Femtosecond Laser Direct Writing of Optical Overpass

Round 1

Reviewer 1 Report

For the study of femtosecond laser fabrication and its applications, the authors may refer this more recent paper: Nano-Micro Lett. 14,97,2022. The authors addressed the most issues well and it could be accepted.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

 

The author present a method of producing jumper wires and optical overpass based on fs laser irradiation on polymer composite. This would allow o overcome the current technical difficulties in waveguide crossing and, which limits the high integration of photonic chips.

While the method is interesting for researchers working in the fabrication of photonic integrated structures a few points need to be clarified

1.       It is not clear if the process of cross-linking happens under two-photon irradiation. While this non linear process can in principle provide sub diffraction sized structures, the wires fabricated and shown have micron sized dimension. Can the author comment on that?

2.      How much the insertion losses increase with the bending of the wires and with their size? 

3.       The fabrication method is applicable to polymer materials, while current integrated photonics is largely based on silicon or silicon nitride technology. So the problem of integration on these jumper wires to silicon technology must be solved. Can the author comment on that?

4.       What are the insertion losses of these polymer wires when connected to silicon waveguides?

 

Author Response

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Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Xiaochuan Ding et al. have presented new waveguide structure-photonic jumper wire to solve the technical glitch of waveguide crossing and parallel line wrapping, which is perplexing photonic chips to achieve high integration. Moreover, a more complex on-chip optical cross-connection is achieved. It is novel and a topic of interest to the researchers in the related areas. It could be published after the minor issues addressed.

1. For detailed observation of the Jumper wires, the higher magnification SEM should be given in Fig. 2.
2. The scale bar should be given in Fig. 4a.
3. What is the height of the Jumper wires? Relevant test characterization is required.
4. More relatived paper, regarding on femtosecond laser fabrication, may be introduced, for example: Nanoscale, 2017, 9 (37), 14229-14235
5. The authors should polish their writing. There are some grammar mistakes throughout the paper.

Reviewer 2 Report

The work is in general interesting, since it provides a way of fabricating the equivalent of photonic wire bonds by exploiting two photon polimerization, a result which would allow to interconnect different photonic chips. However, the paper as it is now is poorly written and quite difficult to read. Moreover, the work it presents is not new, since a very similar paper - with much sounder presentation and results - was already published in 2012 (Lindenmann, N., et al. "Photonic wire bonding: a novel concept for chip-scale interconnects." Optics express 20.16 (2012): 17667-17677). Due to the lack of novelty, unfortunately I cannot accept the publication of the paper.

Nevertheless, I am still providing the authors with my comments about the paper, in case they would like to resubmit the work after overcoming the novelty issue, e.g. by presenting a specific and innovative application of their approach.

1. The authors should provide a more specific introduction about the problems that could be solved by their approach, instead of giving only a too general introduction about the field of integrated photonics. For example, the analogy between electrical bonding and the authors’ work is not so immediate, and should be better highlighted. Moreover, the introduction should provide more sources, since there are several strong claims that are not well justified: it is not sufficient to list several technologies (e.g. the last paragraph of the first page) to increase the number of cited sources.
2. Are the authors sure that two photon polymerization can reach a resolution better than 10 nm? I did not find this value in the cited references.
3. The materials and methods should be revised to be more readable, instead of being only a list of the used elements.
4. The SU-8 deposition process is not completely clear, and should be better explained. What is the material the jumper is fabricated on? First a cover glass is mentioned, but then the authors say that the photoresist is distributed over a silicon wafer. 
5. The characterization of length and height of the jumper would be more complete by evaluating also the insertion loss of the devices, and not only by a SEM inspection. Do the authors have data about it? Moreover, I did not understand which was the best height of the jumper, since it is not clearly mentioned. Is there one? Or is it sufficient that the jumper does not collapse on the substrate?
6. The insertion loss reported in the article should be better contextualized. In particular, do the author have an idea of the loss caused by the presence of the jumper? The 6.3 dB value should be compared with a similar waveguide fabricated in the same chip but without jumper, to understand if the insertion loss is caused by the jumper or by the waveguide itself (a standard value reported in literature is not sufficient, and anyhow the authors do not provide the length of the waveguide, so it is not possible to estimate the contribution of the propagation loss).
7. Do the authors have data about the overpass? Did they measure the transmission of the device?
8. Why do the authors claim, in the conclusion, that the transmission of the jumper is almost the same as a standard waveguide? This is simply not true, as they reported in the section before.
9. Since this work highlights the importance of jumpers and overpass in integrated photonics and optical interconnects, do the authors have an idea about how to integrate their approach with the most used platforms, such as silicon photonics? In this case, I would expect them to exploit the evanescent coupling between the silicon waveguide and the SU-8 one. Do the authors think that the different in mode dimension (hundreds of nm versus few um) could be a challenge that should be overcome?