PMD estimation and its enabled feedforward adaptive equalization based on superimposed FrFT training sequences.

Fractional Fourier transform (FrFT) training sequences (TSs) have been proposed to superimpose with signals to achieve time synchronization and polarization mode dispersion (PMD) estimation simultaneously before equalization. The feasibility of the proposed scheme is verified by experiment of a 30 GBaud/s PDM quadrature phase shift keying (QPSK) system. Without sacrificing spectrum efficiency, the proposed PMD estimation is robust against amplifier spontaneous emission (ASE) noise and has steady performance in a wide PMD range from 0 ps to 108 ps. The root mean square error (RMSE) of PMD is 5.6 ps by experimental verification. Further, the PMD estimated before equalization is utilized to adjust the numbers of taps in a subsequently tap-varied equalizer, which achieves feedforward adaptive equalization (FFD-AEQ). The number of the AEQ taps is adjusted to an optimal value with the prior-PMD information estimated by FrFT TSs. The experimental results show that the tap-varied AEQ has 52.6% power consumption decrease compared to the tap-fixed AEQ with 13 taps.

Improved receiver of ICI compensation for a spectral efficient frequency division multiplexing IM/DD system.

Spectral efficient frequency division multiplexing (SEFDM) can greatly increase the spectral efficiency for intensity modulation/direct detection (IM/DD) optical communication systems. The sphere detection algorithm (SD) is the most efficient way to get the maximum likelihood (ML) solution to solve the inter-carrier interference (ICI) induced by the bandwidth compression of SEFDM system. However, SD algorithm is restricted by the numbers of subcarriers for SEFDM system, especially for larger numbers. Therefore, a sorted Gram-Schmidt (SGS) orthogonal decomposition algorithm, which can be applied to any subcarrier numbers, is proposed to overcome this restriction. To the best of our knowledge, the searched paths of FSD are researched for the first time to balance performance and complexity. Based on the analysis, a soft-tree-width sphere detection algorithm (SSD) is proposed and demonstrated by simulation and experiment. The results show that the proposed algorithm can greatly reduce the computational complexity (at least 40%) with the same system performance. The proposed algorithms are a promising candidate for flexible and efficient SEFDM systems. The SEFDM with the proposed detector is significant for the IM/DD optical systems.

Joint Fiber Nonlinear Noise Estimation, OSNR Estimation and Modulation Format Identification Based on Asynchronous Complex Histograms and Deep Learning for Digital Coherent Receivers.

In this paper, asynchronous complex histogram (ACH)-based multi-task artificial neural networks (MT-ANNs), are proposed to realize modulation format identification (MFI), optical signal-to-noise ratio (OSNR) estimation and fiber nonlinear (NL) noise power estimation simultaneously for coherent optical communication. Optical performance monitoring (OPM) is demonstrated with polarization mode multiplexing (PDM), 16 quadrature amplitude modulation (QAM), PDM-32QAM, as well as PDM-star 16QAM (S-16QAM) for the first time. The range of launched power is -3 to -2 dBm with a fiber link of 160-1600 km. Then, the accuracy of MFI reaches 100%. The average root mean square error (RMSE) of OSNR estimation can reach 0.37 dB. The average RMSE of NL noise power estimation can reach 0.25 dB. The results show that the monitoring scheme is robust to the increase of fiber length, and the solution can monitor more optical network parameters with better performance and fewer training data, simultaneously. The proposed ACH MT-ANN has certain reference significance for the future long-haul coherent OPM system.

Fabrication and transmission of optical polymer waveguide backplane for high - performance computers.

In this paper, a high-speed, large-capacity and compact optical backplane architecture for high-performance computers (HPC) is proposed and designed. The MT couplers is designed without additional mirror to divert the light from vertical cavity surface emitting laser (VCSEL) array by 90°. The light is then coupled into the optical waveguide through the MT. A bidirectional 8 channels polymer optical waveguide array with low insertion loss is designed and fabricated. The waveguides are embedded in the printed circuit board. We test the performance of 8 channels for the optical waveguide backplane. In the entire optical backplane, the averaged insertion loss of optical backplane with 50 µm input fiber is 1.62 dB with 850 nm VCSEL. The misalignment loss is 0.5 dB when the misalignment between waveguide and multi-mode fiber is 8.5 µm. Then, the transmission performances of 4 channels with different speed signals is demonstrated by off-line experiment. The optical backplane network can achieve 15 G data error-free transmission in the range of [-2 dBm, -10 dBm] received optical power (ROP). Based on VCSEL arrays, the optical interconnection network system can achieve 8 channels parallel signal transmission. In the optical backplane, the 10 Gbit / s data generated / processed by field programmable gate array (Xilinx Kintex-7) chips can realize error-free transmission. In large-capacity, high-speed parallel HPC, the designed optical backplane system can facilitate the establishment of a large number of parallel transmissions.

Bistatic radar scheme based on the digital-analog hybrid chaos system.

Bistatic radar with the separate transmitter and receiver has some attractive merits, thus obtaining many attentions. However, in a traditional bistatic radar system, there are still many problems that hinder it from practical applications. Here we introduce a bistatic radar scheme using a well-designed optical chaos system, from which the analog chaos signal could be determined by generated random binary sequence. The broad bandwidth analog signal is used as surveillance signal and the digital signal are transmitted to the receiver by optical fibers. Finally, high spatial and velocity resolutions of radar system could be obtained by using the analog chaos signal. A high-quality regeneration of the reference signal at different locations can be established by transmitting the digital sequence. Moreover, the mutual interferences could be concealed since the analog surveillance signal and the transmitted digital sequence are delivered by different paths. These could be advantageous for radar applications.

Electro-optic chaotic system based on the reverse-time chaos theory and a nonlinear hybrid feedback loop.

A novel electro-optic chaos source is proposed on the basis of the reverse-time chaos theory and an analog-digital hybrid feedback loop. The analog output of the system can be determined by the numeric states of shift registers, which makes the system robust and easy to control. The dynamical properties as well as the complexity dependence on the feedback parameters are investigated in detail. The correlation characteristics of the system are also studied. Two improving strategies which were established in digital field and analog field are proposed to conceal the time-delay signature. The proposed scheme has the potential to be used in radar and optical secure communication systems.

Research of two-dimensional beam steering in LCOS-based wavelength selective switch.

The phenomenon of two-dimensional beam steering in liquid crystal on silicon (LCOS) is demonstrated. We propose a novel flexible wavelength selective switch (WSS) using LCOS two-dimensional beam steering. The cross talk of adjacent channel in the WSS is theoretically small, while the measured insert loss is lower than 15 dB.

Rapid and accurate chromatic dispersion measurement of fiber using asymmetric Sagnac interferometer.

We propose and experimentally demonstrate an advanced method for chromatic dispersion measurement of fiber. This technique is based on spectral interferometry by using an asymmetric Sagnac loop and broadband optical source. The chromatic dispersion can be directly obtained from the spectral interferogram seen from an optical spectral analyzer. This method is rapid (<1 s), accurate, simple, low cost, and can provide a large dispersion measurement range.

Chromatic dispersion measurement using single sideband spectrum phase difference detection for OOK signal link.

In this paper, we theoretically and experimentally demonstrated the residual chromatic dispersion (CD) measuring of 10Gbit/s NRZ and 40Gbit/s NRZ/RZ links by using a novel single sideband (SSB) spectrum phase difference detection technology. This method can differentiate positive and negative residual CD of the fiber link, the measuring range is dependent on frequency difference (FD) of two local oscillator (LO) and the FD can be adjusted up to CD range. This method is independent on data rate for intensity modulation direct detection (IM/DD) links. In condition of FD of 4 GHz, the measuring range was around ± 1000 ps/nm and resolution was better than 5 ps/nm for 10G NRZ link.

Design and fabrication of an optical interleaver with cascaded multimode interference couplers.

A lattice-form optical interleaver was designed and fabricated with a silicon-based silica waveguide. Cascaded multimode interference couplers were first employed in the lattice circuit and helped to relax the fabrication tolerance. The device shows good performance, the insertion loss is less than 2.25 dB, the passband ripple is less than 0.15 dB, the cross talk is less than -18.9 dB, and the 0.5 dB passband is more than 100 GHz.