A Low-Cost and High-Stability Demodulation Method for Photothermal Spectroscopy Gas Detection in Power Transformer

Dissolved gas analysis is a critical component of transformer defect diagnosis. Oil-dissolved gas sensing based on fiber-optic photothermal spectroscopy was proposed for it is immune to electromagnetic interference. Traditional methods apply sinusoidal modulation to the pump light wavelength. However, sinusoidal phase noise is also generated in the absence of gas due to the laser absorption by fiber splice points, flanges, and other components. This noise makes it difficult to further lower the detection limit. In this article, a pump light frequency modulation method combining a sine wave with a sawtooth wave was introduced. The gas concentration was determined by detecting the amplitude of the second harmonic wave, which addressed the aforementioned issues. However, these systems are highly sensitive to temperature, vibrations, and other factors, leading to poor stability. In addition, the industry application is limited by the high cost of the demodulation system. Thus, this article proposed an acetylene sensing topology based on heterodyne interference and Mach-Zehnder interferometric structure. The sensing phase signal was modulated by a 100-MHz modulation, achieved by introducing an acousto-optic modulator (AOM) into the reference arm, which reduced low-frequency noise interference. Then, a low-cost demodulation method was also proposed based on mixing and down conversion. This method lowered the carrier frequency from 100 MHz to 100 kHz, reducing demodulation costs by about 91%. The proposed photothermal spectroscopy gas detection system achieved a detection limit of 0.256 ppm for acetylene.