There are two main classes of IR detectors: thermal and photonic detectors. Thermal detectors operate on the thermoelectric effect, where temperature is converted to an electrical voltage via a thermocouple. Essentially, there is a temperature differential within the detector that generates a voltage. Examples of thermal detectors are thermopiles, pyroelectric detectors, and bolometers. Photonic detectors on the other hand operate on the same principle as silicon photodiodes and APDs, where the photons of the IR light interact directly with the charge carriers in the detector via the photoelectric effect and generate the electrical signal that way. However, since silicon is not sensitive to MIR wavelengths, we need to use compounds such as indium arsenide antimonide (InAsSb), mercury cadmium telluride (MCT), and lead sulfide / lead selenide (PbS/PbSe). The reason we use these compounds for MIR detectors, rather than silicon, is that the band gap is smaller and is able to absorb the longer wavelengths, which allows for sensitivity to MIR wavelengths. Examples of photonic MIR detectors include InAsSb, MCT, and PbS/PbSe.
Infrared light is ideal for detecting gases for three primary reasons.
· Sensitivity: Gases of interest have much higher absorption coefficients within the MIR spectrum, which allows us to detect them with higher sensitivity in the MIR region, from 2.5 to 14 microns.
· Selectivity: The absorption wavelengths for different gases are unique, creating a “fingerprint region” for each gas. This makes it easier to discriminate between the various gases. If we take a look at the graphic below (Fig. 1), we can see that below 2.5 micron the absorption peaks are much closer to each other, whereas above 2.5 micron (in the MIR region) the absorption peaks have less overlap, which gives us the ability to use the unique peaks to identify specific gas species.
· Water: Water has a lower absorption coefficient in the MIR spectrum; therefore, we see less of an impact on our measurement from moisture in the air, allowing for a more accurate measurement. Grey regions in the graphic below indicate which wavelengths water highly absorbs.
Figure 1. Sample of gas absorption wavelengths
Because of these factors, we are able to use IR light to measure gas concentrations very accurately using non-dispersive infrared spectroscopy, Fourier transform infrared (FTIR) spectroscopy, cavity ring-down spectroscopy, and tunable diode laser spectroscopy. These methods use an MIR source paired with an MIR detector to measure the absorption of light by gas molecules. The amount of absorption correlates to the concentration of a given gas in the environment. With these methods, we can actually achieve ppm (parts per million) sensitivity of gas concentrations or even up to ppb (parts per billion) in some cases where the IR detector has a high enough D*. To achieve ppb sensitivity, we usually need to use a cooled detector to increase the D* to the necessary level.
Optical MIR detectors can also be used for temperature sensing as heat changes the signal.
Matt Seeley is an Applications Engineer with a drive to share his photonics knowledge. While he originally studied mechanical engineering, Matt has since seen the light that is photonics and has found a renewed passion for engineering with photonics at Hamamatsu. When he isn’t putting around with photons, Matt enjoys playing golf and spending as much time as possible outside under the New Jersey sun.
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