Off-Axis Cavity-Enhanced Absorption Spectroscopy of ¹⁴NH₃ in Air Using a Gain-Switched Frequency Comb at 1.514 μm

Area of Science:

• Spectroscopy
• Laser Technology
• Environmental Monitoring

Background:

• Ammonia (NH3) detection is crucial for environmental and industrial applications.
• Cavity-enhanced spectroscopy offers high sensitivity for trace gas analysis.
• Frequency comb spectroscopy provides a broadband, high-resolution light source.

Purpose of the Study:

• To develop and characterize a custom-designed gain-switched frequency comb (GSFC) source for ammonia detection.
• To integrate the GSFC with a medium finesse optical cavity in an off-axis configuration.
• To establish the detection limit and performance of the developed spectrometer for ammonia in dry air.

Main Methods:

• A custom-designed GSFC source was passively coupled to a medium finesse (F ≈ 522) optical cavity.
• Off-axis configuration was employed to enhance light-matter interaction.
• Fourier transform detection was used to measure ammonia absorption in the near-infrared (6604-6607 cm⁻¹).

Main Results:

• Over 30 GSFC lines (2.5 GHz FSR) overlapped with strong 14NH3 ro-vibrational absorption features.
• An ammonia detection limit of approximately 3.7 ppmv was achieved within 20 seconds.
• The system demonstrated effective ammonia detection in static dry air within a laboratory setting.

Conclusions:

• The developed GSFC-cavity system provides a sensitive method for ammonia detection.
• The off-axis cavity configuration and Fourier transform detection scheme are effective for this application.
• The prototype spectrometer shows promise for future advancements in trace gas sensing.