Electronic System of Remote Optical Control of LiNbO3 Mach-Zehnder Modulator Operating Point

Round 1

Reviewer 1 Report

The authors present a system for remote control and stabilization of the operating point of integrated optical  Mach–Zehnder modulators. Some modifications are required as follows;

1.       The authors do not mention and talk about Fig. 1 (a)

2.       The proposed scheme in Fig. 1 (b) is required a lot of details.

3.       What is the main idea behind employing different laser wavelengths (1310 and 1550 nm), although an RoF 1550 nm is used.

4.       Is this method working with respect to the variation of the temperature

5.       Is this method valid for multiple MZM in the meanwhile since there are some applications such as “Microwave Photonic MIMO Radar for Short-Range” employing multiple MZMs.

Author Response

We would like to express our sincere gratitude to the Reviewer for your attention to our manuscript and valuable criticisms that, no doubt, will contribute to a better presentation of our study. The following pages  provide our point-by-point responses to each of the comments of the Reviewer.

1} The authors do not mention and talk about Fig. 1 (a). 

The text was added at lines 37 to describe conventional photonic link “An example of microwave photonic link between CS and RAU is shown in Fig. 1a. An optical carrier from a distributed feedback laser diode (DFB LD) at 1550 nm is delivered to the remote LNMZM over a single mode optical fiber (SMF), and the modulated optical signal is returned back over another SMF.” 

2) The proposed scheme in Fig. 1 (b) is required a lot of details. 

Figure 1(b) was thoroughly redrawn to be more clear: input and output waveguides for LNMZMs` Y couplers were added, output directional coupler of DOLNMZM was drawn more clearly, green arrows that show input and output RF signals were replaced with brown ones to discriminate colors better, 1310 nm laser and 1550 nm laser were interchanged to match position of 1550 nm laser at panel (a), function of the optical fibers marked with different colors were described.  Starting at line 72 to line 97 folloing texet was modified “The proposed scheme  of remote control of DOLNMZM operating point is shown in Fig. 1b. A low-power (average power ~1 mW) laser diode (LD)  at a wavelength of 1310 nm was derectly modulated by an alternating current at a frequency f of several kilohertz. The light at 1310 nm was  launched to the same SMFas optical carrier of the 1550 nm RoF signal by a 1310/1550 fiber optic wavelength division multiplexer (WDM)  and transmitted to the RAU . Any other wavelength from the SMF transparency bands can potentially be used with a spectral spacing that ensures no crosstalk. Our choice was based on the commercial availability and cost of fiber optic components. In addition, such widely spaced wavelength multiplexing provides transparency for various modulation formats, flexibility, and extended functionality. At the modulator end, 1310 laser light was extracted by complementary 1310/1550 WDM and sent to photodiode that was operating in photovoltaic mode. Then the voltage from the output of the photodiode was fed to flyback transformer  A rectified DC voltage from the transformer was supplied to the bias electrode of DOLNMZM. Thus the change in the intensity modulation index of 1310-nm LD resulted in  a change   in the bias voltage which could be varied from 0 to 15.2 V.  Optically carried RoF signals at the wavelength of 1550 nm   were transmitted from DOLNMZM outputs to CS and detected by balanced photodetector. The DC components of the photocurrents of the two photodiodes of the balanced detector were processed to calculate current operating point of the modulator and close a feedback loop by adjusting the amplitude of the 1310 nm LD modulation index. For example, the system will aim for equal DC currents components in order to set the modulator to the quadrature operating point.” 

3) What is the main idea behind employing different laser wavelengths (1310 and 1550 nm), although an RoF 1550 nm is used.  

Following text was added to line 78 to clarify using of different wavelength bands: “Any other wavelength from the SMF transparency bands can potentially be used with a spectral spacing that ensures no crosstalk. Our choice was based on the commercial availability and cost of fiber optic components. In addition, such widely spaced wavelength multiplexing provides transparency for various modulation formats, flexibility, and extended functionality.” 

4)  Is this method working with respect to the variation of thetemperature. 

Following text was added to line 242 to discuss experimental results on heating PVVFS together with predicting behaviour at low temperatures: “As RAU can to be placed outdoor, it is useful to address PFVVS operation in different temperature regimes. Heating test of PFVVS was done using the setup shown in Fig. 5b. The circuit was placed in aluminum container and heated using hot air gun. VOA attenuation was set so that at 25 °C output voltage was 10.8 V. At 54 °C output voltage was found to be 10.3 V, and 9.3 V at 95 °C. We attribute drop in output voltage to increased leakage of the rectifier diode. At low temperature main factor will be decrease of transformer inductance due to loss of magnetic permeability of T35 ferrite. This will cause increase of quasiresonant frequency. To mitigate this effect, ferrite with more stable permeability in wide range of temperatures such as N96 can be used.”

5) Is this method valid for multiple MZM in the meanwhile since there
are some applications such as "Microwave Photonic MIMO Radar for
Short-Range" employing multiple MZMs. 

To address this question following sentence was added to line 80: “Multiple PFVVSs for remote control of modulators can be multiplexed using WDM technique in a similar manner to the information RoF channels. Thus proposed approach could be extended for control of multiple LNMZMs. Note that each of the modulators will corresponds to an independent PFVVS operating at a certain wavelength, since in the general case the voltage values of the operating points of different modulators, as well as their drift, are not correlated with each other.”

Reviewer 2 Report

Figure 1 caption is cut off.

SPICE is not defined on line 124.

commas are used for numbers where periods should be used.

line 127 Fig. 3, line 153 Fig. 4b, line 170 Fig. 5a, line 180 Fig. 5c, line 187 Fig. 5b, line 193 Fig. 6a

line 168 change 'ration' to 'ratio'

line 241 "XX" needs to be replaced with a number.

line 267 change 'quadratyre' to 'quadrature'

English grammar errors need correction in multiple locations

Author Response

Comments of the Reviewer were highly insightful and enabled us to greatly improve the quality of our manuscript. The following pages  provide our point-by-point responses to each of the comments of the Reviewer.

1) Figure 1 caption is cut off.

This problem may arise due to some compatibility issues with our version of software. We tried to follow all guidelines for manuscript preparation.  

2) SPICE is not defined on line 124.  

Name of specific SPICE, i.e. Microcap 9 was added. 

3) commas are used for numbers where periods should be used.  

Commas was replaced with periods as a radix symbol everywhere, including figures. 

4) line 127 Fig. 3, line 153 Fig. 4b, line 170 Fig. 5a, line 180 Fig. 5c, line
187 Fig. 5b, line 193 Fig. 6a

Writing of the names of the figures with multiple pannels was changed.

 5) line 168 change 'ration' to 'ratio' Done 6) line 241 "XX" needs to be replaced with a number.. 

XX was replaced with “8 V” at line 283. Also sentence “The output voltage range was less than what the PDC can generate (Fig. 6a) due to the limited output voltage of the PCM1753 DAC.” was added for clarity.

 7) line 267 change 'quadratyre' to 'quadrature' 

Done 

8) English grammar errors need correction in multiple locations 

We have revised the English grammar by employing the services of a professional translator. 

Round 2

Reviewer 1 Report

Thanks for the authors which improve the paper by their responses.