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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.
1. A method for obtaining power-maximized continuous tuning for a coupled cavity laser system, the steps comprising:
a) powering a quantum cascade laser (QCL) gain chip to provide a source of multiwavelength laser light at a first power level;
b) selectively reflecting said multiwavelength laser light with a diffraction grating in a cavity back to said QCL, gain chip to select a first laser wavelength;
c) adjusting a position of said diffraction grating with a piezoelectric translator (PZT) to adjust 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, said position of said diffraction grating, selectively altered on the order of a few wavelengths of said first laser frequency to find said maximum output distance;
d) making a fine angular displacement step with said diffraction grating to select a new laser wavelength and repeating steps b and c in an ongoing fashion for each new laser wavelength to obtain sufficient data to determine Fabry-Perot (FP) mode comb peaks for said QCL gain chip at said first power level over a desired wavelength range;
e) determining a maximizing power level for said first wavelength by determining a mode comb power level of the QCL gain chip that causes a mode comb wavelength spike of said QCL gain chip to coincide with said first laser wavelength, said maximizing power level achieved by determining a first power level change to shift one of said laser output peaks a first known fractional FP mode distance to determine ΔIFSR, selecting a wavelength at which the laser will operate, determining a second fractional FP mode distance said selected wavelength is from a first output peak, and powering said QCL gain chip by a current equal to
where I0—starting/maximum current (mA), v0—starting frequency (cm−1), v—frequency (cm−1) at the current I (mA), and ΔIFSR—current change (mA) necessary to shift a Fabry-Perot comb of said source by exactly one free spectral range (ΔVFSR cm−1) in the vicinity of v0, and determining maximizing power levels for each of the other wavelengths in said wavelength range in a similar manner;
f) incrementing or decrementing a selected determined 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;
g) adjusting a position of said diffraction grating at said maximum output distance with said piezoelectric translator (PZT) 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; whereby
the coupled cavity laser system operates at a maximum for a selected wavelength within said wavelength range by selecting and ensuring 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, the steps comprising:
a) powering a source of multiwavelength laser light at a first power level, said source of multiwavelength laser light including a quantum cascade laser (QCL) gain chip;
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 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;
wherein said step of determining a maximizing power level further comprises:
(i) determining a first power level change to shift one of said laser output peaks a first known fractional Fabry-Perot (FP) mode distance to determine ΔIFSR;
(ii) selecting a wavelength at which the laser will operate;
(iii) determining a second fractional FP mode distance said selected wavelength is from a first output peak; and
(iv) powering said QCL gain chip by a current equal to
where I0—starting/maximum current (mA), v0—starting frequency (cm −1), v—frequency (cm−1) at the current I(mA), and ΔI FSR—current change (mA) necessary to shift a Fabry-Perot comb of said source by exactly one free spectral range (ΔvFSR cm−1) in the vicinity of v0.
Applicant(s) and/or Inventor(s) hereby rescind any disclaimer and/or any arguments made in any prior related application. Such disclaimer(s) and/or argument(s) as well as any prior art relevant to such disclaimer(s) and/or argument(s) may need to be revisited by the Examiner.
This patent application is related to and claims priority from 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 filed Dec. 8, 2006 for High Sensitivity Detection of TNT, which applications are incorporated herein by this reference thereto.
The technology disclosed herein was supported at least in part by NIST/ATP Grant 70NANB3H3026 and DARPA (Defense Advance Research Projects Agency) Contract HR0011-04-C-0102 (Approved for Public Release, Distribution Unlimited).
Portions of the disclosure of this patent document may contain material which is subject to copyright and/or mask work protection. The copyright and/or mask work owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright and/or mask work rights whatsoever. 37 C.F.R. §1.71(d).
1. Field of the Invention
This invention is directed 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.
2. Description of the Related Art
Mid and long wave infrared quantum cascade lasers (QCLs) cover a very important spectral region from about 3 μm to 15 μm 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.
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.
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.
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.
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.
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.
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.
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.
At least three key innovations are provided the technology disclosed herein.
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.
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.
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.
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.
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.
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 and/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.
The masculine pronoun is generally used herein to indicate the generic individual and as a matter of convention and convenience.
The present invention resides in several embodiments.
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.
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.
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.
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.
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.
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.
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
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 (
To go from the multiwavelength output shown in
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.
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.
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.
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.
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.
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
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.
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−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−1. Practicably attainable gain chip temperature range imposes a similar tunability restriction.
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.
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.
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.
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.
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.
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 are acceptable.
Gain Chip Current Control: Harnessing Saturation
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.
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
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 be kept constant as the refractive index is changed via changing the current.
Therefore, the refractive index, n, changes.
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.
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
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.
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.
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
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-μm-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° C., measured by an integrated thermistor, with the copper cooling block maintained at 20° C.
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
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.
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 (
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 μm 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.
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.
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−1). Gain chip modes are roughly 15 GHz (0.5 cm−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.
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.
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.
From the spectral positions of these maxima, the law of grating movement for single mode continuous tuning is determined by a polynomial fit:
where GP—grating angle position (mm) at a desired frequency v (cm−1), GP0—starting grating position (mm) at starting frequency v0 (cm−1), A and B—polynomial fit coefficients.
The equation for chip current change is:
where I0—starting/maximum current (mA), v0—starting frequency (cm−1), v—frequency (cm−1) at the current I (mA), and ΔIFSR—current change (mA) necessary to shift the gain chip's Fabry-Perot comb by exactly one free spectral range (ΔvFSR cm−1) in the vicinity of v0.
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, ΔIFSR=60.8 mA and ΔVFSR=0.49189 cm−1 at I0=1294.4 mA. The v0/v term corrects for the fact that the Fabry-Perot comb does not really shift but rather expands the comb from zero frequency.
The free spectral range of the gain chip, ΔvFSR, 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 ΔIFSR. 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).
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 (
As of the filing 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.
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 cavaity modes is much smaller than the characteristic width of a spectroscopic feature studied, the laser source can be treated as continuously tunable.
In our long cavity system, a room temperature QCL with the gain region centered at 6.3 μm was mounted and operated in the manner similar to that described above. Chip length was 4 mm and crystal facets were not coated.
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 μm. 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 ˜350 nm. The single mode CW output power in the center of gain curve at the maximum current was around 300 mW (
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.
We obtained an overall tuning range of 350 nm centered at 6,300 nm with maximum single frequency optical power of 300 mW (
The 6,300 nm single wavelength tunable QCL was used for high resolution spectroscopy (
Sensitive Detection of Explosives
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
Triacetone triperoxide (C9H18O6, 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° 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.
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).
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.
To convert the TATP temperature in to an equivalent vapor pressure, we use the data published by Oxley.
To estimate the ultimate TATP detection capability provided by QCL-PAS measurements, we show the L-PAS noise floor on
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.
Detection of a Conventional Explosive, TNT
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 (C7H5N3O6) is almost exactly identical with the molecular weight of nitroglycerine (C3H5N3O9) 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 2×10−4 torr at 25° 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° C. is possible.
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° C. The gas transport lines from the TNT sample chamber to the photoacoustic cell and the photoacoustic cell were maintained at 60° 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° C. but is not a limitation for future operation of the cell at higher temperatures by using appropriate high temperature microphones.
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 ˜7380 nm that matches the expected absorption feature of TNT and the second centered at ˜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° C. for 48 hours while flushing the sample with clean dry air.
The third aspect of the measured spectra (in
From the lowest temperature (24° 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° C. (˜30 pg cm−3). Comparing the L-PAS signal with the noise floor shown in the
QCL-PAS can Distinguish Between Various Explosives
The spectral features of TNT and TATP are so distinctly different that the system should be able to detect each of these three components unambiguously at very low concentrations (
All-Mechanical Control of QCL Wavelength and Cavity Length
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.
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 μm.
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 μm. 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.
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.
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.
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.
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 (
The grating feedback into the gain element is different from zero only for a narrow wavelength region centered around
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 λm(θ). But since this effect is not present in our method, we omitted the corresponding term in equation (6).
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 (
Inserting in equation (2) and requiring that λm(θ)=λG(θ), one gets AB=md/k and L0=ABsinθ0. These relations are equivalent. One can be obtained from the other using the initial condition λm(θ)=λG(θ0). Adding ΔL on both sides of the second relation, one gets
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.
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 I of the cavity (see
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.
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 ˜1% of the center wavelength. An improved version included the possibility of rotating the translation axis a (see
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.
In order to take into account linear dispersion in our cavity, we introduce a dependence of L0 on λ:
with b=dL0/dλ=Σi(dni/dλ)li. The substitution of the expression (6) in (2) leads to:
Requiring λm(θ)=λG(θ) as before, we get AB=(m 2b)d/k and L0 bλ0=ABsinθ0. Inserting the definitions of L0 and b, the quantity Lg=L0(λ0) bλ0 can be written as
One can recognize the expression of the group refractive index ng=n(λ) λdn/dλ. The result of preceding paragraph obtained in (9) has thus to be modified in presence of dispersion as follows:
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.
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 ΔD, i.e., that the cavity length is equal to L(θ)=ΔD+ABsinθ. In this case, the number of half wavelengths (μ) selected in the cavity by the grating,
is no longer constant. The lasing mode is the Fabry-Perot mode λm whose wavelength is the closest to λG, that is the one for which m=integer(μ). The mode-hop free tuning range is given by the condition that the variation of μ has to be smaller than one (1). Introducing the expressions for L(θ) and λG(θ) in equation (10), one gets:
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 λG at its extremities where the above requirement no longer holds.
This equation gives the mode-hop free tuning range (in wavenumbers) as a function of the departure ΔD:
This result is represented graphically in
Written in an alternative form, equation (16) gives the relative tuning range as a function of the center wavelength λ:
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.
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.
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
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.
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.
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.
The procedure for the "smart grid" algorithm includes the following steps:
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 μm region. 1000 scans were performed using the full available range (7.0-7.5 μm), and then another 1000 scans were performed using a smart grid of frequency points, which we identified (see
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
A similar simulation was carried out for the 8.0 to 8.5 μm region where water has a much lower absorption (
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.
The use of the Smart Grid algorithm is suitable for all other sensors based on spectral data and where interferences are present.
Performance Optimization
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.
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.
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.
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.
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.
To achieve optimized performance, the following procedure may be used with the following inputs and steps.
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.
We identified 3 regions for analysis. The first range we called "L-PAS Range 1" 120 (
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
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
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.
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 are set forth for the purposes of example only and not those of limitation.
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.
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Brigitte's House Padang, West Sumatera, Indonesia Experience a unique place to stay in Padang. A great place to relax and enjoy an extraordinary atmosphere. Our friendly and helpful staff are more than happy to help you make the most out of
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Cockroach Control Methods in California
One of the most common pests that humans deal with are cockroaches. These insects are so common because they are very difficult to get rid of.
While there are a lot of treatments that can kill off cockroaches, the most effective way to prevent and eliminate a cockroach infestation is thorough sanitation. Roaches are drawn into a home when searching for food. Proper sanitation will eliminate food sources, making it more difficult for roaches to find food and encouraging them to establish residence somewhere else.
Another approach to cockroach control is to find ways to prevent the cockroach from getting into the home in the first place. A proper home inspection will identify areas where the roaches can sneak into the home. However, roaches are often able to enter a home through bags and food containers, there is no way to completely prevent a cockroach infestation.
Even those who have excellent sanitation, the homeowner sometimes needs to take more aggressive measures. Two of the most common and non-toxic methods of eliminating cockroaches is the use of insecticidal soaps and boric acid. Insecticidal soaps cause the contents of the insect cells to leak from the insect. This leads to the cockroach dying rather quickly. Boric acid is another approach acts as an abrasive to the cockroach exoskeleton, causing the insect to dry out. When ingested, the boric acid acts as a stomach poison. Since the roaches will carry the bait back to the nest, applications of boric acid can wipe out the entire infestation.
Other methods of cockroach control include insecticidal sprays and baits. Some baits are designed to trap the roaches so that they starve. These baits are helpful because they help the homeowner know the extent of the roach infestation. However, even when every method of roach control is implemented, the roaches can still survive in the home for a long time.
Professional cockroach control in California will not only do a better job locating the primary source of the roach infestation, but are also able to find extermination methods that are safe for the family and environmentally friendly. Many chemicals designed to kill cockroaches can also be toxic to humans in large quantities. For this reason, professional extermination services are the best option.


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Established by the State Board of Education in 1996, the Vermont Common Assessment System (CAS) evaluates student performance in the state's schools, based on Vermont's Framework of Standards and Learning Opportunities, with the goal of improving teaching and learning. Statewide assessments include:
Use the following links to view:
Assessment Results
Note: 2013 NECAP Results
For the 2013-2014 school year 27 schools participated in the SBAC field test in lieu of administering the fall NECAP assessments. Participation in the SBAC field test was voluntary. While the state is required to report statewide results, there is evidence to suggest that the exclusion of these 27 schools makes statewide results unrepresentative of all students in the state. Therefore, 2013 NECAP reading, writing and math assessments do not accurately reflect state-level achievement and should be excluded in any state-level comparisons over time.
It is appropriate to make comparisons at the school level for those schools that did administer the fall 2013 NECAP assessments. It is possible for schools that did participate in the SBAC field to have limited results for the fall 2013 NECAP assessments. This occurs because results are often reported by the school where the learning occurred (teaching school) and not the school where the test was administered (testing school).
2015 National Assessment of Educational Progress (NAEP) Assessment WindowThe National Assessment of Educational Progress (NAEP) will be administered to samples of fourth-or-eighth-grade students from January 26 through March 6. Students will be assessed in mathematics, reading, or science. Students will spend about 90 minutes completing the assessment, including answering questions in one subject area and answering questions about themselves and their educational experiences. Each school principal and the teachers of the subjects being assessed will also complete a questionnaire. As principals, you are essential partners in NAEP. You make an important contribution to NAEP by working closely with your students, teachers and staff and explaining why the assessment is important.
Assessment of Educational Progress


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Faithful servant
When you go to the polls at election time, you are often focused on very specific issues that face your immediate community. However, more often than not, there are so many other factors outside the powers of your mayor and City Council that affect your city more than you know. That is why I am urging you to re-elect Carl Morehouse to the Ventura City Council.
I have had the pleasure of serving with Carl at both the League of California Cities and at the Southern California Association of Governments (SCAG), and I can assure you that Carl has been extremely engaged in monitoring and participating in statewide and regional actions that will affect Ventura in the long run. He takes the time to listen, digest and analyze proposals that will have effects on local government and represents your interests well in those settings.
It takes a long time to learn about these organizations, what they do, how they operate and to build relationships with others in these organizations so that your ideas and votes can be effective.
While your City Council seats are nonpartisan, I can tell you that Carl and I do have different party affiliations, but he is living proof that relationships are important and that we can "work across the aisles" to achieve mutually beneficial outcomes for our cities.
I heartily endorse and strongly urge you to vote for Carl Morehouse for Ventura City Council. He serves your city well.
- Ron Garcia,
Councilmember, city of Brea
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Title: Green Tree Python Jigsaw Puzzle
About: Solve the puzzle and see the green tree python coiled up on a branch waiting for it's next meal to come along. This snake is a non-venomous python species found in New Guinea, various islands in Indonesia, and the Cape York Peninsula in Australia. It's completely arboreal and has a striking green color.
Puzzle of the day on: 12-May-2011
Image Credit: Frank Wouters
All our puzzles can be played in full-screen mode on your desktop, smart phone or tablet( iPad/iPhone(iOS) or Android powered devices). DailyJigsawPuzzles.net is your free iPad (or Android tablet) jigsaw puzzle app. Just visit using your mobile device and play our HTML5 powered puzzles.


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Ramp closures set for Loop 306 work
SAN ANGELO, Texas - The $2.9 million project to resurface and re-stripe a two-mile portion of Loop 306 will continue to disrupt traffic for some time, a Texas Department of Transportation spokeswoman said.
This "fairly high traffic area" will see construction through the first week of June, said Karen Threlkeld, spokeswoman for TxDOT.
The overall resurfacing plan includes all frontage roads and ramps between Sherwood Way and Knickerbocker Road.
Hot-mix Superintendent Israel Castillo from Price Construction Ltd. said motorists should expect ramp closures in the coming week.
"This week everything is open," Castillo said. "But next week there are going to be some ramps closures. Monday there's going to be the on-ramp from College Hills to the loop that will be closed. They're going to place the delineators on that guardrail."
The amount paid to date on the Loop 306 project is $606,997, which includes project engineering at $24,073, construction at $549,530 and construction engineering at $33,394, according to the TxDOT website.
Price Construction Ltd. has 25 workers, under general contract, out at the construction site and about 10 with subcontractors.
Remaining work on the two-mile project includes striping of the frontage roads and main lanes, and the installation of traffic signal loops at the Knickerbocker exit, Threlkeld said.
"They're really close to being done," Castillo said. "They're just working on striping. Everything is on schedule right now."
The transportation department advised motorists who use the affected stretches of road to observe the lowered speed limit on the Loop — 60 mph instead of the usual 65 mph. Drivers should also use caution as construction equipment will be entering and exiting the roadway, according to a news release.
Upcoming construction projects on Loop 306 include resurfacing more of the roadway and constructing new roadway lanes. The bid dates on these projects are in July and September, with an estimated cost of $2.2 million and $12.8 million, respectively.
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The Lowcountry division of the March of Dimes is trying to raise even more money during its annual March for Babies fundraising campaign.
In 2012, the drive raised about $410,000. This year, Todd Gallati, CEO of Trident Health and a chairman for March for Babies, called on teams in the greater Charleston area to raise $450,000.
This year marks the 75th anniversary of the March of Dimes organization.
"We're trying to raise a little bit more than last year because we have more need," Gallati told other organizers at a Tuesday kickoff event at Trident Medical Center in North Charleston. "We want to make more of an impact."
The proceeds from the March for Babies are used to help fund research to prevent birth defects and assist families with premature babies.
The campaign will culminate with a 3-mile walk starting at 9 a.m. April 27 in Cannon Park.
Meredith Repik, executive director of the March of Dimes' Charleston office, encouraged team organizers to set strategic goals.
"It is very hard to achieve a goal when you don't know what that goal is," Repik said.
During the 2012 March for Babies fundraising campaign, Publix grocery stores contributed more money than any other local team, raising more than $97,000 during a three-week period in March. Other top local contributors included the Medical University of South Carolina, Bosch, Kmart and Roper St. Francis Healthcare.
During the campaign kickoff, several team organizers pledged to raise more money than last year.
Tim Thomas, district manager for Publix, said their team plans to raise $110,000.
"We're just very humbled to be here and to be able to serve the March of Dimes," Thomas said.
Harbor National Bank pledged to raise $5,000 — double its 2012 goal. Gallati said Trident Health could raise at least $18,000 to rank among the top five local contributors.
For more information about the campaign, visit marchforbabies.org.
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C. Michael Ming currently serves as the Oklahoma Secretary of Energy, appointed by Governor Mary Fallin in January, 2011. He formerly served as the President of the Research Partnership to Secure Energy for America, RPSEA, and as an independent natural gas producer as a Managing Member and principal of K. Stewart Energy Group and K. Stewart Petroleum Corp. He holds a BS degree with distinction in Petroleum Engineering and an MS degree in Engineering Management, both from Stanford University, and is a Registered Professional Engineer in Oklahoma. Mr. Ming is an emeritus member and past Chairman of the Petroleum Investments Committee at Stanford University. He is actively involved in the University's energy programs and co-sponsored the MAP/Ming Visiting Professorship on Energy and the Environment. He currently serves on advisory boards for the Stanford School of Earth Sciences, the Bureau of Economic Geology at the University of Texas, and MAP. He formerly served on the MIT Future of Natural Gas Study, the Oklahoma Clean Energy Independence Commission, as an adjunct professor in Energy Management at the University of Oklahoma, and as an oil & gas strategic planning advisor to the Department of Energy.


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With some help and a victory of their own, the London Knights can punch their ticket to the 2013 Ontario Hockey League post-season as early as Thursday night.
The Knights, who lead the OHL with 81 points through 51 games and have had playoff tickets on sale since Jan. 19, welcome the Sarnia Sting to town on Thursday. The Windsor Spitfires, ninth in the Western Conference with 49 points through 51 games, also play that night.
If Windsor loses and London wins, the Spitfires mathematically cannot catch the Knights in the standings — even in the extremely unlikely scenario where Windsor wins out and London loses out.
This rosy reality won't push the Knights into cruise control, says assistant coach Dylan Hunter. He knows better.
London's 2005 team, which Hunter was an integral part of, was 38-2-2-0 going into a game on Jan. 19. One point would have sealed the deal on a playoff berth.
But, they dropped the contest in Belleville, 2-1, and followed it by getting drubbed the next night in Peterborough, 6-2.
"I remember that, because they weren't happy at all," said Hunter, shaking his head at how worked up the coaches were afterwards. "It humbles you."
The star-studded lineup recovered fairly quickly, and eventually hoisted the Memorial Cup. Hunter would prefer to skip the drama this time around, though.
"As a coaching staff, we're lucky (to have gone through a successful OHL title run last year). Guys who were 16, 17 last year are now 17, 18," he said.
The 2011-12 squad clinched a playoff spot on Feb. 12, locked up the division title on March 8, then claimed first overall on the last day of the regular season, March 17.
Justin Sefton, one of four new faces on London's backend this season, says the post-season is "in the back of our heads, but everyone shows up everyday and goes to work."
Sefton was in the doghouse last Friday, sitting on the bench for a large chunk of the game. With a lesson learned, the San Jose Sharks prospect played normal minutes the next night.
"It kind of called me out a little bit," Sefton said, "(made me) bring my game up."
2014 Memorial Cup bid update
The London Knights were putting the finishing touches on its 2014 Memorial Cup bid on Tuesday.
Other teams supposedly interested in throwing their hat into the ring in hopes of hosting the event are the Windsor Spitfires, Guelph Storm, and Barrie Colts. The submission deadline is next Monday.
Knights governor Trevor Whiffen says $10 million to $14 million in spinoffs was generated when London hosted the national championship nine years ago.
Although a nine-year gap between events is relatively small and will draw criticism, the cities of Montreal and Portland, Ore., have managed to host a pair of Cups each over three-year periods in the 1970s and 1980s, respectively.


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ABCbodybuilding - View Single Post - The Journey of a pHD bodybuilder - Train, Eat, and Think like a Pro!
The Journey of a pHD bodybuilder - Train, Eat, and Think like a Pro!
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is training, researching, or talking on ABC
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Join Date: Apr 2001
Location: The Pain Zone
Posts: 19,500
I think that is a really good idea.
I think HIIT is great, but the main issue is overtraining with HIIT. It is basically like doing weights. One method I use to avoid the problems with HIIT in terms of overtraining are to seperate them from my leg workouts by 48 hours. That way I will be recovered for legs. I.E. doing hill sprints the day before a leg workout is not too smart!
I was actually bout to post this, but I am starting to cut tomorrow and pretty psyched. My cardio I am thinking will be:
1. NEAT - this will be most of my low intensity. I pretty easily get 10-15 thousand steps a day just by taking the stairs, or walking to class. I do not believe this type of cardio causes any concurrent training effect. It is just a good way to metabolize some exra cals.
2. I will do weights am and cardio pm to minimize interference with metabolic adaptations from weights.
3. My plan is to do 10-20 minutes of sprints or occluded cardio followed by 30 mins low intensity cardio at night. Except day before legs I might just do some low intensity cardio. Jake and I propsed a while ago that doing HIIT followed by low I cardio is optimal because you peak the catecholamines, which causes a lot of lypolysis, but less fat oxidation at higher intensities. But when you lower the intensity, the rise in lipids in the blood stream are quickly oxidized. It's explained in this article,
and studies have since supported our hypothesis.
So that's my plan for cardio.
As far as the apparatus, when jake and I were reseaching this it seamed that doing the bike caused the least concurrent training effects. While jogging caused a lot. We think because jogging may cause biomechanical issues due to less ROM during the movement; particularly during hip flexion. Also in my experience it causes a lot of muscle damage and if I do it a lot, especially on hard surfaces, I am at a much higher risk for injuries. So I will mostly be doing the bike and elliptical.
Gabriel "Venom" Wilson, Ph.D. Nutritional Sciences
B.S. (Hons) & M.S. in Kinesiology, CSCS
Vice President, ABCbodybuilding
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Matthew 7:20
And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove;
and nothing shall be impossible unto you.
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