Polarization-multiplexed Quadrature Amplitude Modulation Research Articles

Bidirectional Cladding-Pumped Multicore EDFA and its Optimization for Energy Efficiency by Tuning Pump Condition

This work investigates the feasibility of improving the energy efficiency of a cladding-pumped multicore (MC) EDFA by utilizing bidirectional (BD) pumping. We combined a conventional forward-cladding-pumped MC-EDFA with backward cladding pumping and tuned the pump-gain characteristics and gain shape by controlling the optical pump power ratio of its forward and backward pumps. Since EDFAs have wavelength dependence, output power equalization by each wavelength channel is extremely important for amplifying the wavelength division multiplexing (WDM) signal. The standard method of current equalization is to adjust the attenuation by the output power of each wavelength channel. The main issue with the current standard equalization is the indispensable optical power loss caused the reduced optical amplification efficiency of the EDFA. Our previous work demonstrated that the optical power loss due to equalization can be reduced by controlling the optical power ratio of the forward and backward pumps. This leads to improved energy efficiency of the signal amplification and equalization of both the WDM and spatial division multiplexing (SDM). <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In this paper, we report our detailed investigation into the tuning amplification characteristics of the BD-MC-EDFA. Our main focus was the gain, noise figure (NF), and gain shape of a 7-core BD-MC-EDFA prototype when forward and backward optical pump power was changed. Under 8 W of total optical pump power, the percentage of 60% forward cladding optical pump power was found to be optimal for efficiency considering the minimum output optical power loss due to equalization. This condition translates into a 1.7% pump conversion efficiency (PCE) improvement, which corresponds to a 24% reduction of the total optical pump power compared to conventional MC-EDFA with only forward cladding pumping. Although the NF deteriorated by 1.0 dB in wavelength average in the 4.5-THz signal bandwidth in exchange for the efficiency improvement, degradation of the 34.2-GBaud polarization multiplexed 16 quadrature amplitude modulation (PM-16QAM) signal amplified by the BD-MC-EDFA operated under the optimized condition was negligible.

Independent and mixed transmission of 166.5 Gb/s PM‐8QAM and 222 Gb/s PM‐16QAM Nyquist‐wavelength division multiplexed superchannel for long‐haul metro network

SummaryThis paper investigates the Nyquist‐wavelength division multiplexed (Nyquist‐WDM) terabit superchannel by employing independent and mixed line rate (MLR) transmission of different polarization‐multiplexed quadrature amplitude modulation (PM‐QAM) subcarriers. Here, terabit superchannel transmission including forward error correction overhead is successfully achieved by employing either 7 × 166.5 Gb/s PM‐8QAM or 5 × 222 Gb/s PM‐16QAM or both 2 × 166.5 Gb/s PM‐8QAM and 3 × 222 Gb/s PM‐16QAM simultaneous transmission. Considering linear filtering penalties and nonlinear propagation effects, the proposed superchannel is transmitted over standard single‐mode fiber (SSMF) and pure silica core fiber (PSCF) along with erbium‐doped fiber amplifier (EDFA) at 19.8‐ and 15.12‐dB span loss for SSMF and PSCF, respectively. The performance of both terabit superchannels is analyzed in terms of maximum reach, good spectral efficiency, highest transmission capacity, and optimum launch power with variation in channel spacing from 50‐GHz fixed ITU‐T grid to 29 GHz, at an acceptable bit error rate (BER). To achieve BER 4 × 10−3, MLR‐based 166.5 Gb/s PM‐8QAM and 222 Gb/s PM‐16QAM suffer a penalty of 5.5‐ and 9.2‐dB OSNR over Nyquist‐only 7 × 166.5 Gb/s PM‐8QAM and 5 × 222 Gb/s PM‐16QAM transmission at the advent of a flexible multi‐flow transponder. It is observed that at 33.33‐GHz Nyquist spacing over PSCF, 5 × 222 Gb/s PM‐16QAM and 7 × 166.5 Gb/s PM‐8QAM superchannels deliver a reach up to 1800 and 2970 km, while MLR approach results in extreme reach of 2250 and 3600 km over PSCF as per MLR PM‐16QAM and PM‐8QAM subcarriers, respectively. Hence, an appreciable spectral efficiency–distance product (SEDP) is achieved due to PSCF at Nyquist spacing of 33.33 GHz with 5.25 bit/s/Hz spectral efficiency. A good link power budget is also calculated for the proposed MLR‐based Nyquist‐WDM superchannel. MLR‐based PM‐16QAM and PM‐8QAM subcarriers deliver excellent SEDP of 11,812 and 18,900 bit/s/Hz·km, respectively, which is, to the best of author's knowledge, better on comparing with the other existing literature for single‐core channel explorations.

Statistical properties of nonlinear distortion of a polarization-multiplexed OFDM signal in long-haul fiber links.

We investigate nonlinear transmission regimes of a polarization-multiplexed 16-quadrature amplitude modulation (PDM-16-QAM) orthogonal frequency-division multiplexed (OFDM) signal in a long-haul optical link. We study the dependence between the strength of nonlinear distortion and statistical properties of a PDM-OFDM signal. We also consider the constellation shaping-based solutions that allow to significantly reduce the bit error rate (up to three to 12 times at the cost of 5%-20% signal redundancy) and propose a method to analytically optimize the symbol distribution for higher-order modulation formats using only the initial signal.

Beyond 1 Tb/s Intra-Data Center Interconnect Technology: IM-DD OR Coherent?

We discuss technology options and challenges for scaling intra-datacenter interconnects beyond 1 Tb/s bandwidths, with focus on two possible approaches: pulse amplitude modulation (PAM)-based intensity modulation-direct detection (IM-DD) and baud-rate sampled coherent technology. In our studies, we compare the performance of various orders of PAM modulation (PAM4 to 8). In addition to these fixed PAM signaling options, a flexible PAM (FlexPAM) technique leveraging granularity in spectral efficiency (SE) is proposed to maximize link margin. For baud-rate sampled coherent technology, we propose a simplified digital signal processing (DSP) architecture to bring down power consumption of the coherent approach closer to that of IM-DD PAM. We also propose two new phase noise tolerant 2D coherent modulation formats to relax the laser linewidth requirement. In closing, a comparative study of fixed IM-DD PAM versus coherent polarization multiplexed-quadrature amplitude modulation (PM-QAM) is presented for a 1.6 Tb/s solution (200 Gb/s per dimension), with consideration of link loss/reach budget, power consumption, implementation complexity, as well as fan-out granularity.

QC-LPDC Code modulation with probabilistic shaping for atmospheric turbulence channel

In this paper, we propose two succinct probabilistic shaping (PS) schemes with 18.8% and 37.5% overhead for quasi-cyclic low-density parity-check code to transmit polarization-multiplexed 16 quadrature amplitude modulation (PM-16QAM) signal over the turbulent free space optical (FSO) links. We theoretically analyze irradiance fluctuation within the cross section of received beam after propagating through the Gamma–Gamma channel, of which the refractive-index property is related to Kolmogorov spectrum. The Gamma–Gamma channel model is used to simulate the effects of atmosphere turbulence on PS PM-16QAM FSO communication system. Simulation results show that the proposed PS schemes provide a more efficient way to compensate scintillation effects compared with uniform distribution in PM-16QAM FSO communication system under different turbulence strength regimes. Furthermore, indoor short-range experiments have been carried out to verify the effectiveness of the proposed probabilistic shaping schemes

5×222 Gb/s PM-16QAM Nyquist-WDM Superchannel

This paper reports terabit Nyquist-wavelength division multiplexing (Nyquist-WDM) superchannel transmission using five subcarriers. Each sub-carrier is 222 Gb/s polarization multiplexed- 16 quadrature amplitude modulation (PM-16QAM) advance modulation format based transmitter. Each subcarrier is Nyquist-spaced using a pre filter of bandwidth 33.33 GHz, which is 1.2 ×symbol rate of the PM-16QAM based transmitter. Performance of the proposed superchannel is realized by means of bit error rate (BER), optical signal to noise ratio (OSNR), transmitted power, optical spectrum of the superchannel and individual filtered subcarrier. The proposed work exhibits reference BER of value 4×10-3 at OSNR 21 dB, which is quite remark-able. The proposed Nyquist-WDM superchannel delivers Terabit transmission capacity of 1.11 Tb/s and excellent spectral efficiency of 6.66 bit/s/Hz. Presence of Nyquist-WDM approach makes the system inter channel interference (ICI) tolerant. Hence, the proposed work is efficient enough for the next generation optical network, ultra long haul transmission and elastic net-work.

A Novel Four-Dimensional Modulation Format Based on Subset Selection

Coherent optical transmission systems have a four-dimensional (4-D) signal space (two quadratures in two polarizations), which can be used to create 4-D modulation formats that have better sensitivity than traditional two-dimensional modulation signal. In this paper, we propose an interesting four-dimensional modulation format based on subset selection for coherent optical fiber communications. We compare the proposed modulation format to the polarization multiplexed quadrature amplitude modulation(PM-QAM) and the set-partitioning M polarization-multiplexed QAM (M-SP-QAM). Simulation results show that for the same spectral efficiency, the proposed modulation format can get SNR gains of up to1dB compared with PM-MQAMs at 7% forward exchange control (FEC) threshold at a BER of 1e-3 in back-to-back system. After 800 km transmission distance, the signal-to-noise ratio (SNR) gain is 1.05dB. It is also shown that the mapping process of the proposed modulation format has lower latency and occupies less memory units compared with the commonly used look-up-tables mapping method.

OSNR monitoring technique based on multi-order statistical moment method and correlation function for PM-16QAM in presence of fiber nonlinearities

Abstract We propose an improved optical signal-to-noise ratio (OSNR) monitoring technique in presence of optical fiber nonlinearity-induced distortions, in which traditional multi-order statistical moments based (MOSMB) OSNR monitoring method is corrected by a calibration factor α and correlation functions between neighboring received symbols. This OSNR monitoring technique is performed for the first time in a 224-Gb/s polarization-multiplexed quadrature amplitude modulation (PM-16QAM) system. We explore relationships between α , transmission distances and channel launch powers for further convenient usage considerations. The proposed monitoring technique shows much better performances than existing OSNR monitoring techniques in highly nonlinear systems, where OSNR estimation errors are below 1 dB with actual OSNR ranging 18–28 dB and the channel launch powers reach as highly as 4 dBm and the transmission distances are up to 2000 km. Besides, this proposed technique is applicable to different modulation formats and corresponding performances are compared.

Spectrally efficient 1.55 Tb/s Nyquist-WDM superchannel with mixed line rate approach using 27.75 Gbaud PM-QPSK and PM-16QAM

This paper investigates the transmission performance of a 1.55-Tb/s Nyquist-wavelength division multiplexing superchannel along with a mixed line rate approach using 4 × 111 Gb / s polarization-multiplexed quadrature phase shift keying (PM-QPSK) and 5 × 222 Gb / s polarization-multiplexed quadrature amplitude modulation (PM-16QAM) formats. The system transmission performance with acceptable bit error rate of 4 × 10 − 3 is analyzed over standard single mode fiber (SSMF), large effective area fiber (LEAF), and pure silica core fiber (PSCF). It is observed that SSMF increases 45% reach over LEAF, whereas PSCF increases 66% to 84% reach over LEAF for 4.5 b/s/Hz spectral efficiency of the system. Also, spectral efficiency × distance product of 24,750 and 10,350 b/s/Hz.km are obtained for PM-QPSK and PM-16QAM, respectively, which is, to the best of our knowledge, better in comparison with the other existing literatures for single core channel investigations.

Joint Tracking of Polarization State and Phase Noise Using Adaptive Cascaded Kalman Filtering

We propose a cascaded Kalman filtering (CKF) technique, a series of extended Kalman filter (EKF), and a linear Kalman filter, for joint tracking of phase and amplitude noise as well as polarization state. We further propose an adaptive CKF (A-CKF), in which the process noise covariance has been estimated adaptively, using covariance matching method. Numerical results on back-to-back and transmission scenarios on 28-Gbd polarization-multiplexed 16-quadrature amplitude modulation system justify that our proposed CKF as well as A-CKF outperforms the earlier proposed EKF and the unscented Kalman filter (UKF) with an improved tolerance toward polarization rotations and phase noise besides less computational effort.

Crosstalk Reduction With Bidirectional Signal Assignment on Square Lattice Structure 16-Core Fiber Over WDM Transmission for Gradual Upgrade of SMF-Based Lines

C-band and L-band wavelength division multiplexing (WDM) transmission performance was improved by using bidirectional signal assignment (BSA) to reduce crosstalk (XT) in square lattice structure 16-core fiber. Transmission of 16 WDM channels with 256-Gb/s polarization multiplexed-16 quadrature amplitude modulation signals on a 50-GHz grid with a net spectral efficiency of 4 b/s/Hz over 55-km of 16-core fiber spans was achieved. Use of BSA to reduce XT extended the transmission distance to 880 km in both types of transmission. Furthermore, transmission was achieved over eight mixed spans consisting of 16-core fiber and conventional single-mode fiber (SMF), demonstrating the possibility of gradually upgrading standard SMF-based lines to multicore fiber-based lines. With BSA, six 16-core fiber spans can be acceptable if the Q penalty from XT is below 0.5 dB.

Subcarrier multiplexing based self-heterodyne coherent detection for PM-16QAM format

We propose and demonstrate subcarrier multiplexing (SCM) based self-heterodyne (SH) coherent detection for polarization-multiplexed 16 quadrature amplitude modulation (PM-16QAM) format. Three closely spaced subcarriers carrying pilot-tone (PT) and two 16QAM signals, respectively, are generated via subcarrier modulation technique. An effective digital signal processing (DSP) scheme is proposed to eliminate image frequency interference (IFI) arising in such SH coherent detection. Meanwhile, a closed-form analytical BER formula is verified by numerical simulations with a deviation less than 0.3dB at BER=10−3. The concept of optimum modulation index (MI) ratio between data and PT signal is derived and verified, in order to reduce the distortion of 16QAM constellation. Meanwhile, we also explore the effects of optical filter bandwidth and subcarrier frequency spacing on the quality of PT signal. Finally, we are able to obtain several optimum parameters for the proposed SH coherent detection.

Periodic Training Sequence Aided In-Band OSNR Monitoring in Digital Coherent Receiver

We propose and demonstrate a new data-aided in-band optical-signal-to-noise ratio (OSNR) monitoring method, which employs the periodic property of a specially de- signed short training sequence to differentiate the signal power from noise and is proved to be insensitive to the first-order polarization mode dispersion (PMD). This method is verified in a typical 112-Gb/s polarization multiplexed-quadrature phase shift keying and a 224-Gb/s polarization multiplexed-16 quadrature amplitude modulation coherent optical transmission system. The results show that the method is highly accurate, with the moni- toring error less than 1 dB in the OSNR range of 7-30 dB under different linewidths and has good robustness to a large amount of residual accumulated CD and first-order PMD.

Unrepeatered 256 Gb/s PM-16QAM transmission over up to 304 km with simple system configurations.

We study unrepeatered transmission of 40x256 Gb/s systems with polarization-multiplexed 16-quadrature amplitude modulation (PM-16QAM) channels using simple coherent optical system configurations. Three systems are investigated with either a homogeneous fiber span, or simple two-segment hybrid fiber designs. Each system relies primarily on ultra-low loss, very large effective area fiber, while making use of only first-order backward pumped Raman amplification and no remote optically pumped amplifier (ROPA). For the longest span studied, we demonstrate unrepeatered 256 Gb/s transmission over 304 km with the additional aid of nonlinear compensation using digital backpropagation. We find an average performance improvement in terms of the Q-factor of 0.45 dB by using digital backpropagation compared to the case of using chromatic dispersion compensation alone for an unrepeatered span system.

GN-Model Validation Over Seven Fiber Types in Uncompensated PM-16QAM Nyquist-WDM Links

We report on a set of transmission experiments over seven different uncompensated fiber links carrying a 22-channel Nyquist-WDM signal comb based on polarization-multiplexed 16-quadrature amplitude modulation at RS=15.625 GBd(RB=125 Gb/s). The channel spacing was Δf=16 GHz. The experimental transmission performance was compared with analytical GN-model predictions showing a good agreement for all seven considered fiber types. Moreover, we used the experimental results for the validation of a fiber figure of merit based on the maximum reach, showing its effectiveness for approximate fiber performance prediction. Within the experimental tests, a maximum transmission distance of 3810 km at a bit error-rate of 1.5×10-2 was achieved using a pure-silica-core fiber with 150 μm2 effective area, at a net spectral-efficiency-distance product of 24700 (b·km)/(s· Hz), in a good agreement with the GN-model forecast.

112-Gb/s RF-Assisted Multicarrier DP-16-QAM Optical Transmission Over Field Deployed Fiber Link

A 112-Gb/s radio-frequency (RF)-assisted dual carrier, polarization-multiplexed 16-quadrature amplitude modulation (QAM) transmitter and receiver are evaluated using a 542-km field deployed fiber link. The introduction of multicarrier radio technologies in transmitter and receiver showed no significant transmission penalty.

Analyses of wavelength- and polarization-division multiplexed transmission characteristics of optical quadrature-amplitude-modulation signals

We theoretically study optical transmission characteristics of wavelength-division multiplexed (WDM) and polarization-multiplexed (POLMUX) signals using high-order optical quadrature-amplitude-modulation (QAM) formats up to 256. First, we conduct intensive computer simulations on bit-error rates (BERs) in WDM POLMUX QAM transmission systems and find maximum transmission distances under the influence of nonlinear impairments. Next, to elucidate the physics behind such nonlinear transmission characteristics, we calculate the distribution of constellation points for QAM signals as functions of the the launched power, the transmission distance, and the symbol rate. These results lead to a closed-form formula for BER of any QAM formats. From such formula, we derive simple laws that determine the maximum transmission distance and the optimum power as functions of the QAM order and the symbol rate. These laws can well explain the simulation results.

Transmission of 9$\,\times\,$138 Gb/s Prefiltered PM-8QAM Signals Over 4000 km of Pure Silica-Core Fiber

We experimentally demonstrated the application in ultrahigh spectral density long-haul optical systems of tight prefiltering to generate “quasi-Nyquist wavelength division multiplexing” signals together with the use of pure-silica core fiber and hybrid Raman/erbium-doped fiber amplification. The transmission of polarization-multiplexed quadrature-amplitude modulation with 8 symbols (PM-8QAM) at 138 Gb/s and channel spacing equal to 1.22 × (symbol rate) was demonstrated, reaching over 4000 km at net spectral efficiency (SE) of 4.1 b/s/Hz. In our experiment, we achieved to date highest per-carrier symbol rate (23 Gbaud), SE (4.1 b/s/Hz), and (SE) × (distance) product (16655 b/s/Hz·km) for non-orthogonal frequency-division multiplexing transmission using PM-8QAM modulation.