Mid-IR Spectroscopy


Traditional security measures at airports have included the use of metal detectors to identify weapons that may be concealed in conjunction with X-ray machines for viewing the contents of bag- gage. One major problem is that explosive substances are often not easily detectable using conventional approaches and in this context many terrorist groups have adapted to avoid the use of metallic objects. Approaches to detect volatile substances, such as ion mobility spectrometry in conjunction with swabbing, whilst in routine use—are largely only suitable for the screening of items of hand baggage.

For the detection of traces of explosive substances in the air, issues related to the low vapour pressures of many explosives are only exacerbated further when these explosives are wrapped or packaged to avoid detection. The increasing use of peroxide-based explosives has led to much research into detection of this group of explosive substances, the issue being that many current chemical identification techniques are based on the nitrogen and carbon content of a substance for identification and this practice is not suitable for peroxide explosives.

Mid-IR spectroscopy is a linear spectroscopy method used for detection in the defense & security industry. It has been seen that this method has high efficiency for standoff threat detection. Mid-IR spectroscopy is similar to nonlinear spectroscopy since it also targets molecular vibrations.The vast majority of gaseous chemical substances exhibit fundamental vibrational absorption bands in the mid-infrared spectral region (≈ 2–25 μm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. Mid-IR spectroscopy is generally best suited for materials identification; process monitoring through increase or decrease of a functional absorption band or determination of a specific component in a mixture.

Genia Photonics’ synchronized laser system provides a solution to generate mid-IR frequencies. The outputs of both internal lasers are combined into a single output beam and the common electronics ensure that the pulse stream of each laser remains synchronized throughout the entire wavelength sweep cycle. The synchronized beam is then sent through a nonlinear crystal (DFG crystal) to generate the difference frequency output (DFG), hence achieving the Mid-IR frequencies corresponding to the targeted wavenumbers. One of the main advantage of the Mid-IR source is the fast tunability feature of the programmable laser. This feature allows the user to rapidly tune within a range of wavelengths in either sequential or arbitrary manner. Another key advantage to Genia Photonics’ laser system for standoff explosive detection is that the power level remains constant throughout the sweep cycle.

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