Dual-polarization Quadrature Phase Shift Research Articles

Fast-Tuning Coherent Burst-Mode Receiver for Metropolitan Networks

We demonstrate a fast wavelength-tunable burst-mode receiver for coherent 28 GBd dual-polarization quadrature phase shift keying signals using a commercially available DS-DBR laser as the local oscillator (LO). We find that the frequency drift and linewidth of the LO must be precisely controlled to prevent bit-error-ratio (BER) penalties when the signal undergoes transmission distances typical of metropolitan-area networks. We demonstrate reduction of the LO frequency drift and linewidth after wavelength switching by compensation using the phase section of the laser and achieve negligible BER penalty with burst-mode reception for single-mode fiber transmission up to 1280 km. We also measure the sensitivity of the receiver to LO detuning when multiple wavelength-division multiplexed channels are received simultaneously as in a broadcast-and-select network architecture.

Analog domain adaptive equalizer for low power 40 Gbps DP-QPSK receivers

Electrical domain equalization of chromatic and polarization mode dispersion is attractive in coherent optical communication links. Digital coherent receivers used for this purpose are based on high speed ADCs followed by DSP, which dissipate excessive amount of power and are very costly to implement. We propose analog coherent receiver to drastically reduce the power consumption, size and cost. An adaptive feed forward equalizer for 40 Gbps dual polarization quadrature phase shift keying (DP-QPSK) systems, which processes signals in analog domain itself, is demonstrated using circuit and system simulations. The equalizer, designed in 90 nm CMOS technology, consumes 450 mW of power and occupies 1.8 mm × 1.1 mm chip area. System simulations are used to show that blind equalization is also possible when this approach is used in decision directed mode.

100 Gb s−1 coherent dense wavelength division multiplexing system reach extension beyond the limit of electronic dispersion compensation using optical dispersion management

We report numerical investigations of increasing system reach beyond the limit imposed by the state-of-the-art digital signal processing (typically 70 ns nm−1 or 4000 km of standard single mode fiber (SSMF)) by using partial optical dispersion compensation in wavelength division multiplexing 100 Gb s−1 dual polarization quadrature phase shift keying coherent systems. Our numerical model has been verified by the comparison of simulation results with the straight-line real-time transmission experiment. We find that for a legacy fiber-based (non-zero dispersion-shifted fiber) link the transmission distance can be extended to 7000 km with either hybrid spans consisting of SSMF and non-zero dispersion-shifted fiber, or by using partial dispersion compensation employing dispersion-compensating fiber (DCF)with a 1.5 and 0.6 dB optical signal-to-noise ratio (OSNR) margin respectively.

An Effective Integration Solution for 100 Gb/s DP-QPSK Coherent Receiver With a Small Skew

We propose and develop a simple hybrid integration solution for a 100 Gb/s dual polarization quadrature phase shift keying coherent receiver. The cost-effective, precise, and passive alignment technique can be used to couple four output waveguides of planar lightwave circuits (PLCs) based 90 ° optical hybrid (OH) with a 1 × 4 photodiode (PD) array directly, adopting an infrared charge-coupled device camera to monitor the light spots, a beam of infrared light to illuminate the light spots and the active areas of PD array, and an epoxy glue to adhere the PLC-based 90 ° OH to the heat sink. Based on this proposed coupling scheme, the responsivity variation for the coherent receiver is less than ±0.18 dB for all PD channels in an environment temperature range of -5°C-80 °C and the skew of this coherent receiver is only 1.96 ps.

Gain-switched multicarrier transmitter in a long-reach UDWDM PON with a digital coherent receiver

The authors report on the downlink performance of a 10 Gb/s long-reach and ultra-dense wavelength-division multiplexed passive optical network, based on a multicarrier transmitter realized by using an externally injected gain-switched distributed-feedback laser diode. Each of the comb channels, spaced by 10 GHz, is modulated with a 3 Gbaud dual polarization quadrature phase shift keying signal that included a 20% overhead for forward error correction. Frequency selectivity and enhanced receiver sensitivity is achieved by employing a digital coherent receiver to receive the signal. Experimental results achieved in a back-to-back and 100 km transmission scenarios show an excellent worst case receiver sensitivity of -44 dBm.

Phase noise tolerant inter-carrier-interference cancellation for WDM superchannels with sub-Nyquist channel spacing

We propose and demonstrate a novel multi-input multi-output (MIMO) equalization based inter-carrier-interference (ICI) cancellation approach employing constant modulus algorithm (CMA) for supperchanels with sub-Nyquist channel spacing, where optical combs are used as optical sources. Compared with the least mean square (LMS) algorithm based ICI canceller, the proposed approach has comparable capability to accomplish the ICI mitigation for 56 Gbaud dual-polarization quadrature phase shift keying (DP-QPSK) signals with tight channel spacing till 50 GHz. In particular, compared with the LMS-MIMO based ICI canceller, the optical linewidth tolerance of 6 MHz is relaxed to 20 MHz given a 1dB required optical signal-to-noise ratio (OSNR) penalty for the CMA-MIMO based ICI canceller. Meanwhile, the CMA-MIMO based ICI canceller is ideal for real-time processing, since the number of parallel processing pipelines can be greater than 240 even in the presence of large linewidth.

Hybrid EDFA/Raman Amplification Topology for Repeaterless 4.48 Tb/s (40 x 112 Gb/s DP-QPSK) Transmission Over 302 Km of G.652 Standard Single Mode Fiber

In this work we carry out a comprehensive experimental study on hybrid optical amplification topologies (based on first order Raman and Erbium doped fiber amplifiers) for repeaterless transmission of 112 Gb/s dual polarization-quadrature phase shift keying (DP-QPSK) enabling a 4.48 Tb/s (40 × 112 Gb/s) transmission over 302 km of standard single mode fiber with coherent detection. Hybrid optical amplification was employed for signal boosting and pre-amplification and repeaterless transmission is assured without the use of neither in-line amplification nor remote optical amplifiers. The goal was to achieve the highest bandwidth-distance product for repeaterless DWDM transmission systems over legacy fiber without any in-line amplification technology. The achieved result is a significant milestone, when compared to recent state-of-the-art investigations.

Application of a Novel Modulation Scheme of PS-RZ-QPSK Signal in High-Speed Optical Transmission System

Modulation principle of dual-polarization quadratu-re phase shift keying signal and polarization-switched QPSK signal are demonstrated and advantages of PS-QPSK are proved in theoretically. A novel modulation scheme of PS-RZ-QPSK signal is proposed in this paper. The scheme reduces transmitter cost by less use of Mach-Zehnder modulator, but also presents similar performance as traditional structure for PS-RZ-QPSK. The simulate result indicates PS-RZ-QPSK can achieve a better transmission performance than DP-RZ-QPSK at the same bit rate (84Gb/s) and baud rate (28GBd), and proves showing the feasibility of novel modulation scheme.

Single DPLL Joint Carrier Phase Compensation for Few-Mode Fiber Transmission

A carrier phase estimation and compensation algorithm using a digital phase locked loop is proposed which exploits the common-mode impairment from the shared local oscillator at the receiver side in a few-mode fiber transmission system. We verify that the proposed algorithm is more tolerant to phasemismatches than earlier work. The carrier phase estimation (CPE) scheme is evaluated for a 3 × 112 Gbit/s dual polarization quadrature phase shift keying mode division multiplexed transmission over 80 km including an in-line multimode erbium doped fiber amplifier. The digital signal processing computational complexity is reduced in comparison to a traditional CPE stage. In addition, the proposed scheme is shown to perform similarly with respect to the 3.8 × 10-3forward error correcting limit optical signal to noise ratio tolerance of the traditional CPE scheme.

Maximum-Likelihood Detection in Few-Mode Fiber Transmission With Mode-Dependent Loss

In this letter, the performance of maximum-likelihood (ML) detection is evaluated and compared with the zero-forcing (ZF) and minimum mean square error (MMSE) equalizers for a 3 × 158-Gb/s mode-division multiplexed dual-polarization quadrature phase shift keying orthogonal frequency division multiplexing transmission impaired by mode-dependent loss. The receiver schemes are compared in terms of performance and complexity. The simulations show that the ML approach outperforms both ZF and MMSE equalization for strongly coupled modes. However, the complexity of the ML detection is significantly higher and increases faster with the number of modes used for transmission and the modulation format order.

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.

On the Energy Efficiency of Modulation Formats for Optical Communications

An information theoretic framework for performance evaluation of high-dimensional modulation formats in an optical communication link is proposed. The spectral and energy efficiency of different candidates for future coherent optical communications are analyzed to explore the advantages of exploiting different degrees of freedom for symbol coding, construction, and transmission. Our analysis demonstrates the advantage of fully utilizing both polarization states from the spectral and energy efficiency perspective. In particular, at the same spectral efficiencies of 2.5 and 2.8 b/s/Hz, dual polarization quadrature phase shift keying (DP-QPSK) has, respectively, 0.8 and 1.6 dB energy efficiency advantage over polarization switched QPSK (PS-QPSK).

In-band OSNR monitor with high-speed integrated Stokes polarimeter for polarization division multiplexed signal

An in-band optical signal-to-noise ratio (OSNR) monitor is proposed, based on an instantaneous polarization state distribution analysis. The proposed monitor is simple, and is applicable to polarization division multiplexed signals. We fabricate a high-speed Stokes polarimeter that integrates a planar lightwave circuit (PLC) based polarization filter, high-speed InP/InGaAs photodiodes and InP hetero-junction bipolar transistor (HBT) trans-impedance amplifiers (TIA). We carry out proof-of-concept experiments with the fabricated polarimeter, and successfully measure the OSNR dependent polarization distribution with 100-Gb/s dual polarization quadrature phase shift keying (DP-QPSK) signals.

Application of a digital non-linear compensation algorithm for evaluating the performance of root-raised-cosine pulses in 112 Gbit s−1 DP-QPSK transmission

In this paper, we numerically investigate the non-linear tolerance of root-raised-cosine (RRC) pulse shaping by interpolating finite impulse response (FIR) filters in conjunction with digital backward propagation (DBP) in coherent 112 Gbit s−1 dual-polarization quadrature phase shift keying (DP-QPSK) transmission. The results depict that RRC pulses are more tolerant to intra-channel non-linearities, i.e. self-phase modulation (SPM), as compared to standard RZ-33 and NRZ pulses. The non-linear threshold point is improved by using RRC pulses by a factor of 2 dB signal input power as compared to RZ pulses and by 4 dB signal launch power as compared to NRZ pulses. The behavior of RRC pulses is also investigated with standard single mode fiber (SMF), non-zero dispersion shifted fiber (NZDSF) and next-generation large Aeff pure silica core fiber (LA-PSCF). Most importantly multi-span DBP is implemented and in the case of RRC pulses the computational efforts of the conventional DBP algorithm are reduced by 80% with a diminutive Q-penalty of 0.74 dB. The duty cycle of the RRC pulses is further optimized for efficient system performance. We have also compared the performance of single-channel transmission with the multi-channel transmission, where the performance is limited due to inter-channel non-linear effects. Furthermore, the non-linear tolerance of RRC pulses is investigated with; (a) different amplifier spacing and (b) variation in transmission link design information for the DBP algorithm.

Influence of Fiber-Bragg Grating-Induced Group-Delay Ripple in High-Speed Transmission Systems

The implementation of a chirped fiber-Bragg grating (FBG) for dispersion compensation in high-speed (up to 120 Gbit/s) transmission systems with differential and coherent detection is, for the first time, experimentally investigated. For systems with differential detection, we examine the influence of group-delay ripple (GDR) in 40 GBd 2-, 4-, and 8-ary differential phase shift keying (DPSK) systems. Furthermore, we conduct a nonlinear-tolerance comparison between the systems implementing dispersion-compensating fibers and FBG modules, using a 5×80 Gbit/s 100-GHz-spaced wavelength division multiplexing 4-ary DPSK signal. The results show that the FBG-based system provides a 2 dB higher optimal launch power, which leads to more than 3 dB optical signal-to-noise ratio (OSNR) improvement at the receiver. For systems with coherent detection, we evaluate the influence of GDR in a 112 Gbit/s dual-polarization quadrature phase shift keying system with respect to signal wavelength. In addition, we demonstrate that, at the optimal launch power, the 112 Gbit/s systems implementing FBG modules and that using electronic dispersion compensation provide similar performance after 840 km transmission despite the fact that the FBG-based system delivers lower OSNR at the receiver. Lastly, we quantify the GDR mitigation capability of a digital linear equalizer in the 112 Gbit/s coherent systems with respect to the equalizer tap number (Ntap). The results indicate that at least Ntap=9 is required to confine Q-factor variation within 1 dB.

Multi-span digital non-linear compensation for dual-polarization quadrature phase shift keying long-haul communication systems

We have numerically investigated a method to reduce the complexity of the digital backward propagation algorithm (DBP). A filtered logarithmic step-size based split-step Fourier method (SSFM) is investigated in this paper to digitally compensate chromatic dispersion (CD) and non-linearities (NL) in dual-polarization quadrature phase shift keying (DP-QPSK) systems. The algorithm was evaluated for coherently-detected multiple channel DP-QPSK system over un-compensated transmission links with diverse baud-rates i.e. 14GBaud, 28GBaud and 56GBaud. The results depict efficient mitigation of CD and NL, therefore improving the non-linear threshold point (NLT) by 4dB. Furthermore by implementing a low-pass-filter (LPF) in each DBP stage, the required number of DBP stages are significantly reduced (multi-span DBP) by 75%. The results delineate improved system performance of logarithmic step size based filtered DBP (FL-DBP) both in terms of efficiency and complexity which will be helpful in future deployment of DBP algorithm with real-time signal processing modules for non-linear compensation.

Impact of channel baud-rate on logarithmic digital backward propagation in DP-QPSK system with uncompensated transmission links

We numerically investigate the impact of channel baud-rate on the performance of logarithmic step-size based spilt-step Fourier method (SSFM). This algorithm is used to implement digital backward propagation (DBP) to efficiently compensate fiber chromatic dispersion (CD) and non-linearities (NL). The DBP method is implemented in N-channel dual-polarization quadrature phase shift keying (DP-QPSK) transmission over 2000 km standard single mode fiber (SMF) with no in-line optical dispersion compensation. We investigate the same-capacity and same-bandwidth transmission systems with 56 Gbit/s/ch (14 GBaud), 112 Gbit/s/ch (28 GBaud) and 224 Gbit/s/ch (56 GBaud). Each system has the bandwidth occupancy of 500 GHz with a total transmission capacity of 1.12 Tbit/s. Moreover, we have also compared the multiple channel transmission performance with single channel transmission to quantify the impact of inter-channel (cross-phase modulation ‘XPM’ and four-wave mixing ‘FWM’) and intra-channel (self-phase modulation ‘SPM’) non-linearities. The logarithmic step-size based DBP algorithm (L-DBP) depicts efficient mitigation of CD and NL impairments. The benefit of the logarithmic step-size is the reduced complexity and computational time for higher baud-rate transmission systems.

Coherent DWDM technology for high speed optical communications

The introduction of coherent digital optical transmission enables a new generation of high speed optical data transport and fiber impairment mitigation. An initial implementation of 40 Gb/s coherent systems using Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) is already being installed in carrier networks. New systems running at 100 Gb/s DP-QPSK data rate are in development and early technology lab and field trial phase. Significant investment in the 100 Gb/s ecosystem (optical components, ASICs, transponders and systems) bodes well for commercial application in 2012 and beyond. Following in the footsteps of other telecommunications fields such as wireless and DSL, we can expect coherent optical transmission to evolve from QPSK to higher order modulations schemes such as Mary PSK and/or QAM. This will be an interesting area of research in coming years and poses significant challenges in terms of electro-optic, DSP, ADC/DAC design and fiber nonlinearity mitigation to reach practical implementation ready for real network deployments.

WDM Performance and PMD Tolerance of a Coherent 40-Gbit/s Dual-Polarization QPSK Transceiver

We report the measured wavelength-division-multiplexing (WDM) performance and polarization-mode dispersion (PMD) tolerance of a coherent 40-Gbit/s dual-polarization quadrature phase shift keying (DP-QPSK) transceiver at 50-GHz minimum channel spacing in a 40-channel 40-span test bed comprised of 3200 km of uncompensated G.652 fiber. We also evaluate the impact of polarization-dependent loss (PDL) on system performance and present the measured tolerance to frequency misalignment between the transmitter and a multiplexer/demultiplexer filter pair.

Real-time measurements of a 40 Gb/s coherent system

Continuous real-time measurements are shown from a coherent 40 Gb/s transmission system that uses Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) modulation. Digital compensation is used for dispersion and polarization effects, with little performance degradation created by 150 ps of rapidly varying 1st-order PMD.