VCSEL-based calibration-free carbon monoxide sensor at 2.3 μm with in-line reference cell

Abstract

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.

Low-level and ultralow-volume hollow waveguide based carbon monoxide sensor

“…Applying the recently developed 2:3 μm vertical-cavity surface-emitting lasers (VCSELs) [5,6], an absorption line in the first-overtone band of CO is sampled, which is approximately 160 times stronger (peak absorption coefficient α peak ≈ 0:43 cm −1 ) than its second overtone band at 1:5 μm (α peak ≈ 0:0027 cm −1 ). Utilizing a 10 cm single-reflective cell, a sensor resolution of 3 parts in 10 6 (ppm) (1σ at 1 Hz) is already achieved at 2:3 μm [7]. Using a 3 m hollow fiber, a sensitivity improvement of a factor of 30 is expected.…”

“…2), the wavelength scale is calibrated inherently without using an external reference cell. In [7] this was introduced for a single-reflective cell. Furthermore, it enables simultaneous tracking of the position of the CO line and determination of the linear and quadratic current tuning coefficient of the laser.…”

“…According to experiments (Fig. 2), the fiber background has a standard deviation of 8 × 10 −5 (absorbance) without vibration and 10 −5 with vibration, which is the absorbance resolution achievable with a conventional single-reflective cell [7]. Therefore, the improvement of sensitivity over conventional cells is expected to be linearly scaled with the enhancement factor of the optical path length [see Eq.…”

“…Compared to a single-reflective cell based sensor [7], the fiber sensor has a 16-fold improvement in concentration resolution with a 30 times enhancement of the optical path length, while the needed sample volume is simultaneously lowered by several orders of magnitude to 1:3 ml. This clearly demonstrates the potential of laser spectroscopic hollow capillary fiber based sensors.…”

Field laser applications in industry and research

“…The integrated cavity output spectroscopy technique, applied to the isotopic ratio measurement of atmospheric methane, is the subject of the paper by Witinski, Sayres and Anderson [12]. The final four papers deal with devices for atmospheric monitoring: Chen and coworkers propose a self calibrating carbon monoxide analyzer with a reference cell embedded in the optical detector [13]; McDermitt and coworker developed a fast and precise open path analyzer with very low power consumption for the measurement of ecosystem fluxes of methane in remote areas [14]; Sonnenfroh and Parameswaran describe a high precision CO 2 analyzer for applications in distributed measurement networks [15]; Lee and coworkers have developed an analyzer for fast and simultaneous detection of HONO and NO 2 for monitoring applications [16].…”