Unrepeatered 240-km 64-QAM Transmission Using Distributed Raman Amplification over SMF Fiber^†

Round 1

Reviewer 1 Report

This paper report experimental results and simulations regarding single channel unrepeated transmission of a 64-QAM channel using second order Raman amplification and digital back propagation in off-line DSP.

The authors report results up a distance of 240 km of SMF but under FEC-threshold BER results have been attained only up to 200 km.

The authors then report simulations showing that with an improved transmitter (in respect of the one used in their experimental setup) under FEC-threshold transmission would be possible up to an unrepetead span of 240 km.

The paper suffer from a number of weak points which are summarized in the following:

Reasons and novelty of the paper results are not adequately discussed in the introduction Fig7 is not even discussed in the text Claim of demonstrating 240 km transmission in the conclusion is not completely correct as this comes out only by simulations after a number of assumption and after not showing such a result in the experimental results Generally speaking the paper is not extensively commented and discussed.

Author Response

1.“The authors report results up a distance of 240km of SMF but under FEC- threshold BER results have been attained only up to 200 km.

In the first lines in the Abstract section we specifically state: “We present a theoretical and experimental investigation of unrepeatered transmission over standard SMF-28 fibre using several schemes of distributed Raman amplification”. We agree that experimental results for 240 km case are below FEC -we also say it in the manuscript:

“Although higher order amplification has been proven [3-5] to be superior in long-haul and unrepeatered transmissions, we could not achieve the BER performance that was below FEC limit in a 240 km experiment using any of the proposed higher order, bidirectional and hybrid Raman pumping schemes”

 and follow with the explanations and simulation results confirming our hypothesis:

“Based on previous experimental knowledge in unrepeatered experiments with lower order modulation formats (QPSK and 16QAM) using the proposed amplification scheme [5, 15, 16], we believe that with improved back-to-back performance a successful 240 km SMF–28 transmission with BER below FEC is achievable.” 

To summarize: we first adopt all simulation variables to match the experimental results, this is shown in Fig.2. Based on complete match between theoretical and experimental results we show through simulations the achievable distance in 240 km in the assumption with improved back-to-back performance of the transmitter.

Both experimental and simulation results presented in the manuscript are valid, regardless of the actual quality outcome of the BER measurement, but in order to avoid confusion, we have modified the abstract, replacing "our experimental work confirmed by simulations demonstrates transmission up to 240 km span length" with "our work demonstrates the possibility of transmission up to 240 km span length with a total span loss of 52.7 dB".

2. Reasons and novelty of the paper are not adequately discussed in the introduction

Again, in the Abstract we state: “We present a theoretical and experimental investigation of unrepeatered transmission over standard SMF-28 fibre using several schemes of distributed Raman amplification, including first, second and dual order.”

The same we state in the Introduction: “In this paper, we experimentally investigate the performance of a single channel 28GBaud 64-QAM transmission with coherent detection over different Raman amplification schemes and confirm the results with numerical simulations.” 

Following with the explanations and  benefits of such approach, we have modified the introduction so it now states: “Our results illustrate the potential improvement attainable just through show the benefits of employing digital back propagation (DBP) adequately combined with careful distributed amplifier design in an unrepeatered standard single mode fibre (SMF-28) span without the need for remote optically-pumped amplifier (ROPA), large effective area or ultra-low loss fibre that surely could extend the transmission distance even further.”.  

We also compare with the state-of-art research and reason our motivation in the paper: “Here, we can refer to [10] where authors presented unrepeatered transmission up to 370 km using combinations of different low loss/effective area fibres, forward and backward ROPA combined with amplification map optimization. By employing only low-loss fibre (0.169 dB/km) into our system, theoretically, we could extend our achievable distance from 200 km (41 dB) up to 242 km, that gives almost 25% of total improvement, without forward and backward ROPA, using simple design of distributed Raman amplifiers [9] instead”   

 

Fig7 is not even discussed in the text

This is an informative figure showing the nonlinear distortion, and it is now commented in the text as:

"Fig. 7 shows , for illustration, simulation results of the impact of nonlinear distortion only without ASE and chromatic dispersion for the three transmission distances considered."

This is Claim of demonstrating 240 km transmission in the conclusion is not completely correct as this comes out by simulations after number of assumptions and after not showing such results in the experimental results

To make it clear we now modified the Conclusion section to:

“The possibility of unrepeatered transmission of a 28 Gbaud Nyquist-shaped optical 64-QAM signal up to 240 km has been demonstrated by combining experimental and numerical results. Theoretical investigation through numerical simulations shows considerable improvement in the valid achievable distance below FEC limit if the hardware constraints in the transmitter were suppressed."

Reviewer 2 Report

Report on manuscript applsci-684323 Rosa:

In the manuscript entitled "Unrepeatered 240 km 64-QAM transmission using distributed Raman amplification over SMF fibre" by Rosa and his/her coworkers, the authors have carried out an experimental investigation on unrepeatered transmission of a 28 Gbaud Nyquist-shaped optical 64-QAM signal over standard SMF-28 fibre with different designs of distributed Raman amplification up to 240 km. Their numerical simulations confer with the experimental results and also indicate considerable improvement in the achievable distance by suppressing the hardware constraints in the transmitter. In particular, this distance could be enlarged in low-loss fibre by using Raman amplification based DFB fibre laser with a single pump wavelength.

The manuscript is written well and organized well. The content can be easily accessible without difficulty. Although the work meets the standards of Applied Science and the results are suitable for the journal, yet, before its formal acceptance, I would like to invite the authors to consider the following issues which exist in the current manuscript:

1)  For the experimental results, it would be helpful if the authors could provide further details on the parameters involved. For example, the launch powers.

2)  What are the dimensions for axises in Figures 5, 7 and 8?

3)  Some abbreviations are lack of introductions.

Author Response

All pump and signals launch powers used in experiments are shown in figures 3 and 4. This is now clarified in the text:

 

Backward pumping (BW) was provided by two Raman pumps at 1366 nm and 1455 nm respectively. All pump and signals launch powers used in experiments are listed in Fig. 3 and Fig. 4 in Section 3.To combine and demultiplex the Raman pumps and the signal, we used 1×3 WDM couplers at the beginning and at the end of the span. In all setups, there was an EDFA implemented before and after the transmission line.

 

Dimensions for axis in Fig. 5, 7 and 8

 

Thank you for spotting it. This is a relative unit of measure, we now corrected the graphs with the [arbitrary unit] dimensions.

 

Missing abbreviations

 

Thank you, this is now fully corrected in the manuscript.

Round 2

Reviewer 1 Report

I expected more anyway Authors improved the paper to an acceptable level