Enhancing the sensitivity of mid-IR quantum cascade laser-based cavity-enhanced absorption spectroscopy using RF...

Area of Science:

• Spectroscopy
• Laser Physics
• Quantum Cascade Lasers

Background:

• Mid-infrared quantum cascade laser (QCL) off-axis cavity-enhanced absorption spectroscopy (CEAS) sensitivity is often limited by cavity mode structure and diffraction losses.
• Existing methods to mitigate these limitations may not be robust or sufficiently sensitive.

Purpose of the Study:

• To enhance the sensitivity of mid-IR QCL-CEAS by applying broadband radio frequency (RF) noise to the laser current.
• To demonstrate a more sensitive and robust alternative to single-frequency noise perturbation.

Main Methods:

• Applied broadband RF noise to the current of a mid-IR QCL used in an off-axis CEAS setup.
• Utilized pump-probe measurements to confirm the increase in laser linewidth and reduction in mode structure.
• Analyzed CO(2) absorption features at 1890 cm⁻¹ with and without RF perturbation.

Main Results:

• Broadband RF noise addition effectively increased the laser linewidth, reducing detrimental mode structure in CEAS.
• Achieved a minimum detectable absorption of 5.5×10⁻³ Hz⁻¹/² for CO(2) with RF perturbation, a nearly tenfold improvement.
• Demonstrated a minimum acquisition time of 100 ms, suitable for real-time breath analysis.

Conclusions:

• Broadband RF noise perturbation is a highly effective method for enhancing QCL-CEAS sensitivity.
• This technique offers a robust and significantly more sensitive approach compared to unperturbed CEAS.
• The rapid measurement capability makes this enhanced CEAS suitable for breath-by-breath gas concentration monitoring.