Multigas Sensing Based on Wavelength Modulation Spectroscopy Using Frequency Division Multiplexing Combined With Time Division Multiplexing

We proposed a six-gas simultaneous detection system for methane (CH ₄ ), ethane (C ₂ H ₆ ), acetylene (C ₂ H ₂ ), ethylene (C ₂ H ₄ ), carbon monoxide (CO) and carbon dioxide (CO ₂ ) based on wavelength modulation spectroscopy (WMS). Five different near infrared distributed feedback (DFB) lasers combined with a custom $5\times1$ wavelength division multiplexer were used to detect the six gases. The lasers were modulated at five different sinusoidal signals and the detection light intensity signals were demodulated with corresponding sinusoidal signals, which can be named frequency division multiplexing (FDM). Among them, CH ₄ and C ₂ H ₆ were measured with one laser based on time division multiplexing (TDM). The principle of four multi-gas detection based on FDM was also derived. To miniaturize the device, a Raspberry Pi was used for data processing based on python, and the detected gas concentrations can be displayed directly on the screen. The six-gas sensing system exhibits detection limits of 247 ppb, 1.28 ppm, 126 ppb, 982 ppb, 2.55 ppm and 4.87 ppm with average time of 1 s for CH ₄ , C ₂ H ₆ , C ₂ H ₂ , C ₂ H ₄ , CO and CO ₂ , respectively. With an average time extended to 100 s, the measurement precision achieves 24 ppb, 135 ppb, 14 ppb, 141 ppb, 495 ppb and 608 ppb, respectively. With a fast response time, compact size and high detection sensitivity, the detection system is promising for dissolved gas analysis (DGA) in transformer oil and other material aging process.