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Fields of applications
Why the Mid-infrared
Detection techniques
Our Customers

Fields of applications

Quantum cascade lasers have been proposed in a wide range of applications where powerful and reliable mid-infrared sources are needed. Examples of applications are:

Industrial process monitoring:

Contamination in semiconductor fabrication lines, food processing, brewering, combustion diagnostics.
Life sciences and medical applications

Medical diagnostics, biological contaminants.

Law enforcement

Drug or explosive detection.

Military

Chemical/biological agents detection, counter measures, covert telecommunications.

Why the Mid-infrared

Because most chemical compounds have their fundamental vibrational modes in the mid-infrared, spanning approximately the wavelength region from 3 to 15µm, this part of the electromagnetic spectrum is very important for gas sensing and spectroscopy applications. Even more important are the two so-called atmospheric windows at approximately 3-5µm and 8-12µm. The high transparency of the atmosphere in these two windows allows remote sensing and detection. As an example, here are the relative strengths of CO2 absorption lines as a function of frequency:
Wavelength (µm) Relative absorption strength
1.432 1
1.602 3.7
2.004 243
2.779 6800
4.255 69000

Approximate relative line strengths for various bands of the CO2 gas.

Moreover, because of the long wavelength, Rayleigh scattering from dust and rain drops will be much less severe than in the visible, allowing applications such as radars, ranging, anti-collision systems, covert telecommunications and so on. As an example, Rayleigh scattering decreases by a factor 104 between wavelengths of 1µm and 10µm.

Detection techniques

Direct absorption

In a direct absorption measurement, the change in intensity of a beam is recorded as the latter crosses a sampling cell where the chemical to be detected is contained. This measurement technique has the advantage of simplicity. In a version of this technique, the light interacts with the chemical through the evanescent field of a waveguide or an optical fiber.


Some examples of use a direct absorption technique:

Frequency modulation technique (TILDAS)

In this technique, the frequency of the laser is modulated sinusoidally so as to be periodically in and out of the absorption peak of the chemical to be detected. The absorption in the cell will convert this FM modulation into an AM modulation which is then detected usually by a lock-in technique.

The advantage of the TILDAS technique is mainly its sensitivity. First of all, under good modulation condition, an a.c. signal on the detector is only present when there is absorption in the chemical cell. Secondly, this signal discriminates efficiently against slowly varying absorption backgrounds. For this reason, this technique will usually work well for narrow absorption lines, requiring also a monomode emission from the laser itself. This technique has already been successfully applied with Distributed Feedback Quantum Cascade Laser (DFB-QCL). Some examples in the literature include:

Photoacoustic detection

In the photoacoustic technique, the optical beam is periodically modulated in amplitude before illuminating the cell containing the absorbing chemical. The expansion generated by the periodic heating of the chemical creates an acoustic wave which is detected by a microphone. The two very important advantages of photoacoustic detection are
  • a signal is detected only in the presence of absorption from the molecule
  • no mid-ir detectors are needed.
For these reasons, photoacoustic detection has the potential of being both cheap and very sensitive. However, ultimate sensitivity is usually limited by the optical power of the source.

Photoacoustic detection has already been used successfully with unipolar laser, see
  • Paldus et al., Optics Letters ...
For further details see also: University of Neuchâtel (new browser window).

Customers

Our list of customers includes:
Jet Propulsion Laboratory (USA),
Vienna University of Technology (Austria),
Fraunhofer Institute (Germany),
Georgia Institute of technology (USA),
ETHZ (Switzerland),
Physical Sciences Inc. (USA),
first QCL based product,
Aerodyne (USA),
Daylight Solutions (USA),
Neoplas Control (Germany),
Scuola Normale de Pisa (Italy),
Cascade Technologies (Scotland)

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