Dual-polarization Quadrature Phase-shift Keying Signals Research Articles

All-fiber mode exchange of high-speed DP-QPSK signals using mechanical long-period fiber grating

Mode exchange (ME) can effectively improve the utilization of network resources in short-distance mode division multiplexing (MDM) communication systems. In this paper, a mechanical long-period fiber grating (LPFG) mode converter is used to realize the channel exchange of LP01- and LP11-mode dual-polarization (DP) signals, with the channel crosstalk of −14 dB or less. We experimentally tested the receiver sensitivities of two DP channels, which are dependent on channel crosstalk and polarization-dependent loss (PDL), and demonstrated the practicability of the real-time data transmission and exchange for 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) in multiple-input multiple-output free (MIMO-free) MDM system. The experimental results show that the ME system can provide the optical power margin of about 25 dB, capable of effectively supporting the metropolitan transmission and exchange between the data centers.

Investigation of 340-Gbps terrestrial FSO link incorporating spectral-efficient DP-QPSK-PolSK hybrid modulation scheme

A high-capacity spectral-efficient dual-polarization quadrature phase-shift keying (DP-QPSK)-polarization shift-keying (PolSK) hybrid modulation scheme for terrestrial free-space optics (FSO) transmission link is proposed and investigated. A DP-QPSK signal modulated at 300 Gbps and a PolSK signal modulated at 40 Gbps are simultaneously transmitted using a single optical carrier over the FSO link. The proposed link performance is investigated under different weather conditions, where the bit error rate metric is used to evaluate the performance of the PolSK modulated signal and the error vector magnitude parameter is used for the DP-QPSK signal. The FSO link range and the required received power are carefully explored. The conducted numerical simulations of the proposed system showed reliable 340-Gbps data transmission over link ranges varying from 1.6125 to 50 km depending on the weather conditions. The impact of the channel scintillation due to atmospheric turbulence is also investigated. The proposed high-speed FSO transmission system offers a promising solution for high-bandwidth hungry systems used for the internet of things, 5G, and smart cities. It can also be used in developing fronthaul/backhaul links for future wireless networks and optical access networks. The performance of the proposed transmission system is compared with recently published work in the literature.

Optical Performance Monitoring in Mode Division Multiplexed Optical Networks

This article considers, for the first time, optical performance monitoring (OPM) in few mode fiber (FMF)-based optical networks. 1-D features vector, extracted by projecting a 2-D asynchronous in-phase quadrature histogram (IQH), and the 2D IQH are proposed to achieve OPM in FMF-based network. Three machine learning algorithms are employed for OPM and their performances are compared. These include support vector machine, random forest algorithm, and convolutional neural network. Extensive simulations are conducted to monitor optical to signal ratio (OSNR), chromatic dispersion (CD), and mode coupling (MC) for dual polarization-quadrature phase shift keying (DP-QPSK) at 10, 12, 16, 20, and 28 Gbaud transmission speeds. Besides, M-ary quadrature amplitude modulation (M = 8 and 16) is considered. Also, the OPM accuracy is investigated under different FMF channel conditions including phase noise and polarization mode dispersion. Simulation results show that the proposed 1D projection features vector provides better OPM results than those of the widely used asynchronous amplitude histogram (AAH) features. Furthermore, it has been found that the 2D IQH features outperform the 1D projection features but require larger number of features samples. Additionally, the effect of fiber nonlinearity on the OPM accuracy is investigated. Finally, OPM using the 2D IQH features has been verified experimentally for 10 Gbaud DP-QPSK signal. The obtained results show a good agreement between both simulation and experimental findings.

Simultaneous polarization separation and switching for 100-Gbps DP-QPSK signals in backbone networks**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFC0803900) and the National Natural Science Foundation of China (Grant No. 9163801).

We propose a novel scheme of simultaneous polarization separation and switching, based on the orthogonally-polarized four-wave mixing (FWM) effect, for ultra-high-speed polarization multiplexing (Pol-MUX) fiber networks such as 100-Gbps and 400-Gbps backbone networks. We use theoretical and experimental analysis of the vector theory of FWM to successfully achieve polarization separation and all-optical switching by utilizing a 100-Gbps dual polarization-quadrature phase shift keying (DP-QPSK) signal and two orthogonally-polarized pumps. Both of the polarization-separated QPSK signals have clear constellation diagrams, with root mean square (RMS) error vector magnitudes (EVMs) of 14.32% and 14.11% respectively. The wavelengths of idlers can be created at 30 different wavelengths, which are consistent with International Telecommunication Union-Telecommunication (ITU-T) wavelengths, by flexibly changing the wavelength of the pump light. Moreover, the idlers that have distinct wavelengths have power distributed in a range from −10 dBm to −15 dBm, which can support error-free transmission. The power penaltyis 5 dB lower than that of back-to-back (BTB) signal for both the X- and Y-polarization components measured at a bit error ratio (BER) of 3.8×10−3. Our experimental results indicate that this scheme has promising applications in future backbone networks.

First Demonstration of Subsystem-Modular Optical Cross-Connect Using Single-Module 6 × 6 Wavelength-Selective Switch

We demonstrate a highly scalable subsystem-modular optical cross-connect (OXC) that uses sub-OXCs based on a newly developed single-module 6 × 6 wavelength-selective switch (WSS). Network simulations in dynamic-traffic scenarios assure the good routing performance of the architecture. Its technical feasibility is confirmed by transmission experiments using 96-wavelength 32-Gbaud dual-polarization quadrature phase-shift keying (DP-QPSK) signals. The developed 6 × 6 WSS allows each DP-QPSK signal to traverse 12 nodes when the internode distance is 100 km.

Demonstration of L-band DP-QPSK transmission over FSO and fiber channels employing InAs/InP quantum-dash laser source

The next generation of optical access communication networks that support 100 Gbps and beyond, require advances in modulation schemes, spectrum utilization, new transmission bands, and efficient devices, particularly laser diodes. In this paper, we investigated the viability of new-class of InAs/InP Quantum-dash laser diode (Qdash-LD) exhibiting multiple longitudinal light modes in the L-band to carry high-speed data rate for access network applications. We exploited external and self injection-locking techniques on Qdash-LD to generate large number of stable and tunable locked modes, and compared them. To stem the capability of each locked mode as a potential subcarrier, data transmission is carried out over two mediums; single mode fiber (SMF) and free space optics (FSO) to emulate real deployment scenarios of optical networks. The results showed that with external-injection locking (EIL), an error-free transmission of 100 Gbps dual polarization quadrature phase shift keying (DP-QPSK) signal is demonstrated over 10 km SMF and 4 m indoor FSO channels, with capability of reaching up to 128 Gbps, demonstrated under back-to-back (BTB) configuration. On the other hand, using self-injection locking (SIL) scheme, a successful data transmission of 64 Gbps and 128 Gbps DP-QPSK signal over 20 km SMF and 10 m indoor FSO links, respectively, is achieved.

Bit-Error Ratio Performance Improvement Using Iterative Decoding for Polybinary-Shaped Super-Nyquist Wavelength Division Multiplexed Signals

In super-Nyquist wavelength division multiplexed (WDM) systems with frequency spacing smaller than the signal baudrate, the maximum-likelihood (ML) decoder in the receiver is usually introduced to compensate for intersymbol interference due to tight spectral filtering, such as polybinary shaping. After the ML decoder, symbol errors tend to propagate, causing excess continuous errors. Considering that forward error correction (FEC) is commonly introduced, the excess continuous errors degrade bit-error ratio (BER) performance after FEC, so-called post-FEC BER. In order to suppress the performance degradation, we introduce iterative decoding between the first ML decoder for polybinary shaping and the second FEC decoder in the receiver. First, we calculate BER characteristics of polybinary-shaped super-Nyquist WDM quadrature phase-shift keying (QPSK) signals. The results show that iterative decoding is effective for improving post-FEC BER performance. A lager pre-FEC BER threshold for post-FEC BER <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> is obtained in super-Nyquist WDM case than in the Nyquist WDM case, although a higher signal-to-noise ratio (SNR) is required. Next, we measure the BER characteristics of three-channel duobinary-shaped super-Nyquist WDM 12.5-Gbaud dual-polarization QPSK signals. The iterative decoding reduces the optical SNR penalty by 0.8 dB. A larger pre-FEC BER threshold of 3.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> is obtained in the duobinary-shaped super-Nyquist WDM case, compared with the threshold of 2.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> in the Nyquist WDM case.

Demonstration of 100-Gb/s/λ-Based Coherent WDM-PON System Using New AGC EDFA Based Upstream Preamplifier and Optically Superimposed AMCC Function

In this paper, we present the first 100-Gb/s/λ-based coherent wavelength division multiplexing passive optical network (WDM-PON) prototype system, highlighting a real-time 100-Gb/s coherent transceiver with a simplified digital signal processing suitable for access spans, and a new automatic-gain-controlled erbium-doped fiber amplifier based preamplifier with an amplified spontaneous emission compensation function for the upstream to improve the minimum receiver sensitivity for coherent detection, especially at very low received signal power. Thanks to our proposed technologies, this first demonstration achieved an increased bidirectional loss budget of more than 39.1 dB, which supports 80 km transmission with eight optical network unit splits, with an improved upstream receiver sensitivity of -38.1 dBm. In addition, to show the feasibility of the proposed 100-Gb/s/ λ-based coherent WDM-PON as a promising candidate for forthcoming 5G mobile fronthaul networks, we further investigate experimentally a 128-kb/s auxiliary management and control channel (AMCC) superimposed on the 100-Gb/s dual polarization quadrature phase-shift keying (DP-QPSK) signals employed by the 100-Gb/s/λ -based coherent WDM-PON system. By setting the modulation index of the AMCC signals between 5% and 40%, we also successfully demonstrated 100-Gb/s DP-QPSK signal transmission with only a small power penalty of 0.2 dB due to the introduction of the AMCC signal.

Optimized WDM Transmission Impairment Mitigation by Multiple Phase Conjugations

Multiple phase conjugations within a transmission-optimized fiber link enable significant nonlinearity cancellation in the wavelength division multiplexed (WDM) systems. In particular, we demonstrate the fiber impairment mitigation of WDM $24\times 48$ Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) signals using 12 optical phase conjugations (OPC). The power excursion and dispersion-tailored transmission line in which the phase conjugations acted periodically, enabled an 8-dB higher nonlinearity threshold for the WDM 1.1-Tb/s DP-QPSK signals. With the nearly perfect nonlinearity cancellation, large input powers per span became available. Under these circumstances, we also present a first approach to the analysis of the impact of multiple phase conjugations in a distributed Raman amplified link. The analysis considers the performance of the experimentally measured OPC subsystem within an optimized transmission line limited only by an amplified spontaneous emission noise. This raw estimate gives insight into the linear noise performance limit of the employed phase conjugator when inserted multiple times in such a transmission system.

Field trial over 820 km installed SSMF and its potential Terabit/s superchannel application with up to 57.5-Gbaud DP-QPSK transmission

In this paper, we report the result of a field trial of 56-Gbaud (224-Gbit/s) and 57.5-Gbaud (230-Gbit/s) dual-polarization quadrature phase shift keying (DP-QPSK) coherent optical transmission over 820km installed standard single mode fiber (SSMF). Offline digital signal processing (DSP) was applied for signal recovery and bit-error-rate (BER) counting in our field trial experiments, and BER performance well below the 7% overhead hard-decision forward error correction (FEC) error-free threshold (4.5×10−3) at 231-1 pseudo random bit sequence (PRBS) pattern length has been achieved, with the best achievable BERs of 2×10−4 (56-Gbaud) and 3×10−4 (57.5-Gbaud), respectively. In parallel a 1.15-Tbit/s (5×230-Gbit/s) quasi-Nyquist spaced wavelength division multiplexing (WDM) superchannel transmission over the same 820km optical field link (FL) was also investigated through numerical simulations based on the same 57.5-Gbaud DP-QPSK signal using 1% roll-off Nyquist pulse shaping with 60-GHz channel spacing, and the results indicate that the BER performance well below the 7% overhead hard-decision FEC error-free threshold (4.5×10−3) for the 1.15-Tbit/s DP-QPSK superchannel transmission can be achieved.

1.2 Tb/s Superchannel Transmission Over 80$\,\times\,$100 km ULAF Using Nyquist FDM DP-QPSK

Employing 8-bit 65GSps digital-to-analog converters with pre-equalization and Nyquist pulse-shaping, we generated a terabit superchannel based on both low-baud-rate Nyquist frequency-division-multiplexing (FDM) dual polarization quadrature-phase-shift-keying (DP-QPSK) and high-baud-rate Nyquist DP-QPSK signals. The 1.2-Tb/s superchannel transmission over 80 × 100 km ultra-large-area fiber with effective area (Aeff) of 125 μm2 was demonstrated using erbium doped fiber amplifier only spans. We further discussed the roll-off factor (α) in Nyquist pulse-shaping on the system optimization. It has been found that the Nyquist FDM DP-QPSK signal outperforms the high-baud-rate Nyquist DP-QPSK. Given the same data capacity, the Nyquist FDM DP-QPSK scheme exhibits better tolerance to fiber nonlinearities.

An Ultracompact DP-QPSK Demodulator Based on Multimode Interference and Photonic Crystals

We propose a novel scheme that combines a polarization-beam splitter (PBS) and a 90° hybrid into a single device to demodulate the dual-polarization quadrature phase-shift keying (DP-QPSK) signal by introducing a photonic crystal structure into a 4 × 4 multimode interference coupler. Since no separated PBS is utilized, the demodulator has much smaller chip size compared with a conventional DP-QPSK demodulator which contains two PBSs and two 90° hybrids. Simulation with 2-D finite-difference time-domain method indicates that the demodulator can operate very well for demodulating the DP-QPSK signal. The numerical simulation demonstrates a common-mode rejection ratio larger than 20 dB and phase errors smaller than ±5° of the device over the C-band wavelength range.

All-in-One 112-Gb/s DP-QPSK Optical Receiver Front-End Module Using Hybrid Integration of Silica-Based Planar Lightwave Circuit and Photodiode Arrays

We propose a new hybrid integrated optical front-end module for a 100-Gb/s-class coherent receiver using silica-based planar lightwave circuit (PLC) technology. We have developed an integrated structure composed of multichannel microcollimator optics between PLC waveguides and photodiodes (PDs), and compact hermetically sealed optical-to-electrical converters with a PD array and a transimpedance amplifier array to maintain reliability. We used the technology to fabricate an all-in-one optical front-end module for a dual polarization quadrature phase-shift-keying (DP-QPSK) coherent receiver, and confirmed that the module operated successfully for 112-Gb/s DP-QPSK signal detection.