Supercontinuum intensity noise coupling in Fourier transform photoacoustic spectroscopy

Abstract

We investigate the noise transfer mechanism from the light source intensity fluctuations to the acoustic signal in Fourier transform photoacoustic spectroscopy (FT-PAS). This noise coupling is expected to be reduced in FT-PAS compared with conventional Fourier transform spectroscopy, as only the specific spectral components that are absorbed by the probed sample contribute to the noise level. We employ an incoherent supercontinuum (SC) light source in our experiments and observe a linear relation between the sample gas concentration and the detected noise level, which significantly reduces the influence of the SC noise on the detection limit. Based on our experimental results, we derive a model for the noise level, which establishes the foundation for practical sensitive implementation of FT-PAS.

Sensitive multi-species photoacoustic gas detection based on mid-infrared supercontinuum source and miniature multipass cell

“…A long-pass filter with a cut-off at 2989 nm (3346 cm À1 , Northumbria Optical Coatings SLWP-2989) removes optical power from spectral regions not under interest and thus suppresses excess absorption from water (H 2 O) molecules in the gas cell. As we have shown previously, 9 eliminating unutilized absorption decreases the noise level significantly, as the intensity noise of the SC is coupled to the PA signal via light absorption by gas molecules. The filtered SC light with 21 mW of optical power is directed into a Fourier transform spectrometer (FTS, Bruker IRCube), whose maximum spectral resolution is 1 cm À1 .…”

Sensitive multi-species photoacoustic gas detection based on mid-infrared supercontinuum source and miniature multipass cell

“…A long-pass filter with a cut-off at 2989 nm (3346 cm −1 , Northumbria Optical Coatings SLWP-2989) removes optical power from spectral regions not under interest and thus suppresses excess absorption from water (H 2 O) molecules in the gas cell. As we have shown previously [9], eliminating unutilized absorption decreases the noise level significantly, as the intensity noise of the SC is coupled to the PA signal via light absorption by gas molecules. The filtered SC light with 21 mW of optical power is directed into a Fourier transform spectrometer (FTS, Bruker IRCube), whose maximum spectral resolution is 1 cm −1 .…”

“…For the multipass enhanced system, we estimated the limit of detection as LOD = 3σc/S, where c is the gas concentration (36 ppm), S is the strongest spectral peak evaluated at 3017 cm −1 and σ is the noise level calculated from a separate 40 seconds long recording with only N 2 in the gas cell, as this represents the noise level at low CH 4 concentrations [9]. Due to a small H 2 O residual in the N 2 measurement, the noise level was calculated as the standard deviation of the baseline at 2200-2800 cm −1 .…”

“…We studied the effect of the gas concentration on the amplitude of the measured photoacoustic signal and the noise level. The concentration-amplitude dependence is used for calibration, which is always required in PAS, and the noise relation gives insight into the unique noise mechanism of FT-PAS studied in more detail elsewhere [9]. Samples of C 2 H 4 with varying concentration between 6 and 900 ppm were prepared by diluting C 2 H 4 from a 1% gas bottle to N 2 .…”

“…In FT-PAS, a scanning interferometer modulates each wavenumber of a broadband light source at a distinct acoustic frequency, after which all the pressure waves at these frequencies are simultaneously detected in an acoustically non-resonant gas cell. Broadband thermal emitters are typically used as the light source in FT-PAS, but recently supercontinuum (SC) sources and frequency combs have been shown to yield significantly better performance [8][9][10][11][12].…”