David S. Millar

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.

Digital Coherent Receivers for Long-Reach Optical Access Networks

The relative merits of coherent-enabled optical access network architectures are explored, with a focus on achievable capacity, reach and split ratio. We review the progress in implementing the particular case of the ultra dense wavelength division multiplexed (UDWDM) passive optical network (PON), and discuss some challenges and solutions encountered. The applicability of digital signal processing (DSP) to coherent receivers in PONs is shown through the design and implementation of parallelized, low-complexity application-specific digital filters. In this work, we focus on mitigating the impact of local oscillator laser (LO) relative intensity noise (RIN) on receiver sensitivity, and propose an algorithm which compensates for this impairment. This phenomenon is investigated theoretically and then experimentally by evaluating the sensitivity of a coherent receiver incorporating different tunable light sources; a low-RIN external cavity laser (ECL) and a monolithically integrated digital supermode distributed Bragg reflector (DS-DBR) laser. It is shown that the RIN of the signal laser does not significantly contribute to the degradation of the receiver sensitivity. Finally, a 10 Gbit/s coherent PON is demonstrated using a DS-DBR laser as the LO laser. It is found that a receiver sensitivity of -38.8 dBm is achievable assuming the use of hard-decision forward error correction.

Blind adaptive equalization of polarization-switched QPSK modulation

Coherent detection in combination with digital signal processing has recently enabled significant progress in the capacity of optical communications systems. This improvement has enabled detection of optimum constellations for optical signals in four dimensions. In this paper, we propose and investigate an algorithm for the blind adaptive equalization of one such modulation format: polarization-switched quaternary phase shift keying (PS-QPSK). The proposed algorithm, which includes both blind initialization and adaptation of the equalizer, is found to be insensitive to the input polarization state and demonstrates highly robust convergence in the presence of PDL, DGD and polarization rotation.

Generation and long-haul transmission of polarization-switched QPSK at 42.9 Gb/s

We demonstrate, for the first time, the generation and transmission of polarization-switched QPSK (PS-QPSK) signals at 42.9 Gb/s. Long-haul transmission of PS-QPSK is experimentally investigated in a recirculating loop and compared with transmission of dual-polarization QPSK (DP-QPSK) at 42.9 Gb/s per channel. A reduction in the required OSNR of 0.7 dB was found at a BER of 3.8 x 10(-3), resulting in an increase in maximum reach of more than 30% for a WDM system operating on a 50 GHz frequency grid. The maximum reach of 13640 km for WDM PS-QPSK is, to the best of our knowledge, the longest distance reported for 40 Gb/s WDM transmission, over an uncompensated link, with standard fiber and amplification.

Widely Tunable Burst Mode Digital Coherent Receiver With Fast Reconfiguration Time for 112 Gb/s DP-QPSK WDM Networks

A widely tunable burst mode digital coherent receiver is implemented in a 112 Gb/s DP-QPSK WDM system. The receiver performance is validated in a 24-channel WDM test-bed using a commercially available DS-DBR laser as the local oscillator. It is demonstrated that the wavelength tunable laser can switch to any one of the 24-channels in less than 130 ns, thus enabling the dynamic reception of 5 μ s optical bursts. The performance of the DS-DBR local oscillator laser is commensurate with burst mode coherent reception when differential decoding is employed and the parallel DSP implementation does not impair the polarization and frequency tracking performance of a digital coherent receiver under burst mode operation. The worst case reconfiguration time of the burst mode receiver, which is a combination of the laser switching time and the CMA convergence time, is less than 410 ns when switching from a single channel to any other channel in the WDM grid. It is shown that the variation in reconfiguration time is dependent on the convergence time of the CMA equalizer, which is adversely affected by certain input states of polarization.

Burst Mode Receiver for 112 Gb/s DP-QPSK with parallel DSP.

We demonstrate a burst mode 112Gb/s DP-QPSK digital coherent optical receiver with parallel DSP suitable for implementation in a CMOS ASIC (437.5MHz clock speed). A convergence time of less than 200ns is reported.

Characterization of long-haul 112Gbit/s PDM-QAM-16 transmission with and without digital nonlinearity compensation

In this paper long-haul, single channel, polarization multiplexed 16-state quadrature amplitude modulation (PDM-QAM-16) transmission at 112 Gbit/s is investigated. Novel digital signal processing techniques are used to perform carrier phase estimation and symbol estimation, in combination with nonlinear digital backpropagation. The results obtained demonstrate that the use of digital nonlinear backpropagation increases the optimum launch power from -4 dBm to -1 dBm with a consequent increase in maximum reach from 1440 km to 2400 km, which is a record transmission distance for QAM-16 reported to date for an SMF link with EDFAs only. Furthermore, experimental measurements are supported by simulations, based on the link used in the experiment.

Cycle Slip-Mitigating Turbo Demodulation in LDPC-Coded Coherent Optical Communications

We show that an iterative demodulation with soft-decision feedback information from FEC decoder can efficiently mitigate cycle slips. With 3% pilot insertion, the turbo QPSK demodulation achieves 1.05dB gain even in the presence of frequent cycle slips.

Influence of Pulse Shape in 112-Gb/s WDM PDM-QPSK Transmission

In this work, we investigated the influence of pulse shape on the transmission performance of polarization-division-multiplexed quadrature-phase-shift keying modulation format at 112 Gb/s in a ten-channel wavelength-division-multiplexed (WDM) transmission experiment with 50-GHz channel spacing. Nonreturn-to-zero (NRZ) and return-to-zero with 50% duty cycle (RZ50) were compared. RZ50 was found to have better performance in both single-channel and WDM experiments. Compared with NRZ, the use of RZ50 yielded an increase in reach from 6560 to 7760 km in the single-channel experiment (corresponding to an increase in reach by 18%); in the case of WDM. the reach was extended from 5920 to 7360 km (corresponding to a 24% increase in reach).

A 24-Dimensional Modulation Format Achieving 6 dB Asymptotic Power Efficiency

We propose modulation using the extended Golay code over the 24D hypercube, achieving 6 dB asymptotic power efficiency with 1 b/s/Hz/pol spectral efficiency. Noise tolerance is improved by 3 dB over DP-BPSK at a BER of 10−3.