# CO2 Sensing Using Symmetrical Three-Wavelength Precompensated Current-Modulated Tunable Diode Laser Absorption Spectroscopy

## TL;DR

A new CO2 sensing method using three wavelengths improves long-term stability and reduces temperature effects compared to traditional two-wavelength approaches.

## Contribution

A symmetrical three-wavelength TDLAS method with precompensated current modulation is introduced for improved CO2 sensing stability.

## Key findings

- Fourfold improvement in reproducibility over 24 days compared to two-wavelength TDLAS.
- Seven times weaker correlation between CO2 concentration and temperature.
- Achieved 0.6 pm wavelength stability with 2 kHz update rate using a single laser source.

## Abstract

A third, symmetrically placed wavelength was added to the differential absorption spectroscopy archetype to obviate baseline instability when remotely measuring CO2 concentrations. An appropriate current shaping was used to offset the diode laser’s slowest time constants, achieving an update rate of 2 kHz with a single laser source. The method was compared to the standard two-wavelength differential absorption spectroscopy approach.

What are the main findings?
Fourfold improvement in stability over 24 days.Eight times weaker correlation between concentration and temperature.

Fourfold improvement in stability over 24 days.

Eight times weaker correlation between concentration and temperature.

What are the implications of the main findings?
Substantial improvement in long-term stability with matched short-term performance.Demonstrated a reduced effect of baseline instability on concentration measurements.

Substantial improvement in long-term stability with matched short-term performance.

Demonstrated a reduced effect of baseline instability on concentration measurements.

In this paper, a novel symmetrical three-wavelength toggling archetype for measuring the concentration of gases using a tunable diode laser absorption spectroscopy (TDLAS) system is introduced and demonstrated. The system was operated at 1.5714 µm with a 2 kHz update rate, targeting an absorption line of gaseous CO2. Precompensated diode–current pulses are introduced to offset the inherent thermal time constants of the diode laser by orders of magnitude. Here, repetition rates matching that of contemporary methods can be achieved, while simultaneously providing a noteworthy wavelength stability of 0.6 pm for the three targeted wavelengths that are approximately 70 pm apart (142 pm maximum wavelength excursion). A 10 Hz current loop locks one of the wavelengths to a CO2 absorption peak, thus providing an absolute and stable wavelength reference. The flexibility in choosing the shape and repetition frequency of the current pulses makes this approach easily adaptable to other gases and/or absorption lines, since wavelength filters are avoided. The new method is benchmarked against a two-wavelength precompensated continuous-wave TDLAS technique, revealing a fourfold improvement in reproducibility with system restart over the span of 24 days, while outperforming other widespread spectroscopic techniques applied to comparable transmittance levels. The effect of the analytical model was further studied by thermally inducing baseline changes, showing a 7.9 ± 0.2 times weaker correlation between concentration and temperature with respect to the one observed using the two-wavelength TDLAS archetype. These results demonstrate the system’s suitability for sensitive applications.