High-resolution and high-sensitivity absorption spectroscopy in external optical resonators with fast frequency...

Area of Science:

• Molecular Spectroscopy
• Laser Physics
• Cavity-Enhanced Absorption Spectroscopy

Background:

• Studying weak optical absorption lines requires sensitive techniques.
• High-quality optical resonators enhance light-matter interactions.
• Laser frequency tuning dynamics can influence spectral line shapes.

Purpose of the Study:

• To investigate the effect of laser frequency tuning rate on weak absorption line profiles.
• To analyze the spectral behavior of water vapor in an optical resonator under varying tuning conditions.
• To understand the limitations imposed by tuning rates on quantitative absorption spectroscopy.

Main Methods:

• Utilized a diode laser and a high-finesse optical cavity (reflectivity up to 99.98%).
• Employed off-axis cavity enhanced absorption spectroscopy (off-axis ICOS) for high spectral resolution.
• Studied water vapor at reduced pressures (0.03-1 Torr) with tuning rates from 10^2 to 10^3 cm^-1 s^-1.

Main Results:

• Observed a shift and asymmetry in the Doppler absorption profile with increasing tuning rate.
• Demonstrated that the observed effects were symmetrical when the tuning direction was reversed.
• Measurements aligned with theoretical calculations considering photon lifetime and laser coherence.

Conclusions:

• The laser frequency tuning rate significantly distorts weak absorption line profiles.
• Finite photon lifetime and laser coherence length are critical factors in spectral line shape analysis.
• Established limitations for quantitative molecular concentration measurements using rapid tuning methods.