Photoacoustic Hydrocarbon Spectroscopy Using a Mach-Zehnder Modulated cw OPO

Abstract

This paper describes a new photoacoustic spectrometer for the investigation of hydrocarbons based on a continuous wave optical-parametric oscillator (OPO). Two modulation methods for the generation of the photoacoustic signal are compared. In addition to the traditional modulation by a mechanical chopper a Mach-Zehnder modulator was set up for this wavelength range and used to shape the OPO beam. Spectra of three hydrocarbon test gases (methane, ethane and propane in nitrogen) were measured between 3200 nm and 3500 nm. The differences between the two modulation methods are explained and the advantages of the newly introduced Mach-Zehnder modulator are elaborated. In particular the frequency fluctuation and complexity of both methods are set in contrast with each other. The measured methane spectrum is compared against data from the HITRAN database.

Keywords

• Chopper
• Mach-Zehnder interferometer
• Optical-parametric oscillator
• Hydrocarbons
• Methane
• Ethane
• Propane
• Photoacoustic spectroscopy.