Abstract
A custom-designed gain-switched frequency comb (GSFC) source was passively coupled to a medium finesse (F ≈ 522) cavity in off-axis configuration for the detection of ammonia (14NH3) in static dry air. The absorption of ammonia was detected in the near infrared spectral region between 6604 and 6607 cm−1 using a Fourier transform detection scheme. More than 30 lines of the GSFC output (free spectral range 2.5 GHz) overlapped with the strongest ro-vibrational ammonia absorption features in that spectral region. With the cavity in off-axis configuration, an NH3 detection limit of ∼3.7 ppmv in 20 s was accomplished in a laboratory environment. The experimental performance of the prototype spectrometer was characterized; advantages, drawbacks and the potential for future applications are discussed.
Highlights
The usefulness of the number of spaced, phase-coherent, narrow-band spectral lines of frequency combs (FC) has long been recognized for state-of-the-art applications in gas-phase laser spectroscopy [1,2,3,4]
We demonstrate that by passively coupling the light from a gain-switched frequency comb (GSFC) to an optical cavity in off-axis configuration, absorption spectra can be measured without active mode stabilization or locking
Conventional single pass absorption measurements were taken in this narrow-band region with the gain-switched frequency comb (GSFC) in order to enable a quantitative comparison of conventional
Summary
The usefulness of the number of spaced, phase-coherent, narrow-band spectral lines of frequency combs (FC) has long been recognized for state-of-the-art applications in gas-phase laser (absorption) spectroscopy [1,2,3,4]. The reduction of complexity by eliminating any active mode matching schemes, the potential miniaturization of the GSFC (photonic integration) in conjunction with a robust cavity alignment scheme and the competitive cost of components represent ingredients for a compact, robust and commercially viable trace gas sensor for field application. In this proof-of-concept study, a GSFC with an FSR of 2.5 GHz was designed in a spectral region where significant absorption features of ammonia (14 NH3 ) are located around 1514 nm.
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