Narrow-Linewidth Tunable Fiber Laser Based on Laser-Induced Graphene Heated Fiber Bragg Grating with Low Voltage
Round 1
Reviewer 1 Report
Thank you for submitting your manuscript. The manuscript reports a tunable laser based on graphene-heated FBG and this manuscript is suggested for publication after minor revision.
1. What is the advantage of the proposed laser when compared with other types of tunable laser?
2. The author mentioned that the proposed laser is low-cost and compact. However, it requires another pump LD, multiple optical components, and expansive graphene material, which might be high-cost and less compact compared with the semiconductor laser. Could the author justify the claim?
3. Has the author studied the stability of the proposed laser?
4. Could the author explain the relatively large power fluctuation across different lasing wavelengths in Fig. 4(c)? What will be the impact on the actual application with those power fluctuations?
5. Please go through the manuscript to correct all the typos such as the “circulator” in Fig. 3(a).
6. More sensing applications scenarios (1. Cao, R. et al ,Fiber optical sensor for methane detection based on metal-organic framework/silicone polymer coating. In CLEO: QELS_Fundamental Science (pp. JW2A-168); 2. Lyu, Shuhan, et al. "Optical Fiber Biosensors for Protein Detection: A Review." In Photonics, vol. 9, no. 12, p. 987. MDPI, 2022) could be introduced in the introduction section for a comprehensive understanding of the significance of the role laser plays in sensing.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The author describes a narrow-linewidth tunable fiber laser based on laser-induced graphene paper heated (LIG-H) Fiber Bragg Grating (FBG) with low voltage. The linewidth is less than 600 Hz and the central wavelength of the output laser can be continuously adjusted from 1549.5 nm to 1552 nm. This method introduces the integration of LIG-H into wavelength tunable fiber lasers are cheap and simple, which show great relative applications. Therefore, I support the publication of the manuscript in Photonics. However, some modifications taking into account my following concerns.
1. The tuning range of temperature heating tuning is relatively small, so tunable grating can be considered to increase the tuning range
2. EDF2 can be further calculated and adjusted to the optimal length, which can improve the filtering mode selection ability, and achieve higher OSNR and narrower linewidth
3. Why the linewidth spectrum in Fig 3(f) is not smooth enough.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
The article presents the use of a laser-induced graphene (LIG) heater to thermally tune the wavelength of a fiber Bragg grating (FBG), which control the wavelength emission of a ring cavity laser around 1550 nm. The use of LIGs in general is recent and has not been applied, to my knowledge, to the tuning of FBGs in a laser cavity. The article starts with a concise (but perhaps a little too short) introduction on some methods used to tune the wavelength of fiber lasers and the recent fabrication of LIG. Half of the article is then dedicated to the method of fabrication of such LIG, then the rest is on the detail of the laser cavity and its tuning properties. I found that the article is similar to an article published in JLT in 2019 by other authors (https://doi.org/10.1109/JLT.2019.2892964), where a similar ring cavity design is demonstrated and the wavelength tunability is achieved by optically “pumping” a graphene-coated FBG. Still, I think that the present article is worth publishing due to the novel and simple tuning scheme that is much less complicated, after the following comments have been answered and the text revised (the comments are provided in order that they appear in the text):
· I find that the introduction is short, especially on LIG. Some acronyms are defined there, such as OTBF, FP-TF, IMZI, but are never used in the text. A spacing should be added before adding the reference number (good example [1], not good example[2]). The authors propose several wavelength tuning methods, but should also compare the tuning range and speed between them. For example, stretching and compressing a FBG can give a tunability of several tens of nanometer when properly packaged with minimal insertion losses, but Mach-Zehnder interferometer can go extremely fast since they have no moving parts. The first citation is a conference proceeding, but a quick search on Google Scholar shows that the authors of the proceeding published an article on the same subject in Optics Express, which might be more suitable for citing. LIG-H is defined and used in the introduction, but then only LIG is used in the rest of the article. PI and LIG-H are both defined in the abstract and both used in the introduction, however LIG-H is defined again in its first appearance in the introduction, while PI is only defined in the second section.
· On line 37, it is said that one of the benefits of LIG is the flexibility of its shape. On line 51, the LIG are produced by exposing a polyimide paper with a CO2 laser. From my experience, polyimide is quite flexible (at least in curvature). How does the physical properties of the polyimide sheet change once the LIG is fabricated? Does it remain flexible, or does it become brittle? Instead of using the paper flat on a glass substrate, could it be roll on itself to form a cylinder in which the fiber could be inserted? The reason I am asking this is because while reviewing the literature on LIG and FBG, I found this article that might interest you that has just been published where they directly transform the polyimide coating of an standard optical fiber into LIG, directly on a FBG (https://doi.org/10.1016/j.optlastec.2022.109047). This could reduce the size of the device significantly.
· On line 56, the speed and the power of the laser used to produce the LIG-H are provided. However, it is missing one critical element which is the spot size of the laser on the polyimide sheet. More detail should be given in the fabrication of LIG. How wide is a single line? What is the overlap between each line? How long does it take to fabricate a sample (for example the 20x10 one).
· On line 57, it is mentioned that “copper electrodes were attached to both sides of the sample and the gaps were filled with conductive silver paint, as shown in Figure 1(b).” As in Figure 1(a) and (c), could you indicate where each one is in the figure? How are the copper electrodes attached to the sample? What is the conductive silver paint that was used?
· Feel free to keep it the same, but I would prefer if Figure 1 was split in two figures: a first figure that includes (a), (b) and (c), showing the fabrication of the LIG, and a second figure that includes (d), (e) and (f), showing the characterisation of the LIG.
· The scales in Figure 1(d) and (e) are hard to see (black text on dark gray background).
· On line 68, “revealing” is not in the right tense (“revealed” would be more suitable).
· On lines 76-77, the origin of the two peaks seen in Figure 1(f) (top) at 23° and 43° are explained. Please add them on the figure, like how it’s done in the bottom part of the figure for the Raman shift.
· It is not clear if the five infrared thermographies in Figure 2(a) are made at the same voltage (5 V) as in (b)? If so, right now, the colormap is scale from the minimum to the maximum of each one, but I think it would be clearer if the colormap was the same for all five (from 19°C to 177°C), making it apparent that the 20x10 mm2 LIG-H is heating more than the other.
· On lines 98-99, the equations are referenced as Eq1 and Eq2, while on line 104, it is eq 1. I would go with Eq. (1), but please check in the authors’ instruction what the correct formatting is for this journal.
· On line 110, there is a typo: “to place the hole FBG” should be “to place the whole FBG”.
· On line 113, replace “of the voltage is 3 V” with “when the voltage is 3 V”?
· On lines 113-114, the following is mentioned: “It is obviously observed that the temperature rises fast”. I am not sure we can consider the 5-10 seconds (0.1-0.2 Hz) it takes to go from one step to the next fast, especially when physically tuning the FBG by stretching can reach kHz and interferometric techniques can reach GHz… Heating a FBG is one of the slowest (if not the slowest) methods to tune a FBG. Also, Eq. (1) is defined, but not used. There is “tau” in it, the time constant of the LIG, which would give how “fast” the LIG is heating. You should fit an exponential on your curve and provide its value. Also, while the glass slide underneath the LIG reduces heat loss (as mentioned on line 60), I think it also add heat capacity to the assembly, thus slowing a little its heating and its cooling, especially the latter.
· On line 114, it is mentioned that “a long-term heating and cooling” cycling is done in Figure 2(e). Again, I am not sure we can consider 20 cycles as “long-term”.
· On lines 118-120, about Figure 2(e), it is stated that “the profile clearly indicates that the temperature heats from ∼25 to ∼220 °C during the powered stage; and it is back to the original level after the cooling stage.”. Please reformulate this sentence, since on the figure, the cooling stage never goes bellow ~80°C because another heating cycle takes place before the LIG-H can reach room temperature.
· Please add a space between the text and the parenthesis in your figures. For example, in Figure 2(e), it should be “Time (s)” and “Temperature (°C)”, not “Time(s)” and “Temperature(°C)”.
· What is the resistance of each LIG-H? How much current is it drawing? How much power? It is mentioned in the conclusion that it could be battery-operated. Please provide values that confirm this statement. How does it compare to other methods used for tuning?
· There is a lot of labeling errors in Figure 3(a), which shows the components of the ring cavity laser. On line 127, it is said that the pump is labelled “LD”, but in the figure it is labelled as “976nm pump”. In the text, the optical isolators are labelled as IOS instead of ISO. Also, I don’t understand why ISO2 is needed in the ring cavity. In the figure, EDF1 is instead labelled as EDF2. The final “r” from circulator is missing in the figure (“Circulato”), the same thing for PC (“P” in the figure).
· How the OCs are drawn in Figure 3(a) and 3(d) is a little bit misleading. In 3(d), the "input" port is the middle one and the "output" ports are the two on the side. However, in 3(a), the middle port is now one of the output ports and one of the side ports is now the input port. Consider drawing them more like “--<“ or “—{“ (see for example Thorlabs website on 1x2 fiber coupler), or adding an arrow that clearly indicate the input port.
· What are EDF1 and EDF2? Are they homemade or bought from a manufacturer. My guess is they were bought since you provided the absorption coefficient at 1530 nm. Please provide the fiber model and the manufacturer. The length of EDF2 should be provided. Also, I am wondering why EDF2 is not the same fiber as EDF1? Is there any reason? Why not use a length of EDF1 that is 1/3 the length of EDF2 that you have use, thus giving the same absorption (7 dB/m divide by 20 dB/m is roughly 1/3)?
· What is the FBG? Was it written in-house? Was it bough (if so, provide information)? In what fiber? How long is it? UV-written or fs-written? Is it still coated? The latter is important because you are heating it up to 200°C and if it is still coated with acrylate, the coating will degrade in the long term. What about long-term operation of this FBG at 200°C? Was the FBG thermally annealed to stabilize its reflectivity?
· I am not an expert in self-heterodyne measurement, but from my understanding, 40 km is not long enough to correctly characterize the 600 Hz linewidth of a laser? (L = c/(n df))? When both arms recombine (the one with the delay fiber and the one with the AOM), they are still coherent with themselves and I think you are underestimating the real linewidth of your laser and another method should be used instead, like using a recirculating loop on your measurement setup. This might also be why the measured trace in Figure 3(f) have a sharper peak than the fit and two sharp side-lobed on both side of it. Can you comment on that? Also, what are the coupling ratio of OC1 and OC2 in Figure 3(d)? Since the delay fiber will have at least 8 dB of loss (0.2 dB/km x 40 km), I guess the optimal would be something like a 90:10 coupler for OC1 (90% for the delay fiber, 10% for the AOM), such that the incident power on OC2 (50:50) is the same for both arms? Note that I am not saying that your linewidth is off by several orders of magnitude, but more that I doubt the accuracy of it.
· “ESM” in Figure 3(d) is not defined anywhere (ESA is defined on line 149).
· On lines 165-167, I don’t understand what you mean by sentence: “Because the variation of neff is much lower than Λ in the thermal expansion model of FBG, central wavelength shows a linear relationship with Λ[17].”
· A curve is fitted in Figure 4(b). You don’t talk about it, but you provide Eq. (5). What is the fitted eta0?
· On line 179, it is said “when the FBG is placed on the LIG-H”. Can you provide more details on that? Is it just placed on top of it? Taped? Glued? How do you assure a good thermal contact?
· “4. Discussion” should be renamed “4. Conclusion”.
· In the Author Contributions, it is stated: “Conceptualization, S.G.”. Do you mean B.G.?
· In the Acknowledgements section, it is said “We are deeply grateful to my teachers […]”. Consider reformulating this sentence to indicate who the “my” refer to, such as “We are deeply grateful to Baoshan Gu’s teachers […]” or whoever the “my” refer to.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
The authors have answered to all my comments in my previous report with either good or excellent answers. I think that the readers could benefits from some of the answers that the authors gave me that have not been added in the text, but overall, it is my opinion that the manuscript is good enough to be accepted in its present form.
