Comparison of a Quantum Cascade Laser and an Interband Cascade Laser for the Detection of Stable Carbon Dioxide Isotopes Using Tunable Laser Absorption Spectroscopy

Quantum cascade lasers (QCLs) and interband cascade lasers (ICLs) are widely used as light sources in tunable laser absorption spectroscopy because they emit in the mid-infrared region where many strong and characteristic absorption bands are present. In this paper, we compare the performance of these lasers emitting at about 2310.1 cm^-1 to determine an optimal light source for detecting isotopic ratios of carbon dioxide (CO₂). Our results show that the QCL has a higher relative intensity noise of up to 15 dBc/Hz compared to the ICL over the entire measured frequency range. In addition, it has a higher frequency fluctuation. However, the maximum tuning range of the QCL is up to 5.2 cm^-1 compared to up to 3.8 cm^-1 for the ICL. Both lasers lose more than half of their tuning range when the tuning rate is increased to 10 kHz. When measuring the isotope ratio of CO₂, an uncertainty in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>δ</mml:mi><mml:mn>13</mml:mn></mml:msup></mml:math> value of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>σ</mml:mi><mml:mrow><mml:mrow><mml:mn>13</mml:mn><mml:mi>C</mml:mi><mml:mo>,</mml:mo><mml:mi>min</mml:mi></mml:mrow></mml:mrow><mml:mrow><mml:mrow><mml:mi>ICL</mml:mi></mml:mrow></mml:mrow></mml:msubsup><mml:mo>=</mml:mo><mml:mn>0.17</mml:mn></mml:math>‰ was achieved with the ICL and of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>σ</mml:mi><mml:mrow><mml:mrow><mml:mn>13</mml:mn><mml:mi>C</mml:mi><mml:mo>,</mml:mo><mml:mi>min</mml:mi></mml:mrow></mml:mrow><mml:mrow><mml:mrow><mml:mi>QCL</mml:mi></mml:mrow></mml:mrow></mml:msubsup><mml:mo>=</mml:mo><mml:mn>0.42</mml:mn></mml:math>‰ with the QCL, both at an integration time of 0.2 s. In summary, the QCL is more appropriate for applications that require a larger spectral tuning range, such as the measurement of a complex gas mixture, while the ICL has an excellent signal-to-noise ratio and is therefore better suited for applications that require higher precision.