Optical frequency comb photoacoustic spectroscopy

TL;DR

This paper introduces a novel photoacoustic spectroscopy technique using optical frequency combs, achieving high spectral resolution, broad spectral coverage, and sensitive detection of trace gases in small samples.

Contribution

It is the first demonstration of OFC-based photoacoustic spectroscopy, combining high resolution and broadband detection with a cantilever-enhanced PA detector.

Findings

Achieved spectral resolution two orders of magnitude higher than previous broadband PAS methods.

Detected methane at 0.8 ppm with a 200 s measurement time.

Normalized noise equivalent absorption of 8×10^{-10} W cm^{-1} Hz^{-1/2}.

Abstract

We report the first photoacoustic detection scheme using an optical frequency comb - the optical frequency comb photoacoustic spectroscopy (OFC-PAS). OFC-PAS combines the broad spectral coverage and the high resolution of OFCs with the small sample volume of cantilever-enhanced PA detection. In OFC-PAS, a Fourier transform spectrometer (FTS) is used to modulate the intensity of the exciting comb source at a frequency determined by its scanning speed. One of the FTS outputs is directed to the PA cell and the other is measured simultaneously with a photodiode and used to normalize the PA signal. The cantilever-enhanced PA detector operates in a non-resonant mode, enabling detection of broadband frequency response. The broadband and the high-resolution capabilities of OFC-PAS are demonstrated by measuring the rovibrational spectra of the fundamental C-H stretch band of CH${_4}$, with no instrumental line shape distortions, at total pressures of 1000 mbar, 650 mbar, and 400 mbar. In this first demonstration, a spectral resolution two orders of magnitude higher than previously reported with broadband PAS is obtained, limited by the pressure broadening. A limit of detection of 0.8 ppm of methane in N${_2}$ is accomplished in a single interferogram measurement (200 s measurement time, 1000 MHz resolution, 1000 mbar total pressure) for an exciting power spectral density of 42 {\mu}W/cm${^{-1}}$. A normalized noise equivalent absorption of 8x10${^{-10}}$ W cm${^{-1}}$ Hz${^{-1/2}}$ is obtained, which is only a factor of three higher than the best reported with PAS based on continuous wave lasers. A wide dynamic range of up to four orders of magnitude and a very good linearity (limited by the Beer-Lambert law) over two orders of magnitude are realized. OFC-PAS extends the capability of optical sensors for multispecies trace gas analysis in small sample volume with high resolution and selectivity.