Abstract
A sensing system in the near infrared region has been developed for ammonia sensing based on the wavelength modulation spectroscopy (WMS) principle. The WMS is a rather sensitive technique for detecting atomic/molecular species, presenting the advantage that it can be used in the near-infrared region by using the optical telecommunications technology. In this technique, the laser wavelength and intensity were modulated by applying a sine wave signal through the injection current, which allowed the shift of the detection bandwidth to higher frequencies where laser intensity noise was typically lower. Two multi-pass cells based on free space light propagation with 160 cm and 16 cm of optical path length were used, allowing the redundancy operation and technology validation. This system used a diode laser with an emission wavelength at 1512.21 nm, where NH3 has a strong absorption line. The control of the NH3 gas sensing system, as well as acquisition, processing and data presentation was performed.
Highlights
Ammonia (NH3) is a colourless gas composed of nitrogen and hydrogen with a sharp, penetrating odour
The wavelength modulation spectroscopy (WMS) technique has been demonstrated in a system using a distributed feedback (DFB) laser diode with an emission wavelength at 1532 nm in conjunction with hollow optical waveguides [17]
An NH3 sensor based on the combination of resonant photo acoustic spectroscopy and direct absorption spectroscopy techniques with a DFB laser diode operating at 1532 nm, was described [18]
Summary
Ammonia (NH3) is a colourless gas composed of nitrogen and hydrogen with a sharp, penetrating odour. An instrument based on off-axis integrated cavity output spectroscopy and room-temperature near infrared diode lasers, with an emission wavelength at 1532 nm, was applied for measurement of several gas species [20]. In this case, the combination of high-finesse optical cavities with the simplicity of a direct-absorption-spectroscopy technique results in fast, sensitive, and absolute gas measurement. The current work reports the development of an NH3 gas sensing system based on the WMS principle This system uses a diode laser with an emission wavelength at 1512.21 nm, where the absorption coefficient of NH3 is approximately twice its value at 1532 nm [22]. The proposed sensing system allows the selection of two multi-pass cells based on free space light propagation with 160 cm and 16 cm of the optical path length
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