Open-Path 3.3-μm ICL Cavity Ring-Down Spectroscopy of Ambient Methane with FPGA Event-Driven Acquisition and Optical-Feedback Control

Ping-Chun Chen¹, Khemendra Shukla^1*, Tzu-Ling Chen¹

¹Department of Photonics, National Yang Ming Chiao Tung University, Hsinchu City, Taiwan

* Presenter:Khemendra Shukla, email:khemendrashukla@yahoo.in

We present an open-path cavity ring-down spectroscopy (CRDS) instrument targeting the CH₄ ν₃ band near 3.3-μm using an interband-cascade laser (ICL) and an FPGA-based, event-driven acquisition scheme. The cavity length (L ≈ 10 cm) is scanned at 10 Hz by a PZT, producing ≈ 20 resonance crossings per sweep (≈200 ring-down events s^-1). The FPGA applies a voltage-threshold trigger to capture only 3-5 μs waveforms at each resonance, minimizing idle data and streaming time-stamped snippets for τ fitting; a Thorlabs OSA207 wavemeter provides scan-synchronous wavenumber referencing. Ambient spectra around 3094.8-3095.6 cm^-1 are retrieved with HITRAN-guided multi-species Voigt models that co-fit H₂O interference, supported by linewidth (pressure/Doppler) calculations. We quantify and mitigate cavity-induced optical feedback: without an isolator, strong back-reflection increases power but induces frequency excursions; adding an isolator reduces feedback to a benign level and improves short-term stability as confirmed by Allan deviation. With Allan-optimized averaging, we obtain a minimum detectable absorption $\alpha_min ≈ 4.4×10^-8 cm^-1 and a noise-equivalent absorption NEA ≈ 6.22×10^-9 cm^-1 Hz^-1/2 at ƒ_rep ≈ 10 Hz. To our knowledge, this is the first demonstration of open-path CRDS at the 3.3-μm CH₄ fundamental using an ICL, combining strong-band access, compact low-power hardware, and FPGA event capture for fieldable methane monitoring.

Keywords: Cavity Ring Down Spectroscopy, Optical Cavity, Methane Monitoring