Auxiliary management and control channel aided crosstalk online monitoring in multi-core fibers.

As a stochastic perturbation, the inter-core crosstalk (IC-XT) severely distorts the signal in multi-core fibers (MCF), especially for long-haul transmission. How to quickly measure and monitor the IC-XT online for an MCF-based space division multiplexing (SDM) system is of special importance. In this paper, we introduce the technology of auxiliary management and control channel (AMCC) to online monitor the IC-XT of MCF, in which the unique advantage of low-frequency auxiliary management and control signal is fully utilized with the limited influence on high-speed data transmission. Specifically, two orthogonal sequences are chosen as monitoring signals for the signal-channel core and the crosstalk-channel core, respectively, followed by digital signal processing (DSP) for the received signal to evaluate the real-time crosstalk accurately. The experimental verifications of XT online monitoring confirm the effectiveness of our proposed method with very small monitoring error (mostly < 0.5 dB) for both heterodyne XT and homodyne XT in the C and L bands, showing its great potential for future SDM systems.

On the performance stability of MCF-based SDM system affected by inter-core crosstalk fluctuation.

The inter-core crosstalk (IC-XT) of multi-core fiber (MCF) limits the capacity of space division multiplexing system (SDM) fundamentally. We develop a closed-form expression of the magnitude of IC-XT for various types of signals, which can well explain the mechanism of different fluctuation behaviors of the real-time short-term average crosstalk (STAXT) and bit error ratio (BER) for the optical signals with and without strong optical carrier. The experimental verifications with the real-time measurement of the BER and outage probability in a 7 × 10-Gb/s SDM system agree well with the proposed theory and confirm that the unmodulated optical carrier plays a substantial role in fluctuation of BER. The range of fluctuation can be reduced by 3 orders of magnitude for the optical signal without optical carrier. We also investigate the effect of IC-XT in a long-haul transmission system based on a recirculating 7-core fiber loop and develop a frequency-domain IC-XT measurement technique. Longer transmission distance is shown to have a narrower BER fluctuation range, since IC-XT is no longer the only dominant factor on transmission performance.

Generalized model of optical single sideband generation using dual modulation of DML and EAM.

The dual modulation of a directly modulated laser (DML) and an electro-absorption modulator (EAM) is known to be an effective way to generate an optical single sideband (SSB) signal in highly compact and cost-effective fashions. Two theoretical models were developed for this dual modulation scheme under the assumptions of negligible intensity modulation of DML and/or zero chirp of EAM. In this paper, we develop a generalized model of dual modulation scheme without those assumptions. We show theoretically that the previous models can be unified as special cases of our generalized model. We evaluate the validity of our model using numerical simulations and experiments. The results show that our model estimates the modulation conditions for optical SSB generation accurately over wide ranges of modulation frequency and EAM's chirp. We also show experimentally that we can achieve an optical sideband suppression ratio higher than 40 dB by using our generalized model. This is >25 dB higher than the ratios obtained from the previous models.

Low-complexity nonlinear equalizer based on absolute operation for C-band IM/DD systems.

The intensity-modulation/direct-detection transmission system operating in the C-band suffers from nonlinear waveform distortions induced by fiber chromatic dispersion due to the square-law detection. The Volterra nonlinear equalizers (VNLEs) can be used at the receiver to compensate for such distortions. However, the major concern about the equalizers is their huge implementation complexity. In this paper, we propose and demonstrate a low-complexity nonlinear equalizer based on the absolute operation for a cost-sensitive IM/DD system. In this equalizer, the cross-beating product terms (required in VNLE) are replaced with the absolute operation of the sum of two input samples. We evaluate the performance of the proposed equalizer over a 56-Gb/s 4-ary pulse amplitude modulation transmission system implemented by using 1.5-µm directly modulated laser or electro-absorption modulated laser. The results show that the proposed equalizer performs similar to the 2nd-order diagonally-pruned VNLE, but lowers the implementation complexity by >20%. We also show that the proposed equalizer outperforms the VNLE when the implementation complexities of the two nonlinear equalizers are similar.

Reduced-state MLSE for an IM/DD system using PAM modulation.

The performance of a high-speed intensity-modulation (IM)/direct-detection (DD) transmission system could be limited by the bandwidth of optical transceivers. One popular way to cope with this performance limitation is to utilize the maximum likelihood sequence estimation (MLSE) at the receiver. However, a practical problem of MLSE is its high implementation complexity. Even though the channel impulse response can be truncated by using a two-tap filter before applying the MLSE, it still faces an implementation problem when used for multi-level modulation formats. In this paper, we propose and demonstrate a reduced-state MLSE for band-limited IM/DD transmission systems using M-ary pulse amplitude modulation (PAM-M) formats. We use a conventional Viterbi algorithm to search a reduced-state trellis, which is constructed by using the coarse pre-decision of the signal equalized by a feed-forward equalizer. Thus, the proposed MLSE reduces the implementation complexity significantly. We evaluate the performance of the proposed reduced-state MLSE over 100∼140-Gb/s PAM-4/6/8 transmission systems implemented by using a 1.3-µm directly modulated laser. The results show that the proposed MLSE achieves almost the same performance as the conventional MLSE but reduces the implementation complexity by a factor of 4∼10 when the complexity is assessed by the number of multiplications and additions.

Probabilistically shaped coded modulation for IM/DD system.

The probabilistic constellation shaping (PCS) technology has recently gained a great deal of attention for coherent optical communication systems since it allows us to approach the Shannon capacity limit by varying the symbol distribution adaptively to the signal-to-noise ratio (SNR). However, there is a lack of literature on how to apply this technology to intensity modulation (IM)/direct detection (DD) systems. In this paper, we propose and demonstrate an efficient way to apply the PCS technology for IM/DD systems. In the mapping of forward error correction-encoded bits onto the pulse amplitude modulation (PAM) symbols, we assign the uniformly distributed bits to the least significant bit of binary reflected Gray coding. Then, we have a pairwise distribution of symbol amplitude, where two adjacent symbols have the same probability. Although this distribution deviates from the optimum distribution (such as Maxwell-Boltzmann distribution), we show that the SNR penalty caused by this discrepancy is negligible. We evaluate the performance of the proposed scheme through simulation by measuring the achievable rate and frame error ratios after inverse distribution matcher. The results show that the proposed scheme provides a shaping gain larger than the time-division hybrid modulation. We also carry out the experimental demonstration of the proposed scheme using a 10-Gbaud PAM-8 signal. By using the proposed scheme, we improve the receiver sensitivity by 0.9 dB when compared with the uniformly distributed PAM-8 signal.

Kramers-Kronig receiver operable without digital upsampling.

The Kramers-Kronig (KK) receiver is capable of retrieving the phase information of optical single-sideband (SSB) signal from the optical intensity when the optical signal satisfies the minimum phase condition. Thus, it is possible to direct-detect the optical SSB signal without suffering from the signal-signal beat interference and linear transmission impairments. However, due to the spectral broadening induced by nonlinear operations in the conventional KK algorithm, it is necessary to employ the digital upsampling at the beginning of the digital signal processing (DSP). The increased number of samples at the DSP would hinder the real-time implementation of this attractive receiver. Hence, we propose a new DSP algorithm for KK receiver operable at 2 samples per symbol. We adopt a couple of mathematical approximations to avoid the use of nonlinear operations such as logarithm and exponential functions. By using the proposed algorithm, we demonstrate the transmission of 112-Gb/s SSB orthogonal frequency-division-multiplexed signal over an 80-km fiber link. The results show that the proposed algorithm operating at 2 samples per symbol exhibits similar performance to the conventional KK one operating at 6 samples per symbol. We also present the error analysis of the proposed algorithm for KK receiver in comparison with the conventional one.

Common phase error estimation in coherent optical OFDM systems using best-fit bounding box.

In this paper, we investigate and characterize a new approach of adopting best-fit bounding box method for common phase error estimation in coherent optical OFDM systems. The method is based on the calculation of the 2-D convex hull of the received signal constellation, which is generally adopted in image processing area to correct the skew of images. We further perform detailed characterizations including root mean square error analysis, laser linewidth tolerance, noise tolerance, and computation complexity analysis, via numerical simulations and experiments. The results show the proposed method achieves much improved spectral efficiency and comparable system performance than the pilot-aided method, while it exhibits good estimation accuracy and reduced complexity than the blind phase searching method.

Flexible and scalable wavelength multicast of coherent optical OFDM with tolerance against pump phase-noise using reconfigurable coherent multi-carrier pumping.

Recently the ever-growing demand for dynamic and high-capacity services in optical networks has resulted in new challenges that require improved network agility and flexibility in order for network resources to become more "consumable" and dynamic, or elastic, in response to requests from higher network layers. Flexible and scalable wavelength conversion or multicast is one of the most important technologies needed for developing agility in the physical layer. This paper will investigate how, using a reconfigurable coherent multi-carrier as a pump, the multicast scalability and the flexibility in wavelength allocation of the converted signals can be effectively improved. Moreover, the coherence in the multiple carriers prevents the phase noise transformation from the local pump to the converted signals, which is imperative for the phase-noise-sensitive multi-level single- or multi-carrier modulated signal. To verify the feasibility of the proposed scheme, we experimentally demonstrate the wavelength multicast of coherent optical orthogonal frequency division multiplexing (CO-OFDM) signals using a reconfigurable coherent multi-carrier pump, showing flexibility in wavelength allocation, scalability in multicast, and tolerance against pump phase noise. Less than 0.5 dB and 1.8 dB power penalties at a bit-error rate (BER) of 10-3 are obtained for the converted CO-OFDM-quadrature phase-shift keying (QPSK) and CO-OFDM-16-ary quadrature amplitude modulation (16QAM) signals, respectively, even when using a distributed feedback laser (DFB) as a pump source. In contrast, with a free-running pumping scheme, the phase noise from DFB pumps severely deteriorates the CO-OFDM signals, resulting in a visible error-floor at a BER of 10-2 in the converted CO-OFDM-16QAM signals.