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 obtaining power-maximized continuous tuning for a coupled cavity laser system, the steps comprising:
 a) powering a source of multiwavelength laser light at a first power level;   b) selectively reflecting said multiwavelength laser light in a cavity back to said source to select a first laser wavelength;   c) adjusting a distance of said cavity to obtain a maximum output distance for said first laser wavelength and said first power level, said maximum output distance maximizing output of the laser system at said first laser wavelength;   d) repeating steps b and c for other laser wavelengths to obtain sufficient data in to determine laser output peaks for said source at said first power level over a desired wavelength range; and   e) determining a maximizing power level for each wavelength in said wavelength range;   whereby the coupled cavity laser system operates at a maximum for a selected wavelength within said wavelength range by selecting said maximum output distance and said maximizing power level for said selected wavelength.   
     
     
         2 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 1 , where said source of multiwavelength laser light includes a quantum cascade laser (QCL) gain chip. 
     
     
         3 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 1 , further comprising selectively reflecting said multiwavelength laser light with a diffraction grating. 
     
     
         4 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 3 , further comprising adjusting said cavity distance by adjusting a position of said diffraction grating. 
     
     
         5 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 4 , further comprising adjusting said position of said diffraction grating with a piezoelectric translator (PZT). 
     
     
         6 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 1 , further comprising adjusting said distance by selectively altering said distance on the order of a few wavelengths of said first laser frequency. 
     
     
         7 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 1 , further comprising repeating steps b and c by selecting a later second laser wavelength in step b that departs minimally from said earlier first laser wavelength. 
     
     
         8 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 7 , further comprising obtaining said later second laser wavelength by making a fine angular displacement step with a diffraction grating. 
     
     
         9 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 1 , where said output peaks correspond to a Fabry-Perot mode comb for said source of multiwaveleneth laser light at said first power level. 
     
     
         10 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 9 , where said maximizing power level is determined for said first laser wavelength by determining a mode comb power level that causes a mode comb wavelength spike of said source to coincide with said first laser wavelength. 
     
     
         11 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 10 , further comprising:
 incrementing or decrementing said maximizing power level to match a second selected laser wavelength within said wavelength range; and   shifting said maximizing power level a free spectral range amount when needed to maintain said maximizing power level within preferred power limits of said source while simultaneously matching said first laser wavelength.   
     
     
         12 . A method for obtaining power-maximized continuous tuning for a coupled cavity laser system as set forth in  claim 1 , further comprising:
 selecting a wavelength at which the laser system will operate, said wavelength selected from said wavelength range;   further adjusting a distance of said cavity at said maximum output distance to ensure that said selected maximum output distance for said selected wavelength is as much a maximum output distance as possible to enable maximum output of the laser system at said selected laser wavelength.