Yi Weng

A fast convergence frequency domain least mean square algorithm for compensation of differential mode group delay in few mode fibers

We investigated the convergence speed of a modified frequency-domain least-mean-square algorithm in few-mode fiber transmission systems. This algorithm improves the convergence speed by about 30% with negligible increase in implementation complexity.

A Step-Size Controlled Method for Fast Convergent Adaptive FD-LMS Algorithm in Few-Mode Fiber Communication Systems

Space-division multiplexing using few-mode fibers (FMF) has emerged as a promising technology to overcome the next capacity crunch. The key challenges of FMF systems are inter-modal crosstalk due to random mode coupling and large differential mode group delay (DMGD). Adaptive frequency domain least mean square (FD-LMS) algorithm has been proposed as the most hardware efficient multi-input multi-output equalization method for compensating large DMGD. Except for hardware complexity, the convergence speed of the adaptive FD-LMS algorithm is another important consideration. In this paper, we propose a noise power spectral density (PSD) directed adaptive FD-LMS algorithm, which adopts variable step size to render the posterior error of each frequency bin convergent to the background noise in an additive white Gaussian noise (AWGN) channel. In a 3000 km six-mode transmission system with 35 ps/km DMGD, compared with signal PSD dependent FD-LMS method and conventional FD-LMS method, our new algorithm could improve the convergence speed by 36.1% and 48.3%, but their hardware complexity will only increase 10.7% and 17.2%, respectively.

Theoretical analysis and numerical simulation of inter-modal four-wave-mixing in few mode fibers

Analogous to single-mode fiber, nonlinear effects may eventually limit the information capacity in next generation long-haul few-mode fiber (FMF) transmission systems. In this paper, we theoretically and numerically investigate inter-modal four-wave mixing (IM-FWM) among up to four spatial modes in FMFs. Different forms of multi-mode non-linear Schrödinger equation (MM-NLSE) for two, three and four spatial modes are derived, along with up to six types of wave/mode distributions and their corresponding phase matching conditions. We analyze the mixed effects of differential mode group delay (DMGD), chromatic dispersion (CD), random mode coupling (RMC) and wavelength separation upon IM-FWM efficiency in FMF.

Investigation of the nonlinearity in few mode fibers

Nonlinear effects in few-mode fibers (FMF) have been explored in the early years of optical communication. Due to the development of space-division multiplexing (SDM) technology for fiber transmission in FMF, the nonlinearity in FMF has attracted significant research interests recently. Analogous to single-mode fiber, nonlinear effects may eventually limit the information capacity in FMF transmission systems. In this paper, we first reviewed the past and current research in this area. The paper then concentrates on the theoretical and numerical investigation of inter-modal four-wave mixing (IM-FWM) among up to four spatial modes in FMFs. Different forms of multi-mode non-linear Schrödinger equation (MM-NLSE) for two, three and four spatial modes are derived, along with up to twelve types of wave/mode distributions and their corresponding phase matching conditions. We analyze the mixed effects of differential mode group delay (DMGD), chromatic dispersion (CD), random mode coupling (RMC) and wavelength separation upon IM-FWM efficiency in FMF. In addition, intermodal nonlinear effects in FMFs may potentially provide a variety of innovative optical signal processing functionality for fiber network applications through this additional spatial dimension. For example, we show simultaneous mode and wavelength conversion based on IM-FWM, and explore the effects of group velocity dispersions in order to achieve higher conversion efficiency.

Non-data-aided chromatic dispersion estimation for Nyquist spectrally shaped fiber transmission systems

We investigate a non-data-aided chromatic dispersion estimation based on a polynomial fitting algorithm for root-raised-cosine (RRC) Nyquist spectrally shaped fiber transmission systems. We compare the performances of the estimation method on RRC spectrally shaped systems with regular non-return-to-zero systems. At a same symbol rate, a signal with a narrower spectrum has a lower estimation accuracy using a same estimation time window. The estimation accuracy can be improved by increasing the time window. With an OSNR at a bit error rate at a FEC-limit or 3.8×10-3, 112 Gbit/s Nyquist PDM-QPSK with RRC 0.1 spectral shaping requires 8192 symbols to achieve a measured standard deviation below 200 ps/nm, and 224 Gbit/s Nyquist PDM-16QAM with the same spectral shape requires 4096 symbols to achieve a measured standard deviation below the same level.

Frequency domain equalizer in few-mode fiber space-division-multiplexing systems

Few-mode fiber (FMF) communication system has been emerging as a promising space-division-multiplexing (SDM) technology to overcome the next-generation capacity crunch. The key challenges of FMF system are inter-modal crosstalk due to random mode coupling and large differential mode group delay (DMGD). Adaptive frequency domain least mean square (FD-LMS) algorithm has been proposed and demonstrated as the most hardware efficient method in compensating large DMGD and random mode coupling. In this paper, we propose a noise power directed adaptive FD-LMS algorithm, which adopts variable step size to render the posterior error of each frequency bin converge to the background noise level in the additive white Gaussian noise (AWGN) channel. Our simulation result shows that, in a 3000 km two-mode transmission system with 35 ps/km DMGD, noise power directed algorithm can improve the convergence speed by 34% and 54% compared to signal power spectrum density (PSD) dependent adaptive FD-LMS method and conventional fixed step-size adaptive FD-LMS method, with the hardware complexity (number of complex multiplication) increased by only 5.7% and 8.1% respectively. We also propose to use a single-stage adaptive equalizer for compensating both chromatic dispersion (CD) and DMGD simultaneously for further decreasing the overall hardware complexity of digital signal processor (DSP) in coherent receivers. We show that such single-stage equalizer may have a slower convergence speed due to a larger mean square error (MSE) induced by uncompensated CD in equalizers initial condition. We extend the proposed noise power directed algorithm to increase the convergence speed of the single-stage equalizer; and the simulation results show that the noise power directed algorithm can achieve 51% faster convergence speed than conventional algorithm in a 3000 km transmission system with DMGD of 35 ps/km and CD of 20 ps/nm/km.