A., “Diode Laser-Based 

Detector for Fast Detection of Binary Gas Mixtures,” Proceedings of the 1999 Halon 

Options Technical Working Conference, Albuquerque, 1999. 

McNesby, K.L., Wainner, R.T., Miziolek, A.W., Jackson, W.M., McLaren, I.A., "High

Sensitivity Laser Abs

In

September 2000. 

McNesby, K.L., Miziolek, McLaren I.A., “Rapid High Sensitivity L

M

Proceedings of Laser Applications To Chemical and Environmental Analysis Topical 

Meeting, p. 80, 2000. 

K

W.M. Jackson, and I.A. McLaren, “Detection and measurement of middle distillate fue

vapors using tunable diode lasers”, Applied Optics, Vol. 40, pp. 840-845 (2001). 

K

absorption measurements of broad band absorbers in the near infrared spectral region"

Laser Applications to Chemical and Environmental Analysis, OSA (Optical Society of 

America) Technical Digest, pp. 80-83 (2000) 

A.W. Miziolek, K.L. McNesby, R.T. Wainner, R.S. Harmon, P.D. French, "LIBS: An

emerging chemical sensor technology for field use in warfare and peacetime

applications", 2000 Army Science conference.

K.L. McNesby, R.G. Daniel, A.W. Miziolek, R.T. Wainner, W.M. Jackson, I.A. 

McLaren, "Measurement of middle distillate fuel vapor concentrations using diode lase

spectroscopy", Proceedings of Halon Options Technical Working Conference (HOTW

May, 2000. 

30
K.L. McNesby, "Flame and Temperature Measurement Using Vibrational Spectroscopy

Encyclopedia of Sp

",

ectroscopy and Spectrometry, pp. 548 to 559 (Academic 

r-ir lasers for oxygen detection. 

ition of LIBS sensor technology from ARL to ATC. 

tatus: Unmet. After testing of LIBS for characterization of suppressants, fuels, and 

down

. Measurement of near infrared absorption survey spectra (using a FT-NIR 

avelength is coincident with an absorption 

ansition in these gases as determined by the survey spectra. 

ens-tipped fibers, and InGaAs detectors

r multipoint detection. 

easurements. 

Press,London, 2000). 

K.L. McNesby and A.W. Miziolek, “Fourier Transform Laser Spectroscopy”, accepted

for publication in Applied Optics, July, 2000. 

6. Milestones 

 FY 97 Milestones 

 

1. Complete comparison of LIBS vs nea

Status: Met 

2. Evaluate LIBS for P, Si, -NHx, and metal-containing suppressants, as well as other

new fire suppression chemicals of tri-service interest. 

Status: Met 

3. Procure near-ir lasers for two additional gases. 

Status: Met 

4. Complete installation of additional near-ir lasers at ATC. 

Status: Met 

 

5 Complete trans

S

oxidizers, it was found that current fiber coupled laser systems did not meet sufficient

durability requirements for transmission of laser radiation necessary for air break

(approximately 100 mJ/pulse, 10 nsec pulse length). 

FY 98-99 Milestones 

1

spectrometer) of heptane and JP-8 at several temperatures. 

Status: Met 

 

2. Procurement of laser diodes whose output w

tr

Status: Met 

 

3. Procurement of fiber signal splitters, GRIN l

fo

Status: Met 

 

4. Procurement of additional lasers and equipment for multiplexing applications and for 

multipoint m

31
Status: Met

itional fibers for multiple point LIBS.

tatus: Unmet. Suitable fibers for transmission of pulsed laser radiation were 

aluation of demodulation techniques for multiplex TDLAS. 

tatus: Partially met. Modulation techniques for measurement of gases with unstructured 

ecame a main focus of this project. The work performed in this area 

sections 3d and 3e of this document), although the 

suitably

ve. Real time display used commercial 

ftware (LabView), employing custom VI interface programs. 

ent of demodulation electronics. Demonstration of continuous 10 ms time 

solution, data storage capabilities. 

spectra for O2, heptane, and JP-8. 

referenced

ng

eptane/air and JP-8/air mixtures. 

omparison testing against the NIST "phi meter". 

tatus: Partially met. JP-8/air mixtures were successfully measured in lab. Comparison 

etection caused the mixed 

lose modulation balance. We were not able to correct this within the 

mixtures (laboratory 

termination of flammable mixture detection. 

Single point detection successful. Use of the splitter required for 

hin the project duration.

fuel/oxygen sensor installation, testing at ATC facility. Initial LIBS 

installation, testing at ATC facility. 

 

5. Procurement of add

S

unavailable. 

 

6. Complete ev

S

absorption spectra b

resulted in three publications (see

authors believe the effort at measuring broad band absorbers has not been

concluded. 

7. Software development for multiplex spectral measurement, demodulation, and real 

time display of concentrations of fuel and oxidizer. 

Status: Partially met. See milestone 6 abo

so

 

8. Procurem

re

Status: met 

9. Laboratory calibration of diode laser 2f

Status: Partially met. Oxygen calibration was straightforward, and was

against an Oxygraph oxygen meter. Middle distillates and heptane were measured usi

the “broadband absorber” techniques described in sections 3d and 3e of this document.

 

10. Initial testing of the fuel/oxygen sensor on static h

C

S

testing with NIST phi-meter was never performed. 

11. Extension of initial sensor to multipoint detection capability. 

Status: Unmet. Use of the splitter required for multipoint d

laser beam to

project duration. 

 

12. Complete initial multipoint measurements of fuel/oxidizer

based) for de

Status: Partially met.

multipoint detection caused the mixed laser beam to lose modulation balance. We were 

not able to correct this wit

13. Initial

32
Status: Partially met. Initial fuel/air testing at ATC successful for single point 

measurement. Multipoint installation failed because splitter required for multipoint 

sed the mixed laser beam to lose modulation balance. We were not able to 

orrect this within the project duration. 

e

ission of pulsed laser radiation were 

ltiple point detection of FM-200 in crew 

ulsed laser radiation were

– Discussion and Recommendations 

en successful, although the achieved results differed 

from those outlined in the initial proposals. The project fell short of initial 

sers are currently fielded at ATC (Aberdeen Test Center) 

d

al structure), we have been unable to achieve sensitivities 

were unable to maintain the 

not exhibit well resolved vibrational 

rement of these heavy hydrocarbon vapors requires nearlasers must be custom

copy (LIBS) has never been transitioned to field 

(3M, “Powercore” fibers, up to 80

nsec) may enable LIBS of gases at remote locations. 

detection cau

c

 

 

14. Initial field testing of LIBS for multipoint measurement of halon alternativ

compounds. Comparison with DIRRACS method. 

Status: Unmet. Suitable fibers for transm

unavailable. 

 

15. Demonstration of multplex detection of fuel/oxygen mixtures during real scale fire 

suppression testing. 

Status: Partially met. Single point detection only. 

 

16. Demonstration of LIBS for mu

compartment. 

Status: Unmet. Suitable fibers for transmission of p

unavailable. 

7. Significant Project Results

We believe this project to have be

appreciably

expectations in that diode la

only for HF and oxygen measurement. Real time fuel measurement, demonstrate

successfully in the laboratory at the ARL (Army Research Laboratory), has not been

successfully transitioned to ATC. The reasons for this are twofold. First, although we 

developed two methods for laser based measurements of “broadband absorbers” (gases 

with poorly resolved vibration

for these gases similar to those attainable for detection of light hydrocarbons, e.g., 

methane. Additionally, for multipoint detection we

modulation balance on which our first technique was dependent. We are still engaged in 

refining our techniques for measuring gases that do

structure. Second, measu

infrared DFB lasers near a wavelength of 1.7 micrometers. These

fabricated. For such custom orders, it is necessary to wait until several customers need

lasers in this wavelength region, making the wafer growth economically feasible. 

Laser induced breakdown spectros

use within armored vehicles at ATC. The main reason for this is that optical fibers 

necessary for delivering pulsed radiation to the measurement site were found to be 

unreliable (failure at less than 200 shots). It’s worth noting that several manufacturers 

are now offering fibers for delivering pulsed radiation at 1.06 micrometers. The current 

highest power delivery fiber of which we are aware

mJ/pulse at 10

33
Given these shortcomings, we still believe the project to be successful. First, we 

tive

er

rometer, but with no moving parts, and with the 

urier frequencies decoupled from the light wavelengths. Secondly, and perhaps even 

strated the use of laser induced breakdown 

this

chnique into investigations of

.

greater understanding of toxic gas formation 

laser measurements and of extractive FT-IR measurements during 

Tunable diode laser measurements 

how the rate of formation of HF gas is affected by varying fire suppressants, 

. Conclusion

We believe microphotonic sensors, employing TDL spectroscopy and LIBS, will play 

a m

demonstrated the first measurement of “broadband absorbers” using phase sensi

detection. Additionally, the project resulted in development of Fourier transform las

spectroscopy, in which an array of lasers are used to create an addressable source that 

mimics the output of a Michelson interfe

Fo

more important, the project has demon

spectroscopy (LIBS) for quantitative measurement of gases. The use of LIBS for

project provided much impetus for expanding this te

detection methods for toxic and explosive materials, including chem./bio agents

Thirdly, the project has resulted in a

(specifically HF gas) during fire suppression onboard armored vehicles. The 

combination of diode

real scale testing has lent credence to both methods.

have shown

and by chemicals added to decrease HF formation. 

8

ajor role in the next generation of sensor equipment. Equipment design and 

experimental results from this project have enhanced the effort towards this next 

generation of sensors. We hope to continue our work in TDL spectroscopy and LIBS,

and to continue development of sensors of use to the Army. 

34