System for Stabilizing an Operating Point of a Remote Electro-Optical Modulator Powered by Optical Fiber

Round 1

Reviewer 1 Report

Dear editor,

In this paper, authors present the possibility of using signal optical radiation to implement the electrical power supply for a stabilization system of the operating point of a remote integrated optical Mach-Zehnder modulator based on lithium niobate. A scheme for bias control and stabilization with low power consumption was developed. This work is meaningful. I have some issues as follows.

1. Authors may illustrate Fig. 3 in detail as it’s the core of this paper.
2. I am interesting in the algorithm for stabilizing the operating point. Please add it if possible.
3. It seems that authors should do related experiments to verify the significance of optical fiber.

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

We are sending a revised version of " System for stabilizing an operating point of a remote electro-optical modulator powered by optical fiber" (reference number 978977) by Aleksei Petrov, Aleksandr Tronev, Peter Agruzov, Aleksandr Shamrai and Vladimir Sorotsky for publication in Electronics. Comments of the reviewers were highly insightful and enabled us to greatly improve the quality of our manuscript. In the following pages are our point-by-point responses to each of the comments of the reviewers.

Point 1: Authors may illustrate Fig. 3 in detail as it’s the core of this paper.

Response 1: A more detailed description of the characteristics presented in graph 3 has been added to the text of the article. (Lines 223-242)

«Fig. 3 demonstrates the successful start of the stabilization system from an optical power of 15 mW at the input of the photovoltaic converter. The blue curves represent the voltage at the output of the DC-DC converter supplying the operational amplifier. The converter did not start at 10 mW optical input power and provided stable output voltage about 8 volts at 15 mW. The corresponding electrical power consumption could be estimated from known photovoltaic converter efficiency of 30% and was smaller than 5 mW. The gray curves correspond to the voltages at the output of the photovoltaic converter when the input optical power is applied. The red curves in Fig. 3 shows the response of the output voltage of the stabilization system when it is turned on. Zero control signal was at 10 mW optical power. The exit to the working point and its stabilization was observed at 15 mW optical power.»

 

Point 2: I am interesting in the algorithm for stabilizing the operating point. Please add it if possible.

 

Response 2: The algorithm has been added (Lines 155-161)

«The algorithm of the operating point stabilization system can be briefly presented as follows:

1. Calculation of the signal error function by analyzing the difference between the input and the reference signal.
2. Formation of the control signal as the sum of three components (proportional, integral and differential components).
3. Subtracting the control signal from the input signal.
4. Re-analysis of the error and repetition of the cycle from 1-3.»

 

Point 3: It seems that authors should do related experiments to verify the significance of optical fiber.

 

Response 3: The comment is not completely clear. Several studies were carried out to identify the factors limiting the transmission of energy through optical fiber, as well as to determine the maximum optical power that can be transmitted through a single-mode optical fiber. Some references to the related papers was given in the “Introduction” [11 – 16]. The goal of this work was to minimize optical power needed for powering of the stabilization system. The wavelength range about 1550 nm was chosen, corresponding to the minimum optical loss, a comment has been added to the text. (Lines 59-66) The ranges of the paper do not allow more detailed description.

 

«In this paper, we consider the possibility of using signal optical radiation to implement the electrical power supply for a stabilization system of the operating point of a remote integrated optical Mach-Zehnder modulator based on lithium niobate. The goal was a minimization of the optical power delivered for stabilization system powering. The choice of the optical range near 1550 nm for power transmission is determined by the requirement to minimize losses in the optical fiber when transmitting power over long distances (more than 1 km). A scheme for bias control and stabilization with low power consumption was developed and stable operation of the modulator at an optical power level of less than 20 mW at its input was demonstrated.»

 

Reviewer 2 Report

In this paper, the authors demonstrate a system for stabilizing the operating point of Mach-Zehnder modulators remotely. The authors claim the demonstration low power consumption of less than 5 mW and illustrate improved common mode noise suppression using an additional Mach-Zehnder modulator. The paper has some merit but must be improved significantly. Here are my comments:

1. The introduction needs to be improved as in the present form it is more like a historical account of fiber-optic communication. Please describe the problem and the method you are trying to solve. The last two paragraphs in the Introduction should be expanded.
2. References are missing. For example, the authors say that modulators usually work with polarized light at the input (line 69). Give references to support this.
3. Rather than using "laid" on line 78, please consider using the phrase "are located".
4. On lines 109-110, please provide a reference for the integrated optical modulator.
5. Figures of the set-up would strengthen this paper, but all that is shown is schematics. Please consider adding photos of the set-up.
6. The authors say that the photodiodes had a large internal capacitance. How much was it, and what would be the highest operating frequency?
7. Figure 2 is not necessary. Consider replacing it with a graph of how the parameters for the PID were optimized.
8. The paper claims that lower than 5 mW power was used. Where is this power and how was it estimated?
9. What are the numbers in Figure 4?
10. Figure 5 is drawn without explanation. Were the measurements taken at 0.5 GHz intervals as the line appears to have more points from 0 to 1 GHz and becomes more discrete after 1 GHz. How does the commom-mode rejection value compare to previous works? Please provide comparison and what could be done to improve it further.

Author Response

Dear reviewer,

We are sending a revised version of " System for stabilizing an operating point of a remote electro-optical modulator powered by optical fiber" (reference number 978977) by Aleksei Petrov, Aleksandr Tronev, Peter Agruzov, Aleksandr Shamrai and Vladimir Sorotsky for publication in Electronics. Comments of the reviewers were highly insightful and enabled us to greatly improve the quality of our manuscript. In the following pages are our point-by-point responses to each of the comments of the reviewers.

 

Point 1: The introduction needs to be improved as in the present form it is more like a historical account of fiber-optic communication. Please describe the problem and the method you are trying to solve. The last two paragraphs in the Introduction should be expanded.

 

Response 1: A more complete description of the works aimed to the transmission of energy through optical fiber was given in the “Introduction” and related references were added [11 – 16].

 

Point 2: References are missing. For example, the authors say that modulators usually work with polarized light at the input (line 69). Give references to support this.

 

Response 2: We added following references (Line 73)

«19. Chang, W. S. C. et al. (ed.). RF photonic technology in optical fiber links. Cambridge University Press, 2002.

20. Chen A., Murphy E. (ed.). Broadband optical modulators: science, technology, and applications. – CRC press, 2011.»

 

Point 3: Rather than using "laid" on line 78, please consider using the phrase "are located".

 

Response 3: The authors agree with the remark of the reviewer, corrections were made in the text of the article (Line 82)

 

Point 4: On lines 109-110, please provide a reference for the integrated optical modulator.

 

Response 4: We added following references (Line 114)

«24. Petrov, A., Lebedev, V., Agruzov, P., Ilichev, P., Shamrai, A., Liokumovich, L., Kudryashova, T., Sorotsky, V. Lithium Niobate Direction Coupler Modulator for Linearization of Analog Optical Signal Transmission In 2020 IEEE International Conference on Electrical Engineering and Photonics (EExPolytech). 2020. IEEE: Saint Petersburg, Russia, pp. 80-84.

25. Petrov, A., Parfenov, M., Lebedev, V., Ilichev, I., Agruzov, P., Tronev, A., Shamrai, A. Dynamic Range Improvement of Broad-band Analog Fiber Optic Links with Special Lithium Niobate Integrated Optical Modulators. In Internet of Things, Smart Spaces, and Next Generation Networks and Systems. 2020. Springer, Cham.»

 

Point 5: Figures of the set-up would strengthen this paper, but all that is shown is schematics. Please consider adding photos of the set-up.

 

Response 5: The photos were added to Figure 4 (Line 262)

 

Point 6: The authors say that the photodiodes had a large internal capacitance. How much was it, and what would be the highest operating frequency?

 

Response 6: Clarification was made in the text (Lines 150-153).

 

«Photodiodes with a rather large internal capacitance about 2 pF, which determined their bandwidth (< 1 GHz), were used for the stabilization system. The bandwidth of the feedback was advisedly reduced to 10 MHz to maintain the system’s stability when a high-frequency baseband signal is applied to the RF input of the optical modulator.»

 

Point 7: Figure 2 is not necessary. Consider replacing it with a graph of how the parameters for the PID were optimized.

 

Response 7: We agree that Figure 2 presenting general scheme of a PID controller could be omitted. However, we decide leave this figure for the reader specializing in optical telecommunication who have not experience in automatic control systems. Addition explanation about PID parameters choice was made (Lines 195-200).

 

«The contributions from the components are determined by the values of the corresponding coefficients, which are set taking into account the characteristics of a particular system. The values of the PID controller coefficients were searched consequently and ensured stability of the system, as well as minimum settling time of 20 seconds from the initial position to the quadrature point. Recursive methods were used for calculating the control voltage of the controller to increase the speed of the stabilization system.»

 

Point 8: The paper claims that lower than 5 mW power was used. Where is this power and how was it estimated?

 

Response 8: In the experimental demonstration the incident optical power needed for operating point stabilzation was measured using an optical power meter at the input of the photovoltaic converter (15mW), the efficiency of photovoltaic converters was about 30%, respectively, the output electrical power was about 5mW. The comments were added to the text (Lines 223-242).

 

«Fig. 3 demonstrates the successful start of the stabilization system from an optical power of 15 mW at the input of the photovoltaic converter. The blue curves represent the voltage at the output of the DC-DC converter supplying the operational amplifier. The converter did not start at 10 mW optical input power and provided stable output voltage about 8 volts at 15 mW. The corresponding electrical power consumption could be estimated from known photovoltaic converter efficiency of 30% and was smaller than 5 mW. The gray curves correspond to the voltages at the output of the photovoltaic converter when the input optical power is applied. The red curves in Fig. 3 shows the response of the output voltage of the stabilization system when it is turned on. Zero control signal was at 10 mW optical power. The exit to the working point and its stabilization was observed at 15 mW optical power.»

 

Point 9: What are the numbers in Figure 4?

 

Response 9: Figure 4 has been changed and components of the setup was denoted directly on the picture (Line 262).

 

Point 10: Figure 5 is drawn without explanation. Were the measurements taken at 0.5 GHz intervals as the line appears to have more points from 0 to 1 GHz and becomes more discrete after 1 GHz. How does the commom-mode rejection value compare to previous works? Please provide comparison and what could be done to improve it further.

 

Response 10: Figure 5 was replaced with more accurate data in all frequency range 0.1 – 5 GHz taking into account the comments of the reviewer. The deterioration of noise suppression with increasing frequency is associated with an increase in requirements for alignment of optical paths rather than failure of the operating point stabilization. To improve suppression in a wider frequency range alignment of the optical cable lengths with greater precision is required. It should be noted that the goal of this work was to demonstrate the accuracy of the stabilization system, and the fact that it does not introduce measurable noise, and the suppression of common mode noise was given for this purpose. Corresponding comments were added to the text (Lines 271-281).

 

« Figure 5. Common mode noise suppression by balanced detection. “1det” - system transmission coefficient curve with a single detector connected (first one); “1det_2” - system transmission coefficient curve with a single detector connected (the second one); “2det” curve - difference between two channels; “Projected transmission” – system transmission coefficient curve with balance detection; “Suppression” - system common-mode rejection with balance detection.

 

It has been demonstrated that it is possible to effectively reject common-mode interference by more than 14 dB in the frequency bandwidth up to 3 GHz. To further improve noise suppression, it is required to align the optical cable lengths with greater precision. It has also been established that the stabilization system of the operating point powered by optical radiation at the signal wavelength can be used, accurately setting and maintaining the quadrature value at a high level of intensity noise in the optical radiation at the modulator input.»

Round 2

Reviewer 2 Report

The authors have answered my questions clearly and justified their responses with data and updated figures.