Characterization of Laser Systems at 1550 nm Wavelength for Future Gravitational Wave Detectors

Round 1

Reviewer 1 Report

The paper is very interesting and well written. It describes an important characterization of the status of the art for the 1550nm laser systems. This study will be an important reference for the development of injection system design for the third generation of gravitational wave detectors.

Few minor comments:

• line 95, it is not clear how the alignment control is designed. At first it seems that the sensing uses the demodulation/modulation technique, as for the longitudinal control, but then it is described as beam pointing (which does not need the demodulation). How is this alignment working?
• Figure 2. What is the continuous black line in the plot? May be I missed the explanation
• line 194, typo in the citation
• Figure 14, the relative power noise requirements could be added to the plot.

Author Response

Please see the attachment.

Reviewer 2 Report

The manuscript presents a comprehensive study of the noise and modulation characteristics of different combinations of 1550nm wavelength seed lasers, pre-amplifiers, and power amplifiers. A specialized DBB is used to investigate frequency and intensity noise, as well as pump diode current modulation transfer functions for multiple injection points. I found the draft overall well-written, the methodology is clear, well-motivated from the side of gravitational wave instrumentation, and the results are presented comprehensively. Systematic studies like these are extremely valuable for researchers who work on developing instrumentation that relies on low noise laser sources, as they go much beyond the information that is typically available for commercial laser systems.

I have a few suggestions how to make the presentation clearer and easier to follow, with more detailed comments further below. At the top-level, I find that there are many long run-on sentences that could be split into parts to make reading easier, or use commas to create breaks. The draft is written predominantly in American English, but at times British Spelling is used. For example, in line 335 both AE and BE spellings of polarization are found in the same sentence. Besides AE/BE discrepancies I found very few typos. There are also formatting inconsistencies, such as having a space before a citation bracket or not, and occasionally missing spaces after commas or full-stops.

Detailed comments:

Line 9: make … to … cannot be used for what the authors want to say here. Scrapping the ‘to’ is a possible way out, but there are others.
Lines 34-36: This sentence brushes over details for the design of the Einstein Telescope (ET). While this information isn’t exactly vital to the presented work, it is lacking a bit of context, such as why three detectors, what does it mean to have a low-frequency detection band. This is more a comment for the authors that this stood out to me while reading the draft.
Lines 44-46: Here it is stated that the laser performance up to 200 MHz is important to assess, but none of the measurements shown go that high in frequency
Line 51: Typo in stabilization
Line 80: It is stated here that spatial mode properties were analyzed but unless I overlooked it there are no results presented anywhere in the draft except for a mention in line 146. Since all of the lasers and amplifiers are fiber-coupled, this will of course depend on the fiber NA and collimation package.
Line 82: To my knowledge a Fabry-Perot resonator is a standing wave linear cavity (possibly folded), but not a traveling wave triangular cavity.
Line 127: Is the optical booster amplifier the BOA?
Line 140: The PD manufacturer is likely Perkin Elmer
Line 142: Is there a reason only the in-loop detector was outfitted with this electronic modification?
Line 148: Is the Grande laser special wrt to the statement in this sentence? It is said further up that it uses an Orion-equivalent seed but is a fair bit different from the Orion-only noise.
Line 186: I am not so happy with the statement that “a small overlap” “compensates for calibration uncertainties”. I fully understand the whitening argument but if I take the calibration uncertainty statement at face value, what dose that mean for the individual traces?
Figure 2: I just want to bring to attention that this is a crowded figure and even in color I need to take a very close look to tell some of the traces apart. In B+W print this would be even worse.
Line 193: the word ‘maybe’ adds too much uncertainty, I suggest replacing it with ‘could ‘be caused’ or something similar.
Line 194-197: Something doesn’t add up here: the noises are at the same level, yet stabilising the pump diodes doesn’t reduce the output noise much. I guess if they are truly just in the same ballpark that is a possibility, however, there is no statement on how much the power noise of the pump diodes was suppressed. If it was only a factor of 2 then the result wouldn’t be surprising (although I do believe it was significantly more than that to warrant the statement in this sentence)
Line 200: typo: decreaseS
Line 216: While it does follow from context, I suggest explicitly stating that what is modulated is the pump current of the preamplifier pump diodes
Line 217: Personally, I would prefer the modulation to be stated in units of modulation per mA, which makes it independent of any scaling of the modulation signal by the current driver modulation input.
Figure 7: One can see a phase lead behaviour in the Orion+fiber pre-amp at 100kHz without much change in the amplitude response, which may be relevant for stabilisation efforts. I wonder what the physical explanation could be for this.
Line 248: Instead of ‘size’ I think using ‘magnitude’ or something equivalent is better.
Line 272: Is it ‘appropriate’ rather than ‘appropriated’?
Line 274: analyser is BE
Line. 276: at least the lowpass limitation could be relaxed by using higher modulation frequencies, unless PD bandwidth becomes an issue.
Line 297-299:  Looking at figure 2, I don’t think this statement fully holds. The adjustik trace there is quite a bit lower than the noise ‘floor’ in figure 12 and it was measured with the DBB as well in my understanding.
Line 305-307: I have re-read this sentence and the sections around it several times and I just don’t understand what it is supposed to tell me. Are there words missing? ‘The noise was below 10kHz’, which is over an order of magnitude below shot noise?
Line 326: typo: another
Line 343: The high loop gain presumably refers to the power stabilisation, figure 13, not the frequency? There are two loops discussed in this paragraph.

Author Response

Please see the attachment the response to the comments of reviewer 1 and reviewer 2.

Author Response File: Author Response.pdf