Optical cavity for auto-referenced gas detection.

An enhanced optical system design for NDIR gas detection is presented. Multiple paths lengths within the same cavity are used to auto reference the system. The system has good thermo-mechanical stability: it requires no special thermal stabilization, shows no sensitivity to thermal emitter drift and has no moving parts involved. Long term stability, virtually no zero-drift and sub-ppm level gas detection were achieved using commercial thermopile sensors and a thermal emitter modulated at low frequency (~0.5 Hz). Experimental tests were performed using carbon monoxide (CO) and a 30.5 cm cavity length. The design can be extended to allow multiple gas detection within the same optical cavity.