Sources and detectors used in the mid-IR

There exists another way, chosen by a few manufacturers, to incorporate a delay into one of the optical paths when obtaining an interferogram. The principle relies upon the light passing through a crystal having two refractive indices, which constitutes a polarization interferometer. 

11 Gas flow cell with a fixed I fipi and direct optical path  

The detection of photons in the infrared region was difficult to achieve for a long time and a major cause of this was the mediocre sensitivity of the first spectrophotometers. 

The pyroelectric detector contains a mono-crystal of deuterated triglycine sulfate (DTGS) or lithium tantalate (LiTaOj), sandwiched between two electrodes, one of which is semi-transparent to radiation and receives the impact of the optical beam. It generates electric charges with small temperature changes. The crystal is polarized proportionally to the radiation received and it acts as a capacitor. 

The photodiode detector contains a semiconductor, which is used either in photovoltaic mode (no voltage ramp) or in photoconductor mode (with voltage ramp) and contains a P-N junction, which, under the effect of radiation, liberates electron/hole pairs which create a PD measurable in an open circuit. 

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