Chirp modulation improves accuracy and capacity in photoacoustic spectroscopy

Photoacoustic spectroscopy, which involves irradiating intermittent light onto samples and detecting the resulting temperature variations as pressure fluctuations, offers an indispensable method for measuring trace amounts of gasses at part-per-billion and even part-per-trillion levels. But, among other issues, the technology’s sensitivity to temperature tends to cause detuning and create errors with resonant photoacoustic cells – and frustration for scientists and researchers.

To overcome the challenges associated with high-precision hydrocarbon gas measurement, and to improve the technology specifically for dissolved gas analysis of power transformers, Wu et al. introduced chirp modulation into the conventional wavelength modulation method. The chirp-wavelength combined modulation method produces a continuous sweep in the frequency domain and effectively decreases the temperature sensitivity of the photoacoustic measurement system.

“The resonant frequency drift of the acoustic resonance cavity due to temperature is difficult to solve by conventional methods,” said author Zhicheng Wu. “Our study of frequency domain signals helped us to generate the idea of merging chirp and wavelength modulation.”

The scientists performed a detailed mathematical-theoretical analysis of the method to obtain a parameter setting strategy, and they conducted a verification experiment with acetylene measurements to demonstrate efficacy. Their results revealed the method can effectively reduce the temperature sensitivity of a high-Q resonator, thereby enhancing accuracy and detection limits in trace gas detection.

“Our work effectively reduces the drift of resonant frequencies due to temperature sensitivity in photoacoustic cells.” said author Qiaogen Zhang, “We believe this technique has important implications for the accuracy of resonant enhancement devices and high-quality factor acoustic signal enhancement techniques.”

Source: “Reducing temperature sensitivity of gas measurement using chirped-modulated photoacoustic spectroscopy,” by Z. Wu, Y. Jiao, F. Liu, Z. Ai, and Q. Zhang, Review of Scientific Instruments (2022). The article can be accessed at http://doi.org/10.1063/5.0106669 .