Robert I. Killey

Electronic compensation of chromatic dispersion using a digital coherent receiver

Digital signal processing (DSP) combined with a phase and polarization diverse coherent receiver is a promising technology for future optical networks. Not only can the DSP be used to remove the need for dynamic polarization control, but also it may be utilized to compensate for nonlinear and linear transmission impairments. In this paper we present results of a 42.8Gbit/s nonlinear transmission experiment, using polarization multiplexed QPSK data at 10.7GBaud, with 4 bits per symbol. The digital coherent receiver allows 107,424 ps/nm of chromatic dispersion to be compensated digitally after transmission over 6400km of standard single mode fiber.

Electronic dispersion compensation by signal predistortion using digital Processing and a dual-drive Mach-Zehnder Modulator

We propose and investigate a novel electronic dispersion compensation technique, in which signal precompensation is achieved using a dual-drive Mach-Zehnder modulator, driven by adaptive nonlinear digital filters. The results demonstrate effective compensation of over 13600 ps/nm, equivalent to 800 km of standard single-mode fiber, at 10 Gb/s.

Reduction of intrachannel nonlinear distortion in 40-Gb/s-based WDM transmission over standard fiber

Intrachannel cross-phase modulation and four-wave mixing in high-bit-rate WDM transmission systems employing standard single-mode fiber are investigated. The effects of imperfect third-order dispersion compensation are included in the study and analytical expressions giving optimum values of dispersion precompensation minimizing the distortion due to the intrachannel nonlinear effects are derived.

Mitigation of Fiber Nonlinearity Using a Digital Coherent Receiver

Coherent detection with receiver-based DSP has recently enabled the mitigation of fiber nonlinear effects. We investigate the performance benefits available from the backpropagation algorithm for polarization division multiplexed quadrature amplitude phase-shift keying (PDM-QPSK) and 16-state quadrature amplitude modulation (PDM-QAM16). The performance of the receiver using a digital backpropagation algorithm with varying nonlinear step size is characterized to determine an upper bound on the suppression of intrachannel nonlinearities in a single-channel system. The results show that for the system under investigation PDM-QPSK and PDM-QAM16 have maximum step sizes for optimal performance of 160 and 80 km, respectively. Whilst the optimal launch power is increased by 2 and 2.5 dB for PDM-QPSK and PDM-QAM16, respectively, the Q-factor is correspondingly increased by 1.6 and 1 dB, highlighting the importance of studying nonlinear compensation for higher level modulation formats.

Spectrally Shaped DP-16QAM Super-Channel Transmission with Multi-Channel Digital Back-Propagation

The achievable transmission capacity of conventional optical fibre communication systems is limited by nonlinear distortions due to the Kerr effect and the difficulty in modulating the optical field to effectively use the available fibre bandwidth. In order to achieve a high information spectral density (ISD), while simultaneously maintaining transmission reach, multi-channel fibre nonlinearity compensation and spectrally efficient data encoding must be utilised. In this work, we use a single coherent super-receiver to simultaneously receive a DP-16QAM super-channel, consisting of seven spectrally shaped 10GBd sub-carriers spaced at the Nyquist frequency. Effective nonlinearity mitigation is achieved using multi-channel digital back-propagation (MC-DBP) and this technique is combined with an optimised forward error correction implementation to demonstrate a record gain in transmission reach of 85%; increasing the maximum transmission distance from 3190 km to 5890 km, with an ISD of 6.60 b/s/Hz. In addition, this report outlines for the first time, the sensitivity of MC-DBP gain to linear transmission line impairments and defines a trade-off between performance and complexity.

Digital Equalisation of 40Gbit/s per Wavelength Transmission over 2480km of Standard Fibre without Optical Dispersion Compensation

We demonstrate transmission and demodulation of 40Gbit/s per wavelength data, using 10Gbaud polarisation multiplexed QPSK. The digital coherent receiver includes carrier recovery and equalisation of all linear impairments, including chromatic dispersion and polarisation mode dispersion.

Performance of single-mode fiber links using electronic feed-forward and decision feedback equalizers

The performance limits of optically amplified links using electronic feed-forward equalizers and decision feedback equalizers are experimentally and numerically investigated. A 10-Gb/s transmission over 140 km of standard single-mode fiber with 22.7-dB optical signal-to-noise ratio sensitivity at 10/sup -9/ bit-error rate is demonstrated, allowing single-span operation with 4-dB margin.

On the performance of multichannel digital backpropagation in high-capacity long-haul optical transmission

The performance of digital backpropagation (DBP) equalization when applied over multiple channels to compensate for the nonlinear impairments in optical fiber transmission systems is investigated. The impact of a suboptimal multichannel DBP operation is evaluated, where implementation complexity is reduced by varying parameters such as the number of nonlinear steps per span and sampling rate. Results have been obtained for a reference system consisting of a 5×32 Gbaud PDM-16QAM superchannel with 33 GHz subchannel spacing and Nyquist pulse shaping under long-haul transmission. The reduction in the effectiveness of the algorithm is evaluated and compared with the ideal gain expected from the cancellation of the nonlinear signal distortion. The detrimental effects of polarization mode dispersion (PMD) with varying DBP bandwidth are also studied. Key parameters which ensure the effectiveness of multichannel DBP are identified.

Generation of optical OFDM signals using 21.4 GS/s real time digital signal processing

We demonstrate a field programmable gate array (FPGA) based optical orthogonal frequency division multiplexing (OFDM) transmitter implementing real time digital signal processing at a sample rate of 21.4 GS/s. The QPSK-OFDM signal is generated using an 8 bit, 128 point inverse fast Fourier transform (IFFT) core, performing one transform per clock cycle at a clock speed of 167.2 MHz and can be deployed with either a direct-detection or a coherent receiver. The hardware design and the main digital signal processing functions are described, and we show that the main performance limitation is due to the low (4-bit) resolution of the digital-to-analog converter (DAC) and the 8-bit resolution of the IFFT core used. We analyze the back-to-back performance of the transmitter generating an 8.36 Gb/s optical single sideband (SSB) OFDM signal using digital up-conversion, suitable for direct-detection. Additionally, we use the device to transmit 8.36 Gb/s SSB OFDM signals over 200 km of uncompensated standard single mode fiber achieving an overall BER<10(-3).

Electronic dispersion compensation by signal predistortion

We review recent work on electronic dispersion precompensation, and present the results of simulations investigating the compensation of chromatic dispersion and intra-channel fibre nonlinearity in 10 Gb/s systems using a programmable look-up table and linear finite impulse response filters