Optimised Performance of Non-Dispersive Infrared Gas Sensors Using Multilayer Thin Film Bandpass Filters

Abstract

In this work performance improvements are described of a low power consumption non-dispersive infrared (NDIR) methane (CH4) gas sensor using customised optical thin film bandpass filters (BPF). BPF’s shape the spectral characteristic of the combined mid infrared III-V based light emitting diode (LED)/ photodiode (PD) light source/detector optopair, enhancing NDIR CH4 sensor performance. The BPF, deposited using a novel microwave plasma assisted pulsed DC sputter deposition process, is deposited at room temperature directly onto the temperature sensitive PD heterostructure. BPF’s comprise germanium (Ge) and niobium pentoxide (Nb2O5) alternating high and low refractive index layers respectively. Two different optical filter designs are progressed; with BPF bandwidths (BWs) of 160 nm and 300 nm. Comparison of modelled and measured NDIR sensor performance is described, highlighting maximized signal to noise ratio (SNR) and minimized cross talk performance benefits. BPF spectral stability for various environmental temperature and humidity conditions is demonstrated.  

High throughput low cost closed field magnetron sputter deposition of durable reflectors based on dielectric overcoated metal for application in non-dispersive infrared gas sensors

“…Non-dispersive Infrared (NDIR) gas sensors typically comprise an infrared light source/photodetector optopair, an optical bandpass filter to select emitted/ detected light at a wavelength tuned to the absorption of a specific gas in the optical path between source and photodetector [1]. Gas absorption wavelengths are typically in the mid-infrared (MIR) waveband (3 to 5 µm).…”

“…Current NDIR gas sensors typically utilise injection moulded reflective optics [1] to guide light through the gas cell pathlength. Typically, gold thin films (thickness ≥200 nm) are used as the reflective surfaces providing a MIR high reflectance [1,3,4]. However, high price of gold material is not compatible with volume manufacture of low cost NDIR sensors.…”