Off-Axis Cavity-Enhanced Absorption Spectroscopy of 14NH3 in Air Using a Gain-Switched Frequency Comb at 1.514 μm

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.

Wavelength Tunable 100 MHz Gain Switched Optical Frequency Comb for Cavity-Enhanced Absorption Spectroscopy

“…The number density of NH3, n, was evaluated to be 2.7810 15 cm -3 (=mixing ratio 111 ppmv at 294 K) for the measurement shown, based on a non-linear least square fit of eq. ( 2) to the measured absorption coefficient [9] (red trace in Fig. 5(b)).…”

“…( 2). The uncertainty reported for the NH3 absorption cross-section is ~15% in this spectral region [9,19]. Other factors contributing to the error of the mixing ratio are the uncertainty of the reflectivity, R (ΔR=8.3%), and the cavity length, d (Δd = 0.2%).…”

“…One of the main challenges in CEAS using OFCs is the effective coupling of the OFC to the optical cavity by matching the narrow band comb lines with the mode structure of a high-finesse optical cavity. As demonstrated in [9], by passively coupling the light from an OFC to an optical cavity in an off-axis configuration, the absorption spectra can be measured without active mode stabilization or locking. Since cavities in off-axis alignment are quite stable and less prone to misalignment, this approach significantly simplifies the application of OFCs in the context of optical cavity applications.…”

Wavelength Tunable 100 MHz Gain Switched Optical Frequency Comb for Cavity-Enhanced Absorption Spectroscopy

“…4) was ~4 mm. The experimental procedure in this experiment follows that reported in [9]. Prior to each individual measurement, the cavity cell was evacuated (including the inlet line and regulator valve) to a pressure of < 0.1 mbar.…”

“…Since cavities in off-axis alignment are quite stable and less prone to misalignment, this approach significantly simplifies the application of OFCs in the context of optical cavity applications. In [9] and [10], externally injected gain-switched OFCs, with free spectral ranges (FSRs) of 10 GHz and 2.5 GHz respectively, were used. However, OFCs with an FSR of 10 GHz are spaced too widely in order to capture typical absorption features completely; for narrow spectral (isolated) lines, comb line spacings of 2.5 GHz still do not map the spectral signatures comprehensively, especially not at low pressure detection applications.…”