Sergejs Makovejs

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.

A long-reach ultra-dense 10 Gbit/s WDM-PON using a digital coherent receiver

We investigate the impact of channel spacing and nonlinear transmission over 120 km of standard single mode fiber for a 10 Gbit/s long-reach wavelength division multiplexed passive optical network (WDM-PON). We employed polarization division multiplexed quadrature phase shift keying (PDM-QPSK), which allowed data transmission at 3.125 GBaud, including a 25% overhead for forward error correction. To receive this spectrally efficient modulation format, a digital coherent receiver was employed, allowing for both frequency selectivity and an increased sensitivity of -45 dBm (25 photons/bit).We investigated a channel spacing as low as 5 GHz, for which the loss budget was 48.6 dB, increasing to 54.0 dB for a 50 GHz grid.

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.

Pulse-shaping versus digital backpropagation in 224Gbit/s PDM-16QAM transmission

We investigate the transmission performance of 224Gbit/s polarization-division-multiplexed 16-state quadrature amplitude modulation (PDM-16QAM) for systems employing standard single mode fiber (SSMF) and erbium doped fiber amplifiers (EDFAs). We consider the effectiveness of return-to-zero (RZ) data pulses with varying duty cycles and digital backpropagation (DBP) in reducing nonlinear distortion in wavelength-division- multiplexed (WDM) links with 3, 5, 7 and 9 channels. Similar improvement in transmission reach of 18-25% was achieved either by pulse-carving at the transmitter or by DBP, yielding maximum transmission distances of up to 1760km for RZ-pulse-shapes and 1280km for NRZ.

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.

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).

Record 500 km unrepeatered 100 Gb s−1 transmission

In this work we experimentally demonstrate 500 km unrepeatered transmission of a single-channel 100 Gb s−1 dual polarization quadrature phase shift keyed (DP-QPSK) signal. Such long distance transmission is achieved through the use of an advanced configuration of remotely pumped optical amplifiers (ROPAs), chromatic dispersion pre-compensation and ultra-low-loss Corning® SMF-28® ULL optical fiber. Excellent long-term bit error ratio (BER) performance is observed. To the best of our knowledge this is the longest unrepeatered 100 Gb s−1 transmission reported to date.

Long-Haul Transmission of PS-QPSK at 100 Gb/s Using Digital Backpropagation

We experimentally investigate the generation and long-haul transmission of polarization-switched quadrature phase shift keying (PS-QPSK) at an uncoded datarate of 112 Gbit/s (37.4 GBd). In what is the first demonstration of nonlinearity compensation for PS-QPSK, digital backpropagation is applied in order to evaluate performance improvements. We find that, even at this high symbol rate, the maximum transmission distance of this format can be increased by 20% by employing digital backpropagation.

Unrepeatered Nyquist PDM-16QAM transmission over 364 km using Raman amplification and multi-channel digital back-propagation

Transmission of a net 467-Gb/s PDM-16QAM Nyquist-spaced superchannel is reported with an intra-superchannel net spectral efficiency (SE) of 6.6 (b/s)/Hz, over 364-km SMF-28 ULL ultra-low loss optical fiber, enabled by bi-directional second-order Raman amplification and digital nonlinearity compensation. Multi-channel digital back-propagation (MC-DBP) was applied to compensate for nonlinear interference; an improvement of 2 dB in Q(2) factor was achieved when 70-GHz DBP bandwidth was applied, allowing an increase in span length of 37 km.

Amplification Schemes and Multi-Channel DBP for Unrepeatered Transmission

The performance of unrepeatered transmission of a seven Nyquist-spaced 10 GBd PDM-16QAM superchannel using full signal band coherent detection and multi-channel digital back propagation (MC-DBP) to mitigate nonlinear effects is analysed. For the first time in unrepeatered transmission, the performance of two amplification systems is investigated and directly compared in terms of achievable information rates (AIRs): 1) erbium-doped fibre amplifier (EDFA) and 2) second-order bidirectional Raman pumped amplification. The experiment is performed over different span lengths, demonstrating that, for an AIR of 6.8 bit/s/Hz, the Raman system enables an increase of 93 km (36 %) in span length. Further, at these distances, MC-DBP gives an improvement in AIR of 1 bit/s/Hz (to 7.8 bit/s/Hz) for both amplification schemes. The theoretical AIR gains for Raman and MC-DBP are shown to be preserved when considering low-density parity-check codes. Additionally, MC-DBP algorithms for both amplification schemes are compared in terms of performance and computational complexity. It is shown that to achieve the maximum MC-DBP gain, the Raman system requires approximately four times the computational complexity due to the distributed impact of fibre nonlinearity.