U.S. patent application number 13/042248 was filed with the patent office on 2011-06-30 for tunable quantum cascade lasers and photoacoustic detection of trace gases, tnt, tatp and precursors acetone and hydrogen peroxide.
Invention is credited to Ilya Dunayevskiy, Rowel C. Go, Richard Maulini, C. Kumar N. Patel, Manu Prasanna, Michael Pushkarsky, Alexei Tsekoun.
Application Number | 20110158270 13/042248 |
Document ID | / |
Family ID | 39583923 |
Filed Date | 2011-06-30 |
United States Patent
Application |
20110158270 |
Kind Code |
A1 |
Patel; C. Kumar N. ; et
al. |
June 30, 2011 |
Tunable Quantum Cascade Lasers And Photoacoustic Detection Of Trace
Gases, TNT, TATP and Precursors Acetone And Hydrogen Peroxide
Abstract
Methods and apparatus for broad tuning of single wavelength
quantum cascade lasers and the use of light output from such lasers
for highly sensitive detection of trace gases such as nitrogen
dioxide, acetylene, and vapors of explosives such as
trinitrotoluene (TNT) and triacetone triperoxide (TATP) and TATP's
precursors including acetone and hydrogen peroxide. These methods
and apparatus are also suitable for high sensitivity high
selectivity detection of other chemical compounds including
chemical warfare agents and toxic industrial chemicals. A quantum
cascade laser (QCL) system that better achieves single mode,
continuous, mode-hop free tuning for use in L-PAS (laser
photoacoustic spectroscopy) by independently coordinating gain chip
current, diffraction grating angle and external cavity length is
described. An all mechanical method that achieves similar
performance is also described. Additionally, methods for improving
the sensor performance by critical selection of wavelengths are
presented.
Inventors: |
Patel; C. Kumar N.; (Los
Angeles, CA) ; Dunayevskiy; Ilya; (Los Angeles,
CA) ; Prasanna; Manu; (Marina del Rey, CA) ;
Go; Rowel C.; (Carson, CA) ; Tsekoun; Alexei;
(Los Angeles, CA) ; Pushkarsky; Michael; (San
Diego, CA) ; Maulini; Richard; (Los Angeles,
CA) |
Family ID: |
39583923 |
Appl. No.: |
13/042248 |
Filed: |
March 7, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11821528 |
Jun 22, 2007 |
7903704 |
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13042248 |
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60816245 |
Jun 23, 2006 |
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60873649 |
Dec 8, 2006 |
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Current U.S.
Class: |
372/29.021 |
Current CPC
Class: |
H01S 5/1092 20130101;
B82Y 20/00 20130101; H01S 5/06255 20130101; G01N 21/1702 20130101;
H01S 5/3401 20130101; H01S 5/141 20130101; H01S 5/0654
20130101 |
Class at
Publication: |
372/29.021 |
International
Class: |
H01S 3/13 20060101
H01S003/13 |
Goverment Interests
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED
RESEARCH AND DEVELOPMENT
[0002] The technology disclosed herein was supported at least in
part by NIST/ATP Grant 70.NANB3F13026 and DARPA (Defense Advance
Research Projects Agency) Contract HR0011-04-C-0102 (Approved for
Public Release, Distribution Unlimited).
Claims
1. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system, the steps comprising: a) powering a
source of multiwavelength laser light at a first power level; b)
selectively reflecting said multiwavelength laser light in a cavity
back to said source to select a first laser wavelength; c)
adjusting a distance of said cavity to obtain a maximum output
distance for said first laser wavelength and said first power
level, said maximum output distance maximizing output of the laser
system at said first laser wavelength; d) repeating steps b and c
for other laser wavelengths to obtain sufficient data in to
determine laser output peaks for said source at said first power
level over a desired wavelength range; and e) determining a
maximizing power level for each wavelength in said wavelength
range; whereby the coupled cavity laser system operates at a
maximum for a selected wavelength within said wavelength range by
selecting said maximum output distance and said maximizing power
level for said selected wavelength.
2. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 1, where said
source of multiwavelength laser light includes a quantum cascade
laser (QCL) gain chip.
3. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 1, further
comprising selectively reflecting said multiwavelength laser light
with a diffraction grating.
4. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 3, further
comprising adjusting said cavity distance by adjusting a position
of said diffraction grating.
5. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 4, further
comprising adjusting said position of said diffraction grating with
a piezoelectric translator (PZT).
6. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 1, further
comprising adjusting said distance by selectively altering said
distance on the order of a few wavelengths of said first laser
frequency.
7. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 1, further
comprising repeating steps b and c by selecting a later second
laser wavelength in step b that departs minimally from said earlier
first laser wavelength.
8. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 7, further
comprising obtaining said later second laser wavelength by making a
fine angular displacement step with a diffraction grating.
9. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 1, where said
output peaks correspond to a Fabry-Perot mode comb for said source
of multiwaveleneth laser light at said first power level.
10. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 9, where said
maximizing power level is determined for said first laser
wavelength by determining a mode comb power level that causes a
mode comb wavelength spike of said source to coincide with said
first laser wavelength.
11. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 10, further
comprising: incrementing or decrementing said maximizing power
level to match a second selected laser wavelength within said
wavelength range; and shifting said maximizing power level a free
spectral range amount when needed to maintain said maximizing power
level within preferred power limits of said source while
simultaneously matching said first laser wavelength.
12. A method for obtaining power-maximized continuous tuning for a
coupled cavity laser system as set forth in claim 1, further
comprising: selecting a wavelength at which the laser system will
operate, said wavelength selected from said wavelength range;
further adjusting a distance of said cavity at said maximum output
distance to ensure that said selected maximum output distance for
said selected wavelength is as much a maximum output distance as
possible to enable maximum output of the laser system at said
selected laser wavelength.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This patent application is a continuation and claims the
benefit of U.S. patent application Ser. No. 11/821,528 filed Jun.
22, 2007 for Tunable Quantum Cascade Lasers and Photoacoustic
Detection of Trace Gases, TNT, TATP and Precursors Acetone and
Hydrogen Peroxide, which itself claims priority to U.S. Provisional
Patent Application Ser. No. 60/816,245 filed Jun. 23, 2006 for
Sub-ppb Level Detection of NO2 using Room Temperature Quantum
Cascade Lasers as well as U.S. Provisional Patent Application Ser.
No. 60/873,649 tiled Dec. 8, 2006 for High Sensitivity Detection of
TNT. Each of these applications is incorporated here by this
reference.
TECHNICAL FIELD
[0003] This invention relates to highly sensitive or ultrasensitive
equipment and methods that detect trace amounts of gases by refined
optical illumination and response detection as well as
photoacoustic methods.
BACKGROUND ART
[0004] Mid and long wave infrared quantum cascade lasers (QCLs)
cover a very important spectral region from about 3 .mu.m to 15
.mu.m where most of the important trace gas pollutants, chemical
warfare agents, toxic industrial chemicals, and vapors of
explosives exhibit their characteristic infrared fingerprint
absorption. Use of these QCLs for sensitive spectral analysis of
the target gases requires a broad tunability of essentially single
wavelength radiation and techniques, for detection of the target
gases at very low concentrations in the presence of normally
occurring interferent gases whose infrared absorption fingerprints
overlap with those of the targets.
[0005] Quantum cascade lasers as fabricated operate as Fabry-Perot
cavity lasers formed by the end facets of the semiconductor laser
chips and produce a multiwavelength output covering some hundreds
of nanometers. The spectral position of each of the independently
lasing wavelengths is determined by the Fabry-Perot cavity modes of
the laser chip and the wavelength spread is determined by the gain
width of the laser. Such broad spectral output is virtually useless
for the highly sensitive and selective detection of the target
gases. The laser output needs to be essentially one single
wavelength and mechanisms are needed for broad tunability so that
the fingerprint characteristic of the target gas absorption can be
accurately measured. The broad tuning necessary for the sensitive
and selective target gas detection has led to both software and
hardware innovations.
[0006] Further, use of broadly tunable single wavelength radiation
for sensitive detection of the target gases in a sample is
complicated by the presence of other constituents, often called
interferents, in the sample. The overlapping spectra signatures of
the interferents and the target gases, obtained using broadly
tunable single frequency lasers can be deconvolved using algorithms
and techniques described in an earlier patent application, U.S.
patent application Ser. No. 11/256,377 filed Oct. 21, 2005 for
System and Method for High Sensitivity Optical Detection of Gases
which shares co-inventors with this instant patent document and
which is incorporated herein by this reference thereto.
DISCLOSURE OF INVENTION
[0007] In view of the foregoing disadvantages inherent in the known
types of gas detectors now present in the prior art, the present
invention provides a new and more sensitive gas detection system
wherein the presence of selected gases can be readily determined at
very low levels with a high degree of rejection of false signals
arising from interferents.
[0008] The general purpose of the present invention, which will be
described subsequently in greater detail, is to provide reliable
and highly sensitive gas detection systems which are not
anticipated, rendered obvious, suggested, taught, or even implied
by any of the prior art gas detection systems, either alone or in
any combination thereof.
[0009] In one embodiment of the present invention, a method for
leveling the output of a multiwavelength laser system over a number
of lasing wavelengths in a timing window is set forth with the
steps including the providing a laser gain chip and determining a
first current change value needed to shift the laser gain chip's FP
mode comb exactly one free spectral range. The laser system is then
tuned with monitoring occurring for a second current change value
from the initial point during the tuning. Remapping then occurs
with the remapping of the second current change to the first
current change when the second current change exceeds the first
current change. Remapping takes place by subtracting the second
current change from first current change such that laser output
power varies minimally with laser wavelength as the second current
change value for a tuned wavelength of the multiwavelength laser
system is remapped onto the first current change value.
[0010] In another embodiment, a laser illumination system for
providing laser light over a multiwavelength spectrum includes a
multiwavelength laser light source emitting light and a
wavelength-selective reflector in optical communication with the
source. A translator coupled to the reflector displaces the
reflector according to a first signal and controls a distance
between the reflector and the source. A rotation stage is coupled
to the translator. The rotation stage rotates the reflector
according to a second signal and controls an angle between the
reflector and the source such that single mode, continuous,
mode-hop free tuning is provided by the laser illumination
system.
[0011] In a third embodiment, a method for more quickly determining
the presence of a target gas includes the steps of identifying and
selecting regions in a frequency range of a selectable wavelength
light source which meets all the following criteria: the target gas
has large absorption in at least some frequencies in the frequency
range, expected interferents have low absorption at their expected
concentrations, and a detectable signature of the target gas is
linearly independent of signature of interferents. A sample of gas
is collected for testing of the target gas and a scan is performed
across the identified and selected regions with the collecting of
photoacoustic data from the scan. The photoacoustic data is
linearly deconvolved against a standardized library of the target
gas and list of expected interferents to obtain a gas concentration
measurement for the target gas.
[0012] At least three key innovations are provided the technology
disclosed herein.
[0013] The first is a software-implemented algorithm (or method)
that provides tuning capability of the single wavelength QCL output
Such that under computer control, the laser wavelength can be
reproducibly tuned over a very broad spectral region without
experiencing any mode hops or jumps. The computer simultaneously
controls the QCL drive current, the angle of the wavelength
selective grating and the over-all cavity length. Furthermore, an
all-mechanical system is provided that automatically and in a
highly-coordinated fashion provides the cavity length adjustment as
the angle of the grating is changed. When performed in tandem,
these coordinated actions enable selection of the desired operating
wavelength of the QCL.
[0014] Second, a laser wavelength tuning algorithm/method is
achieved by creating a "Smart Grid" of interrogating wavelengths
that simultaneously focus on the unique features of the fingerprint
absorption of the desired targets while eliminating those
wavelengths at which the key interferents may have their sharp
absorption features. Use of appropriate Smart Grids improves the
ROC (receiver operational characteristic) curves, i.e., the
characteristic describing the detection threshold versus
probability of false alarms (PFA) and reduces the time it takes to
make a measurement.
[0015] Third, performance optimization of the tunable QCL based
trace gas detection scheme is achieved by iteratively changing the
measurement algorithms in a learning mode as the instrumentation
carries out the trace gas detection.
[0016] Additionally, for speeding up the process of data collection
without jeopardizing the selectivity desired in the target gas
detection, described by receiver operational characteristic (ROC)
curves, we describe schemes that avoid measurement of the sample
gas absorptions at wavelengths that correspond to strong but
sharply defined absorptions of the interferent gases. One such
scheme is called the Smart Grid algorithm and has proven to be very
successful in the detection of traditional and homemade explosives
and their precursors.
[0017] Other embodiments of the present invention are set forth in
more detail, below, and the embodiments set forth above are made
for purposes of example only and not of limitation.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 is a diagram of an output spectrum of the Fabry-Perot
geometry 7.3 .mu.m QCL.
[0019] FIG. 2 is a diagram of light-current (LI) characteristics of
semiconductor lasers: a) in the standard, linear operating regime;
and b) in the operating regime utilizing power saturation.
[0020] FIG. 3 is a labeled schematic of a two-segment QCL chip.
[0021] FIG. 4 is a diagram showing operating point 1 of a
two-segment QCL such as that shown in FIG. 3.
[0022] FIG. 5 is a diagram complementing that of FIG. 4 showing
operating point 2 of a two-segment QCL.
[0023] FIG. 6 is a cross-sectional view of the external grating
cavity quantum cascade laser (EGC-QCL) system described herein.
[0024] FIG. 7 is a diagram showing tuning characteristics of
continuous wave room temperature Operation of the 7.3 .mu.m EGC
QCL.
[0025] FIG. 8 is a comparative diagram showing high resolution
HITRAN simulated absorption spectrum of acetylene (top trace) and
measured QCL-PAS spectrum of 10 ppm acetylene in CDA at a total
pressure of 300 torr (bottom trace).
[0026] FIG. 9 is a coordinated diagram showing acetylene L-PAS
measurements as a function of acetylene concentration. The figure
also shows the lowest detectable of level for acetylene
detection.
[0027] FIG. 10 is a diagram showing tuning characteristics of
continuous wave room temperature operation of the 6.3 .mu.m EGC
QCL. The highest measured single-mode power is .about.300 mW.
[0028] FIG. 11 is a comparative diagram of L-PAS scans across the
selected NO.sub.2 spectral feature in clean dry air. The top trace
shows 10.4 ppm NO.sub.2 and the step changes in the signal
intensity arc due to external cavity mode hops. The bottom trace
shows five-point averaged 100 ppb NO.sub.2 data.
[0029] FIG. 12 is a diagram showing NO.sub.2 L-PAS linearity. Unity
signal to noise ratio corresponds to 0.5 ppb NO.sub.2 detection
limit (grayed-out area below 0.01 L-PAS Signal).
[0030] FIG. 13 is a diagram showing comparative laser photoacoustic
signals measured from triacetone triperoxide (TATP) sample vapors
at several temperatures.
[0031] FIG. 14 is a diagram of a laser photoacoustic spectrum of
TATP at -3.degree. C.
[0032] FIG. 15 is a diagram showing TATP L-PAS signal strength and
TATP vapor pressure as a function of temperature along with a
Clapeyron fit to the measured TATP vapor pressure (vapor pressure
data from Oxley). The QCL-PAS data are fitted at T=25.degree. C.
point on the Clapeyron fit.
[0033] FIG. 16 is a diagram showing QCL-PAS signal for 1.4 ppm
acetone (a precursor for synthesizing TATP) shown along HITRAN
acetone absorption simulation.
[0034] FIG. 17 is a diagram showing QCL-PAS signal for acetone as a
function of acetone concentration with the L-PAS noise floor shown
below 2 a.u. on the y-axis.
[0035] FIG. 18 is a diagram comparatively showing measured QCL-PAS
absorption spectra of TNT at four different temperatures.
[0036] FIG. 19 is a diagram of a QCL-PAS spectrum of "cleaned up"
TNT sample at three different temperatures using smart grid tuning
algorithm.
[0037] FIG. 20 is a diagram showing calculated vapor pressure of
TNT (using Clapeyron fit) versus temperature and the measured
photoacoustic signal strength at various temperatures.
[0038] FIG. 21 is a diagram comparatively showing QCL-PAS
absorption spectra of TATP (T=25.degree. C., first peak left to
right), acetone (1.4 ppm, second peak), and TNT (T=60.degree. C.,
third peak).
[0039] FIG. 22 shows a schematic cross section of a configuration
for mode-hop-free tuning system. The point B moves along the
optical axis b and the point A moves along a perpendicular axis
a.
[0040] FIG. 23 shows a schematic cross section of the required
cavity setting for mode-hop free tuning. The projection of the
segment AB on the optical axis has to be equal to the optical
length of the cavity L.
[0041] FIG. 24 is a graph showing mode hop free tuning range as a
function of the departure of distance D from its ideal value,
deviation .DELTA.D (.mu.m) spanning from -150 to 150.
[0042] FIG. 25 shows in graphic form mode hop free tuning range in
percent of the center wavelength as a function of the departure of
distance D from its ideal value. The y-axis of the graph spans from
0 to 100 logarithmically
[0043] FIG. 26 is a diagram showing a simulation absorption
spectrum of acetone (ascending 3-peak curve) and water (7-7.5
.mu.m)
[0044] FIG. 27 is a diagram showing PFA for acetone in the presence
of 10,000 ppm of water using the full scan (lower curve) and smart
grid (upper curve) (7.2 mm-7.5 mm).
[0045] FIG. 28 is a diagram showing simulated absorption spectrum
of acetone (ascending 3-peak curve) and water (descending
multi-peak curve) (8.0 .mu.m-8.5 .mu.m).
[0046] FIG. 29 is a diagram showing PFA for acetone in the presence
of 10,000 ppm of water using the full scan (lower curve) and smart
grid (upper curve) (8.0 .mu.m-8.5 .mu.m).
[0047] FIG. 30 is a diagram showing L-PAS absorption spectra of TNT
indicating different ranges used for analysis.
[0048] FIG. 31 is a diagram showing TNT Detection threshold vs.
Probability of False Alarms for the different spectral ranges seen
in FIG. 30.
BEST MODE FOR CARRYING OUT THE INVENTION
[0049] The detailed description set forth below in connection with
the appended drawings is intended as a description of
presently-preferred embodiments of the invention and is not
intended to represent the only forms in which the present invention
may be constructed or utilized. The description sets forth the
functions and the sequence of steps for constructing and operating
the invention in connection with the illustrated embodiments.
However, it is to be understood that the same or equivalent
functions and sequences may be accomplished by different
embodiments that are also intended to be encompassed within the
spirit and scope of the invention.
[0050] The present invention resides in several embodiments.
[0051] In one embodiment of the present invention, a method for
leveling the output of a multiwavelength laser system over a number
of lasing wavelengths in a tuning window is set forth with the
steps including the providing a laser gain chip and determining a
first current change value needed to shift the laser gain chip's FP
mode comb exactly one free spectral range. The laser system is then
tuned with monitoring occurring for a second current change value
from the initial point during the tuning. Remapping then occurs
with the remapping of the second current change to the first
current change when the second current change exceeds the first
current change. Remapping takes place by subtracting the second
current change from first current change such that laser output
power varies minimally with laser wavelength as the second current
change value for a tuned wavelength of the multiwavelength laser
system is remapped onto the first current change value.
[0052] In another embodiment, a laser illumination system for
providing laser light over a multiwavelength spectrum includes a
multiwavelength laser light source emitting light and a
wavelength-selective reflector in optical Communication with the
source. A translator coupled to the reflector displaces the
reflector according to a first signal and controls a distance
between the reflector and the source. A rotation stage is coupled
to the translator. The rotation stage rotates the reflector
according to a second signal and controls an angle between the
reflector and the source such that single mode, continuous,
mode-hop free tuning is provided by the laser illumination
system.
[0053] In a third embodiment, a method for more quickly determining
the presence of a target gas includes the steps of identifying and
selecting regions in a frequency range of a selectable wavelength
light source which meets all the following criteria: the target gas
has large absorption in at least some frequencies in the frequency
range, expected interferents have low absorption at their expected
concentrations, and a detectable signature of the target gas is
linearly independent of signature of interferents. A sample of gas
is collected for testing of the target gas and a scan is performed
across the identified and selected regions with the collecting of
photoacoustic data from the scan. The photoacoustic data is
linearly deconvolved against a standardized library of the target
gas and list of expected interferents to obtain a gas concentration
measurement for the target gas.
[0054] Referring to the drawings, where like numerals of reference
designate like elements throughout, it will be noted that at least
three key innovations are provided the technology disclosed
herein.
[0055] The first is a software-implemented algorithm (or method)
that provides tuning capability of the single wavelength QCL output
such that under computer control, the laser wavelength can be
reproducibly tuned over a very broad spectral region without
experiencing any mode hops or jumps. The computer simultaneously
controls the QCL drive current, the angle of the wavelength
selective grating and the over-all cavity length. Furthermore, an
all-mechanical system is provided that automatically and in a
highly-coordinated fashion provides the cavity length adjustment as
the angle of the grating is changed. When performed in tandem,
these coordinated actions enable selection of the desired operating
wavelength of the QCL.
[0056] Second, a laser wavelength tuning algorithm/method is
achieved by creating a "Smart Grid" of interrogating wavelengths
that simultaneously focus on the unique features of the fingerprint
absorption of the desired targets while eliminating those
wavelengths at which the key interferents may have their sharp
absorption features. Use of appropriate Smart Grids improves the
ROC (receiver operational characteristic) curves, i.e., the
characteristic describing the detection threshold versus
probability of false alarms and reduces the time it takes to make a
measurement.
[0057] Third, performance optimization of the tunable QCL based
trace gas detection scheme is achieved by iteratively changing the
measurement algorithms in a learning mode as the instrumentation
carries out the trace gas detection.
Broad Tuning of Single Wavelength Quantum Cascade Lasers
[0058] A QCL with no additional controls, other than the
reflectivity provided by the end-facets, acts as a Fabry-Perot
laser that lases on the longitudinal modes of the laser cavity
formed by the two end facets (assuming that the lateral dimension
of the gain region is small to support only one transverse mode).
All the longitudinal modes falling under the QCL gain curve and for
which the QCL gain exceeds the cavity round trip losses, could lase
to create laser light, producing a comb of laser wavelengths
separated by the longitudinal cavity mode spacing (FIG. 1).
[0059] To go from the multiwavelength output shown in FIG. 1 to a
single wavelength output that is necessary for the measurement of
the fingerprint absorption of the target gases, a wavelength
selective loss element needs to be incorporated within the QCL
cavity. The simplest such wavelength selective element is the
incorporation of a distributed feedback (DFB) grating in the active
layer of the QCL. Such DFB lasers do provide single wavelength
output at the grating wavelength. However, tunability of the laser
wavelength is limited and is provided by temperature changes of the
QCL (which also changes the refractive index and the grating
dimensions). Typically, DFB laser wavelength tunability is limited
to about 20-30 nm, far less than what would be needed for
interrogating a target gas whose absorption feature(s) may span
hundreds of nanometers (nm). Broad tunability, necessary for trace
gas detection, is obtained by having a wavelength selective
component that is external to the QCL. Such configuration, in the
infrared region, uses an external reflection from a diffraction
grating in Littrow (or other) configuration. In the simplest
analysis, the angle of the grating with respect to the laser axis
determines the lasing wavelength.
[0060] In a general case, an external grating cavity (EGC) tunable
semiconductor laser consists of a gain chip and a
wavelength-selective element optically coupled to one of the facets
of the gain chip. In a system with no AR (anti-reflective) coatings
on the gain chip facets as the most general case, one encounters
the well-described coupled cavity problem. This produces
single-mode output radiation and to wavelength-tune such a system
continuously, the longitudinal modes of the gain chip have to match
those of the external cavity and the loss minimum of the wavelength
selective element. Therefore, such a system has to have three
controls to change all parameters independently.
[0061] The external cavity configuration described above is very
complex due to the necessity to control three (3) parameters
synchronously, namely 1) the wavelength grating angle selection, 2)
the overall cavity length, and 3) the gain chip's FP (Fabry-Perot)
mode comb. In addition to the mechanical and electronic complexity
that negatively affects reliability, such configuration exhibits
several performance-based negative characteristics which are all
constructively addressed by the system and methods disclosed
herein.
[0062] First, since the FP mode comb of the gain chip is adjusted
by changing the laser drive current (or alternatively by adjusting
gain chip temperature), it is impossible to avoid output power
changes as the laser tunes. This can be undesirable for some
applications. Further, the tuning process is slow due to the
adaptive nature of the novel tuning algorithm/method set forth
herein that finds external cavity modes by tuning its length and
finding power maxima.
[0063] One simplification to the system results from the
elimination of the gain chip's FP etalon effect, which can be
achieved by applying a high performance (<<1%) antireflection
(AR) coating to the output facet of the gain chip facing the
external cavity. With an AR-coated gain chip, the requirement to
match the modes of the external cavity to those of the gain chip is
eliminated, and therefore only two controls remain--the wavelength
selection, and the overall cavity length control to align a given
EC (external cavity) longitudinal mode with the loss minimum of the
diffraction grating.
[0064] In an external cavity configuration utilizing a diffraction
grating as the wavelength selective element, grating angle is
controlled mechanically. Cavity length can be controlled (1) via
change of physical length, (2) optical control via refractive index
change, or (3) a novel mechanical control.
[0065] Optical cavity length control relies on the change in the
optical path length of the overall cavity via changing the
refractive index of one or more intracavity components without
changing its physical length. Numerous such schemes have been
developed over the years, including the use of separate
electro-optic crystals, fabrication of integrated electro-optical
modulators adjacent to the gain chip, etc. Since electro-optics for
the mid-IR and far IR is in its infancy, methods for optically
changing the overall cavity length via changes to the refractive
index of the gain chip itself are set forth herein.
Gain Chip Current Control: Periodic Current or Temperature
Tuning
[0066] A well-known approach to Changing the refractive index of
the gain chip is changing the injection current or gain chip
temperature, thus taking advantage (indirectly or directly) of the
refractive index's temperature dependence.
[0067] However, this approach contains a serious limitation on the
overall tuning range, namely the limited dynamic range of the laser
gain chip control parameters, including laser injection current and
temperature. For some typical QCL gain chips, the total current
span between laser threshold and the maximum rated laser current is
only sufficient to shift the FP comb by less than 3 cm.sup.-1, thus
putting an upper limit on the mode hop-free tuning range. In a
practical instrument, where a reasonably high laser power level
needs to be maintained in order to achieve the desired sensitivity,
the actual tuning range becomes less than 1 cm.sup.-1. Practicably
attainable gain chip temperature range imposes a similar tunability
restriction.
[0068] The problem posed by this limitation has been overcome by
inventing the algorithm/method set forth herein where the current
(or temperature) is periodically changed to maintain high power
operation.
[0069] First, determination is made of the current change value
need to shift the gain chip's FP mode comb exactly one free
spectral range, which will be called hereafter the current
periodicity value.
[0070] This could be achieved, for example, by analyzing high
resolution FTIR (Fourier transform infra-red) spectra of the gain
chip in FP configuration at varying injection currents, or by
spectroscopic means as described below with respect to the
experimental demonstration.
[0071] Next, as the laser begins tuning, the algorithm keeps track
of the current value change from the initial current set point.
When that change exceeds the previously determined current
periodicity value, the algorithm subtracts that value from the next
desired set point, and thus remaps the current change span back
onto the original region of operating current. If one sets the
maximum allowed current as the upper bounding value, this results
in the highest average output power across the tuning window.
[0072] Further, the same approach can be employed if one uses gain
chip temperature instead of the injection current as the control
parameter for refractive index. However, temperature is typically
much slower to respond, making current control the preferred
embodiment.
[0073] Finally, the periodic current/temperature change algorithm
is independent of whether one desires a true continuous tuning
(where the overall cavity length needs to be changed with spectral
tuning to maintain the selected FP mode of the external cavity at
the minimum of loss), or quasi-continuous tuning of an
intentionally long cavity where mode hops over external cavity
modes arc acceptable.
Gain Chip Current Control: Harnessing Saturation
[0074] Any standard gain chip index control method, including
periodic current control described above, has the undesirable
property that the output power of the laser varies with the change
of the wavelength. This property is undesirable for many
applications, where constant output power needs to be maintained as
the laser is tuned. A method that minimizes such changes through
the use of the power saturation region of the light-current (LI)
curve characteristic of QCLs may be used to resolve this
problem.
[0075] FIG. 2 illustrates this concept. FIG. 2a shows the standard
operating regime for controlling gain chip refractive index with
injection current. Since one uses the linear portion of the LI,
there is an appreciable change in output power (P.sub.lin) as one
varies the index by the needed amount. However, QCLs are
characterized by a noticeable saturation portion of their LI
curves. Saturation is caused by either thermal effects or by
electronic band misalignment under higher bias conditions, and is
reproducible and completely reversible. Therefore, one is free to
select the laser operating point to minimize optical power changes
for the same change of current (FIG. 2b). It is obvious that the
"saturation power change" is much less than in the linear case
(P.sub.sat<<P.sub.lin).
[0076] A QCL gain chip for this application may be specifically
designed to exhibit a saturation region that is as flat as possible
since this will minimize optical power changes. In the ideal case
of a completely flat saturation region (which we have already
observed in some devices), the optical power change with laser
tuning in this configuration is effectively eliminated.
Gain Chip Current Control: 2-Segment QCL
[0077] Even if the saturation region of the gain chip's LI curve
cannot be made completely flat, we have devised a system where the
total output power can he kept constant as the refractive index is
changed via changing the current.
[0078] FIG. 3 shows a single QCL chip with 2 independently
addressable electrical contacts. The operating points of the two
(2) sections may be chosen to be on the opposite sides of their
respective maximum power points of their LI curves (FIG. 4). Then,
as the laser is tuned, the current of both sections is increased so
that the front section moves closer to the maximum power point, and
the rear section moves away from it (FIG. 5). The total current
through the device will be increasing, thus monotonously changing
the refractive index of the two (2) sections, and therefore
properly varying the overall cavity length. However, due to the
existence of optical power saturation, the optical power of the
front section will increase, while that of the rear section will
decrease. The size of the respective current changes can be
selected such that the overall optical power is kept constant as
the laser is tuned (Equations 1 and 2).
I.sub.1(F)+I.sub.1(R)>I.sub.2(F)+I.sub.2(R) (1)
[0079] Therefore, the refractive index, n, changes.
[0080] However,
P.sub.1(F)+P.sub.1(R)>P.sub.2(F)+P.sub.2(R)P.sub.output= (2)
[0081] In this configuration, the total optical power is lower than
if both sections were tuned around the maximum optical power point
(which in effect is equivalent to the previous solution, titled
"Harnessing Saturation"). Therefore, such a 2-segment QCL provides
operational flexibility where one can either tune the laser source
with constant power at a somewhat lower overall level, or accept
power variations and generate maximum possible power at every
spectral point.
[0082] Finally, even though described for the case of a QCL, this
invention will be applicable to any semiconductor laser that
exhibits such reproducible and reversible power saturation with
increased current.
Gain Chip Current Control: Power Equalization Across the Gain Curve
with the 2-Segment QCL
[0083] Output power change due to the variation in injection
current necessary to change the overall external cavity length
(addressed immediately above) is one of the two main mechanisms
responsible for output power variation as the laser source is
broadly tuned. The second mechanism is the decrease of the laser
gain as one spectrally tunes the source away from gain maximum,
towards the edges of the tunability window.
[0084] Using the 2-segment QCL approach, this second source of
power variability can also be suppressed, and in some cases
completely eliminated, depending on the actual characteristics of a
given system (namely, the shapes of the gain and LI curves). This
will be accomplished by changing the spread (or current difference)
between the current setpoints of the two QCL sections. The spread
value controls the total output power at any given setpoint. The
lower the spread, the higher will be the total power. Therefore, as
the laser is tuned away from the center of its gain curve, the
spread will begin to decrease in order to compensate for the
decrease in gain by pumping the laser harder.
[0085] The extent to which this method will be able to stabilize
output power depends on the actual system characteristics. On the
other hand, this approach gives the system designer wide
flexibility in selecting system operating modes and parameters,
allowing the tradeoff between maximum output power and maximum
power stability, as well as allowing tailoring of the actual
dependence of power on wavelength to suit any particular
application.
Experimental Demonstration of Computer Control of QCL Wavelength
Through Simultaneous Control of the Grating Angle, QCL Current and
Cavity Length
[0086] In our case, a quantum cascade laser (QCL) gain chip was
incorporated into an external grating cavity (EGC) to produce
single mode continuous mode hop free tuning. The 7,300 nm QCL epi
material was grown using molecular-beam epitaxy. After cleaving,
the 3-mm-long, 10.6-.mu.m-ridge-width chips were mounted epi-side
down on an AlN (aluminum nitride) substrate by using Au--Sn
(gold-tin) eutectic solder. The chip on the submount was integrated
onto a copper heat-spreading pyramid with a thermoelectric cooler
(TEC) and a miniature copper cooling block for heat removal from
the hot side of the TEC. The cooling block is water-cooled but can
be modified for forced-air cooling. The gain chip was operated at
an actively controlled submount temperature of 25.degree. C.,
measured by an integrated thermistor, with the copper cooling block
maintained at 20.degree. C.
[0087] Operated in Fabry-Perot (FP) geometry, the 7,300 nm QCLs
(uncoated facets) generated multimode continuous wave (CW), room
temperature (RT) power output of 80 mW per facet. See FIG. 1 for a
spectral analysis of the output.
[0088] To collimate the input and output beams, two collimator
lenses were used. The collimators were ZnSe (zinc selenide)
aspheric, 6 mm diameter, f/0.7 lenses with 5500 to 7300 nm AR
coatings. The output beam from the collimator was approximately 4
mm in diameter.
[0089] FIG. 6 shows a cross-sectional view of the EGC-QCL (external
grating cavity--quantum cascade laser) system 100.
[0090] The external cavity included an output laser facet with no
coating and a diffraction grating 102 with 240 grooves/mm as a
second mirror and wavelength selective element (FIG. 6). Alignment
quality of the EGC is judged by the strength of feedback it
provides to the gain chip, measured by threshold current reduction.
For the 7300 nm laser described here, threshold current dropped
from 850 mA in FP geometry to 730 mA in EGC configuration (14%
reduction). The entire setup was enclosed into a plastic box and
filled with dry nitrogen to prevent water absorption inside the
external cavity, which is strong in this spectral region.
[0091] The aforementioned diffraction grating 102 was mounted on a
piezoelectric translator (PZT) 104 which is computer controlled by
changing the piezo voltage and capable of approximately 30 .mu.m
linear displacement. The PZT 104 controls the length of the
external cavity by moving the grating 102 along the optical axis.
Change of external cavity length allows control of the external
cavity modes. The grating-PZT assembly is in turn mounted on a
computer-controlled rotation stage 106. This control provides the
overall wavelength selection by changing the grating angle and thus
controlling the frequency that is reflected back to the gain
chip.
[0092] Between the grating-PZT assembly and the rotation, stage
106, both the external cavity modes and the overall wavelength
selection can be independently controlled in a coordinated
fashion.
[0093] The single pass external cavity length for our system was
approximately 4 cm, yielding an external cavity mode spacing of
around 3.75 GHz (0.125 cm.sup.-1). Gain chip modes are roughly 15
GHz (0.5 cm.sup.-1) apart. Since our lasers did not have AR
coatings, the system exhibited coupled cavity behavior. Therefore,
it required three controls to ensure single mode, continuous,
mode-hop free tuning. Gain chip current controls its FP mode comb,
grating angle controls center wavelength, and PZT controls external
cavity FP mode comb. To have reproducible single mode operation,
the desired gain chip mode has to be aligned with the proper
external cavity mode and with the central wavelength of the
diffraction grating. After all modes are aligned properly, an FTIR
scan has to be performed to measure wavelength at given control
positions. To tune such a system continuously, starting from this
pre-aligned point, chip current, PZT displacement and grating angle
should be changed simultaneously and in a synchronized fashion.
[0094] To align all modes in the first place, the following
approach was developed. Laser current is set to its maximum value,
and then the grating angle is scanned with a fine step across the
entire gain curve (allowed spectral output region) of the chip.
[0095] For each grating step, the PZT scans the distance equal to a
few wavelengths, and system output power is continuously measured.
After the PZT ramp is finished, maximum value of power is recorded.
After that, the grating moves to next step. As a result of such a
scan, a curve with distinct, periodic power maxima and minima with
changing wavelength is acquired. Each maximum position of the curve
corresponds to a best match between grating angle, EC length, and
gain chip current.
[0096] From the spectral positions of these maxima, the law of
grating movement for single mode continuous tuning is determined by
a polynomial fit:
GP=GP.sub.0+A(v-v.sub.0)+B(v-v.sub.0).sup.2 (3)
[0097] where GP is the grating angle position (mm) at a desired
frequency v (cm.sup.-1), GP.sub.0 is the starting grating position
(mm) at starting frequency v.sub.0 (cm.sup.-1), and A and B are the
polynomial fit coefficients.
[0098] The equation for chip current change is:
I = I 0 - .DELTA. I FSR v 0 v mod ( v - v 0 .DELTA. v FSR , 1 ) ( 4
) ##EQU00001##
where I.sub.0 is the starting/maximum current (mA), v.sub.0 is the
starting frequency (cm.sup.-1), v is the frequency (cm.sup.-1) at
the current I (mA), and .DELTA.I.sub.FSR is the current change (mA)
necessary to shift the gain chip's Fabry-Perot comb by exactly one
free spectral range (.DELTA.v.sub.FSR cm.sup.-1) in the vicinity of
v.sub.0.
[0099] The PZT 104 finds its position adaptively by finding maximum
power at each point of the scan. As one can see from the expression
for current, every time the frequency moves one free spectral range
of a chip, the current jumps back to maximum value and then starts
going down with the frequency. This type of periodic current
tracking, described above, allows us to remove the output power
dynamic range limitation that would otherwise be imposed on the
tunability window, and to maintain high output power across the
entire gain curve of the chip. For the laser described above,
.DELTA.I.sub.FSR=60.8 mA and .DELTA.V.sub.FSR=0.49189 cm.sup.-1 at
I.sub.0=1294.4 mA. The v.sub.0/v term corrects for the fact that
the Fabry-Perot comb does not really shift but rather expands the
comb from zero frequency.
[0100] The free spectral range of the gain chip, .DELTA.v.sub.FSR,
can be determined by taking a high resolution FTIR scan of the gain
chip in FP configuration and measuring the distance between
adjacent modes. We have devised two independent ways of determining
.DELTA.I.sub.FSR. One way is to take FTIR scans of the gain chip in
FP configuration for different currents and linearly fit data. A
second approach is to utilize a photoacoustic gas cell and manually
find a reference gas line by adjusting grating angle and gain chip
current to obtain maximum photoacoustic signal. During this
process, the PZT should be constantly moving to average out the
effect of the external cavity mode mismatch. After an absorption
line is found and optimized, the grating angle stays constant and
the laser current is changed until the photoacoustic signal is
maximized again, which means that current was shifted exactly one
FSR (free spectral range).
[0101] For our short cavity setup, we obtained an overall tuning
range of 350 nm, centered around 7,350 nm with highest single
frequency optical power of nearly 200 mW (FIG. 7). At the edges of
the gain curve, laser output goes to Fabry-Perot mode even in the
presence of feedback. A proof of continuous single mode, mode hop
free tuning over 200 nm range and of the very narrow output
linewidth while tuning was obtained by measuring PAS (photoacoustic
spectroscopy spectrum of 10 ppm acetylene in 750 Torr of CDA (clean
dry air) (FIG. 8). The near perfect match between the measured line
positions and linewidths and those obtained from HITRAN
simulations, provides convincing proof that computer-based
algorithm provides a mode hop free tuning system even with an
uncoated facet QCL gain chip. By carrying out QCL-PAS on one of the
features of acetylene as a function of the partial pressure of
acetylene, we obtained the minimum limit for the detection of this
trace gas (FIG. 9) of 2.5 ppb.
[0102] As of the tiling date of this patent, the website at
http://cfa-www.harvard.edu/hitran// (aliased by www.hitran.com)
indicates that HITRAN is an acronym for high-resolution
transmission molecular absorption database. HITRAN is a compilation
of spectroscopic parameters that a variety of computer codes use to
predict and simulate the transmission and emission of light in the
atmosphere. The database is a long-running project started by the
Air Force Cambridge Research Laboratories (AFCRL) in the late
1960's in response to the need for detailed knowledge of the
infrared properties of the atmosphere.
[0103] The system described above has three controls for single
mode continuous tuning. However, one can remove the need for PZT
control for by relaxing the continuous tuning requirement. To
achieve that, instead of short cavity, a long cavity can be used.
In this case, laser tunes quasi continuously, on a grid determined
by the positions of the FP comb of the external cavity. The
spectral density of the grid is determined by the overall optical
length of the external cavity, and laser will hop from one external
cavity mode to another while tuning. If the spectral distance
between external cavity modes is much smaller than the
characteristic width of a spectroscopic feature studied, the laser
source can be treated as continuously tunable.
[0104] In our long cavity system, a room temperature QCL with the
gain region centered at 6.3 .mu.m was mounted and operated in the
manner similar to that described above. Chip length was 4 mm and
crystal facets were not coated.
[0105] The output from the back facet of the gain chip was
collimated by using either a f/1.0, 25-mm diameter off-axis
parabolic mirror or a f/0.7 aspheric AR coated ZnSe collimating
lens. Beam diameter of collimated beam in the case of a parabolic
mirror was roughly 2.5 mm and in the case of ZnSe lens was roughly
4 mm. The resulting collimated beam was reflected off a planar, 300
grooves-per-mm diffraction grating blazed at 5.4 .mu.m. In the case
of the parabolic mirror, the grating was mounted in the Littrow
configuration. In case of the ZnSe lens, the grating was mounted in
a double pass configuration where the beam was reflected from the
grating and incident on a flat mirror to be reflected back to the
grating and then to the gain chip. Such configuration lets us
compensate for the decrease of grating resolution due to beam size
reduction. The length of the external cavity in both cases was
maintained at around 100 cm, yielding FP mode spacing of
approximately 150 MHz. Gain chip mode spacing was approximately 12
GHz. Resolution of the grating was sufficient to support single
mode tuning over the range of .about.350 nm. The single mode CW
output power in the center of gain curve at the maximum current was
around 300 mW (FIG. 10).
[0106] In the long cavity configuration, only two parameters need
to be controlled simultaneously in order to achieve single mode
tuning of the system. One parameter is laser current, the other
parameter is grating angle. Grating angle is controlled by a linear
actuator driving a rotation stage, as described above. Grating
angle and current change algorithm were the same as described for
the short cavity case.
[0107] We obtained an overall tuning range of 350 nm centered at
6,300 nm with maximum single frequency optical power of 300 mW
(FIG. 10). Continuous tuning was demonstrated by recording actual
gas spectra over approximately 20 nm range with absolute frequency
error of no more than 1 GHz. Further continuous tuning was
confirmed by recording FTIR spectra in several additional randomly
selected regions of the overall tuning window, spanning multiple
free spectral ranges of the gain chip.
[0108] The 6,300 nm single wavelength tunable QCL was used for high
resolution spectroscopy (FIG. 11) and sensitive detection (FIG. 12)
of nitrogen dioxide, pollutant gas that results from industrial
activities as well from automobile emissions. Data in FIG. 12
permits us to extrapolate the measurements to obtain a detectivity
of 0.5 ppb for nitrogen dioxide using the broadly tunable external
grating cavity QCL.
Sensitive Detection of Explosives
[0109] Reliability and reproducibility of the tuning of the single
wavelength QCL permits us to use our QCL based laser photoacoustic
spectroscopy system for the sensitive detection of home-made
explosives such as triacetone triperoxide (TATP), its precursors,
acetone and hydrogen peroxide, and conventional explosives such as
trinitrotoluene (TNT).
Detection of TATP and Acetone
[0110] Triacetone triperoxide (C.sub.9H.sub.18O.sub.6, molecular
mass of 222.24 g/mol) is a powerful explosive that is easy to
synthesize using commonly available household chemicals, acetone
and hydrogen peroxide. Because of the simplicity of its synthesis,
triacetone triperoxide (TATP) is often the explosive of choice for
terrorists including suicide bombers. For providing safety to
population, early detection of TATP and isolation of such
individuals is essential. We report unambiguous, high sensitivity
detection of TATP and its precursor, acetone, using room
temperature quantum cascade laser photoacoustic spectroscopy
(QCL-PAS). The available sensitivity is such that TATP, carried on
a person (at a nominal body temperature of 37.degree. C.), should
be detectable at some distance. The combination of demonstrated
detection of TATP and acetone should be ideal for screening at
airports and other public places for providing increased public
safety.
[0111] Unlike most other high explosives, TATP contains no nitrogen
or nitrates. The absence of nitrates makes it difficult to detect
using technologies that utilize nuclear quadrupole resonance for
the detection of other explosives such as TNT, PETN, RDX, etc., all
of which are nitrate rich. TATP is suspected as being the explosive
that was used in London Underground bombings on Jul. 7, 2005 that
killed more than 50 people and injured more than 700. Countless
number of civilians have been killed by suicide bombers who often
prefer TATP because of the relative simplicity of its synthesis.
The notorious but unsuccessful shoe bomber in 2001 was suspected of
carrying TATP. Finally, TATP can be readily synthesized from
acetone and hydrogen peroxide. The present international air travel
crisis owes much to the simplicity of synthesis of TATP and the
absence of high sensitivity detection of TATP and its liquid
precursors. Once the presence of TATP is established, its presence
is easy to verify using a variety of wet chemical techniques.
However, this adds nothing to public safety because the initial
determination of the TATP is still missing. We demonstrate that
this first crucial step, identification of the presence of TATP and
its precursor, acetone, is now possible using quantum cascade laser
based photoacoustic spectroscopy (QCL-PAS).
[0112] Because of the expected broad absorption feature of TATP,
continuous tuning of the QCL wavelength is not necessary.
Furthermore, use of a "Smart Grid" of wavelengths selected as
described herein avoids measurements at the known wavelengths of
the sharp absorption features corresponding to residual water
vapor. We select the appropriate density of the grid, under
computer control, depending on the nature of the target. For
targets with narrow absorption features, such as acetylene, a dense
grid has been used, while for targets with broad absorption
features, such as TNT and TATP, a sparse grid can be used without
any loss of spectral details.
[0113] FIG. 13 shows the measured QCL-PAS data for TATP at several
different temperatures from 25.degree. C. to -3.degree. C. (the
lower temperature limit is set by the presently used chiller). Very
high signal-to-noise ratio spectra are obtained. To evaluate our
detection sensitivity for identifying TATP, in FIG. 14 we show the
lowest temperature (-3.degree. C.) data on an expanded scale. An
S/N ratio of in excess of 1000 is seen.
[0114] To convert the TATP temperature in to an equivalent vapor
pressure, we use the data published by Oxley. FIG. 15 shows vapor
pressure measurements of Oxley and a Clapeyron equation fit to the
data as a function of temperature. We also show our QCL-PAS data
over the temperature range that now extends below the lowest
temperatures for which vapor pressure data are available. The
QCL-PAS data are fitted to T=25.degree. C. vapor pressure point on
the Clapeyron fit. The QCL-PAS data fit the measured vapor pressure
data of Oxley well in 12.degree. C. to 30.degree. C. region.
However, at lower temperatures, where measured vapor pressure data
arc not available, the L-PAS data diverge form the Clapeyron fit.
Since the L-PAS signal generation process is inherently linear at
low concentrations of absorbers, we believe that the use of L-PAS
data as a surrogate for vapor pressure measurements at low
temperatures would improve the Clapeyron equation fit.
[0115] To estimate the ultimate TATP detection capability provided
by QCL-PAS measurements, we show the L-PAS noise floor on FIG. 15.
We estimate from the extrapolation of QCL-PAS data that TATP at
vapor pressures as low as 10 ppb (which is approximately 10
picograms per cubic centimeter (.about.10 pg cm.sup.-3)) should be
possible with an S/N .about.1. Incidentally, this extrapolation is
independent of the Clapeyron fit. We cannot unequivocally convert
the .about.1 ppb sensitivity into a corresponding TATP temperature
as yet.
[0116] From the current interest in assuring safety of air travel
it is clear that we need to detect not only TATP with very high
sensitivity but also detect at least one of the precursors
necessary to synthesize the explosive. FIG. 16 shows a QCL-PAS
spectrum of 1.4 ppm acetone, one of the two principal ingredients
for synthesizing TATP. Very high signal/noise ratio spectrum is
seen. HITRAN simulation of acetone is also shown on the same figure
and indicates excellent agreement between QCL-PAS measurements and
simulated data. FIG. 17 shows acetone L-PAS signal versus acetone
concentration along with the L-PAS noise floor. We can estimate
that the lowest concentration that can be detected is about 3 ppb
(.about.8 pg cm.sup.-3) in the present preliminary studies. The
acetone detection sensitivity value is comparable to the acetylene
detection sensitivity value (2.5 ppb) reported earlier and is
consistent with the relative absorption strengths of the two gases
as reported in the HITRAN compilation. It should be noted that the
accepted value of odor threshold for acetone is about 13 ppm
although very wide variation in the number exists in the
literature.
Detection of a Conventional Explosive, TNT
[0117] Detection of illegally transported explosives has become
important since the global rise in terrorism subsequent to the
events of Sep. 11, 2001. While not a choice of suicide bombers, TNT
is considered a potent explosive for which techniques for detection
on a person's body or in one's baggage is considered important for
assuring safety of airports and air travel. As with detection of
other similar compounds, such as chemical warfare agents, any
detection scheme that claims to detect these targets must exhibit
acceptable receiver operational characteristic (ROC) that assures
detection at very low levels without unacceptable level of false
alarms. The molecular weight of TNT (C.sub.7H.sub.5N.sub.3O.sub.6)
is almost exactly identical with the molecular weight of
nitroglycerine (C.sub.3H.sub.5N.sub.3O.sub.9) even though the
chemical compositions of the two molecules are very different (TNT:
227.131 Da vs. NG: 227.0872 Da). The nearly same molecular weights
often lead to problems for unambiguous detection of TNT using
techniques that rely on measuring the molecular mass of the
species. On the other hand, the differences in the chemical
structure between TNT and nitroglycerine leads to noticeably
different infrared absorption signatures making it possible to
distinguish between the two. Moreover, the detection of TNT in
vapor phase is made difficult by its low vapor pressure of
approximately 2.times.10.sup.4 torr at 25.degree. C. Nonetheless,
the high sensitivity afforded by L-PAS shows that the vapor phase
detection of TNT at an ambient temperature of approximately or
roughly 25.degree. C. is possible.
[0118] The 7300 nm QCL-PAS system spans a wavelength region from
7150 to 7500 nm and also covers the absorption spectrum of TNT (in
addition to those of TATP and acetone described above). For
exploring detection of TNT we provided a continuous flow of clean
dry air over a sample of TNT and the emerging gas was continuously
analyze by QCL photoacoustic spectrometer. The temperature of the
TNT sample could be controlled from room temperature to 60.degree.
C. The gas transport lines from the TNT sample chamber to the
photoacoustic cell and the photoacoustic cell were maintained at
60.degree. C. to prevent condensation of TNT vapors either in the
transfer lines on in the photoacoustic cell. The upper temperature
limit was set by the photoacoustic cell microphone whose
sensitivity begins to degrade significantly above 60.degree. C. but
is not a limitation for future operation of the cell at higher
temperatures by using appropriate high temperature microphones.
[0119] FIG. 18 shows an L-PAS spectrum obtained when the TNT sample
was kept at 24.degree. C., 35.degree. C., 45.degree. C. and
55.degree. C. respectively. Note should be taken of three specific
aspects of the PA (photoacoustic) spectrum. The first is that a
number of sharp absorption features arising from residual water
vapor in the system (as verified using water vapor absorption
spectra obtained from HITRAN simulations) occur at certain
wavelengths in the same region of wavelengths. These were avoided
by using a smart grid of laser wavelengths that skips these
specific wavelengths as the computer provides the tuning
instructions to the EGC QCL.
[0120] The second is that the QCL-PAS spectrum is significantly
broader than that would be expected. In fact, the QCL-PAS spectrum
consists of two distinct features, one centered at .about.7380 nm
that matches the expected absorption feature of TNT and the second
centered at .about.7300 nm that arises from the yet unknown
impurity in the commercial grade TNT. The unknown impurity was seen
to be located on the surface of the TNT sample and the 7300 nm
feature gradually disappeared as the TNT sample was kept at
100.degree. C. for 48 hours while flushing the sample with clean
dry air.
[0121] FIG. 19 shows the measured L-PAS spectrum of "purified"
sample of TNT vapor in a background of room air with relative
humidity of about 40% at 25.degree. C. The spectrum matches the
expected position and width well. The spectra were taken using 300
discreet wavelengths determined by the computer using the smart
grid algorithm that skips the wavelengths corresponding to the
known strong absorption features of water vapor. We conjecture that
the disappearing peak at 7300 nm could be used in the future to tag
the origin and age of the TNT sample for forensic purposes.
However, a confirmation of the conjecture will have to await
getting samples of different age and origin from NAWS, China Lake.
In either case, the shape and location information provides a
powerful tool for QCL-PAS to uniquely identify TNT and minimize
effects of interference.
[0122] The third aspect of the measured spectra (in FIGS. 18 and
19), which deserves mention, is that the signal feature in the
.about.7380 nm absorption region, grows rapidly as the TNT
temperature is increased from 24.degree. C. to 55.degree. C. as
would be expected form the temperature dependence of the vapor
pressure of TNT shown in FIG. 20. We have plotted the measured
QCL-PAS data for TNT on the same plot, anchoring the 50.degree. C.
QCL-PAS data on the vapor pressure vs. T plot. An acceptable
correlation is seen between the vapor pressure data and the PA
signal amplitude.
[0123] From the lowest temperature (24.degree. C.) at which the
photoacoustic spectrum is shown, we can estimate the detection
sensitivity from known vapor pressure data for TNT. The vapor
pressure of TNT is seen to be (from Clapeyron fit curve) about 3
ppb at 24.degree. C. (.about.30 pg cm.sup.-3). Comparing the L-PAS
signal with the noise floor shown in the FIG. 20, we estimate that
we can detect TNT at a level of 0.1 ppb (.about.1.01 pg cm.sup.-3)
with a S/N of 1 (i.e., TNT at temperatures as low as 5.degree. C.).
It should be noted, however, that the relationship of vapor
pressure and temperature is dependent on the Clapeyron fit to the
measured data.
QCL-PAS can Distinguish Between Various Explosives
[0124] The spectral features of TNT and TATP arc so distinctly
different that the system should be able to detect each of these
three components unambiguously at very low concentrations (FIG.
21). Moreover, the use of an intelligent grid of discrete sampling
wavelengths that avoids the strong, but sharp, absorption features
of water vapor would be immediately deployable for real time
screening of personnel and baggage at airports and other public
places where threat from explosives and/or precursors is perceived.
Such high sensitivity screening should lead to the relaxing of some
of the draconian security measures that govern air travel
today.
All-Mechanical Control of QCL Wavelength and Cavity Length
[0125] A method for mode-hop-free tuning of grating-coupled
external cavity lasers may be based on a Littrow configuration in
which the grating assembly moves along two perpendicular axis,
resulting in simultaneous grating rotation and cavity length
adjustment to track one longitudinal mode of the cavity. This
improves laser power output despite the change in laser operating
frequency that occurs with rotation of the grating. This
configuration requires only one linear actuator and gives an exact
solution of the problem of coordinating grating rotation with
cavity length despite the presence of linear dispersion.
[0126] Electrically pumped semiconductor lasers are divided in two
main categories: diode lasers based on inter-band transitions
(between the conduction and the valence bands) and quantum cascade
lasers based on intersubband transitions between confined states of
the conduction band. Tunable external grating cavity diode lasers
(EGCDLS) have been used for a long time in both laboratory and
industrial applications including optical telecommunication
equipment testing, optical metrology, and gas sensing. EGCDLs are
attractive for the latter application because of their compactness,
ease of use, and cost. However, to date most of the accessible
spectral region has been in the visible and near-infrared (NIR)
portions of the electromagnetic spectrum and has involved the use
of diode lasers. This arrangement allows one to spectroscopically
investigate only overtone and combination vibrational bands of most
molecules. The fundamental bands, which have typically several
orders of magnitude stronger absorption strengths, lie in the mid
wavelength infrared (MWIR) and/or long wavelength infrared (LWIR)
regions, between 3 and 12 .mu.m.
[0127] The invention of the quantum cascade laser (QCL) in 1994 and
its rapid development during the following decade led to room
temperature continuous wave operation with power levels in excess
of 100 mW between 3.8 and 9.6 .mu.m. QCLs opened up a way to the
realization of compact, ultra-sensitive, trace gas sensors based on
absorption spectroscopy. Such trace gas sensors have a very wide
field of applications including industrial process control,
environmental air-quality monitoring, agricultural and industrial
emission monitoring, chemical warfare agent (CWA) detection, and
explosives detection. For the detection of heavy molecules with
broad absorption features, detection of a target species in
presence of interferences, or detection of several species
simultaneously, external grating cavity QCLs (EGCQCLs) are
preferred over the distributed feedback QCLs because of the broader
tuning range that is accessible with a single EGCQCL.
[0128] To obtain mode-hop-free tuning of external grating cavity
lasers, one needs to vary the cavity optical length while rotating
the diffraction grating in order to preserve the coincidence
between the grating-selected wavelength and a single longitudinal
mode of the overall cavity. Without such coordination, mode-hopping
between adjacent or available laser modes can occur on a generally
unpredictable basis. Such coordination between grating angle and
cavity length can be done straightforwardly by having separate
actuators for controlling grating angle and cavity length, at the
price of increased complexity and cost of the system. The optical
length can also be adjusted by varying the injection current, the
temperature, or both, in the case of semiconductor lasers. This
method is intrinsically slow because the primary contribution to
the optical length of the semiconductor laser comes from the
injection current and/or the temperature dependence of the
refractive index of the semiconductor laser material and one has to
wait for the temperature of the active region to stabilize. This is
especially true for QCLs in which the tuning mechanism is
thermo-optic effect.
[0129] A more powerful approach is to design the mechanical
arrangement in such a way that the cavity length and the grating
angle are adjusted simultaneously to track one longitudinal mode by
means of a single actuator. This problem, as applied to dye lasers,
has been investigated in the eighties. McNicholl and Metcalf have
given solutions based on rotation of the grating around a carefully
chosen pivot point for the Littrow grazing incidence with a tuning
mirror (Littmann-Metcalf), and grazing incidence with Littrow
grating configurations. These methods have been successfully
applied to EGCDLs by several authors.
[0130] We have discovered a different method in which the grating
moves along two perpendicular planes and is seen to be especially
suited for making EGCQCLs.
[0131] The example set forth herein uses a system having a laser
consisting of gain element, a collimating lens, and possibly other
elements in which the wavelength selection process is achieved by
means of a diffraction grating in Littrow configuration (FIG. 22).
We assume that the gain element is anti-reflection coated so that
we can neglect the coupled cavity effects. In this case, the lasing
mode is determined by the grating angle and the overall cavity
length only.
[0132] The grating feedback into the gain element is different from
zero only for a narrow wavelength region centered around
.lamda..sub.G(.theta.)=(2d/k)sin .theta. (5)
where d is the grating period, k the diffraction order that is
used, and .theta. the angle between the normal to the grating and
the optical axis. The cavity modes are given by the condition that
the number of half-wavelengths in the cavity is an integer:
L=m.lamda..sub.m/2, where L is the optical length of the cavity and
m is an integer. Let L.sub.0 be the optical length of the cavity at
the starting angle .theta..sub.0. We assume that
.lamda..sub.G(.theta..sub.0) coincides with a longitudinal mode
.lamda..sub.m=2 L.sub.0/m. Experimentally this initial condition
can be obtained by slightly adjusting the injection current or by
small adjustment in the cavity length. In order to preserve the
concurrence of the angle .theta. and the proper cavity length when
the grating is rotated, the cavity length, L, should change by an
amount .DELTA.L(.theta.):
.lamda..sub.m(.theta.)=2[L.sub.0+.DELTA.L(.theta.)]/m (6)
[0133] In cases where the movement of the grating includes a
translation along its own plane, as in the case for the pivot point
methods, it is necessary to take into account the dephasing that
this translation induces in the equation for
.lamda..sub.m(.theta.). But since this effect is not present in our
method, we omitted the corresponding term in equation (6).
[0134] The required angular and linear movement of the grating may
be achieved by means of a mechanical assembly of which a point B
moves along the optical axis b and another point A moves along an
axis a perpendicular to optical axis b and to the grating lines
(FIG. 22). This configuration produces a cavity length variation of
the form
.DELTA.L(.theta.)=AB sin .theta. (8)
[0135] Inserting in equation (2) and requiring that
.lamda..sub.m(.theta.)=.lamda..sub.G(.theta.), one gets AB=md/k and
L.sub.0=AB sin .theta..sub.0. These relations are equivalent. One
can be obtained from the other using the initial condition
.lamda..sub.m(.theta..sub.0)=.lamda..sub.G(.theta..sub.0). Adding
.DELTA.L on both sides of the second relation, one gets
L(.theta.)=AB sin .theta. (8)
[0136] The geometrical interpretation is straightforward: the
projection of the segment AB on the optical axis has to be equal to
the optical length of the cavity.
[0137] This allows us to determine the position of the back mirror
with respect to the axis a. The distance D between these two has to
be equal to the difference between the optical length L and the
physical length l of the cavity (see FIG. 23):
D=L-l=.SIGMA..sub.i(n.sub.i-l)l.sub.i (9)
where the index i runs over the optical elements inside the cavity,
n.sub.i and l.sub.i being the refractive index and length of
element i, respectively. The difference in the optical length and
the physical length arises from the finite refractive index of the
gain clement inside the cavity (not shown). In the specific case
where there are no other sources that change the propagation
properties of the electromagnetic wave, i.e., dispersion, the
problem of concurrent change of cavity length, L and the grating
angle .theta. has now a closed form solution.
[0138] Since semiconductor lasers generally contain a waveguide
structure, they present modal refractive index dispersion. That is,
their modal refractive index, and consequently their optical
length, are not constant as functions of wavelength. To the first
order, this variation can be approximated as linear. However, if
neglected in the design of the system, this dispersion has strong
detrimental consequences on the tuning method set forth herein
which relies on varying the optical cavity length while rotating
the grating in order to preserve coincidence between
grating-selected wavelength and a longitudinal Fabry-Perot mode of
the cavity. There are detectable effects arising from waveguide
dispersion as well as dispersion in other optical elements of the
cavity on tuning method disclosed herein.
[0139] Prior attempts to resolve this dispersion problem did not
take this effect into account. One attempt resulted in an observed
a mode-hop-free tuning range of .about.1% of the center wavelength.
An improved version included the possibility of rotating the
translation axis a (see FIG. 22) to compensate for dispersion. This
approach demonstrated a larger tuning range of more than 5% of the
center wavelength.
[0140] Continuously tuning the laser in the presence of dispersion
may be achieved by positioning the translation axis a at a
different location. This novel solution provides an exact solution
to the problem in the case of linear dispersion (where refractive
index which depends linearly on the wavelength). By providing an
exact solution to the problem, the mode-hop-free tuning range can
be made arbitrarily broad if one can build a sufficiently precise
mechanical setup.
[0141] In order to take into account linear dispersion in our
cavity, we introduce a dependence of L.sub.0 on .lamda.:
L.sub.0(.lamda.)=L.sub.0(.lamda..sub.0)+b(.lamda.-.lamda..sub.0)
(10)
with b=dL.sub.0/d.lamda.=.SIGMA..sub.i(dn.sub.i/d.lamda.)l.sub.i.
The substitution of the expression (6) in (2) leads to:
.lamda..sub.m(.theta.)=2[L.sub.0(.lamda..sub.0)-b.lamda..sub.0+.DELTA.L(-
.theta.)]/(m-2b) (11)
[0142] Requiring .lamda..sub.m(.theta.)=.lamda..sub.G(.theta.) as
before, we get AB=(m-2b)d/k and L.sub.0 b.lamda..sub.0=AB sin
.theta..sub.0. Inserting the definitions of L.sub.0 and b, the
quantity L.sub.g=L.sub.0(.lamda..sub.0) b.lamda..sub.0 can be
written as
L.sub.R=.SIGMA..sub.i[n.sub.i(.lamda..sub.0)-.lamda..sub.0dn.sub.i/d.lam-
da.]l.sub.i (12)
[0143] One can recognize the expression of the group refractive
index n.sub.g=n(.lamda.) .lamda.dn/d.lamda.. The result of
preceding paragraph obtained in (9) has thus to be modified in
presence of dispersion as follows:
D=L.sub.R-1=.SIGMA..sub.i(n.sub.R,l-1)l.sub.i (13)
[0144] D has to be equal to the difference between the group
optical length and the physical length of the cavity. It should be
noted that this method still gives an exact solution of the problem
in this case.
[0145] For mode-hop free tuning range in case of non-ideal
positioning, let us suppose that the distance between the axis a
and the back mirror deviates from the ideal distance, given by
equation (13), by a small amount .DELTA.D, i.e., that the cavity
length is equal to L(.theta.)=.DELTA.D+AB sin .theta.. In this
case, the number of half wavelengths (.mu.) selected in the cavity
by the grating,
.mu.=2L(.theta.)/.lamda..sub.G(.theta.) (14)
is no longer constant. The lasing mode is the Fabry-Perot mode
.lamda..sub.m whose wavelength is the closest to .lamda..sub.G,
that is the one for which m=integer(.mu.). The mode-hop free tuning
range is given by the condition that the variation of .mu. has to
be smaller than one (1). Introducing the expressions for L(.theta.)
and .lamda..sub.G(.theta.) in equation (10), one gets:
.DELTA..mu.=2.DELTA.D[1/.lamda..sub.min-1/.lamda..sub.max].ltoreq.1
(15)
[0146] In this last equation, we have assumed that the tuning range
is much larger that the free spectral range of the cavity and have
also chosen the lasing wavelength by .lamda..sub.G at its
extremities where the above requirement no longer holds.
[0147] This equation gives the mode-hop free tuning range (in
wavenumbers) as a function of the departure .DELTA.D:
.DELTA.v=.lamda..sub.min.sup.-1-.lamda..sub.max.sup.-1=1/2.DELTA.D
(16)
[0148] This result is represented graphically in FIG. 24. It is
interesting to note that a tuning range of one hundred wavenumbers
requires only a tolerance of .+-.50 .mu.m on the position of this
axis a, i.e., in the distance D derived in equation (13).
[0149] Written in an alternative form, equation (16) gives the
relative tuning range as a function of the center wavelength
.lamda.:
.DELTA..lamda./.lamda.=.lamda./2/.DELTA.D (17)
which shows how mode-hop-free tuning is simplified when working at
longer wavelengths. The fundamental reason behind this is that, in
order to avoid a mode hop, the cavity length has to be controlled
to a precision of .+-..lamda./4. FIG. 26 portrays graphically the
tuning Tange in percent of the center wavelength obtained for three
typical QCL wavelengths of 5 .mu.m, 7.5 .mu.m and 10 .mu.m, and a
telecom diode laser wavelength of 1.5 .mu.m, for cavity length
deviation up to 0.1 mm.
[0150] In conclusion, a new method for continuous tuning of
external cavity lasers has been achieved which is fundamentally
different from the prior pivot point methods. It provides an exact
solution of the coordination problem even in the presence of linear
dispersion.
[0151] This novel method is particularly attractive for quantum
cascade lasers (QCLs) because the mechanical tolerances are
somewhat relaxed for mid wave infrared and long wave infrared
wavelengths. In addition, the scheme permits one to get around the
current tuning that is required at present that has proven to be
slow because of the large temperature changes required.
[0152] The method presented herein is in principle applicable to
any kind of external grating cavity laser, but since the size of
the grating assembly grows linearly with the gain medium length, it
is particularly well suited to semiconductor lasers in which the
high gain occurs with chips only a few millimeter long.
Smart Grid Algorithm
[0153] The Smart Grid laser tuning algorithm for detection of a
target species tunes the laser across different wavelengths of the
target species absorption signature, but skips the wavelength
regions in the frequency spectrum where potential interferences
have large absorption features and are expected in high
concentrations. This provides for better interference rejection as
well as improving measurement time.
[0154] Gas detection using photoacoustic spectroscopy involves
tuning a laser across different wavelengths and recording the
acoustic transducer signal from the cell. When this signal is
recorded across different wavelengths, it produces a unique
signature for each molecule. In the real world, gas samples are
composed of many different molecules (a "soup"). When the signature
of the soup is linearly deconvolved against a standardized library
of the target species and a list of expected interferents, it
produces a concentration reading for the target gas as well as for
the interferences. L-PAS is very useful for sub-ppb (parts per
billion) detection of gas species.
[0155] To guarantee high sensitivity measurements in L-PAS, it is
important to consider what wavelengths a laser can tune to obtain a
good photoacoustic signature of the target species. Equally
important is identifying potential interferents in the working
region, and looking at the absorbances of the interferents at their
expected concentrations. In the case that the interferents have
very large absorbances which could potentially drown the signature
of low concentrations of the target species which we wish to detect
in a blanket of noise riding on top of the absorption signature of
the soup, we simply choose to skip this region. Beginning with the
complete laser tuning region where the target has large absorption,
we eliminate all the regions where expected interferents have large
absorption. This elimination is a key step in obtaining a rapid and
sensitive L-PAS measurement. Once all the undesirable regions have
been eliminated from the complete working region of the laser, we
call the left over regions a "smart grid" for L-PAS.
[0156] The procedure for the "smart grid" algorithm includes the
following steps: [0157] 1) Identify and select regions in the
frequency range of the laser which meets all the following
criteria: [0158] a) Target species has large absorption; [0159] b)
Expected interferents have low absorption at their expected
concentrations; and [0160] c) Signature of target is linearly
independent of signature of interferents. [0161] 2) These regions
combined are now called the "smart grid." [0162] 3) Perform a scan
across the smart grid and collect photoacoustic data. [0163] 4)
Linearly deconvolve photoacoustic data against a standardized
library of the target species and list of expected interferents,
and obtain a gas concentration measurement.
[0164] To demonstrate the improvements in PFA (probability of false
alarms) and measurement time (throughput) of the smart grid
algorithm we chose to detect acetone in the presence of water as an
interferent in the 7.0 to 7.5 .mu.m region. 1000 scans were
performed using the full available range (7.0-7.5 .mu.m), and then
another 1000 scans were performed using a smart grid of frequency
points, which we identified (see FIG. 26). We had about 1% noise
riding on top of our signal.
[0165] To characterize the detection limit in parts per billion
(ppb) vs. Probability of False Alarms (PFA), we used 10,000 ppm of
water and plotted ROC curves for acetone. The results were as
expected and the smart grid showed improvement in the PFA by a
factor of 30 for lower detection thresholds (see FIG. 27). The
measurement time (throughput) also improved by a factor of 15 for
smart grid detection.
[0166] A similar simulation was carried out for the 8.0 to 8.5
.mu.m region where water has a much lower absorption (FIG. 28) and
the absorption features for acetone remained similar in shape and
size. Here the smart grid showed improvement in the PFA by a factor
of 40 for lower detection thresholds (see FIG. 29) and the
measurement time (throughput) also improved by a factor of 5.
[0167] L-PAS sensors have applications in the industrial
(petrochemical, semi-conductor industries), security (chemical
warfare agent and explosives detection), and medical (breath
analysis) fields. The smart grid algorithm for L-PAS improves PFA
and measurement time for all L-PAS sensors, regardless of their
application or target species.
[0168] The use of the Smart Grid algorithm is suitable for all
other sensors based on spectral data and where interferences are
present.
Performance Optimization
[0169] An iterative laser tuning algorithm is achievable for
detection of a target species which tunes the laser to the more
distinct (interference free) and large absorption frequency ranges
in the absorption spectrum of the target species first to achieve
fast detection with a high probability and then adaptively move on
to wider frequency ranges when lower False Alarm Rates (FAR) are
required (FAR is frequently referred to as PFA). This algorithm
optimizes laser photoacoustic spectroscopy (L-PAS) throughput vs.
performance in the presence of interferents that cannot be
rejected. The performance optimization algorithm permits a
quantification of the L-PAS instrument performance as a function of
throughput and an optimization of the L-PAS instrument performance
for a given throughput requirement.
[0170] Gas detection using photoacoustic spectroscopy involves
tuning a laser across different frequencies and recording the
acoustic signal from the cell. When this signal is recorded across
different frequencies it produces a unique signature for each
molecule. In the real world, gas samples are composed of many
different molecules (a "soup"). When the signature of the soup is
linearly deconvolved against a standardized library of the target
species, and a list of expected interferents, it produces a
concentration reading for the target gas.
[0171] L-PAS is very useful for sub-ppb (parts per billion)
detection of gas species. However tuning the laser across many
different frequencies and collecting photoacoustic data can be a
time consuming process. L-PAS sensors have a wide variety of
applications, in the industrial (petrochemical, semi-conductor
industries), security (chemical warfare agent and explosives
detection), and medical (breath analysis) fields. Many of these
applications require rapid response times, and immediate action to
be taken when threats are detected. Hence there is a great need to
speed up measurements sometimes even at the expense of higher false
alarm rates.
[0172] The iterative laser tuning algorithm for detection of a
target species tunes the laser to the more distinct (interference
free) and large absorption frequency ranges in the absorption
spectrum of the target species first.
[0173] Quick scans are performed in this range to obtain gas
measurements with a high probability of detection (PD) and
relatively high false alarm rate (FAR). Once the target species is
detected above a certain threshold, the laser is tuned across a
broader frequency range to obtain more interference free
measurements with high PD and lower FAR.
[0174] To achieve optimized performance, the following procedure
may be used with the following inputs and steps.
[0175] Inputs: [0176] 1) Alarm Threshold; and [0177] 2) Minimum
Throughput Rate
[0178] Steps: [0179] 1) Identify and select a region in the
frequency range of the laser which meets all the following
criteria: [0180] a) Target species has large absorption [0181] b)
Expected interferents have low absorption at their expected
concentrations; and [0182] c) Signature of target is linearly
independent of signature of interferents. [0183] 2) Perform a scan
across the region and collect photoacoustic data. [0184] 3)
Linearly deconvolve photoacoustic data against a standard library
of the target species and list of expected interferents, and obtain
a gas concentration measurement. [0185] 4) Record measurement and
determine time taken for the measurement (throughput). [0186] 5) If
the gas concentration is below the alarm threshold continue to Step
without making any changes. [0187] 6) If all the following
conditions are met: [0188] a) Gas concentration is above the alarm
threshold; [0189] b) Throughput rate above minimum throughput rate;
and Selected region is not already the maximum tunable range of the
laser, and select a new broader range which meets all the criteria
mentioned in Step 1 as well as not violating the requirement for
minimum allowable throughput. [0190] 7) Continue to Step 2.
[0191] For demonstration purposes. We chose our target as TNT and
ammonium nitrate to be our interferent. We chose a region in the
TNT spectrum where a broad and strong absorption feature is seen,
and where the absorption feature of ammonium nitrate is relatively
small and linearly independent of the TNT feature. We provided a
continuous flow of CDA over a sample of TNT and the emerging gas
was continuously analyzed by our spectrometer. The gas transport
lines from the TNT sample chamber to the PA cell were maintained at
78 C and the PA Cell was maintained at 60 C.
[0192] We identified 3 regions for analysis. The first range we
called "L-PAS Range 1" 120 (FIG. 30) which consisted of 5 discrete,
frequency points near the peak of the TNT absorption feature to do
our analysis. "L-PAS Range 2" 122 was selected to be a wider region
around the TNT absorption peak consisting of 50 discrete frequency
points. The third region was "Full L-PAS Range" 124 and represented
the full usable tuning range of our laser for TNT detection
consisting of 150 discrete frequencies.
[0193] FIG. 30 shows the TNT absorption spectrum as seen on our
setup and the different ranges we used for analysis of the
spectrum. FIG. 31 shows the Detection Threshold vs. the Probability
of False Alarms for each one of these regions. The figure
demonstrates the tradeoff between FAR and system throughput and
quantitative data for the user.
[0194] L-PAS sensors have applications, in the industrial
(petrochemical, semi-conductor industries), security (chemical
warfare agent and explosives detection), and medical (breath
analysis) fields. Quantifying instrument performance as a function
of throughput and being able to optimize performance for a required
throughput level can make sensors tunable to combat various
scenarios, and can save time, money and lives in numerous
situations. Here we look at 2 scenarios where this algorithm would
prove helpful. This algorithm will prove suitable for all other
optical sensors.
Scenario 1: Baggage Screening Checkpoint at an Airport
[0195] For a more efficient throughput rate at a baggage screening
checkpoint as a first line of defense, explosive detection systems
that detect a wide array of threats, with high throughput and a
higher FAR can be used: The baggage items that fail the initial
screening can be again tested with a second set of sensors with far
lower FARs but with lower throughput.
Scenario 2: Cargo Screening Checkpoint at a Port
[0196] For a cargo screening system to detect Explosives and
Chemical Warfare Agents, L-PAS sensors can be tuned for different
threat levels issued by the Department of Homeland Security. For a
high threat level the Minimum Throughput Rate can be lowered to
achieve a lower FAR. When the threat levels are low the Minimum
throughput rate can be made higher, at the expense of higher
FAR.
[0197] These and other advantages, utilities, applications, and
solutions provided by the present invention will be apparent from a
review of the specification herein and accompanying drawings. The
foregoing are some of but a few of the goals sought to be attained
by the present invention and arc set forth for the purposes of
example only and not those of limitation.
[0198] While the present invention has been described with regards
to particular embodiments, it is recognized that additional
variations of the present invention may be devised without
departing from the inventive concept.
INDUSTRIAL APPLICABILITY
[0199] This invention may be industrially applied to the
development, manufacture, and use of highly sensitive or
ultrasensitive equipment and methods that detect trace amounts of
gases by refined optical illumination and response detection as
well as photoacoustic methods.
* * * * *
References