Abstract: An improved intensity-encoded fiber optic sensor incorporating novel drift correction and filtering means is disclosed. The first embodiments of the invention relate to means for removing unwanted higher-order core and cladding modes from an intensity-encoded signal in an optical fiber using mode strippers and mode filters located strategically at various points in the sensing system. The second set of improvements in the invention relate to an improved technique for long-term temporal drift cancellation in a fiber optic pressure sensor by periodically applying pressure to the sensor tip in order to ascertain the measured voltage at which the sensor diaphragm contacts other elements of the sensor. This measured voltage is subtracted from an initial calibration voltage, and the result is applied to the measured signal as a constant correction term.

Abstract: A non-dispersive infrared gas analyzer especially adapted for measuring the concentrations of HC, CO and CO.sub.2 in a vehicle exhaust includes a sample chamber for holding a sample gas, a radiation emitter for directing a beam through the chamber and a detector for indicating the amount of radiation absorbed. A plurality of filters each of which transmit radiation at an absorption band of a gas component to be detected are alternatingly positioned in the radiation path to produce a time-multiplexed signal having concentration information for all gases. The time-multiplexed detector output is corrected for gain and offset errors "on-the-fly", without requiring demultiplexing, through the use of an analog multiplier circuit. The correction factors are themselves determined without demultiplexing the detector output signal.

Abstract: A non-dispersive infrared gas analyzer especially adapted for measuring the concentrations of HC, CO and CO.sub.2 in a vehicle exhaust includes a sample chamber for holding a sample gas, a radiation emitter for directing a beam through the chamber and a detector for indicating the amount of radiation absorbed. A plurality of filters, each of which transmit radiation at an absorption band of a gas component to be detected, are alternatingly positioned in the radiation path to produce a time-multiplexed signal having concentration information for all gases. A calibration assembly is provided which includes a plurality of calibration cells selectively positionable in the radiation path in order to provide an internal calibration standard for two-point calibration of each gas channel. The cells contain one or more of the gas components to be measured at concentrations and pressures predetermined to compensate for the difference in optical path length between the calibration cells and the sample chamber.