A compact and calibration-free carbon monoxide sensor approach utilizing the wide current-tunability of 2.3 µm VCSELs is reported. A separate reference cell is avoided by filling the reference gas (methane) in the photodetector housing. By applying bandwidth optimized wide/narrow wavelength scan concept, inherent wavelength scale calibration and self-monitoring of the sensor are realized, with which the laser aging process is also under control. An efficient linear least-squares curve fit using an analytical signal model for the narrow scan spectrum is done, utilizing the knowledge of the absolute wavelength scale and also the estimated WMS modulation amplitude obtained from the wide scan. The scan width of the narrow spectrum is optimized aiming at the maximum signal to noise ratio on the determined CO concentration. These concepts are universal and can be utilized for optical sensing of other gases as well and the sensor was tested under diverse applications e.g. fire detection and combustion optimization.
We demonstrate an ultralow sample volume optical carbon monoxide sensor with detection sensitivity of 180 parts in 10 9 (1σ at 1 Hz). The utilization of a 2:3 μm surface-emitting laser directly coupled to a 3 m hollow capillary fiber as the gas cell is proven to be a compact, sensitive, and cost-efficient gas sensing concept. By mechanical vibration of the fiber, an absorbance resolution of 10 −5 is achieved, which is comparable to single-reflective (double-pass) cells. An improvement of sensitivity over the conventional single-reflective cell is thus approximately linearly scaled with the enhancement of the optical path length, which is usually more than 1 order of magnitude.
Two multi-harmonic detection methods for wavelength modulation spectroscopy (WMS) systems are presented and compared. The two possibilities discussed in this paper are: simultaneous curve fitting of multiple harmonic spectra, and reconstruction of the transmission from harmonic coefficients. The optimum number of harmonics is four and 25 harmonics, respectively. Compared with standard single-harmonic curve fitting, the methods give about a factor of 3 better performance than standard secondharmonic curve fitting. Concluding, multi-harmonic detection is better than single-harmonic detection and should be used if the system bandwidth is high enough to allow for proper detection of the higher harmonics.
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.
The frequency response of the current-to-wavelength tuning rate (FM response) was measured for two different vertical cavity surface emitting lasers (VCSELs) up to frequencies of 1MHz: GaAs-based VCSEL (763nm) and InP-based VCSEL (1853nm). Both of them show the same qualitative FM response behavior, which can be described by a square root law and therefore cannot be modeled by a first order low pass. The square root law behavior is a significant advantage for laser spectroscopy applications with VCSELs because the decrease of the current-to-wavelength tuning coefficient is less severe as in the case of the first order low pass.
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