Optopairs with temperature compensable electroluminescence for use in optical gas absorption analyzers

Abstract

Optopair for use in sensors and analyzers of gases such as methane, and a fabrication method therefor is disclosed. It comprises: a) an LED, either cascaded or not, having at least one radiation emitting area, whose spectral maximum is de-tuned from the maximum absorption spectrum line of the gas absorption spectral band; and b) a Photodetector, whose responsivity spectral maximum can be either de-tuned from, or alternatively completely correspond to the maximum absorption spectrum line of the absorption spectral band of the gas. Modeling the LED emission and Photodetector responsivity spectra and minimizing the temperature sensitivity of the optopair based on the technical requirements of the optopair signal registration circuitry, once the spectral characteristics of the LED and Photodetector materials and the temperature dependencies of said spectral characteristics are determined, provides the LED de-tuned emission and Photodetector responsivity target peaks respectively.

Accordingly, and based on the materials described above we claim: 
     
         1 . An optopair for use in optical sensors for analysis of a gas comprising at least one radiation source and at least one radiation detector, said at least one radiation source comprising an LED having at least one active area whose spectral maximum is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         2 . The optopair according to  claim 1  wherein said spectral maximum of said at least one active area of said LED is de-tuned from, and positioned on, the short wavelength side of the maximum absorption spectrum line of the absorption band of the gas being analyzed. 
     
     
         3 . The optopair according to  claim 1  wherein said LED is selected from a group consisting of LEDs having a bulk, quantum well, and super-lattice active areas. 
     
     
         4 . The optopair according to  claim 3  wherein said spectral maximum of said active area of said LED selected from said group consisting of LEDs having a bulk, quantum well, and super-lattice active areas, is de-tuned from, and positioned on, the short wavelength side of the maximum absorption spectrum line of the absorption band of the gas being analyzed. 
     
     
         5 . The optopair according to  claim 3 , wherein said LED selected from the group of LEDs having bulk, quantum well and super-lattice active areas comprises no more than one active area whose spectral maximum is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         6 . The optopair according to  claim 1 , wherein said LED is cascaded. 
     
     
         7 . The optopair according to  claim 6 , wherein said cascaded LED comprises at least two cascades, and said at least one active area is located within at least one of the two cascades and selected from a group consisting of bulk, quantum well, and super-lattice active areas. 
     
     
         8 . The optopair according to  claim 6 , wherein said cascaded LED comprises no more than two cascades, and said at least one active area is located within at least one of the two cascades and selected from a group consisting of bulk, quantum well, and super-lattice active areas. 
     
     
         9 . The optopair according to  claim 6 , wherein said cascaded LED comprises two or more cascades, and said at least one active area is located within at least one cascade and selected from a group consisting of bulk, quantum well, and super-lattice active areas. 
     
     
         10 . The optopair according to  claim 6 , wherein said cascaded LED comprises two or more cascades, each respectively having one active area selected from a group consisting of bulk, quantum well, and super-lattice active areas and having a spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         11 . The optopair according to  claim 1 , wherein said at least one radiation detector is a photodetector selected from the group consisting of photodetectors, each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors, each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         12 . The optopair according to  claim 2 , wherein said at least one radiation detector is selected from the photodetector group consisting of photodetectors each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         13 . The optopair according to  claim 3 , wherein said at least one radiation detector is a photodetector selected from the group consisting of photodetectors, each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors, each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         14 . The optopair according to  claim 4 , wherein said at least one radiation detector is selected from the photodetector group consisting of photodetectors each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         15 . The optopair according to  claim 5 , wherein said at least one radiation detector is selected from the photodetector group consisting of photodetectors, each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         16 . The optopair according to  claim 6 , wherein said at least one radiation detector is a photodetector selected from the group consisting of photodetectors, each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors, each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         17 . The optopair according to  claim 7 , wherein said at least one radiation detector is selected from the photodetector group consisting of photodetectors each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         18 . The optopair according to  claim 8 , wherein said at least one radiation detector is selected from the photodetector group consisting of photodetectors each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         19 . The optopair according to  claim 9 , wherein said at least one radiation detector is selected from the photodetector group consisting of photodetectors each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         20 . The optopair according to  claim 10 , wherein said at least one radiation detector is selected from the photodetector group consisting of photodetectors, each having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors, each having having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         21 . The optopair according to  claim 11 , wherein said selected photodetector comprises a sequence of a contact layer, a middle barrier layer and an n-type photon absorbing layer, said contact layer having a valence band, said n-type photon absorbing layer having a conduction band, said middle barrier layer having an energy bandgap significantly greater than that of the photon absorbing layer, and the top energy of said valence band of said contact layer is not more than the bottom energy of said conduction band of said n-type photon absorbing layer. 
     
     
         22 . The selected photo-detector according to  claim 21  wherein said n-type photon absorbing layer comprises Indium Arsenide. 
     
     
         23 . The optopair according to  claim 13 , wherein said selected photo-detector comprises a photodetector having a sequence of a contact layer, a middle barrier layer and an n-type photon absorbing layer, said contact layer having a valence band, said n-type photon absorbing layer having a conduction band, said middle barrier layer having an energy bandgap significantly greater than that of the photon absorbing layer, and the top energy of said valence band of said contact layer is not more than the bottom energy of said conduction band of said n-type photon absorbing layer. 
     
     
         24 . The optopair according to  claim 16 , wherein said selected photodetector having a sequence of a contact layer, a middle barrier layer and an n-type photon absorbing layer, said contact layer having a valence band, said n-type photon absorbing layer having a conduction band, said middle barrier layer having an energy bandgap significantly greater than that of the photon absorbing layer, and the top energy of said valence band of said contact layer is not more than the bottom energy of said conduction band of said n-type photon absorbing layer. 
     
     
         25 . An improved optical absorption gas analyser for determining the concentration of a target gas in a sample, in which is found a chamber for containing the sample in use; a radiation source assembly arranged to emit radiation into the chamber; a first radiation detector assembly arranged to detect radiation transmitted along a first optical path through the chamber; a second radiation detector assembly arranged to detect radiation transmitted along a second optical path through the chamber, wherein the length of the second optical path which the sample can intercept is shorter than that of the first optical path; and a processor adapted to generate a sensing signal Sx based on the detected radiation transmitted along the first optical path and a reference signal SR based on the detected radiation transmitted along the second optical path, and to determine the concentration of the target gas in the sample based on a comparison of the sensing signal with the reference signal, wherein the improvement comprises an LED acting as said radiation source assembly arranged to emit radiation into the chamber, said LED having at least one active area whose spectral maximum is de-tuned from the maximum absorption spectrum line of the absorption spectral. band of the gas being analyzed. 
     
     
         26 . The optopair according to  claim 25 , wherein the improvement further comprises photodectors as said radiation detectors, at least one of each of said photodetectors being selected from the group consisting of photodetectors, each respectively having a responsivity spectral maximum that is de-tuned from the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed, and photodetectors each respectively having a responsivity spectral maximum that coincides with the maximum absorption spectrum line of the absorption spectral band of the gas being analyzed. 
     
     
         27 . A photo-detector comprising a sequence of a contact layer, a middle barrier layer and an n-type photon absorbing layer, said contact layer having a valence band, said n-type photon absorbing layer having a conduction band, said middle barrier layer having an energy bandgap significantly greater than that of the photon absorbing layer, and the top energy of said valence band of said contact layer is not more than the bottom energy of said conduction band of said n-type photon absorbing layer. 
     
     
         28 . The photo-detector according to  claim 27  wherein said contact layer is a p-type contact layer.