Superluminescent Diode-Based Multiple-Gas Sensor for NH $_{\bf 3}$ and H $_{\bf 2}$ O Vapor Monito

An approach of superluminescent light emitting diode (SLED)-based multiple-gas sensing (NH₃ and H₂O vapor) is proposed and demonstrated by using an absorption spectroscopy technique for emission monitoring (DeNOx process) applications. Such process normally occurs at temperature range of 250-400°C where significant interference effect due to H₂O vapor is expected. Hence, measuring multiple gases and eliminating cross-interference effect is of great importance in sensing trace gases. In this study, an SLED-based sensor with center emission wavelength of 1530-nm region is chosen to measure the concentrations of pure NH₃ and H₂O vapor by probing its overtone and combination bands present in this region. Detection limit for the NH₃ gas accounted in the case of aqua-ammonia (undiluted) is estimated to be 120 ppm ·m. A novel approach of introducing an open-space etalon in the path of the SLED beam is also proposed and demonstrated for adequate isolation from interfering species and improving the detection limit. By using an etalon with a free spectral range of 104 GHz and tuning it to appropriate wavelength, detection limit of about 22 ppm·m is attained for NH₃ gas sensing in the presence of H₂O vapor.