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.

1 . A method for more quickly determining the presence of a target gas, the steps comprising:
 1) Identifying and selecting regions in a frequency range of a selectable wavelength light source which meets all the following criteria:
 a) the target gas has large absorption in at least some frequencies in said frequency range; 
 b) expected interferents have low absorption at their expected concentrations; and 
 c) a detectable signature of target gas is linearly independent of signature of interferents; 
   2) collecting a sample of gas for testing of the target gas;   3) Perform a scan across said identified and selected regions and collecting photoacoustic data from said scan; and   4) Linearly deconvolving said photoacoustic data against a standardized library of the target gas and list of expected interferents, to obtain a gas concentration measurement for the target gas.   
     
     
         2 . A method for more quickly determining the presence of a target gas as set forth in  claim 1 , wherein said selectable wavelength light source further comprises:
 a single mode, continuous, mode-hop free source of laser light.   
     
     
         3 . A method for more quickly determining the presence of a target gas as set forth in  claim 1 , wherein said source of laser light further comprises:
 a multiwavelength laser light source emitting light;   a wavelength-selective reflector in optical communication with said source;   a translator coupled to said reflector, said translator displacing said reflector according to a first signal and controlling a distance between said reflector and said source; and   a rotation stage coupled to said translator, said rotation stage rotating said reflector according to a second signal and controlling an angle between said reflector and said source; whereby   single mode, continuous, mode-hop free tuning is provided by the laser illumination system.   
     
     
         4 . A method for more quickly determining the presence of a target gas as set forth in  claim 1 , further comprising:
 said step 1a) further includes the target gas providing a good signal to noise ration for reliable determination of the target gas; and   5) Recording a measurement of said scan and determining a time taken for said measurement to determine a throughput;   6) If concentration the target gas is below an alarm threshold continue to Step 2 without making any changes;   7) If all the following conditions are met:
 a) target gas concentration is above said alarm threshold; 
 b) said throughput rate is above a minimum throughput rate; and 
 c) a selected region of said selected regions is not already at a maximum tunable range of said light source; then 
   d) select a new broader range which meets all the criteria mentioned in Step 1   8) Continue to Step 2.   
     
     
         5 . A method for more quickly determining the presence of a target gas as set forth in  claim 4 , wherein the step of selecting a broader range which meets all the criteria mentioned in Step 1 further comprises:
 selecting additional data points including prior data points to reduce false alarm rates (FAR) by using additional time to collect said additional data points.   
     
     
         6 . A target gas detection system, comprising:
 a continuously-tunable laser operating at room temperature and tuned for exciting a sample of target molecules among other interferents; and   an L-PAS cell receiving light from said laser and containing gas to be sampled and tested for said target molecules; whereby   detection of said target molecules can be made despite presence of interferent signals.   
     
     
         7 . A gas detection system as set forth in  claim 6 , wherein said laser further comprises:
 a high-power continuously-tunable laser.   
     
     
         8 . A gas detection system as set forth in  claim 6 , wherein said laser further comprises:
 a mode-hop diminished, broadly and continuously tunable continuouswave room-temperature high-power external grating cavity quantum cascade laser system.   
     
     
         9 . A gas detection system as set forth in  claim 8 , wherein said laser system further comprises:
 driving said laser system to simultaneously tune a current driving said laser and an angle at which said grating is disposed, an FP wavelength comb of said laser shifted spectrally in coordination with said grating angle, said grating angle adjusted to keep a selected FP mode at a diminished diffraction grating induced loss to promote selective lasing at said FP mode.   
     
     
         10 . A gas detection system as set forth in  claim 9 , wherein said laser system further comprises:
 driving said laser system and shifting said FP wavelength comb by varying an injection current to said laser.   
     
     
         11 . A gas detection system as set forth in  claim 10 , wherein said laser system further comprises:
 varying said laser injection current by:   determining a first current value necessary to shift said FP comb approximately exactly one free spectral range (FSR) of a gain chip of said laser;   tuning said laser in an external cavity geometry;   calculating a nominal laser current value necessary to have one of said gain chip's FP modes exactly coincide with the desired output frequency in coordination with tuning said laser in said external cavity geometry;   if needed, remapping a nominal laser drive current value onto a value in an optimum current window   recalculating said current and setting it to a value close to maximum as the laser is tuned so that said nominal current value is about to become more than one periodicity value below maximum laser current;   continuing this process periodically as said laser system is broadly tuned; whereby   a tuning range restriction is removed or avoided generally imposed by a limited dynamic range of said current.   
     
     
         12 . A gas detection system as set forth in  claim 10 , wherein said laser system further comprises:
 controlling said FP comb by selectably altering a temperature of said laser.   
     
     
         13 . A gas detection system as set forth in  claim 10  wherein said laser system further comprises:
 an external cavity laser system instead of or in conjunction with said quantum cascade laser.