Laser spectroscopic oxygen sensor using diffuse reflector based optical cell and advanced signal processing

Abstract

In this paper, a VCSEL-based oxygen sensor applied for real-time combustion optimization in furnaces is presented. A diffuse reflector instead of a spherical mirror is used to avoid alignment problems during thermal cycles the sensor is exposed to in the in-situ measurement. In spite of the signal loss, this concept proves to be suitable for measurements in the exhaust of a gas furnace even if severe condensation at the optic surfaces occurs. The optic cell has a background level in the order of 10 −5 in absorbance and is long-term stable. A real-time microcontroller-suited curve fitting algorithm is applied to further improve the long-term stable operation of the whole sensor, because a drift of the spectral background is compensated. Neither absolute wavelength scale, nor absolute wavelength difference between two measurement points is needed for concentration determination. Furthermore, the gas pressure can be simultaneously determined. Kalman filtering is also applied to reduce the concentration measurement noise without losing the reliability and fast response time of the sensor. The techniques applied in the sensor are not limited to oxygen sensing, but also can be applied for sensing of other gas molecules.

Optical gas sensing: a review

“…For some measurements, the target line's position in the scan may be determined by reference to a neighbouring line of a different gas, whose presence can be guaranteed [187] . Line fitting algorithms are available to determine the concentration from the entire lineshape rather than just its peak value [188] . Providing a line scan rather than a single peak value can improve SNRs by enabling post-detection filtering of interference fringes with FSRs that are much narrower or wider than the gas linewidth.…”

“…In reflective cells, the optical configuration is similar to that found in the standoff laser pointers of the previous section, albeit with a fixed, known pathlength. Chen et al have used a diffuse reflector manufactured from anodised aluminium in order to improve sensor robustness in a challenging industrial environment (the exhaust of a combustion furnace), with background fluctuations at the level of ΔI/I ~ 10 -5 [188] . A group at Lund University have used TDLS to explore gas concentrations held within porous and scattering media including pharmaceutical samples [261] and biological tissue [262] .…”

A novel methodology to directly pre-determine the relative wavelength response of DFB laser in wavelength modulation spectroscopy

“…environmental monitoring [1], industrial process control [2,3], isotope detection [4], combustion diagnostics [5], national defense security [6] and pathologic diagnosis [7,8]. Tunable diode laser absorption spectroscopy (TDLAS), either in terms of direct absorption spectroscopy (DAS) or wavelength modulation spectroscopy (WMS), during the last decade, has moved from lab towards practical applications [9][10][11][12].…”

A VCSEL based system for on-site monitoring of low level methane emission

“…Tuneable diode laser spectroscopy (TDLS) offers several advantages over conventional NDIR techniques such as high specificity and sensitivity, lower detection limits, auditable function over its lifetime and quick response times of a few seconds. As a consequence, this technique is gaining increasing popularity for gas monitoring in harsh industrial environments [3][4][5][6] and also for trace gas detection in the atmosphere 7,8 .…”