Twisted optical fibers for polarization preservation

Coherent optical fiber transmission systems, which allow the development of advanced communication systems with superior performance by using high-performance constituent subsystems, such as optical transmitters, fibers and receivers, are discussed. Essential technologies for developing the coherent optical fiber transmission systems are reviewed and system advantages are discussed. The state of the art in transmission experiments is also described, and future areas of concern essential to further development are outlined. >

A brief overview of recent experimental research on coherent optical fiber transmission systems at Queen’s University is presented. Exemplary results are described that exploit real-time signal processing to assess the impact of cascaded optical filtering.

Independent component analysis based digital signal processing in coherent optical fiber communication systems

A tutorial review of techniques for coherent optical fiber transmission systems

A low-loss, wide band, compact, polarization preserving, fiber optic directional coupler (PPFDC) is developed for use in future coherent optical fiber transmission systems or gyroscopes. The PPFDC is made of two 200 μm diameter sapphire ball lenses, a half-mirror and four conical alumina-ceramic ferrules with flat reference plane to preserve polarization at the connection of the fibers. Built-in thermal stress change in the fiber is minimized, because the fiber is polished only for the short length within the straight ferrule. The optical and mechanical axes are tilted, so cross-talk light from the fiber endfaces or lens surfaces is avoided. The optical directivity of the PPFDC is over 50 dB and return loss is over 40 dB in the 1.3 μm wavelength range. The excess loss is under 1 dB in the 1.2 to 1.4 μm wavelength range. The extinction ratio is over 25 dB. The length of the coupling region of the PPFDC is about 1 mm including the two ball lenses and half mirror. Spectral line widths of a fiber interfaced DFB laser diode are measured by a self delayed homodyne detection scheme, using the PPFDC. Experimental results of a novel polarization preserving fiber-optic gyroscope using a phase modulating PPFDC showed reasonable sensitivity and stability. These experimental results verified the above mentioned characteristics of the PPFDC.

In this paper, we propose a promising nonlinear noise power estimation method based on correlation functions using artificial neural networks (ANN), which is robust against both amplifier spontaneous emission noise and the symbol patterns. Error vector correlation (EVC), together with the amplitude noise correlation (ANC) and the phase noise correlation (PNC), is used as the input of ANN in the proposed method. 378 cases of 224 Gb/s polarization-multiplexed 16-quadrature amplitude modulated (PM-16-QAM) signal transmitted over a wide range of conditions by varying launch power, OSNR and transmission distance are used to train and test the ANN. With the launch power varying from 0 to 8 dBm and the transmission distance as long as 2400 km, the results of tested samples demonstrate that the maximum absolute deviation (MAD), mean absolute error (MAE) and root mean square error (RMSE) of the estimated nonlinear noise power are 0.65 dB, 0.20 dB and 0.27 dB, respectively. In order to verify the independence of symbol patterns, 150 new cases with 30 pseudo-random binary sequence (PRBS) seeds are used to test the trained ANN. The results show that the MAE, MAD and RMSE of estimated nonlinear noise power are 0.86 dB, 0.25 dB and 0.33 dB respectively, meaning that the trained ANN is valid even if the test samples are not covered by the trained process. The results in this work verified that the ANN with EVC, ANC and PNC as input can make the trained model feasible to accurately estimate the nonlinear noise power in high speed optical coherent communication systems.

In this paper, a rate adaptive geometric constellaion shaping (GCS) scheme which is fully backward-compatible with existing state of the art bit-interleaved coded modulation (BICM) systems is proposed and experimentally demonstrated. The system relies on optimization of the positions of the quadrature amplitude modulation (QAM) points on the I/Q plane for maximized achievable information rate, while maintaining quantization and fiber nonlinear noise robustness. Furthermore, ‘dummy’ bits are multiplexed with coded bits before mapping to symbols. Rate adaptivity is achieved by tuning the ratio of coded and ‘dummy’ bits, while maintaining a fixed forward error-correction block and a fixed modulation format size. The points' positions and their labeling are optimized using automatic differentiation in a many-to-one fashion such that the performance of the ‘dummy’ bits is sacrificed in favor of the performance of the data bits. The proposed GCS scheme is compared to a time-sharing hybrid (TH) QAM modulation and the now mainstream probabilistic amplitude shaping (PAS) scheme employing a Maxwell-Boltzmann probability mass function. The TH without shaping is outperformed for all studied data rates in a simulated linear channel by up to 0.7 dB. In a linear channel, PAS is shown to outperform the proposed GCS scheme, while similar performances are reported for PAS and the proposed GCS in a simulated nonlinear fiber channel. The GCS scheme is experimentally demonstrated in a multi-span recirculating loop coherent optical fiber transmission system with a total distance of up to 3000 km. Near-continuous zero-error flexible throughput is reported as a function of the transmission distance. Up to 1-2 spans of increased reach gains are achieved at the same net data rate w.r.t. conventional QAM. At a given distance, up to 0.79 bits/2D symbol of gain w.r.t. conventional QAM is achieved. In the experiment, similar performance to PAS is demonstrated.

In this paper, we compare the nonlinearity effects of single mode fiber (SMF) and non-zero dispersion shifted fiber (NZ-DSF) on the performance of optical coherent transmission systems. We estimate the nonlinearity effects of fiber, referred to as nonlinear interference noise (NLIN), by using the so-called enhanced Gaussian noise (EGN) model, and compute signal to noise ration (SNR) of optical coherent transmission system by considering NLIN and optical amplifier noise, referred to as amplified spontaneous emission (ASE) noise. By considering EGN model we optimize the lunch power of optical coherent systems considering SMF and NZ-DSF. Finally, a 100Gbps optical coherent transmission system is simulated and its results are compared with analytical results obtained based on the EGN model. The closeness between analytical and simulation results reveal the accuracy of the EGN model and the power optimization formulations.

Shaping modulation formats in multi-dimensional (MD) space is an effective approach to harvest spectral efficiency gains in both the additive white Gaussian noise (AWGN) channel and the optical fiber channel. In the first part of this paper, existing MD geometrically-shaped modulations for fiber optical communications are reviewed. It is shown that large gains can be obtained by exploiting correlation in the dimensions or/and by increasing the cardinality of the modulation format. Practical limitations and challenges are also discussed together with efficient solutions. In the second part, we extend the recently proposed four-dimensional (4D) modulation format family based on the constraint of orthant-symmetry to high spectrum efficiencies up to 10 b/4D-sym by maximizing generalized mutual information for AWGN channel. Reach increases of up to 25% for a multi-span optical fiber transmission system are reported. Lastly, with the help of a recently introduced nonlinear interference (NLI) model, an optimization for designing nonlinear-tolerant 4D modulation formats is introduced for a single-span optical fiber system. Simulation results show that the proposed NLI model-based 4D modulation format could increase the effective SNRs by 0.25 dB with respect to the AWGN channel-optimal 4D modulation format.

Nonlinear effects have been considered as the major limitations in coherent optical (CO) fiber transmission system. Digital signal processing (DSP)-based CO receiver with digital backpropagation (DBP) method has recently facilitated the compensation of fiber nonlinear impairments as well as compensating dispersion of optical fiber. In this paper, for the first time, a comprehensive design is presented for a newly introduced DSP-based CO receiver with the 8-quadrature amplitude modulation (8-QAM) circular-polarization division multiplexed (CPDM) fiber optic communication (FOC) system to investigate the impact of nonlinearities. We have examined the performance of nonlinearity compensating DSP-based CO receiver with DBP method and demonstrated that it can be effectively employed to mitigate the intrachannel nonlinearities in CPDM 8-QAM FOC system over 800-km SSMF link. Accordingly, by effectively compensating the fiber nonlinearities, we have analyzed the performance of bit error rate, optical signal-to-noise ratio (OSNR), optimum launch power, and investigate the link margin by evaluating OSNR margin for a specific launch power. Moreover, a CPDM technique helps as a very suitable means of maximizing the link capacity as well as enhancing the spectral efficiency of the FOC system.

Polarization dependent loss (PDL) causes imbalanced optical signal to noise ratio (OSNR) of the two polarizations, thus remains one of the major bottlenecks for next-generation polarization-division-multiplexed (PDM) coherent optical transmission systems. In this paper, we investigate Pairwise Coding for adaptive PDL mitigation in PDM coherent optical systems. By pre-coding across two polarizations, the PDL-induced performance degradation can be largely mitigated without any coding overhead. We present details of the coding and de-coding design, and also derive the analytical symbol/bit error rate of the Polarization Pairwise Coding scheme, which can be used to predict the performance gain as well as for optimal rotation angle calculation. Simulation results verify that Pairwise Coding achieves substantial system performance gains over a wide range of PDL values. Compared with other digital coding techniques, Polarization Pairwise Coding shows improved performance than Walsh-Hadamard transform since it maximizes the coordinate diversity; and also Pairwise Coding is computationally much simpler to decode compared with the Golden and Silver Codes, therefore is practical for current 100-Gb/s and future 400-Gb/s and 1-Tb/s digital coherent transceivers.

Coherent optical communication systems became possible with development of the gas laser in the early 1960s. During the decade following development of the laser, Enloe and Rodda showed homodyne detection to be possible when they successfully phase-locked two gas lasers. DeLange and Dietrich showed that stable heterodyne detection was possible over an enclosed optical transmission path. However, not until the 1980s were optical waveguide and semiconductor laser technologies developed to the stage where they could be used for coherent optical fiber transmission.

A high speed optical coherent transmission System using Narrowband Frequency Modulation (NBFM) subcarrier multiplexing (SCM) has been introduced where the FM modulated optical signal for MPSK digital data is transmitted by a single mode fiber. At the receiver, a photodiode (PD) generates the electrical signal including N channel SCM signals and the optical intermediate frequency (IF) from the coupler. The electrical signal from the BPF output is FM demodulated and then divided into the N SCM data.We have analyzed the variance of Gaussian noise at the FM discriminator output for four adjacent sucarrier channels which are degraded by 10.1, 12.0, and 14.5 [nW/Hz] as the subcarrier frequency is increased by 0.5 GHz, respectively. The caculation results of BERs for the M-ary PSK modulators, BPSK, QPSK, 8-PSK, and 16-PSK schemes that shows the SNR differences between the modulation schemes are 1.9, 2.1. 2.3 dB at BER = 109, as the subcarrier frequency is increased by 0.5 GHz, respectively.KeywordsSubcarrier MultiplexingNarrowband Frequency ModulationOptical Coherent Transmission

The increasing shortage of spectrum resources makes researchers pay much attention to how to improve the spectral efficiency. Faster-than-Nyquist (FTN) coherent optical transmission is one of the most promising techniques to improve the spectral efficiency achieved by duobinary shaping. In this work, a FTN coherent optical transmission system is constructed containing transmitting side and receiving side. On the transmitting side, signals are transmitted after QPSK or 16QAM modulation with duobinary shaping. On the receiving side, the laser phase noise is eliminated by a pilot aided blind phase search (BPS) algorithm. Then the duobinary-shaped 16QAM or QPSK signal is processed by the maximum likelihood sequence estimation (MLSE). The constellation diagrams prove the effectiveness and convergence of the proposed algorithms. Moreover, the numerical results verify the relationship between the optical signal to noise ratio (OSNR) and the bit error rate (BER). Meanwhile, it is obvious that the FTN coherent optical transmission with duobinary shaping can improve the spectral efficiency effectively for signals of different modulation formats.

Quadrature duobinary (QDB) spectrum shaping polarization division multiplexed-quadrature phase shift keying (PDM-QPSK) signal with Nyquist and super-Nyquist spectrum efficiency will be a promising candidate for future ultrahigh speed ultrahigh spectrum efficiency coherent optical fiber transmissions systems. Several equalization algorithms including constant modulus algorithm (CMA), CMA plus postfilter, and cascaded multimodulus algorithm (CMMA) have been proposed as effective solutions for QDB-PDM-QPSK signal. For the first time as far as we know, the application conditions and performances for these three algorithms are analyzed and compared. System performances for a 112-Gb/s QDB-PDM-QPSK signal as a function of the optical filtering bandwidth and the optical SNR have been theoretically investigated. The results show that CMA would be the best choice in terms of convergence rate for general filtering. However, CMMA can outperform the other two schemes with a good receiver sensitivity and high-dynamic range of optical signal to noise ratio giving a strong filtering effect of super-Nyquist signaling.