Optical-feedback cavity-enhanced absorption spectroscopy with a quantum cascade laser

Area of Science:

• Spectroscopy
• Quantum Cascade Lasers
• Mid-Infrared Optics

Background:

• Cavity-enhanced absorption spectroscopy (CEAS) is a powerful technique for trace gas detection.
• Mid-infrared (mid-IR) spectral regions offer unique molecular fingerprints but often face challenges with detector sensitivity and laser stability.

Purpose of the Study:

• To demonstrate optical-feedback CEAS in the mid-IR using a quantum cascade laser (QCL).
• To leverage optical feedback for improved laser performance and enhanced detection limits.

Main Methods:

• Utilized a quantum cascade laser emitting at 4.46 μm.
• Implemented selective optical feedback from a resonant cavity to reduce laser linewidth and achieve frequency locking.
• Employed room-temperature detectors to compensate for low light sensitivity.

Main Results:

• Achieved a noise equivalent absorption coefficient of 3 × 10⁻⁹ cm⁻¹ with 1-second averaging.
• Demonstrated a detection limit of 35 parts per trillion for N₂O at 50 mbar.
• Quantified detection limits down to 4 × 10⁷ molecules/cm³ or 1 fmol in the sample volume.

Conclusions:

• Optical-feedback CEAS in the mid-IR is a highly sensitive and practical gas sensing technique.
• Laser linewidth reduction and frequency locking via optical feedback are crucial for high performance in this spectral range.
• The method compensates for detector limitations, enabling sensitive measurements with readily available components.