Diode-laser-based mid-infrared absorption sensor for carbon monoxide

Abstract

An all-solid-state continuous-wave (cw) laser system for measurements of the absorption by the CO molecule of mid-infrared radiation in the 4.3-4.6 μm range is described, wherein a single-mode, tunable output of a 70 mW, 860-nm ECDL is difference frequency mixed with the output of a 550 mW diode-pumped cw Nd:YAG laser in a PPLN crystal to produce approximately 1 μW of tunable cw radiation at 4.5 μm.

1. A continuous-wave laser system for measurements of the absorption by the CO molecule of mid-infrared radiation in the 4.3-4.6 μm range, comprising:
 (a) a first laser for providing a first output beam at nominally 860 nm;  
 (b) a second laser for providing a second output beam at nominally 1064 nm;  
 (c) means for overlapping said first and second beams and difference frequency mixing said first and second beams to produce a tunable third beam in the range of 4.3 to 4.6 μm;  
 (d) means defining a test region for containing a sample for measurement;  
 (e) means for collimating said third beam and means for splitting said third beam into a signal beam and a reference beam and passing said signal beam through said test region; and  
 (f) detector means for comparing said signal beam and said reference beam.  
 
   
   
     2. The system of  claim 1  wherein said first laser is a tunable, single mode external cavity diode laser. 
   
   
     3. The system of  claim 1  wherein said second laser is a diode pumped Nd:YAG laser. 
   
   
     4. The system of  claim 1  wherein said means for overlapping said first and second beams and difference frequency mixing said first and second beams includes a dichroic mirror and focusing lens. 
   
   
     5. The system of  claim 1  wherein said means for difference frequency mixing said first and second beams comprises a periodically poled lithium niobate crystal. 
   
   
     6. The system of  claim 1  wherein said means for collimating said third beam and splitting said third beam into a signal beam and a reference beam includes a CaF 2  lens and a beamsplitter. 
   
   
     7. The system of  claim 1  further comprising first and second half wave plates disposed respectively in the paths of said first and second beams for adjusting the polarization of said first and second beams. 
   
   
     8. A method for measuring the absorption by the CO molecule of mid-infrared radiation in the 4.3-4.6 μm range, comprising the steps of:
 (a) providing a first laser beam at nominally 860 nm;  
 (b) providing a second laser beam at nominally 1064 nm;  
 (c) overlapping said first and second beams and difference frequency mixing said first and second beams to produce a tunable third beam in the range of 4.3 to 4.6 μm;  
 (d) providing a test region for containing a sample for measurement;  
 (e) collimating said third beam and splitting said third beam into a signal beam and a reference beam and passing said signal beam through said test region; and  
 (f) comparing said signal beam and said reference beam.  
 
   
   
     9. The method of  claim 8  wherein said first laser is a tunable, single mode external cavity diode laser. 
   
   
     10. The method of  claim 8  wherein said second laser is a diode pumped ND:YAG laser. 
   
   
     11. The method of  claim 8  wherein the step of overlapping said first and second beams and difference frequency mixing said first and second beams is performed using a dichroic mirror and focusing lens. 
   
   
     12. The method of  claim 8  wherein the step of difference frequency mixing said first and second beams is performed using a periodically poled lithium niobate crystal. 
   
   
     13. The method of  claim 8  wherein the step of collimating said third beam and splitting said third beam into a signal beam and a reference beam is performed using a CaF 2  lens and a beamsplitter. 
   
   
     14. The method of  claim 8  further comprising the step of selectively matching the polarization of said first and second beams.