Recent Progress in Development of Hollow-Core Fibers for Telecommunications and Data Transmission Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript provides a useful snapshot of recent activities in the HCF field but fails to transform that raw data into a valuable scientific review. It lacks the critical analysis, physical insight, and formal tone required for publication. The author's expertise is evident in the collection of data, but it is not effectively applied to synthesize and interpret the information for a scholarly audience. Therefore, I must recommend rejection.

1.The manuscript is predominantly descriptive. Sections like "Reduction of HCF attenuation" and "Fiber manufacturing" read as a catalogue of recent papers, listing loss values and design features without deep physical insight.

2.Section 3.3 describes two 0.05 dB/km fibers from ECOC 2025. The description is superficial: "Fiber A had feature X, Fiber B had feature Y." A proper review would delve into why these designs work. What is the underlying physics of the interstitial tubes in the Linfiber design that allows for larger gaps? How do the thick-glass supports in the YOFC design alter the mechanical or optical properties compared to truncated tubes? The current text simply states the facts as one would in a conference report.

3.Section 9 (Discussion) is the only place where critical commentary appears, but it is too brief and disconnected from the main body. The discussion of manufacturing yield (YOFC's average loss vs. record loss) is excellent, but this kind of analysis should permeate the entire manuscript. Why is the average loss so much higher? What are the specific fabrication tolerances that are hardest to control? This is the kind of question a review should explore.

4."Unfortunately, progress in standardization... was absent," "the author deems desirable," "it seems nobody cares," and "worrisome" are subjective, informal, and lack the objectivity expected in scientific writing.

Author Response

1.The manuscript is predominantly descriptive. Sections like "Reduction of HCF attenuation" and "Fiber manufacturing" read as a catalogue of recent papers, listing loss values and design features without deep physical insight.

Well, this was partly my intent - to gather in a single paper all new important developments in the 2022-2025 period. Still, I have made several changes in the suggested direction, trying to compare the workings of alternative fiber designs, as several incompatible ones have appeared and such differences are important.

2.Section 3.3 describes two 0.05 dB/km fibers from ECOC 2025. The description is superficial: "Fiber A had feature X, Fiber B had feature Y." A proper review would delve into why these designs work. What is the underlying physics of the interstitial tubes in the Linfiber design that allows for larger gaps? How do the thick-glass supports in the YOFC design alter the mechanical or optical properties compared to truncated tubes? The current text simply states the facts as one would in a conference report.

As above, the text was modified according to Your suggestion, and new drawing (fiber loss distribution) added. Hope it helps.

3.Section 9 (Discussion) is the only place where critical commentary appears, but it is too brief and disconnected from the main body. The discussion of manufacturing yield (YOFC's average loss vs. record loss) is excellent, but this kind of analysis should permeate the entire manuscript. Why is the average loss so much higher? What are the specific fabrication tolerances that are hardest to control? This is the kind of question a review should explore.

I made several changes to section 9 in this direction, but please understand that I am not working for a HFC manufacturer and have no access to several types data mentioned. Stil, I tried to make an educated guess supported by (scarcely) published data.

4."Unfortunately, progress in standardization... was absent," "the author deems desirable," "it seems nobody cares," and "worrisome" are subjective, informal, and lack the objectivity expected in scientific writing.

I agree, the wording cited was changed or whole statements deleted. Thank You. 

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents a review of the latest developments in hollow-core fibers, focusing on telecommunications and data transmission. The author reports the advantages of hollow-core fibers compared with other technologies used for data transmission and also highlights the issues that still affect this emerging platform. He compares the approaches and solutions proposed by leading research groups in the field and outlines the future perspectives of HCF technology.

The work is precise and well presented. It will be useful for readers who want to approach the topic and serves as a valuable recap for researchers working in this field.

I would like to point out one mistake in Table 2, line 6: Attenuation at 1550 nm [dB/km] is reported as 0.091 ± 0.10, but it should be 0.091 ± 0.01.

 

Author Response

I would like to point out one mistake in Table 2, line 6: Attenuation at 1550 nm [dB/km] is reported as 0.091 ± 0.10, but it should be 0.091 ± 0.01.

A correction was made. Thank You.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents a review of the evolution of Hollow-Core Fibers (HCF) over the last three-four years. Different characteristics are analyzed, including attenuation, latency, dispersion, and PDM. The authors identify fabrication issues (geometry and contaminants), amplification challenges, and standardization aspects, and discuss installation projects for long-distance transmission. The future perspectives are highlighted due to attenuations of 0.05 dB/km achieved in the laboratory, with theoretical predictions of further reduction.

The work represents a comprehensive bibliographic review, is well-structured and well-written, and may serve as a useful guide for users and developers in the field.

I believe I have identified an error in the manuscript. The fiber attenuation indicated in Table 1 is approximately 4000-5000 microseconds/km, which I believe should be 4-5 microseconds/km. Please review this.

Additionally, I miss in the manuscript a numerical assessment (and comparison with conventional SM fibers) of some key aspects for their use, such as: nonlinearity, splice losses, connectivity, light insertion/extraction losses, and coupling losses to amplifiers.

Author Response

I believe I have identified an error in the manuscript. The fiber attenuation indicated in Table 1 is approximately 4000-5000 microseconds/km, which I believe should be 4-5 microseconds/km. Please review this.

I did, and corrected the numbers.

Additionally, I miss in the manuscript a numerical assessment (and comparison with conventional SM fibers) of some key aspects for their use, such as: nonlinearity, splice losses, connectivity, light insertion/extraction losses, and coupling losses to amplifiers.

OK, a good suggestion. I added (in various places) data on first three parameters. About the last two, I don't have reliable information (especially the last one). Thank You.

Reviewer 4 Report

Comments and Suggestions for Authors

The author provides a comprehensive summary of the recent developments in hollow-core fibers, with a particular focus on the latest advancements made by key research teams and leading enterprises in the industry. The paper offers a wealth of valuable data and examples, evaluates progress, and provides forecasts for future developments. The paper is rich in information, and the extensive use of diagrams enhances the reading experience. I believe the paper can be accepted after addressing the following issue.

 

In Section 6, it is mentioned that "the current status of this technology, in particular the difficult manufacturing of active fibers with high bismuth concentration, severely limits amplifier performance." However, considering the rapid development of BDFA in recent years, significant attention has been directed toward the E- and S-bands. The challenges previously encountered are being overcome, and a number of highly promising results have emerged, which are expected to provide substantial support for the future application of these bands. Below are part of the papers from different institutions for your reference (not limited to these, and their citation is not mandatory):

1. “Ultra-wideband hybrid fiber amplifier achieves 215 nm net gain amplification in the E+S+C+L band”, DOI: https://doi.org/10.1016/j.optlastec.2025.113758
2. “Ultra-Wideband and Flat Gain Bismuth-Doped Fiber Amplifier Based on Double Pass Configuration”, DOI： https://doi.org/1109/LPT.2025.3614677
3. “Ultra-broadband optical amplifiers based on bismuth-doped heterogeneous-core fibers”, DOI：https://doi.org/10.1364/OL.567880
4. “Covering a 1280–1495 nm (215 nm) wideband high-gain bismuth-doped fiber amplifier with only 1240 nm pumping”, DOI：https://doi.org/10.1364/OL.540571
5. “High-gain ultra-wideband bismuth-doped fiber amplifier operating in the O + E + S band”, DOI：https://doi.org/10.1364/OL.525583

 

Author Response

In Section 6, it is mentioned that "the current status of this technology, in particular the difficult manufacturing of active fibers with high bismuth concentration, severely limits amplifier performance." However, considering the rapid development of BDFA in recent years, significant attention has been directed toward the E- and S-bands. The challenges previously encountered are being overcome, and a number of highly promising results have emerged, which are expected to provide substantial support for the future application of these bands. Below are part of the papers from different institutions for your reference (not limited to these, and their citation is not mandatory): [...]

A very useful comment! 
I must admit I was misguided by a quite pessimistic paper from one Russian group (now they write differently...).
I have re-worked and expanded the section 6, adding in particular: (a) descriptions of BDFAs and their properties, with a graph of gain spectrum for differing pump wavelengths, table of optical bands, and a vision of ultra-wideband fiber amplifier: a wavelength-multiplexed combination of BDFAs, EDFAs and TDFAs. Appears doable in the lab, but expensive and difficult to control (to ensure stable gain and flat gain spectrum). Thank You for suggested literature!

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Most of my concerns have been responded to, and it is now acceptable.