Narrow line-width lasers for gas sensing in TO-can package with thermo-electric cooler

Narrow line-width lasers for gas sensing in TO-can package with thermo-electric cooler NEL’s DFB lasers are already being used vastly in gas sensing applications. The most common package type used is the butterfly package with pigtailed fibre output. The advantage is the temperature stabilisation with the integrated thermo-electric cooler and thermistor together with the high reliability hermetic packaging of the DFB laser chip.

Now NEL have developed a TO-can package combining these features with the possibility to employ the laser in industrial free-space applications.

These narrow line-width lasers are used in tunable diode laser spectroscopy (TDLS) to identify individual gases in mixtures of gases.

Gases usually show broad spectra in the far infrared (2.5μm - 30μm) caused by rotations and vibrations of the molecules. Overtones of these spectra can be measured in the near IR (0.8μm - 2.5μm). These bands are made up of individual narrow lines, so that by using a highly monochromatic light source, isolated peaks can be probed individually and thus gases can be distinguished in a mixture of gases. Care must be taken as these lines are subject to temperature and pressure effects. The width of the absorption peak is determined by the ambient temperature and pressure, and the area of the peak by the density of the absorbing species.

By temperature tuning of the chip, the laser emission wavelength is set closely to the absorption line of the target gas. A fine sweep over the absorption line can be realised with a fast diode current ramp. As the current increases, the laser wavelength increases. It is recommended the entire absorption peak is scanned, in order to record the full spectral feature and detect the photocurrent as function of the wavelength, for example with a TO-can InGaAs photo detector with good linearity. The location of the absorption identifies the gas, and the area provides information about the concentration.

Greenhouse effect

Common applications of this technology can be found in ammonia (~1.52µm) or methane (~1.65µm) detection. The global warming potential of methane over a 20-year time frame is 72 times higher than that of carbon dioxide. There is also the increasing use of liquefied natural gas (LNG) to transport energy, and power-to-gas technologies to store energy, such as from hydrogen generation. This is leading to authorities getting more involved in trying to lower the impact of these leakages on the greenhouse effect.

TDLS gas sensing devices allow the identification and quantisation of these leakages.

See our range of gas sensing lasers and the new NLK1xxxAYF.

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