11 Oct 2022
11 Oct 2022
Status: this preprint is currently under review for the journal AMT.

New methods for the calibration of optical resonators: Integrated Calibration by means of Optical Modulation (ICOM) and Narrow Band Cavity ring-down (NB-CRD)

Henning Finkenzeller1, Denis Pöhler1,2, Martin Horbanski1,2, Johannes Lampel1,2, and Ulrich Platt1,2 Henning Finkenzeller et al.
  • 1Institute of Environmental Physics, University of Heidelberg, INF 229, 69120 Heidelberg, Germany
  • 2Airyx GmbH, Eppelheim, Germany

Abstract. Optical resonators are used in spectroscopic measurements of atmospheric trace gases to establish long optical path lengths L(λ) with enhanced absorption in compact instruments. In cavity-enhanced broad band methods, the exact knowledge of both the magnitude of L(λ) and its spectral dependency is fundamental for the correct retrieval of trace gas concentrations. L(λ) is connected to the spectral mirror reflectivity R(λ) which is often referred to instead. L(λ) is also influenced by other quantities like broad-band absorbers or alignment of the optical resonator. The established calibration techniques to determine L(λ), e.g., introducing gases with known optical properties or measuring the ring-down time, all have limitations: Limited spectral resolution, insufficient absolute accuracy and precision, inconvenience for field deployment, or high cost of implementation. Here, we present two new methods that aim to overcome these limitations: (1) Narrow Band Cavity ring-down (NB-CRD) uses Cavity Ring-down and a tunable filter to retrieve spectrally resolved path lengths. (2) Integrated Calibration by means of Optical Modulation (ICOM) allows the determination of the optical path-length at the spectrometer resolution with high accuracy in a relatively simple setup. In a prototype set-up we demonstrate the high accuracy and precision of the new approaches. The methods facilitate and improve the determination of L(λ), thereby simplifying the use of cavity enhanced absorption spectroscopy.

Henning Finkenzeller et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2022-274', Anonymous Referee #1, 01 Nov 2022
  • RC2: 'Comment on amt-2022-274', Anonymous Referee #2, 15 Nov 2022

Henning Finkenzeller et al.

Henning Finkenzeller et al.


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Short summary
Optical resonators enhance the light path in compact instruments, thereby improving the sensitivity. Determining the established path length in the instrument is a prerequisite for the accurate determination of trace gas concentrations but can be a significant complication in the use of such resonators. Here we show two calibration techniques which are relatively simple and free of consumables, but still provide accurate calibrations. This facilitates the use of optical resonators.