A Tunable Resolution Grating Monochromator and the Quest for Transform Limited Pulses
Round 1
Reviewer 1 Report
The authors present an interesting overview over the design for a soft X-ray monochromator for LCLS-II, where two different options can be interchanged, either producing highest resolution or delivering shortest pulses, while both options work as close as possible to the limits set by the Fourier transform.
The paper is very interesting and deserves publication after the authors have adressed some points. In general, there are still several typographic errors in the text and another round of proofreading would certainly help. Some of the errors that I spotted, I detail below. Some of the sentences, especially in the introduction, are also rather long and understanding could be improved by shortening the sentences.
As a further general statement, I want to stress that some of the considerations of the authors have a relevance only in the considered range of photon energies. At lower photon energies, similar grating concepts - even if they stay at the Fourier limit - usually produce pulses that are too long for experiments. At higher photon energies, substantially different monochromators are used and the pulse elongation depends on other parameters.
In this context, I wonder how good the paraxial approximation actually works. From my experience with longer wavelengths, I know that higher order terms in the grating equations yield additional contributions to the tmeporal broadening and I would be interested to learn, if the authors considered higher order terms and did a more rigorous estimate.
Furthermore, the authors stated several additional performance goals in lines 59-64, while they do not adress if their design meets any of these goals and how good those goals are met. I refer especially to goals e,f,g and h, which are nowhere mentioned again.
It further remains unclear, why the authors have chosen a design for the high-resolution case with three different grgatings, although already two of them provide an overlapping energy range. I assume, it is to optimized the energy resolution, but a clarification in the manuscript would be appreciated.
Did the authors consider grating or mirror slope errors or other inaccuracies in their estimate of energy resolution? At the stated resolving power, I presume that other error sources become significant as well.
In general, the paper would also benefit from a concluding and summarizing paragraph at the end.
Now some typos and omissions:
l.23 "upgrade"
l.46 "beamlines"
l.65 "others"
l.67 "constraints"
l.132 to be picky: The energy-time relation is actually a Fourier relation and does not depend on Heisenberg's uncertainty relation.
l.170 "account"
l.204 "7°" (no underlining under the degree symbol)
l.248 beta and alpha missing
l.307 "ratio"
Author Response
Please, see attached document for detailed response to your useful comments and suggestion.
Sincerely Daniele and Josep
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Review of manuscript Photonics #1721060
A tunable resolution grating monochromator and the quest for 2 transform limited pulses
Josep Nicolas and Daniele Cocco
Summary
The article is focused on the theoretical description of a monochromator to be used on a free electron laser source, and more precisely on RIXS beamline. The design is done keeping in mind mainly the focusing properties, the pulse length, the preservation of the coherence and the ability to provide different resolving powers. These points will be solved by the chosen monochromator geometry.
Main comments
I found the article interesting, by describing step by step all the processes leading the proposed design, on a nice project.
In the introduction, among all the requirements, point f) deals with “the heat load generated by the high repetition rate source”. There is after in the text no mention of this aspect. Will the grating be cooled (cryogenically- or water-cooled) in order to avoid thermal bump?
Still in the introduction, point h): “to suppress the 3rd harmonic while operating at the Oxygen edge”. Why the 3rd harmonic, there is no 2nd harmonic with such gratings? And why only for O K-edge?
The article is based on equations, and, to my opinion, it can be more “pedagogic”, especially for the instrumentalists non-expert of these theoretical aspects.
Concerning the monochromator design (§3), a drawing presenting schematically the VLS grating with respect to the surface would be helpful (or integrate this representation to Figure 1) to understand how equation 19 (which differs from the usual way to write the Bragg law) can be obtained.
Line 118 and Figure 2: I do not find the figure helpful for this purpose. Maybe add on it Nσ and other parameters from the equations to better illustrate.
Line 237, the authors are saying that the “selected energy may be unknown”. It is quite problematic for a monochromator… maybe you can explain a little bit more?
The influence of the slits aperture is discussed: maybe schematically local these slits on Figure 3.
Details:
- Line 52: “a FEL”?
- Lines 54-55: the indicated values are not energy resolution but resolving power as indicated on Figure 4
- Figure 1: ‘?’ are superimposed to ‘Exit slits pane’
- Lines 220-222, “The convention adopted is […] on opposite side of the grating normal figure 1)” ?
- Equation 20: what does the index ‘200’ mean?
- Line 248: “cff parameter (cos β/cosα)”
- Line 265: “a small ?? (DE)”
- Table 1: Cff is already defined
- Figure 4 can you write on the figures the names of the gratings, to have a correlation with table 1?
- Line 307: “the ratio between…”
- Line 311: “the beam profile shall stay as close as possible…”?
Author Response
Dear Reviewer. Thanks for the useful comments and suggested corrections. Please see attached the complete list of changes addressing your request.
Sincerely, Daniele and Josep
Author Response File: Author Response.pdf
