Experimental demonstrations of dual polarization CO-OFDM using mid-span spectral inversion for nonlinearity compensation.

We experimentally demonstrate fiber nonlinearity compensation in dual polarization coherent optical OFDM (DP CO-OFDM) systems using mid-span spectral inversion (MSSI). We use third-order nonlinearity between a pump and the signal in a highly nonlinear fiber (HNLF) for MSSI. Maximum launch powers at FEC threshold for two 10 × 80-km 16-QAM OFDM systems were increased by 6.4 dB at a 121-Gb/s data rate and 2.8 dB at 1.2 Tb/s. The experimental results are the first demonstration of using MSSI for nonlinearity compensation in any dual polarization coherent system. Simulations show that these increases could support a 22% increase in total transmission distance at 1.2-Tb/s system without increasing the number of inline amplifiers, by extending the fiber spans from 90 to 110 km. When spans of 80 km are used, simulations reveal that MSSI system performance shows less degradation with increasing transmission distance, and an overall transmission distance increase of more than 70% is expected using MSSI.

Flexible all-optical frequency allocation of OFDM subcarriers.

We investigate the underlying mechanism that allows OFDM subcarriers in an all-optical OFDM system to be assigned to any optical frequency using an optical filter, even if that frequency is not generated by the comb-line source feeding the filters. We confirm our analysis using simulations, and present experimental results from a 252-subcarrier system that uses a mode-locked laser (MLL) as the comb source and a wavelength selective switch. The experimental results show that there is no correlation between the programmed frequency offset between a subcarrier and nearest comb line, and the received signal quality. Thus, subcarriers could be inserted into unused portions of an optical transmission system's spectrum without restriction on their particular center frequencies. Any percentage of cyclic prefix can be added to the OFDM symbol simply by reprogramming the optical filter to give wider subcarrier frequency spacing than the comb line spacing, which is useful for tailoring the CP to the dispersion of various optical transmission paths, to maximize the spectral efficiency. Finally, the MLL's center frequency need not be locked to a system reference.

Improving performance of optical phase conjugation by splitting the nonlinear element.

We show that optical phase conjugation (OPC) based on third order nonlinear effects for mid-span spectral inversion (MSSI) can be improved by splitting the nonlinear element into two parts and adding an optical filter between them. This band-stop filter suppresses the cross-phase-modulation products that are generated around the pump, which, if not removed, will be shifted to fall around the output OPC signal band. Numerical simulations show that this method reduces the fundamental limitations introduced by OPC by 3 dB, which results in improvement of the maximum signal quality, Qmax, by 1 dB in a 10 × 80-km 4-QAM 224-Gb/s CO-OFDM system with MSSI.

An optical FPGA: reconfigurable simultaneous multi-output spectral pulse-shaping for linear optical processing.

We demonstrate a pulse-shaping technique that allows for spectrally resolved splitting of an input signal to multiple output ports. This ability enables reconfigurable creation of splitters with complex wavelength-dependent splitting ratios, giving similar flexibility to a Field Programmable Gate Array (FPGA) in electronics. Our technique can be used to create reprogrammable optical (interferometric) circuits, by emulating their multi-port spectral transfer functions instead of the traditional method of creating an interferometer by splitting and recombining the light with an added delay. We demonstrate the capabilities of this technique by creating a Mach-Zehnder interferometer, an all-optical discrete Fourier transform filter, two nested Mach-Zehnder interferometers and a complex splitter with a triangular-shaped response.

Improved two-stage equalization for coherent Pol-Mux QPSK and 16-QAM systems.

We report a two-stage blind frequency domain equalization method for long-haul coherent polarization-multiplexed (pol-mux) systems using quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (16-QAM). In the first stage, blind CD parameter prediction is conducted prior to a CD equalizer. This supports flexible path switching in optical networks. In the second stage, a frequency-domain multi-modulus algorithm (MMA) equalizer is used to cope with the residual fiber impairments and perform polarization de-multiplexing. Compared with the conventional constant modulus algorithm (CMA), MMA shows advantages including better steady state performance and a faster convergence rate. Furthermore, all the estimation and equalization algorithms are implemented in the frequency domain which potentially provides the least complexity for the pol-mux optical coherent systems. The proposed algorithm is experimentally demonstrated with an 800-km 10 Gbaud coherent optical pol-mux system. For QPSK signal, the proposed method achieves error-free transmission and shows superior convergence speed against CMA, and for 16-QAM signals, the proposed MMA outperforms CMA with more than 1-dB improvement in Q-value.

The validity of "Odd and Even" channels for testing all-optical OFDM and Nyquist WDM long-haul fiber systems.

We investigate experimentally the validity of testing all-optical OFDM and Nyquist WDM systems using interleaved test channels derived from only two data sources. These "odd and even" channels are insufficiently decorrelated, so experiments underestimate the inter-carrier interference (ICI). Additionally, numerical simulations demonstrate that using odd and even channels generates stronger nonlinear distortions during transmission, causing an unrealistically large penalty in the nonlinearity-limited region.

Optical performance monitoring for OFDM using low bandwidth coherent receivers.

We propose using low bandwidth coherent receivers for distributed optical performance monitoring. We demonstrate optical signal-to-noise ratio (OSNR) monitoring of both 20-Gb/s single-polarization and 40-Gb/s polarization-multiplexed coherent optical orthogonal frequency-division multiplexing (CO-OFDM) signals with a 0.8-GHz receiver using both data-aided (DA) and non-data-aided (NDA) approaches. The sampling rate of the performance monitor is much lower than the signal baud rate, so provides a cost-effective solution for distributed optical performance monitoring. The proposed method is demonstrated experimentally and through simulation. The results show that after calibration the OSNR monitoring error is less than 1 dB and the two approaches are not affected by fiber dispersion after 800-km transmission and 30-ps differential group delay (DGD).

Fiber nonlinearity compensation for OFDM super-channels using optical phase conjugation.

We experimentally demonstrate that mid-link optical phase conjugation (OPC) effectively compensates fiber nonlinearity in coherent optical OFDM super-channels. The OPC was produced by pump × subcarrier degenerate four-wave-mixing in a 1-km highly nonlinear fiber. The nonlinear threshold for the 10 × 80-km 604.7-Gb/s 16-QAM test system was increased by 4.8 dB. The performance at the optimum power was only improved by 0.2 dB because the OPC module produces a 1.6 dB penalty for the back-to-back system. FWM theory shows that the 'noise' processes of OPC modules utilizing χ3 nonlinearities could be reduced by increasing the pump power, which will improve back-to-back performance with the OPC module.

Statistical moments-based OSNR monitoring for coherent optical systems.

We present a novel statistical moments-based method for optical signal-to-noise ratio (OSNR) monitoring in polarization-multiplexed (pol-mux) coherent optical systems. This technique only requires the knowledge of the envelope of the equalized signal before phase correction, which can be achieved by using any two arbitrary statistical moments, and it is suitable for both constant and non-constant modulus modulation formats. The proposed estimation method is experimentally demonstrated for 10-Gbaud pol-mux coherent systems using QPSK and 16-QAM. Additionally, numerical simulations are carried out to demonstrate 20-Gbaud systems using 16-QAM and 64-QAM. The results show that the OSNR can be estimated accurately over a wide range of values for QPSK, 16-QAM or 64-QAM systems up to 1920-km long and with up to 50-ps all-order polarization mode dispersion. By setting a proper reference value for calibration, the proposed algorithm also shows good tolerance when the received signal is not well compensated.

Optimizing the subcarrier granularity of coherent optical communications systems.

In this paper, we use numerical simulations to show that the symbol rate has a significant effect on the nonlinearity-limited performance of coherent optical communication systems. We consider the case where orthogonal subcarriers are used to maximize the spectral efficiency. Symbol rates from 0.78125 Gbaud to 100 Gbaud and links of up to 3200 km, without inline dispersion compensation, were simulated. The results show that the optimal symbol rates for the 800-km link and 3200-km link were 6.25-Gbaud and 3.125-Gbaud respectively. The optimal baud rate decreases as the length of the link is increased. After 3200 km, the performance of the 100-Gbaud system was worst in the nonlinearity-limited regime producing a received Q 2.4-dB lower than the 3.125-Gband system. The variation in the nonlinearity-limited performance is explained by using Cross-Phase-Modulation (XPM) theory and by considering the RF spectra of the intensity fluctuations of the signal along the link. The findings of the paper suggest that the maximum capacity of nonlinear dispersive optical links can only be achieved by using multiple subcarriers carrying a few Gbaud each, and not by high symbol rate systems.

Pilot-based cross-phase modulation compensation for coherent optical orthogonal frequency division multiplexing long-haul optical communications systems.

A pilot-based nonlinearity compensator (PB-NLC) is shown in this Letter to be an effective method for compensating cross-phase modulation (XPM) in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. An unmodulated pilot tone is transmitted at the center of each OFDM channel to detect phase errors caused by the Kerr effect, which converts intensity fluctuations from all channels to phase errors. The pilots are then used to cancel the XPM phase errors for each OFDM channel at the receiver after each channel's self-phase modulation (SPM) has been compensated, using its intensity waveform to determine its SPM. Numerical simulations of a 58 Gb/s single polarization 2375 km system with inline dispersion compensation show that the signal quality, Q, at the optimal launch power is increased by 2.4 dB if SPM compensation is used before the PB-NLC. This contrasts with only a 0.9 dB improvement if the PB-NLC is used without an SPM compensator for the same link. This shows the PB-NLC can effectively mitigate XPM but not SPM.

No-guard-interval coherent optical OFDM with self-tuning receiver.

We propose and experimentally demonstrate a no-guard-interval (No-GI) coherent optical orthogonal frequency division multiplexed (CO-OFDM) system that uses Fractionally-Spaced Time-Domain Equalizers (FS-TDE) to simultaneously demultiplex and equalize each subcarrier. Least-mean-squares algorithms (LMS) tune each FS-TDE to a subcarrier. A short unique training sequence is transmitted on each subcarrier, allowing each FS-TDE to lock onto its subcarrier. After the initial training, the adaptive blind equalizers remain tuned to their respective subcarriers. Unlike previous systems, this system does not require digital filtering or mixing of each subcarrier to baseband, so is more computationally efficient. Error-free transmission was measured over 800 km of fiber with a three-subcarrier 30 Gb/s system and a five-subcarrier 33.33 Gb/s system. The required OSNRs for a BER of 10(-3) were 8.6 dB and 9.3 dB respectively, which are within 1.5 dB of the theoretical limit for coherent systems.

Pilot-based XPM nonlinearity compensator for CO-OFDM systems.

We experimentally verify that pilot-based nonlinearity compensation is effective for mitigating XPM in CO-OFDM systems, if SPM is compensated first. A 6-dB increase in the nonlinear limit was produced by pilot-based XPM compensation after a single-step SPM compensator in a 400-km link with periodic dispersion compensation. In addition, we use numerical simulations to show that the required bandwidth of the guard-band around the pilot is almost independent of the bandwidth of the data-carrying sidebands. The optimal ratio of pilot to signal power also decreases for higher bandwidth OFDM signals. Therefore, the overhead associated with transmitting the pilot decreases as the bandwidth of the signal increases.

Improved single channel backpropagation for intra-channel fiber nonlinearity compensation in long-haul optical communication systems.

Backpropagation has been shown to be the most effective method for compensating intra-channel fiber nonlinearity in long-haul optical communications systems. However, effective compensation is computationally expensive, as it requires numerous steps and possibly increased sampling rates compared with the baud rate. This makes backpropagation difficult to implement in real-time. We propose: (i) low-pass filtering the compensation signal (the intensity waveform used to calculate the nonlinearity compensation) in each backpropagation step and (ii) optimizing the position of the nonlinear section in each step. With numerical simulations, we show that these modifications to backpropagation improve system performance, reducing the number of backpropagation steps and reducing the oversampling for a given system performance. Using our 'filtered backpropagation', with four backpropagation steps operating at the same sampling rate as that required for linear equalizers, the Q at the optimal launch power was improved by 2 dB and 1.6 dB for single wavelength CO-OFDM and CO-QPSK systems, respectively, in a 3200 km (40 x 80 km) single-mode fiber link, with no optical dispersion compensation. With previously proposed backpropagation methods, 40 steps were required to achieve an equivalent performance. A doubling in the sampling rate of the OFDM system was also required. We estimate this is a reduction in computational complexity by a factor of around ten.

Improved nonlinearity precompensation for long-haul high-data-rate transmission using coherent optical OFDM.

We show that the performance of precompensation of fiber nonlinearity in coherent optical OFDM systems operating at up to 60 Gbps/polarization can be improved by electrical filtering the precompensation signal. The optimal filter bandwidth is related to the FWM efficiency spectrum when dispersion is considered.

Fiber nonlinearity precompensation for long-haul links using direct-detection optical OFDM.

The use of nonlinearity precompensation in direct-detection optical orthogonal frequency division multiplexed links is investigated by simulation. Because of the presence of a strong optical carrier its performance is poorer than for coherent systems: with compensation the signal quality is found to vary almost periodically across the signal band. We propose and explain the operation of two optical, one electrical and one computational method of removing this periodic variation. Optical filtering of one sideband at the receiver is most effective, but a substantial improvement can be obtained by a simple modification to the precompensation algorithm.