Hisao Nakashima

Fast optical channel recovery in field demonstration of 100-Gbit/s Ethernet over OTN using real-time DSP

A field trial of 100-Gbit/s Ethernet over an optical transport network (OTN) is conducted using a real-time digital coherent signal processor. Error free operation with the Q-margin of 3.2 dB is confirmed at a 100 Gbit/s Ethernet analyzer by concatenating a low-density parity-check code with a OTN framer forward error correction, after 80-ch WDM transmission through 6 spans x 70 km of dispersion shifted fiber without inline-dispersion compensation. Also, the recovery time of 12 msec is observed in an optical route switching experiment, which is achieved through fast chromatic dispersion estimation functionality.

Improvements to Digital Carrier Phase Recovery Algorithm for High-Performance Optical Coherent Receivers

The Viterbi-and-Viterbi (V-V) algorithm is widely used to recover the carrier phase in optical digital coherent receivers. For simplicity, the basic V-V algorithm assumes constant carrier phase within the average duration. However, this basic assumption is probably violated by factors such as laser frequency offset and nonlinear XPM. In order to improve the basic V-V carrier phase recovery, five methods are introduced, verified, and analyzed. All these methods are compatible with parallel implementation that is mandatory for a realistic DSP circuit. The Q-improvement brought by each algorithm is analyzed together with the complexity of each. Among the five methods, the laser frequency offset compensation expands the tolerable frequency offset to 0.37 symbol rate, and the optimum weighted averaging in conjunction with normalization processing improves the Q-value by 2 dB under severe XPM condition.

Hybrid Link/Path-Based Design for Translucent Photonic Network Dimensioning

The advancement of ultralong-haul transmission technology has dramatically enhanced the all-optical reaches. However, the actual situations of installed fiber and sites for terrestrial network often prevent implementing a purely transparent network, and thus, opaque reshaping retiming regenerating (3R) regeneration is required to guarantee optical signal reachability. Since 3R regenerators based on optical/electrical/optical conversion tend to dominate the total network costs, an efficient network design method that allocates a minimum number of 3R regenerators to optimum locations is essential to build a cost-effective photonic network. In this paper, we propose such a network-dimensioning method by combining the advantages of link-based and path-based design approaches. It first guarantees optical signal reachability for any possible traffic demand in each segmented linear link. After combining all the links, excessive regenerators are eliminated based on the optical signal quality check with -factor calculation for each wavelength path. A trial design of a large-scale mesh network demonstrated a significant cost savings of more than 30% in comparison with a conventional link-based design. In the trial, the impact of fiber loss coefficient over the total network cost was investigated quantitatively, addressing the importance of such quantitative modeling and analysis.

Interplay between local oscillator phase noise and electrical chromatic dispersion compensation in digital coherent transmission system

We demonstrate that the interplay between local oscillator phase noise and electrical chromatic dispersion compensation induces timing jitter as well as intensity noise. The interplay is investigated in 3000 km-long transmission (accumulated dispersion of 47,000 ps/nm) without optical dispersion compensation.

Superimposition and detection of frequency modulated tone for light path tracing employing digital signal processing and optical filter

We propose a novel method for light path tracing and demonstrate it in 112Gb/s DP-16QAM transmission. The S/N of the detected label exceeds 24dB after 20span transmission. No crosstalk was observed in neighboring DWDM channels.

Digital clock recovery algorithm for optical coherent receivers operating independent of laser frequency offset

We propose a digital clock recovery algorithm and demonstrate its tolerance to at least 5 GHz laser frequency mismatch in a 43 Gb/s DP-RZ-QPSK receiver after 1200 km transmission.

Network innovations brought by digital coherent receivers

Operational efficiency improvements of optical networks enabled by digital coherent receivers are discussed in view of reliability, agility, performance and reduced operational complexity. Several specific technologies are briefly introduced to deliver some more insight.

Complexity-reduced digital nonlinear compensation for coherent optical system

The high complexity of conventional intra-channel nonlinearity compensation algorithms, such as back-propagation, is considered as the major obstacle for the implementation. To reduce the complexity, perturbation analysis is applied because it considers multi-span transmission as one stage. In those perturbation based algorithms, such as perturbation back-propagation (PBP) and perturbation pre-distortion, the number of required compensation stage is much less than that of conventional back-propagation. To reduce the complexity further, the multi-tap finite impulse response filter (FIR) in PBP is replaced with one-tap infinite impulse response (IIR) filter. The number of required compensation stage of IIR PBP is only 15% of conventional back-propagation, whereas the complexity of each stage is almost same. In perturbation pre-distortion, the proposed perturbation combination reduces the number of terms from 19732 to 41, whereas no performance degradation is observed.

Novel wide-range frequency offset compensator demonstrated with real-time digital coherent receiver

A novel frequency offset estimator is proposed and implemented in a 1 Gbit/s real-time coherent receiver to demonstrate all-digital frequency offset compensation up to plusmn0.4 symbol rate.

Semi-blind nonlinear equalization in coherent multi-span transmission system with inhomogeneous span parameters

Semi-blind sequence is proposed for optimizing the nonlinear equalization parameters from limited prior knowledge of link. Its performance is demonstrated in an extreme case where the fiber type of a span is incorrectly informed.