Fig. 1. (a) Theoretical emitted and received signals of the remote validation system; (b) structure of the sensor. EOM denotes an electro-optical modulator, PBS denotes a polarizing beam splitter, and SPAD denotes a single-photon avalanche diode detector.

Fig. 2. (a) In situ laboratory CH₄ absorption spectroscopy test system based on SPAD. CH₄ absorbance characters of SPAD: (b) NoPrbs mode; (c) Prbs mode.

Fig. 3. Effect of DT on CH₄ absorption for the same laser output power. (a) P₁, NoPrbs; (b) P₂, NoPrbs; (c) P₃, NoPrbs; (d) P₁, Prbs; (e) P₂, Prbs; and (f) P₃, Prbs.

Fig. 4. Outfield experimental setup. (a) Images of the installed equipment; (b) pictorial view of the CH₄ remote-sensing system.

Fig. 5. Raw signals received by the SPAD, bipolar signal, and shifted signal. (a) 15 m; (b) 29 m; (c) cross-correlation results between the original echo signal and reference bipolar sequence under different distances; (d) measured and theoretical CH₄ absorption intensity of 1000 ppm · m.