Non-mechanical optical path switching and its application to dual beam spectroscopy including gas filter correlation radiometry

A gas filter correlation radiometer for determining the concentration of a gas of interest in a particular quantity from a polarized light beam which has passed through the gas of interest in the particular quantity, comprising:

• a polarization modulator for modulating the polarized light beam between two alternating orthogonal polarization components;
• a first polarization beam splitter for switching the modulated beam by transmitting one orthogonal polarization component of the polarization modulated light beam along a first optical path and reflecting the other orthogonal polarization component of the polarization modulated light beam along a second optical path;
• a vacuum cell located in one of the first and second optical paths;
• a spectral notch filter having spectral notches which are coincident with the band structure of the gas of interest, said spectral notch filter being located in the other of the first and second optical paths;
• means for intersecting the two optical paths at an intersection point;
• a second polarization beam splitter located at the intersection point of the first and second optical paths, said second polarization beam splitter transmitting one orthogonal component of the intersected components and reflecting the other orthogonal component, whereby a polarization modulated beam is formed comprising the two orthogonal components in alternate order;
• and
• an intensity detector for sensing a difference in the intensity between the two orthogonally polarized components of the formed beam as an indication of the spectral emission/absorption of the light by the gas of interest in the particular quantity, the spectral emission/absorption being indicative of the concentration of the gas of interest in the particular quantity.