Miniaturized optical gas-sensing

Our society has developed a growing concern for the safety of our environment: we want to eat healthy food, drink uncontaminated water and breathe clean air. Fortunately, there are cost-efficient optical methods  to precisely quantify the air quality.

by Hamamatsu Photonics Belgium

Transmission measurements in the near and mid infrared spectral range easily quantify the air quality. In particular, the mid-infrared (MIR) wavelength range from 3 to 10 µm has recently received a lot of attention and many miniaturized gas-sensing products are under design or already commercially available. They can sense important gases such as CO, CO2, N2O, NO2, CH4, SO3, H2S, NH3, O3, acetone and many more.

Light source and detector are key

The two key photonic elements for such a MIR gas-sensing system are a light source and a detector. Hamamatsu Photonics has a comprehensive program of high-performance semiconductor MIR detectors, both cooled and uncooled. A particularly attractive material is InAsSb because its cutoff wavelength can be extended to well above 10 µm. Hamamatsu also has a comprehensive program in MIR light sources, including LEDs as well as lasers. The Quantum Cascade Lasers (QCLs) cover the MIR wavelength range from 4 to 10 µm, offering outstanding characteristics such as extremely narrow emission linewidths. Infrared LEDs are available for several important NIR/MIR bands, reaching from 1.1 to 5 µm and they are even available as high-power versions for the 3-5 µm range. An interesting new development in MIR light sources is the Xenon flash lamp, capable of providing a very broad emission spectrum reaching beyond 6 µm.

High-end and affordable modules

The costlier QCLs are usually employed for high-performance and high-sensitivity optical gas-sensing systems, mainly used in professional applications. These high-end systems are now complemented with affordable optical gas-sensing modules for consumer applications. They are based on cost-effective broadband MIR light sources, in particular LEDs or Xenon flash lamps, and a dispersive optical element whose function is to provide the selectivity of the measurement setup. Usually optical filters, gratings or tunable interferometers are employed for this task.

The high-end systems are now complemented with affordable optical gas-sensing modules for consumer applications.

Miniaturized MIR gas-sensing solution

Based on this notion, Hamamatsu has developed a unique miniaturized Fabry-Perot interferometer solution including an integrated InGaAs PIN photodiode. This MEMS component allows for the integration of very compact, cost-effective MIR gas-sensing solutions, which will be key subsystems for the forthcoming IoT (Internet of Things) sensor revolution. Currently, the spectral tuning range is restricted to below 1.85 µm but the path to the full MIR spectral range is clearly visible.

It is obvious: MIR optical gas sensing will soon be a commodity, and it would not be surprising if such miniature gas-sensing systems would soon make their way also into our cars, our intelligent homes and even our smartphones.