Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

In this paper, we highlight that it is inadequate to describe the rotation of the state of polarization (RSOP) in a fiber channel with the 2-parameter description model, which was mostly used in the literature. This inadequate model may result in problems in polarization demultiplexing (PolDemux) because the RSOP in a fiber channel is actually a 3-parameter issue that will influence the state of polarization (SOP) of the optical signal propagating in the fiber and is different from the 2-parameter SOP itself. Considering three examples of the 2-parameter RSOP models typically used in the literature, we provide an in-depth analysis of the reasons why the 2-parameter RSOP model cannot represent the RSOP in the fiber channel and the problems that arise for PolDemux in the coherent optical receiver. We present a 3-parameter solution for the RSOP in the fiber channel. Based on this solution, we propose a DSP tracking and equalization scheme for the fast time-varying RSOP using the extended Kalman filter (EKF). The proposed scheme is proved to be universal and can solve all the PolDemux problems based on the 2- or 3-parameter RSOP model and exhibits good performance in the time-varying RSOP scenarios.

Two-parameter-SOP and three-parameter-RSOP fiber channels: problem and solution for polarization demultiplexing using Stokes space.

Photon-added squeezed thermal states: Statistical properties and its decoherence in a photon-loss channel

The discrete symmetries of parity P, time reversal T, and charge conjugation C are used to characterize the properties of chiral systems. The concepts of true chirality (time-invariant enantiomorphism) and false chirality (time-noninvariant enantiomorphism) that emerge provide an extension of Lord Kelvin’s original definition of chirality to situations where motion is an essential ingredient thereby clarifying, inter alia, the nature of physical influences able to induce absolute enantioselection. Only a truly chiral influence lifts the degeneracy of chiral enantiomers and so may induce absolute enantioselection in all circumstances. Chiral enantiomers remain strictly degenerate under a falsely chiral influence, which may therefore not induce absolute enantioselection in a process that has reached thermodynamic equilibrium, but may do so in far-from-equilibrium processes via a breakdown in microscopic reversibility analogous to that observed in elementary particle processes under the influence of CP violation. Parity violation and CP violation are suggested to be manifestations of true and false cosmic chirality, respectively, and their possible roles in abiotic enantioselection discussed.

True and false chirality and absolute enantioselection

Using two typical types of polarization controller, this paper analyses theoretically and experimentally the fact that it is necessary to adjust at least three instead of two waveplates in order to transform any state of polarization to any other output covering the entire Poincare sphere. The experimental results are exactly in accordance with the theory discussed in this paper. It has corrected the conventional and inaccurate point of view that two waveplates of a polarization controller are adequate to complete the transformation of state of polarization.

The number of least degrees of freedom required for a polarization controller to transform any state of polarization to any other output covering the entire Poincaré sphere

This paper investigates the zero dispersion wavelength and dispersion slope control of hollow-core photonic bandgap fibres (PBGFs) by using a full-vector finite element method. By simulation we found that theoretically the zero dispersion wavelength can be tailored by respectively changing the rounded diameter of air holes, pitch, refractive index, normalized thickness of core rings, and hole diameter to pitch ratio. At the same time the tailoring of dispersion slope can also be realized by changing the rounded diameter of air holes or pitch or normalized thickness of core rings. To illustrate the reasonability of fibre designs, this paper also gives the variance of normalized interface field intensity which measures the scattering loss relatively versus wavelength for different designs. From the viewpoint of loss, varying the rounded diameter and the thickness of core ring could shift zero wavelength but it is difficult to get the required parameters within so tiny range in practical drawing of PBGFs, on the other hand, it is possible in practice to respectively alter the pitch and refractive index to shift zero wavelength. But varying hole diameter to pitch ratio is not worthwhile because they each induce large increase of loss and narrowness of transmission bandwidth. The zero dispersion wavelength can be engineered by respectively varying the rounded diameter of air holes, pitch, refractive index, and normalized thickness of core rings without incurring large loss penalties.

Zero dispersion wavelength and dispersion slope control of hollow-core photonic bandgap fibres

Two modified timing synchronization methods are proposed for coherent optical OFDM system to solve the ambiguity problem in Park’s and Minn’s methods. It is confirmed that the proposed modified Park’s method can obtain higher time estimation accuracy, and the modified Minn’s method is more robust both to CD and PMD.

Modified Timing Synchronization Methods for CO-OFDM System Stressed by CD and PMD

A novel dual polarization differential quadrature phase shift keying(DP-DQPSK) system is proposed,whose receiver is the same as the single polarization DQPSK system,while it does not need polarization de-multiplexing like the conventional polarization division multiplexing QPSK(PDM-QPSK).Polarization mode dispersion(PMD) is mainly considered and PMD compensation for the DP-DQPSK system is studied.As the feedback signal for PMD compensation,the degree of polarization of the signals is discussed in detail.The results show that PMD tolerance can be improved by 89 ps within 1 dB optical signal noise ratio penalty after PMD compensation for the DP-DQPSK system.

Zhang Xiao-guang

Papers

The number of least degrees of freedom required for a polarization controller to transform any state of polarization to any other output covering the entire Poincaré sphere

Zero dispersion wavelength and dispersion slope control of hollow-core photonic bandgap fibres

Modified Timing Synchronization Methods for CO-OFDM System Stressed by CD and PMD

Polarization mode dispersion compensation in a novel dual polarization differential quadrature phase shift keying system

Three stage polarization mode dispersion compensator capable of compensating second order polarization mode dispersion