KEYWORDS: Modulation, Convolution, Signal to noise ratio, Digital signal processing, Optical engineering, Education and training, Optical transmission, Optical sensing, Forward error correction, Edge detection
Modulation format identification (MFI) and optical performance monitoring are important for elastic optical networks. We propose a cascaded module-based MFI and optical signal-to-noise ratio (OSNR) estimation method. Polar coordinates were used as features to distinguish the modulation formats (MFs) and estimate the OSNR. Dilated convolution was adopted to increase the receptive field and reduce computational complexity. Four commonly used MFs were investigated: dual-polarization (DP) quadrature phase shift keying, 8QAM, 16QAM, and 32QAM. A 32G baud DP transmission system is established, and the results are discussed. The results show that MFI accuracy can reach 100% when the OSNR is less than the 7% forward error correction limit, and the accuracy of OSNR monitoring is close to 100%. Two factors, sample length and nonlinearity, were further investigated. The results show that the MFI and OSNR estimators can achieve perfect performance when the sample length is 3000. Simultaneously, the accuracy of the MFI remained at 100%, and the accuracy of the OSNR estimator decreased by 1% to 2% when the launch power changed from 0 to 5 dBm. Furthermore, two modules with the same structure, DC-CNN and CNN, were compared. The results show that the two models can achieve similar accuracies and that the DC-CNN has the least number of parameters. Finally, experimental verification was conducted to ensure the practical applicability of the proposed method.
Electrodes have an important influence on the terahertz (THz) radiation efficiency of photoconductive antenna, especially photoconductive antennas combined with nanostructures. The nanostructured photoconductive antennas with rectangular electrodes, traditional electrodes and bow-tie electrodes were simulated by the finite difference time domain method. The simulation results show that the bow-tie electrode is better than the rectangular electrode and the traditional electrode. The research results provide a promising scheme for selecting the appropriate electrode structure of the nanostructured photoconductive antenna.
Modulation format identification (MFI) is widely used in elastic optical networks, such as optical performance monitoring and signal compensation. We propose an MFI scheme, through extraction of the amplitude probability distribution feature of the received signals and an analysis of the relative entropy, which is also known as the Kullback–Leibler divergence (KL). The modulation formats (MFs) can be distinguished via calculation of the KL value. Through this method, four commonly used transmission formats were investigated, namely, polarization-division multiplexing (PDM)-QPSK/16QAM/32QAM and 64QAM. A simulation transmission model was created, and the results were analyzed. The simulation results show that the MFs investigated can be distinguished when the optical signal-to-noise ratio (OSNR) is less than or equal to the corresponding theoretical 7% forward error correction (FEC) limit (bit error ratio = 3.8 × 10 − 3). In addition, two important factors, i.e., fiber nonlinearity and sample size, were investigated in relation to the proposed method. The results show that the method exhibits a certain tolerance to fiber nonlinearity and can be used to identify the MF at the FEC threshold even when the sample number is 1000. Furthermore, the experimental verifications were conducted to ensure the practicality of the proposed method.
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