Diamond-Based Fiber-Optic Fabry–Perot Interferometer with Ultrawide Refractive-Index Measurement Range
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors demonstrated a new type of FPI, fabricated by bonding a thin and flat diamond on the apex of an optical fiber. They utilized this FPI as an refractive index sensor with an ultra-wideband measurement range. Due to the high refractive index of diamond, the proposed sensor shows a good interfernce visibility in the refractive index range of 1-2.2, which is conducive to the accurate measurement of RI in a large RI range. Based on the aforementioned, I am in favor of the publication of this work in Photonics.
Comments:
1. Given that the refractive index of diamond is 2.4, it can be reasonably assumed that the FPI should have some visibility in the refractive range of 2.3 to 2.4. However, the author has stated that the working RI range is 1.0 to 2.2, rather than 1.0 to 2.4. This discrepancy requires further clarification.
2. The temperature measurement results presented in Fig. 7 should be included in the section of the paper designated as "Experiment and Results," rather than in the section designated as "Discussion."
3. In line 184, page7 , the paper title of reference 14 has some error, it should not be "Crystallization-sapphire-derived-fiber-based Fabry–Perot interferometer for refractive index and high-temperature measurement."
Author Response
Comments 1: Given that the refractive index of diamond is 2.4, it can be reasonably assumed that the FPI should have some visibility in the refractive range of 2.3 to 2.4. However, the author has stated that the working RI range is 1.0 to 2.2, rather than 1.0 to 2.4. This discrepancy requires further clarification.
Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have changed the last line in the abstract of the revised manuscript to “The FPI cavity is constructed from a diamond with an RI of approximately 2.4, which theoretically enables the sensor to achieve an ultrawide RI measurement range of 1 to 2.4. ” This change can be found on page 1, lines 4 , 5 and 6.
Comments 2: The temperature measurement results presented in Fig. 7 should be included in the section of the paper designated as "Experiment and Results," rather than in the section designated as "Discussion."
Response 2: Thank you for pointing this out. We agree with this comment. Therefore, we have moved the temperature measurement results to the designated “Experiment and Results” section in the revised manuscript. This change can be found on page 6, lines 133 and 134, which reads ” ...The results shown in Fig.7 indicate that the visibility of the FPI has a maximum fluctuation of approximately 0.0035, corresponding to an RI error of about 8.2 × 10−3.”
Comments 3: In line 184, page7 , the paper title of reference 14 has some error, it should not be "Crystallization-sapphire-derived-fiber-based Fabry–Perot interferometer for refractive index and high-temperature measurement."
Response 3: Thank you for pointing this out. We have corrected the reference error in the revised manuscript, which reads ”...Crystallization-sapphire-derived-fiber-based Fabry-Perot interferometer for refractive index and high-temperature measurement.” This change can be found on page 8, lines 242 and 243.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsI found that this paper has been published online. All the pictures and text are basically the same.
https://preprints.opticaopen.org/articles/preprint/Ultrawide_refractive-index_measurement_range_fiber_optic_Fabry-Perot_interferometer_based_on_diamond/25219217
Comments on the Quality of English LanguageMinor editing of English language required.
Author Response
Comments 1: I found that this paper has been published online. All the pictures and text are basically the same.
https://preprints.opticaopen.org/articles/preprint/Ultrawide_refractive-index_measurement_range_fiber_optic_Fabry-Perot_interferometer_based_on_diamond/25219217
Response 1: Thank you, this is correct, this manuscript is the improved edition of that reprint. We have stated when we submitted this manuscript.
Comments 2: Minor editing of English language required.
Response 2: Thank you, we have edited the English language in the revised manuscript and the changes are highlighted in red.
Author Response File: Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsA simple and interesting scheme is proposed in this manuscript for refractive index measurement within a wide range, though the sensitivity is limited. Here are a few suggestions:
1. According to Fig. 3, the sensitivity curves versus refractive index can be obtained, which helps to visually compare the sensitivity for two cases.
2. As measured reflection spectrum in Fig. 2, the reflection visibility of the diamond FPI is not uniform within the measured wavelength range, please give the explanations, and how to accurately determine the maximum visibility? Does the cavity length of FPI affect the accuracy of determining maximum visibility? Please discuss this issue.
3. In Fig. 5, there is a minor peak near 1565 nm, please explain it.
4. For this case, as shown in Fig. 5, how to determine the maximum visibility of the measured interferometric spectrum of the diamond-based FPI? How to evaluate the uncertainty of the maximum visibility?
Comments on the Quality of English Languagegood
Author Response
Comments 1: According to Fig. 3, the sensitivity curves versus refractive index can be obtained, which helps to visually compare the sensitivity for two cases.
Response 1: Thank you for pointing this out. We have modified Fig.3 and added the interference visibility derivation shown in Fig.3(b) as the sensitivity curves versus the refractive index. We also added the comparisons of the theoretical RI performance of the diamond-based FPI and the air-plus-silica hybrid cavity FPI, which are shown in Fig.3(c) and (d).
Comments 2: As measured reflection spectrum in Fig. 2, the reflection visibility of the diamond FPI is not uniform within the measured wavelength range, please give the explanations, and how to accurately determine the maximum visibility? Does the cavity length of FPI affect the accuracy of determining maximum visibility? Please discuss this issue.
Response 2: Thank you for your comments. We have explained the non-uniformity of reflection visibility caused by the non-uniformity of light source intensity. And we identified the peak with the highest intensity and its neighboring valley to accurately determine the maximum visibility. As the light beam divergence in the diamond, the cavity length increase will reduce the reflected light intensity coupling into the optical fiber, thus its visibility will decrease. We have added the discussions in the revised manuscript on pages 3 and 4, lines 70-78, which is marked in red and reads as follows
“It can be observed that the FPI interference spectrum exhibits varying degrees of visibility at different wavelengths. This phenomenon can be attributed to the non-uniformity of the light source intensity. The resonance peaks form a shape similar to that of the light source intensity spectrum. Furthermore, as the divergence of the light beam increases in proportion to the length of the cavity, an increase in the thickness of the diamond results in a reduction in the intensity of the reflected light coupled back to the optical fiber, thereby diminishing its visibility. To accurately determine the maximum visibility of the interference spectrum, we selected the peak with the highest intensity as Ir.max and its neighboring valley as Ir.min in Eq.(4).”
Comments 3: In Fig. 5, there is a minor peak near 1565 nm, please explain it.
Response 3: Thank you for pointing this out. The minor peak near 1565 nm is caused by the light source because the light source we used has a minor peak near 1565 nm. We have added the explanation in the revised manuscript on page 5, lines 115 and 116, which is marked in red and reads as follows
“The minor peak observed near 1565 nm is attributed to the light source, as the light source, exhibits a similar minor peak at approximately 1565 nm”
Comments 4: For this case, as shown in Fig. 5, how to determine the maximum visibility of the measured interferometric spectrum of the diamond-based FPI? How to evaluate the uncertainty of the maximum visibility?
Response 4: Thank you for your comments. For the Fig.5 case, we basically used the aforementioned methods: we selected the peak with the highest intensity as Ir.max and its neighboring valley as Ir.min in Eq.4. For the Fig.5 , Ir.max is the fourth peak near 1595 nm and Ir.min is the third valley near 1584 nm. And the uncertainty is evaluated by repeated measurements. We have added these explains in the revised manuscript on page 5, lines 116-120, which is marked in red and reads as follows
“ The visibility of the FPI is calculated using Eq. 4 by selecting the peak with the highest intensity as Ir.max, which is the fourth peak near 1595 nm, and its neighboring valley as Ir.min, which is the third valley near 1584 nm in Fig.5. The uncertainty associated with these measurements can be evaluated by conducting repeated measurements. ”
Author Response File: Author Response.docx
Reviewer 4 Report
Comments and Suggestions for AuthorsThis manuscript proposed a fiber Fabry-Perot interferometer sensor based on thin diamond film. The fabrication method and principle for the proposed sensor is quite simple. However, there are areas where the logical flow and coherence can be improved to enhance readability and overall quality. Here are some suggestions:
1. The abstract is detailed but slightly verbose and repetitive in places. Please simplify the abstract to emphasize the methodology, key results, and conclusions more effectively.
2. How is the experimental result of the sensor with different thickness of the diamond film. More contrast experiments with different fabrication parameters are need.
3. Why the temperature response for the sensor is non-linear.
4. Authors are need to stress the novelty of the paper in the discussion
5. In Conclusions, the author should describe the practical value of presented results and the direction of your further work.
Comments on the Quality of English Languagenone
Author Response
Comments 1: The abstract is detailed but slightly verbose and repetitive in places. Please simplify the abstract to emphasize the methodology, key results, and conclusions more effectively.
Response 1:Thank you for pointing this out. We have re-modified the abstract in the revised manuscript, the changes are marked in red and the abstract reads as follows.
“Majority of FPI tip RI sensors utilize silica optical fiber as the cavity material, with an RI 1
of approximately 1.45. This restricts their applicability in measuring the RI of liquids with a RI of approximately 1.45. Here, We propose a fiber optic FPI tip RI sensor by bonding a flat, thin diamond film onto the apex of a single-mode optical fiber. The FPI cavity is constructed from diamond with an RI of approximately 2.4, which theoretically enables the sensor to achieve an ultrawide RI measurement range of 1 to 2.4. A theoretical comparison of its measurement performance was conducted with an FPI tip RI sensor, whose cavity is formed by a silica fiber. Additionally, an experimental examination of the device’s RI measurement performance was conducted. The results show that the sensor has visibility to the RI unit of -0.4362/RIU in RI range of 1.33 to 1.40. Combined with other narrow RI range, high-sensitivity sensors, our proposed RI sensor has the potential to be utilized in a wide range of applications. ”
Comments 2: How is the experimental result of the sensor with different thickness of the diamond film. More contrast experiments with different fabrication parameters are need.
Response 2: Thank you for your comments. As the divergence of the light beam increases in proportion to the length of the cavity, an increase in the thickness of the diamond will result in a reduction in the intensity of the reflected light coupled back to the optical fiber, thereby diminishing its visibility. Accordingly, it can be postulated that a diamond of greater thickness will result in a reduction in visibility, consequently affecting the RI sensitivity of the measurements. It is regrettable that our laboratory does not possess the necessary facilities to prepare diamonds of varying thicknesses.
Comments 3: Why the temperature response for the sensor is non-linear.
Response 3: Thank you for your comments. The temperature non-linear response for the sensor is caused by the UV-curing glue which is not temperature stable, under temperature changes, its affected the coupling efficiency of light between the diamond and optical fiber, thus affected the visibility. We have added these in the revised manuscript on page 6, lines 158-162. The related content are marked in red, which reads as follows
“Our fabricated FPI exhibits temperature-induced non-linear fluctuations in visibility, as shown in Fig.7. This phenomenon can be primarily attributed to the high thermal expansion of the UV-curing adhesive used for bonding the diamond. This expansion, due to rising temperatures, affects the light coupling between the diamond and the optical fiber, thereby influencing the overall performance of the RI sensor. ”
Comments 4: Authors are need to stress the novelty of the paper in the discussion
Response 4: Thank you for pointing this out. We have added these in the revised manuscript on page 7, lines 136-141. The related content is marked in red which reads as follows
“The available RI sensors and instruments are typically designed for short-range RI measurements. For instance, even the most popular RI instrument, the Abbe refractometer, has an RI measurement range of 1.30 to 1.70. In comparison, our proposed diamond-based FPI offers an RI measurement range of 1.0 to 2.4, thereby addressing the shortage of wide-range RI sensors in practical applications. Furthermore, our proposed diamond-based FPI tip RI sensor is straightforward to fabricate and cost-effective.”
Comments 5: In Conclusions, the author should describe the practical value of presented results and the direction of your further work.
Response 5: Thank you for your suggestion. We have rewritten the conclusions and added the practical value of the proposed FPI tip type RI sensor and our future work directions, such as using alternative bonding material or techniques to improve its temperature stability. And also searching for other high RI crystal/glass to optimize the fabrication and performance of the proposed FPI configuration.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors replied to the review suggestions and revised the manuscript. However, the responses and revision are not completely compelling. More importantly, the answers to the previously listed questions demonstrate a lack of rigor in the experiments as well as the unreliability of the experimental findings.
Regarding to the nonuniform visibility of the diamond FPI shown in Fig. 2 and the minor peak near 1565 nm shown in Fig. 5, the authors explained those are caused by the non-uniformity of light source intensity, which implies the experimental data shown in Fig. 2 and Fig. 5 are absolution reflection light intensity, and they are affected by the light source. It should be noted that the Eq. (3) in section of operation principle is "the FPI’s normalized total reflective intensity", i.e., the normalized total reflective intensity can be considered as the reflection of the FPI, and it should not be affected by the light source intensity. To obtain the correct reflection of the FPI, the light source intensity should be previously measured and then be subtracted from the measured absolution reflection light intensity of the FPI. Therefore, the most important experimental data for this manuscript shown in Fig. 6 and 7 which are derived from the absolute light intensity are unreliable.
It is well understood that the cavity length of the FPI effects the FSR of the FPI’s reflection spectrum, which then influences the distance between the adjacent peak and valley. The authors claim that “we selected the peak with the highest intensity as Ir.max and its neighboring valley as Ir.min in Eq. (4)”, then how do you believe the cavity length affects the accurate determination of the maximum visibility?
It is proposed that the authors measure the spectrum of the light source, deduct the influence of the light source while analyzing the reflection of the FPI, and revise the manuscript for resubmission.
Comments on the Quality of English Languagegood
Author Response
Comments 1: Regarding to the nonuniform visibility of the diamond FPI shown in Fig. 2 and the minor peak near 1565 nm shown in Fig. 5, the authors explained those are caused by the non-uniformity of light source intensity, which implies the experimental data shown in Fig. 2 and Fig. 5 are absolution reflection light intensity, and they are affected by the light source. It should be noted that the Eq. (3) in section of operation principle is "the FPI’s normalized total reflective intensity", i.e., the normalized total reflective intensity can be considered as the reflection of the FPI, and it should not be affected by the light source intensity. To obtain the correct reflection of the FPI, the light source intensity should be previously measured and then be subtracted from the measured absolution reflection light intensity of the FPI. Therefore, the most important experimental data for this manuscript shown in Fig. 6 and 7 which are derived from the absolute light intensity are unreliable.
Response 1: Thank you for your comments. We can use "the FPI’s normalized total reflective intensity", and we previously used it to simplify in Eq. (3). However, there is no distinction in the calculation of visibility between the use of "normalized reflective intensity" and "unnormalized reflective intensity." Eq. (4) entails the subtraction of intensity, which ultimately yields a visibility value that is identical to that obtained through normalization. Consequently, we opted for simplicity in our experimental setup by not normalizing the light sources. When employing Eq. (4) to calculate visibility, there is no distinction between normalized and non-normalized light sources. In the revised manuscript, we have restored the original form of both Eq. (3) and Eq. (4) in an unnormalized form ( on page 3, lines 64 to 66).
Comments 2: It is well understood that the cavity length of the FPI effects the FSR of the FPI’s reflection spectrum, which then influences the distance between the adjacent peak and valley. The authors claim that “we selected the peak with the highest intensity as Ir.max and its neighboring valley as Ir.min in Eq. (4)”, then how do you believe the cavity length affects the accurate determination of the maximum visibility?
Response 2: Thank you for your comments. From Eq. (4), it can be seen that the highest visibility is achieved when the reflected light intensity from the two mirrors (the diamond and fiber contact interface and the diamond end surface) are equal. This is R1 equals to R2(1 − α)2(1 − R1)2. However, due to the loss caused by the coupling loss and light beam divergence in the diamond (α), R2(1 − α)2(1 − R1)2 is typically smaller than R1 . Consequently, it is not possible to obtain the maximum visibility of "1". As the cavity length increases, the loss caused by the coupling loss and light beam divergence in the diamond (α) also increases. This is due to a number of factors, including an increase in the size of the light beam, a decrease in the amount of light coupled back to the fiber, and an overall increase in light propagation loss. Consequently, R2(1 − α)2(1 − R1)2 decreases, and the visibility also decreases. We have added the discussions in the revised manuscript on page 4, lines 78-89, which is marked in red and reads as follows
“Another factor that affects the visibility of the FPI is the cavity length or diamond thickness. From Eq. (4), it can be seen that the highest visibility is achieved when the reflected light intensities from the two mirrors, the fiber-diamond interface, and the diamond interface are equal. This is represented by R1 equals to R2(1 − α)2(1 − R1)2. However, due to the losses caused by coupling and light beam divergence in the diamond (α), it is typically the case that R2(1 − α)2(1 − R1)2 is smaller than R1, thus preventing the attainment of the maximum visibility value of 1. As the divergence of the light beam increases in proportion to the length of the cavity, an increase in the thickness of the diamond results in a reduction in the intensity of the reflected light coupled back to the optical fiber, indicated by the increase in the value of α. Consequently, the value of R2(1 − α)2(1 − R1)2 diminishes, and the degree of visibility declines.”
Comments 3: It is proposed that the authors measure the spectrum of the light source, deduct the influence of the light source while analyzing the reflection of the FPI, and revise the manuscript for resubmission.
Response 3: Thank you for your comments. We have added a analyzing explanation in the revised manuscript on page 5, lines 115 and 116, which is marked in red and reads as follows
“It can be observed that the FPI interference spectrum exhibits varying degrees of visibility at different wavelengths. This phenomenon can be attributed to the non-uniformity of the light source intensity. In theory, as indicated by Eq. (4), the light source intensity will ultimately be subtracted, meaning that the visibility will not be contingent on the intensity of the light source. In practice, when the lowest light intensity detectable by the optical spectrum analyzer (OSA) is limited, and when the resonance valley Ir.min falls below the noise floor of the OSA, the only result obtained Ir.min is the noise floor. The resonance peaks Ir.max take on a shape similar to that of the light source’s intensity spectrum. Consequently, as the light source’s intensity decreases, the obtained visibility also decreases, falling below the theoretical visibility.”
Author Response File: Author Response.docx
Reviewer 4 Report
Comments and Suggestions for AuthorsThe manuscript can be pulished in thg Photonics in the present form
Comments on the Quality of English Languagenone
Author Response
The English of the manuscript has been revised, and the changes are indicated in red in the revised manuscript.
Round 3
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors answered the questions mentioned previously and revised the manuscript accordingly, but I do not fully accept the authors’ viewpoints.
1. Regarding to the nonuniform visibility of the diamond FPI shown in Fig. 2 and the minor peak near 1565 nm shown in Fig. 5, the authors explained those are caused by the non-uniformity of light source intensity, which means the reflection light intensity are indeed affected by the light source. Subsequently, the calibration function, as shown in Fig. 6, is not only bound to the sensor but also to a specific light source, which greatly weakens the portability and applicability of the sensor. If a new light source is used, the calibrated sensor must be recalibrated. I still suggest deducting the interference of the light source to explore the characteristics of the sensor itself.
2. Regarding to the effect of cavity length on the accuracy of RI measurement, liquid features with dispersion characteristics, that is, its RI depends on the wavelength. For a FPI with a shorter cavity length, the FSR of its reflection spectrum is larger, and the distance between the adjacent resonance peak and dip is also larger up to tens or even more than a hundred of nanometers. The RI obtained by applying the algorithm in this manuscript is the average RI within this wavelength range, which does not accurately reflect the dispersion characteristics of the liquid. Conversely, for a FPI with a long cavity length, the distance between the adjacent resonance peak and dip is small, the RI of liquid can be determined within a narrow wavelength range, while the RI sensitivity will be decrease since the visibility of the reflection spectrum of the sensor decreases. Therefore, there is a tradeoff between the RI measurement accuracy and the sensitivity, i.e., the cavity length of the FPI should be optimized.
3. Additionally, please provide the actual measured reflection spectra of the sensor in liquids with different RIs to support the results in Fig. 6.
Comments on the Quality of English Languagegood
Author Response
Comments 1: Regarding to the nonuniform visibility of the diamond FPI shown in Fig. 2 and the minor peak near 1565 nm shown in Fig. 5, the authors explained those are caused by the non-uniformity of light source intensity, which means the reflection light intensity are indeed affected by the light source. Subsequently, the calibration function, as shown in Fig. 6, is not only bound to the sensor but also to a specific light source, which greatly weakens the portability and applicability of the sensor. If a new light source is used, the calibrated sensor must be recalibrated. I still suggest deducting the interference of the light source to explore the characteristics of the sensor itself.
Response 1: Thank you for your comments. Unfortunately, the diamond optical fiber interface and diamond-measured sample interface have higher reflection coefficients determined by Eq.(1). After placing the diamond to form the FPI, the reflected intensity increases greatly, which makes the deduction very difficult. The figure below shows the spectrum of the original light source reflected by the flat fiber end, and the diamond-based FPI in air and water, respectively. By Eq.(4), the visibility is in principle independent of the light source intensity, unfortunately, the OSA has its sensitivity limits, when the light source was changed, recalibration is needed.
Comments 2: Regarding to the effect of cavity length on the accuracy of RI measurement, liquid features with dispersion characteristics, that is, its RI depends on the wavelength. For a FPI with a shorter cavity length, the FSR of its reflection spectrum is larger, and the distance between the adjacent resonance peak and dip is also larger up to tens or even more than a hundred of nanometers. The RI obtained by applying the algorithm in this manuscript is the average RI within this wavelength range, which does not accurately reflect the dispersion characteristics of the liquid. Conversely, for a FPI with a long cavity length, the distance between the adjacent resonance peak and dip is small, the RI of liquid can be determined within a narrow wavelength range, while the RI sensitivity will be decrease since the visibility of the reflection spectrum of the sensor decreases. Therefore, there is a tradeoff between the RI measurement accuracy and the sensitivity, i.e., the cavity length of the FPI should be optimized.
Response 2: Thank you for your comments. The dispersion characteristics of the liquid are complex, and more related to the choice of light source and the resolution of the OSA used for the measurements. We agree, that a longer FPI cavity length will narrow the measurement wavelength range, but unfortunately, our FPI RI sensor is more about wide range but not that sensitive. It is better to combine with other narrow-range but more sensitive RI sensors to characterize the dispersion of the measured liquid sample.
Comments 3: Additionally, please provide the actual measured reflection spectra of the sensor in liquids with different RIs to support the results in Fig. 6.
Response 3: Thank you for your comments. We have added the actual measured reflection spectra of the sensor in liquids with different RIs in Fig.6 as Fig.6(a).
Author Response File: Author Response.docx
