Abstract: An optical gas analyzer includes optical, mechanical and electronic systems. The optical system includes a collimator, a light filter, a modulated interference polarization filter (IPF), and a photo detector. The modulated IPF has two polarizers, crossed or parallel to each other, with an optical modulator and a birefringent plate mounted between them. The birefringent plate has a variable thickness which is mechanically driven and controlled by the electronic system via the electro-mechanical driver. The photo detector has its output connected to the electronic system and is placed after IPF. The gas analyzer has the advantage of allowing the minimum detectable limit of the concentration of measured gas to be decreased by greatly reducing the influence of parasitic modulation. Parasitic modulation is induced by the optical modulator into the optical and electronic units of the gas analyzer and affects the output signal.