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

We propose a radial basis function neural network (RBFNN)-based nonlinear equalizer (NLE) for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The hidden layer neuron weights of the RBFNN-NLE are calculated using the $K$ -means clustering algorithm and the output layer weights are updated using the least mean square algorithm. With only 3% overhead in training, the proposed RBFNN-NLE was found to provide up to 4-dB performance improvement in terms of $Q$ -factor for 70-Gb/s 16-QAM CO-OFDM transmission over 1000 km ( $10\times 100$ km) fiber. Numerical results show that the operating data rate is 80 Gb/s at $Q = 6.25$ dB with the proposed RBFNN-NLE, compared with previously reported value of 70 Gb/s with artificial neural network-based NLE.

Radial Basis Function Neural Network Nonlinear Equalizer for 16-QAM Coherent Optical OFDM

This focus issue is a collection of 17 invited papers showcasing the recent advances and the future challenges of optical interconnect technology. These papers represent multiple key areas that include the enabling devices and components, advances in the system and sub-system technologies as well as networking and its applications.

Optical interconnects: recent advances and future challenges.

Recently, a new code structure, zero cross-correlation (ZCC) using anti-diagonal type-identity-column block matrices for spectral amplitude coding-optical code division multiple access (SAC-OCDMA) system, was proposed by K.S. Nisar. The new ZCC code (NZCC) is available for each natural number and has satisfactory code lengths. An effective OCDMA system must have efficient address codes to encode the source with almost zero correlation properties. ZCC eliminates phase-induced intensity noise that consequently improves the bit error rate. In this paper, performance analysis of the NZCC code employing SAC-OCDMA using the direct detection scheme is simulated and compared the efficiency with the other existing codes. The results show that for shorter ranges more number of users accommodated with high data rate by using NZCC code.

Performance evaluation of newly constructed NZCC for SAC-OCDMA using direct detection technique

We propose a joint multi-polarization-effect tracking and equalization method based on two extended Kalman filters, which can cope with state of polarization (SOP) tracing, polarization demultiplexing, equalization for polarization dependent loss (PDL) and polarization mode dispersion (PMD) in PDM-M-QAM coherent optical communication system. The mathematical model of the proposed method is given and analyzed in detail. Through simulation, the proposed method is proved to be very effective in a 28 Gbaud/s PDM-16QAM system. With the proposed method, SOP tracing speed is up to 110 Mrad/s for azimuth angle and 1200 krad/s for phase angle, respectively, and PDL and PMD can be equalized simultaneously in the values of 10 dB and more than half of the symbol period.

Joint tracking and equalization scheme for multi-polarization effects in coherent optical communication systems.

The invention provides a feasible Terabit transmission rate coherent light orthogonal frequency division multiplexing system which is of simpler system structure, and comprises a transmitting end and a receiving end, wherein, the transmitting end comprises a transmitting end optical carrier generation module which comprises a transmitting end laser, an optical comb generator and an optical demultiplexer. In the system, on the basis of single light source, the optical comb generator is utilized to obtain light frequency combs with multiple wavelengths, which are used as optical carriers of base band OFDM signals, and mutually orthogonal OFDM frequency bands are generated on an optical domain, thus a plurality of radio frequency (RF) oscillation sources with complex structures are not need in the prior art. By utilizing the system, an OBM-COOFDM (orthogonal-band-multiplexed-coherent optical orthogonal frequency division multiplexing) optical signal is obtained under the condition of not using the RF oscillation sources, thus greatly reducing the complexity and cost of a transmitter; and the system has better feasibility of the OBM-COOFDM hardware, thus reducing the higher sampling rate requirements of the traditional COOFDM Terabit transceiver on a digital to analog converter (DAC) and an analog to digital converter (ADC).

Terabit transmission rate coherent light orthogonal frequency division multiplexing (OFDM) system based on optical comb

We propose a modified method of constant modulus algorithm (CMA) based on polarization demultiplexing in Stokes space for polarization demultiplexing (SS-PDM) in optical coherent receivers. SS-PDM can tolerate chromatic dispersion (CD) and polarization-dependent loss (PDL), but is sensitive to PMD. On the other hand, the CMA with high-order FIR filters could compensate for polarization-mode-dispersion (PMD) effectively, but suffers from the singularity problem which results from PDL. Therefore, we use the polarization rotation matrix estimated by SS-PDM to set the initial center taps of the CMA. The singularity problem of the CMA is avoided effectively in a much wider range of PDL and its convergence speed is also improved. We demonstrate this modified CMA in a simulation of 100-Gbit/s PDM-QPSK with PDL of 1, 3, and 5 dB, respectively. PDL is emulated by attenuating one polarization tributary before polarization multiplexing and the interaction between PDL and PMD is ignored. The singularity problem disappears as expected. Because the tolerance of PDL is affected by DGD, we further investigate the PDL tolerance under different DGD. Finally, we conduct the simulation of 100-Gbit/s PDM-QPSK with PDL of 3 dB over 3000 km SSMF and 4000 km SSMF separately. The convergence speed accelerates indeed compared with that of the conventional CMA.

Modified Constant Modulus Algorithm With Polarization Demultiplexing in Stokes Space in Optical Coherent Receiver

The embodiment of the invention discloses a method and a device for generating and receiving an OOFDM (Orthogonal Frequency Division Multiplexing) signal and a wavelength division multiplexing system, belonging to the communication field, wherein the method for generating the OOFDM signal comprises the steps of: converting one path of serial high-speed data into N paths of parallel low-speed data; respectively modulating and mapping the N paths of parallel low-speed data to obtain N paths of information sequences; expanding the N paths of information sequences into 2N+2 paths of information sequences with Hermitian symmetrical structures; carrying out rapid inverse Fourier conversion on the 2N+2 paths of information sequences to obtain an OFDM baseband signal; carrying out digital-to-analogue conversion on the OFDM baseband signal to obtain an OFDM analog signal; and modulating the OFDM analog signal to an optical carrier wave to obtain the OOFDM signal. The OOFDM signal generated in the embodiment of the invention is received without tracking and compensating phase and frequency with a complex algorithm, therefore, the cost of realizing the system is lowered.

Method and device for generating and receiving OOFDM (Orthogonal Frequency Division Multiplexing) signal and wavelength division multiplexing system

Self-seeded passive optical networks (PONs) are currently attracting extensive research interest. In this paper, a novel self-seeded PON transmitter is, for the first time, proposed and experimentally demonstrated, which incorporates two face-to-face-positioned reflective semiconductor optical amplifiers (RSOAs) operating at their gain saturation regions: one RSOA directly driven by an upstream electrical signal and the other RSOA biased at a fixed current. Detailed experimental explorations are undertaken of the dynamic performance characteristics of the proposed transmitter. It is shown that, in comparison with previously reported self-seeded transmitters each employing a reflective mirror and a single electrical signal-driven RSOA, the proposed transmitter has a number of salient advantages including, considerably narrowed optical signal spectra, up to 16dB reduction in RINs of intensity-modulated optical signals, and residual intensity modulation crosstalk suppression as high as 10.7dB. The aforementioned features enable experimental demonstrations of real-time self-seeded 10Gb/s optical OFDM (OOFDM) transmitters. In particular, by making use of two low-cost RSOAs having their 3-dB modulation bandwidths as small as 1.125GHz, 10Gb/s over 25km adaptive OOFDM transmissions with power penalties of 0.6dB are experimentally achieved in the simple self-seeded IMDD PON systems.

Self-seeding-based 10Gb/s over 25km optical OFDM transmissions utilizing face-to-face dual-RSOAs at gain saturation

By utilizing digital orthogonal filtering (DOF) in the digital domain, we report, for the first time, experimental demonstrations of aggregated 30.078Gb/s/λ transmissions of DOF-multiplexed spectrally-overlapped and/or frequency gapless six channels over IMDD PON systems incorporating off-the-shelf and low-cost 10G-class optical devices. Experimental results show that simple adaptive channel power loading implemented in the digital domain enables very similar transmission performances of individual channels regardless of their locations in the digital filter space. As a direct result of the interplay between the transmission system-associated negative chromatic dispersion and the intensity modulation-induced frequency chirp, negative power penalties of >0.2dB are experimentally observed for all the involved channels under various transmission system configurations. In addition, excellent performance robustness of the demonstrated systems is also obtainable for various transmission distances up to 45km.

https://research.bangor.ac.uk/portal/files/20475878/2018_Experimental_demonstrations_of_30Gb_digital_orthogonal_filtering.pdf

Experimental demonstrations of 30Gb/s/λ digital orthogonal filtering-multiplexed multiple channel transmissions over IMDD PON systems utilizing 10G-class optical devices.

Mingliang Deng

Papers

Terabit transmission rate coherent light orthogonal frequency division multiplexing (OFDM) system based on optical comb

Modified Constant Modulus Algorithm With Polarization Demultiplexing in Stokes Space in Optical Coherent Receiver

Method and device for generating and receiving OOFDM (Orthogonal Frequency Division Multiplexing) signal and wavelength division multiplexing system

Self-seeding-based 10Gb/s over 25km optical OFDM transmissions utilizing face-to-face dual-RSOAs at gain saturation

Experimental demonstrations of 30Gb/s/λ digital orthogonal filtering-multiplexed multiple channel transmissions over IMDD PON systems utilizing 10G-class optical devices.