Long-path free-space photothermal spectroscopy based on coherent light Fabry-Perot demodulator

Area of Science:

• Spectroscopy
• Laser Physics
• Environmental Sensing

Background:

• Photothermal spectroscopy (PTS) is a sensitive technique for detecting trace gases.
• Conventional PTS systems often face limitations in detection range and sensitivity.
• Improving the resolution and real-time sensing capabilities of PTS is crucial for advanced trace gas analysis.

Purpose of the Study:

• To develop and demonstrate a long-range free-space photothermal spectroscopy (PTS) system.
• To enhance the sensitivity and detection limits for trace gas sensing.
• To provide a high-precision, long-path PTS platform for real-time measurements.

Main Methods:

• Utilized a coherent light Fabry-Pérot (FP) demodulator with a broadband mode-locked pulse laser.
• Implemented fast Fourier transform and Buneman frequency estimation for high-resolution FP interferometer demodulation.
• Achieved sub-nanometer optical path difference resolution at a 17 kHz refresh rate over a 1.8 m optical path.

Main Results:

• Demonstrated real-time sensing of photothermal-induced refractive index variations.
• Achieved a minimum detection limit (MDL) of 3.6 ppm for trace acetylene.
• Obtained a normalized noise-equivalent absorption (NNEA) coefficient of 4.1 × 10^-7 cm^-1·W·Hz^-1/2.

Conclusions:

• The proposed PTS system offers significant sensitivity improvements over conventional white-light demodulators.
• The high-resolution demodulation enables precise detection of trace gases over extended paths.
• This technology presents a promising platform for high-precision, long-path PTS in trace sensing applications.