Laser optical gas sensor by photoexcitation effect on refractive index

Laser optical gas sensors are fabricated by using the crystalline silicon carbide polytype 6H-SiC, which is a wide-bandgap semiconductor, and tested at high temperatures up to 650 degrees C. The sensor operates on the principle of semiconductor optics involving both the semiconductor and optical properties of the material. It is fabricated by doping 6H-SiC with an appropriate dopant such that the dopant energy level matches the quantum of energy of the characteristic radiation emitted by the combustion gas of interest. This radiation changes the electron density in the semiconductor by photoexcitation and, thereby, alters the refractive index of the sensor. The variation in the refractive index can be determined from an interference pattern. Such patterns are obtained for the reflected power of a He-Ne laser of wavelength 632.8 nm as a function of temperature. SiC sensors have been fabricated by doping two quadrants of a 6H-SiC chip with Ga and Al of dopant energy levels E(V)+0.29 eV and E(V)+0.23 eV, respectively. These doped regions exhibit distinct changes in the refractive index of SiC in the presence of carbon dioxide (CO(2)) and nitrogen monoxide (NO) gases respectively. Therefore Ga- and Al-doped 6H-SiC can be used for sensing CO(2) and NO gases at high temperatures, respectively.