Improving spectral efficiency via high-order quadrature amplitude modulation(QAM)formats for high-capacity transmission has been extensively investigated in coherent wavelength division multiplexing(WDM)transmission systems [
Laser & Optoelectronics Progress, Volume. 60, Issue 7, 0736002(2023)
[in Chinese]
We experimentally demonstrate an 80-channel wavelength division multiplexing (WDM) transmission system over a 400 km fiber link. Raman amplification results in a non-flat WDM signal spectrum. Therefore, bit allocation optimization is used to enable different channels to carry different order quadrature amplitude modulation signals according to their optical signal-noise-ratios. A neural network equalizer based on a convolutional neural network (CNN), long short-term memory (LSTM) network, and fully connected (FC) layer structure is adopted in Rx digital signal processing, in which CNN is used for characteristic extraction, LSTM is used for equalization and demodulation, and FC layers are used for output. After transmission, the bit error rate of all channels is below the 25% soft-decision forward error correction threshold, and the line rate reaches 53.76 Tbit/s.
1 Introduction
Improving spectral efficiency via high-order quadrature amplitude modulation(QAM)formats for high-capacity transmission has been extensively investigated in coherent wavelength division multiplexing(WDM)transmission systems [
In this study,we propose an optimization method based on bit allocation that produces a higher exploitable capacity while being straightforward to implement. In the bit allocation optimization method,the QAM format per wavelength channel is identified using the received OSNR. Compared with power optimization based on a mathematical model,bit allocation optimization has very low complexity and does not require detailed knowledge of the characteristics of the network components,especially the parameters of the fiber and optical amplifier. For the first time,we designed a hybrid convolution neural network,long short-term memory network,and fully connected layer(CNN-LSTM-FC)structure for polarization division multiplexing(PDM)QAM signal equalization while incorporating a combination of the advantages attributed to the CNN,LSTM network,and FC layer structure [
2 Experimental setup
The experimental setup is shown in
Figure 1.Experimental setup of 80-channel coherent WDM transmission
The output electrical signals from the four independent channels(Iodd,Qodd,Ieven,
Figure 2.Photographs of the experimental setup comprising 80 ECLs and a 4×100 km fiber link with Raman amplifiers
The optical spectra of WDM signals employing the 256QAM format before and after fiber transmission at 0.02 nm resolution are illustrated in
Figure 3.Spectra. (a) WDM signals fiber launch spectrum; (b) output spectrum after fiber transmission with Raman amplification
After the fiber transmission,a tunable optical filter(TOF)was used to select the received optical signal of each test WDM channel. In addition,a variable optical attenuator(VOA)was added to adjust the input optical power into an integrated coherent receiver. We used another ECL signal as an optical local oscillator(LO)for homodyne detection. After optical-to-electrical conversion,the received PDM signals were captured by an 80 GSa/s sampling rate oscilloscope with a 36 GHz electrical bandwidth. In the offline Rx-side DSP,an NN equalizer based on the CNN-LSTM-FC structure was used after resampling,chromatic dispersion(CD)compensation,constant-modulus algorithm(CMA)equalization,frequency offset,and carrier phase estimation.
3 NN structure
In the NN equalizer,we designed a hybrid CNN-LSTM-FC structure,as illustrated in
Figure 4.Schematic overview of NN equalizer based on CNN-LSTM-FC structure
4 Results and discussions
First,we loaded the same data into the AWG in the transmitter and selected the optical signal of each wavelength channel using the TOF in the receiver. An optical spectrum analyzer was used to capture the selected optical signal and measure its OSNR. The received OSNR after fiber transmission versus the wavelength is shown in
Figure 5.Received OSNR versus wavelength
As shown in
Figure 6.BER of all 80 channels after fiber transmission
5 Conclusions
An 80-channel 50 GHz grid WDM transmission employing 48 Gbaud PDM signals was demonstrated using the bit allocation optimization method. Owing to the NN equalizer based on a hybrid CNN-LSTM-FC structure,53.76 Tbit/s line rate(43.008 Tbit/s net rate)transmission over a 400 km fiber link can be achieved.
[14] Zheng L, Chen Z Y, Wu D M et al. Channel distributions of the transient power overshoot in backward-pumped Raman amplified WDM systems[J]. Chinese Optics Letters, 2, 503-504(2004).
[15] Xue F, Qiu K, Chen Y. Research on WDM optical fiber transmission system based on fiber Raman amplifier[J]. Chinese Optics Letters, 1, 564-566(2003).
Get Citation
Copy Citation Text
Junjie Ding, Chen Wang, Zhou Ju, Bowen Zhu, Bohan Sang, Bo Liu, Jianjun Yu. [J]. Laser & Optoelectronics Progress, 2023, 60(7): 0736002
Category: Information
Received: Dec. 19, 2022
Accepted: Feb. 22, 2023
Published Online: May. 24, 2023
The Author Email: Yu Jianjun (jianjun@fudan.edu.cn)