Improving long-path differential optical absorption spectroscopy with a quartz-fiber mode mixer

Area of Science:

• Atmospheric Chemistry
• Spectroscopy
• Environmental Science

Background:

• Long-path differential optical absorption spectroscopy (DOAS) is crucial for measuring tropospheric trace gases.
• Photodiode array (PDA) detectors offer enhanced sensitivity in DOAS systems due to the multiplex advantage.
• Current PDA-based DOAS systems face limitations due to unwanted spectral structures that exceed photon noise.

Purpose of the Study:

• To investigate the cause of unwanted spectral structures in PDA-based DOAS systems.
• To develop a method to reduce these spectral structures and improve detection limits.
• To enhance the sensitivity and reduce measurement time for long-path DOAS.

Main Methods:

• Development and testing of a quartz-fiber mode mixer.
• Characterization of spectral structures in PDA-based DOAS systems.
• Comparison of PDA detectors with scanning detectors in long-path DOAS.

Main Results:

• Identified angular dependence of PDA response as the cause of spectral structures.
• The quartz-fiber mode mixer reduced unwanted structures by a factor of 10 in lab and field tests.
• Improved detection limits by one order of magnitude, surpassing scanning detector systems.

Conclusions:

• The quartz-fiber mode mixer effectively minimizes spectral noise in PDA-based DOAS.
• This advancement significantly enhances the sensitivity and reduces measurement time for trace gas determination.
• PDA detectors, when optimized, offer superior performance for long-path DOAS applications.