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 laser illumination system for providing laser light over a multiwavelength spectrum, comprising:
 a two-segment QCL chip laser light source emitting light, said chip having an approximately flat saturation region and being powered by a current;   said source powered by a current selected according to the equation   
       
         
           
             
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       where I 0 — starting/maximum current (mA), v 0 —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 (Δv FSR  cm −1 ) in the vicinity of v 0 ;
 a diffraction grating 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; 
 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 such that single mode, continuous, mode-hop free tuning is provided by the laser illumination system; 
 said angle between said reflector and said source is determined according to the equation GP=GP 0 +A(v−v 0 )+B (v−v 0 ) 2 , where GP—grating angle position (mm) at a desired frequency v (cm −1 ), GP 0 —starting grating position (mm) at starting frequency v 0  (cm 1 ), and A and B—polynomial fit coefficients; and 
 said current, said distance, and said angle all coordinated by a process including the steps of:
 a. setting said current to a maximum; 
 b. setting said rotation stage to an initial position to provide an initial angle for said angle; 
 c. ramping said translator through a distance of approximately a few wavelengths of said light; 
 d. measuring resulting laser light output from the illumination system; 
 e. determining a power maximum of said resulting laser light and determining a translator position and a rotation stage position associated with said power maximum; 
 f. setting said rotation stage to a second position to provide a second angle for said angle; and 
 g. repeating steps c-f across an available spectrum for said source such that a curve with distinct, periodic power maxima and minima with changing wavelength is acquired with each maximum position of the curve generally corresponding to a best match between said 
 
 reflector angle, said distance, and said current; whereby 
 continuous single mode, mode hop free tuning of the laser illumination system may be obtained. 
 
     
     
         2 . A method for leveling power for a tunable EGC-QCL system, the steps comprising:
 determining a current saturation peak for a first QCL gain chip system;   determining a first current periodicity value that shifts a Fabry-Perot comb mode of said first QCL gain chip system one free spectral range;   setting a first injection current to a current value near said saturation peak plus or minus approximately 1 to 2 first current periodicity values; and shifting said first injection current by said first current periodicity value to maintain said first injection current within approximately 1 to 2 first current periodicity values of said current saturation peak; whereby   such that said first injection current can be adjusted to selectably shift said Fabry-Perot comb mode with diminished effect upon laser power output of said first QCL gain chip system;   
     
     
         3 . A method for leveling power for a tunable EGC-QCL system, the steps comprising:
 determining a current saturation peak for a QCL gain chip system having first and second segments optically linked to one another;   determining a first current periodicity value that shifts a Fabry-Perot comb mode of said QCL gain chip system one free spectral range;   operating said first segment with said first injection current on one side of said current saturation peak; and   operating said second segment with a second injection current on a second side of said current saturation peak; and   maintaining a current difference between said first and second injection currents, said current difference selected from the group consisting of:
 a) a constant current difference of said first current periodicity value between said first and second injection currents such that a total power output is maintained relatively constant while enabling changes in said first and second injection currents to selectably shifting a refractive index of said QCL gain chip system; and 
 b) a varying current difference between said first and second injection currents such that selected power output for several wavelengths of the tunable EGC-QCL system are leveled with wavelengths having stronger outputs being diminished while wavelengths having weaker outputs being augmented; and 
 c) combinations thereof; whereby 
   said first and second injection currents may be selectably adjusted to shift a Fabry-Perot comb mode of said QCL gain chip system.   
     
     
         4 . A laser illumination system for providing laser light over a multiwavelength spectrum, comprising:
 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.   
     
     
         5 . A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in  claim 4 , wherein said source further comprises:
 a QCL with an approximately flat saturation region.   
     
     
         6 . A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in  claim 5 , wherein said QCL further comprises:
 a two-segment QCL chip.   
     
     
         7 . A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in  claim 4 , further comprising:
 said source powered by a current selected according to the equation   
       
         
           
             
               
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               ; 
             
           
         
         
           where I 0 —starting/maximum current (mA), v 0 —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 (Δv FSR  cm −1 ) in the vicinity of v 0 . 
         
       
     
     
         8 . A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in  claim 4 , wherein said reflector further comprises:
 a diffraction grating.   
     
     
         9 . A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in  claim 8 , wherein said grating further comprises:
 a diffraction grating having approximately 240 grooves/mm.   
     
     
         10 . A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in  claim 8 , wherein said angle between said reflector and said source is determined according to a relationship directly linking the two. 
     
     
         11 . A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in  claim 10 , wherein said relationship is determined according to:
     GP=GP   0   +A ( v−v   0 )+ B ( v−v   0 ) 2 ;   where GP—grating angle position (mm) at a desired frequency v (cm −1 ), GP 0 —starting grating position (mm) at starting frequency v 0  (cm −1 ), and A and B—polynomial fit coefficients.   
     
     
         12 . A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in  claim 4 , further comprising:
 said source powered by a current; and   said current, said distance, and said angle all coordinated by a process including the steps of:   a. setting said current to a maximum;   b. setting said rotation stage to an initial position to provide an initial angle for said angle;   c. ramping said translator through a distance of approximately a few wavelengths of said light;   d. measuring resulting laser light output from the illumination system;   e. determining a power maximum of said resulting laser light and determining a translator position and a rotation stage position associated with said power maximum;   f. setting said rotation stage to a second position to provide a second angle for said angle; and   g. repeating steps c-f across an available spectrum for said source;   whereby   a curve with distinct, periodic power maxima and minima with changing wavelength is acquired with each maximum position of the curve generally corresponding to a best match between said reflector angle, said distance, and said current.   
     
     
         13 . A method for leveling power for a tunable EGC-QCL system, the steps comprising:
 determining a current saturation peak for a first QCL gain chip system;   determining a first current periodicity value that shifts a Fabry-Perot comb mode of said first QCL gain chip system one free spectral range; and   setting a first injection current to a current value near said saturation peak plus or minus approximately 1 to 2 first current periodicity values; whereby said first injection current can be adjusted to selectably shift said Fabry-Perot comb mode with diminished effect upon laser power output of said first QCL gain chip system.   
     
     
         14 . A method for leveling power for a tunable EGC-QCL system as set forth in  claim 13 , the steps further comprising:
 shifting said first injection current by said first current periodicity value to maintain said first injection current within a selected range of said current saturation peak.   
     
     
         15 . A method for leveling power for a tunable EGC-QCL system as set forth in  claim 14 , further comprising:
 said selected range being approximately 1 to 2 first current periodicity values.   
     
     
         16 . A method for leveling power for a tunable EGC-QCL system as set forth in  claim 14 , the steps further comprising:
 providing a second QCL gain chip system optically coupled to said first QCL gain chip system;   operating said first QCL gain chip system with said first injection current on one side of said current saturation peak;   operating said second QCL gain chip system with a second injection current on a second side of said current saturation peak; whereby   said first and second injection currents may be selectably adjusted to shift a Fabry-Perot comb mode of said first and second QCL gain chip systems.   
     
     
         17 . A method for leveling power for a tunable EGC-QCL system as set forth in  claim 16 , the steps further comprising:
 maintaining a constant current difference of said first current periodicity value between said first and second injection currents; whereby   a total power output is maintained relatively constant while enabling changes in said first and second injection current to selectably shifting a refractive index of said first and second QCL gain chip systems.   
     
     
         18 . A method for leveling power for a tunable EGC-QCL system as set forth in  claim 16 , the steps further comprising:
 varying a current difference between said first and second injection currents; whereby   selected power output for several wavelengths of the tunable EGC-QCL system are leveled with wavelengths having stronger outputs being diminished while wavelengths having weaker outputs being augmented.