Review of Kuhn, et al. “Mobile and high spectral resolution Fabry Pérot interferometer

spectrographs for atmospheric remote sensing”

The authors advocate for the increased use of Fabry Perot Interferometers (FPI) in trace gas remote sensing.  Conceptual comparisons are made between FPI and grating spectrometers (GS) that are more widely used for trace gas remote sensing, with some notional quantification of spectral resolution, signal to noise ratio (SNR) and mass/volume.  The authors emphasize the mobility of such an instrument.  The case is made that though the SNR is 1%-10% that of a GS, the spectral resolving power can be 30 times greater.  A prototype FPI is presented with images visually comparing data and calculations.

General Comments:

The authors attempt a thorough presentation of important FPI features related to the optics in order to compare the resolving power of the GS and FPI.  However, for a mobile instrument no mention is made of temperature control/stability of the FPI needed for high resolution systems which would presumably have a significant dependence on the stability of the refractive index of the optics.

The authors fail to mention FPIs are commonly used for observing airglow and auroras.  Fiber Bragg-grating versions are also used in receivers for in-elastic scattering lidars.

The authors do not mention grating-prism instruments (aka grisms) that should offer improvements in resolving power over GS with a small increase in volume/mass.

The estimation of SNR for the FPI and GS is somewhat through, but the following discussion concerning level of detection is more vague.  Do you have a reference for the statement in line 436 “…the sensitivity increase will be almost linear to the increase in spectral resolution…”

Line 440: In this discussion of reducing the temporal resolution of the FPI are you also reducing the temporal resolution of the “moderate (spectral) resolution DOAS measurement?”  Using this dramatic approach to increase the SNR will only work for certain investigations that can afford such coarse knowledge.  Such a direct means of SNR increase does not seem applicable to air-borne based instruments.

There is no dispute that FPIs could be used more widely for trace gas detection.  The practical question to answer is whether or not they can achieve the suggested detection capabilities in practice for the mobile scenarios.  The prototype instrument is interesting and demonstrated quantitative performance characteristics should be included, in addition to the visual side-by-side comparison of measurements and model results shown in Fig. 5.

Minor Comments:

Line 99 is missing a comma before water.

Line 416 should delete “by” to then read “…throughput is about…”

This is an excellent paper describing the advantages of using Fabry-Perots interferometers (FPI) in compact spectrographs for remote sensing measurements in the atmosphere. The authors emphasize that FPI-based instruments provide higher resolving power in comparison to conventional grating spectrographs (GS) of similar physical dimensions. The results are convincing and supported by an elegant prototype demonstration. The paper is very well written and certainly suits the scope of AMT. I have only minor suggestions to be addressed in the manuscript:

1. The authors limit their consideration to the geometrical dimensions of the instrument. However, more factors can and should be considered in the development of the mobile optical instrument. I would name mechanical and thermal stability, cost of production, and maintenance requirements. The paper would benefit if some of these points could be addressed by the authors.
2. Despite being essentially different in principle, laser spectroscopy offers high resolution and selective detection of trace gases and recent developments also show that the laser-based solutions can also be made compact and energy-efficient. The presented development could be put in the context of state-of-the-art mobile/compact laser systems.
3. Line 448, the discussion on isotope detection is somewhat vague. Could you add what isotopes and in which compound you are aiming to detect?
4. Minor technical comment: Line 439 "... will be by a factor of ... 0.04 lower than that of ... " Please check if it is a correct statement.

Again, this is an excellent paper that deserves publication!