Dual-polarization Quadrature Phase-shift Keying Research Articles

Blind third-order dispersion estimation based on fractional Fourier transformation for coherent optical communication

In this paper, we propose a blind third-order dispersion estimation method based on fractional Fourier transformation (FrFT) in optical fiber communication system. By measuring the chromatic dispersion (CD) at different wavelengths, this method can estimation dispersion slope and further calculate the third-order dispersion. The simulation results demonstrate that the estimation error is less than 2% in 28GBaud dual polarization quadrature phase-shift keying (DP-QPSK) and 28GBaud dual polarization 16 quadrature amplitude modulation (DP-16QAM) system. Through simulations, the proposed third-order dispersion estimation method is shown to be robust against nonlinear and amplified spontaneous emission (ASE) noise. In addition, to reduce the computational complexity, searching step with coarse and fine granularity is chosen to search optimal order of FrFT. The third-order dispersion estimation method based on FrFT can be used to monitor the third-order dispersion in optical fiber system.

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.

Design of Low-Power DSP-Free Coherent Receivers for Data Center Links

Coherent detection offers high spectral efficiency and receiver sensitivity, but digital signal processing (DSP)-based coherent receivers may be prohibitively power hungry for data centers even when optimized for short-reach applications, where fiber propagation impairments are less severe. We propose and evaluate low-power DSP-free homodyne coherent receiver architectures for dual-polarization quadrature phase shift keying (DP-QPSK) for inter- and intradata center links. We propose a novel optical polarization demultiplexing technique, for DP-QPSK and higher-order modulation formats, with three cascaded phase shifters driven by marker tone detection circuitry. We consider carrier recovery based on either optical or electrical phase-locked loops (PLLs). We propose a novel multiplier-free phase detector based on XOR gates, which exhibits less than 0.5 dB power penalty relative to a conventional Costas loop phase detector. We also study the relative performance of homodyne DP-differential QPSK, for which carrier phase recovery is unnecessary. Our proposed DSP-free architectures exhibit ∼1 dB power penalty at small chromatic dispersion compared to their DSP-based counterparts. We estimate conservatively that the high-speed analog electronics of an electrical PLL-based coherent receiver consume nearly 4 W for 200 Gbit/s DP-QPSK, assuming a 90-nm complementary metal-oxide semiconductor process.

All-fiber-based selective mode multiplexer and demultiplexer for weakly-coupled mode-division multiplexed systems

We show all-fiber-based selective mode multiplexers and demultiplexers for weakly-coupled mode-division multiplexed systems. We fabricate a set of six-mode multiplexer and demultiplexer based on fiber mode selective couplers, and experimentally evaluate the performance for the six-mode dual-polarization (DP) quadrature phase shift keying (QPSK) optical signals. In the mode multiplexer and demultiplexer, the mode couplings between the lower three modes and the higher three modes are suppressed to be less than −20 dB, which enables us to apply partial 6×6 MIMO equalizers even for the six-mode demultiplexing. For the six-mode DP-QPSK signals, the penalty of optical signal-to-noise ratio by replacing the full 12×12MIMO to the partial 6×6 MIMO is suppressed by less than 1 dB.

Frequency-doubled microwave waveforms generation using a dual-polarization quadrature phase shift keying modulator driven by a single frequency radio frequency signal

A photonic approach to generate frequency-doubled microwave waveforms using an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator driven by a sinusoidal radio frequency (RF) signal is proposed. By adjusting the dc bias points of the DP-QPSK modulator, the obtained second-order and six-order harmonics are in phase while the fourth-order harmonics are complementary when the orthogonal polarized outputs of the modulator are photodetected. After properly setting the modulation indices of the modulator, the amplitude of the second-order harmonic is 9 times of that of the six-order harmonic, indicating a frequency-doubled triangular waveform is generated. If a broadband 90° microwave phase shifter is attached after the photodetector (PD) to introduce a 90° phase shift, a frequency-doubled square waveform can be obtained after adjusting the amplitude of the second-order harmonic 3 times of that of the six-order harmonic. The proposal is first theoretically analyzed and then validated by simulation. Simulation results show that a 10GHz triangular and square waveform sequences are successfully generated from a 5GHz sinusoidal RF drive signal.

Generation of Phase-Coded Microwave Signals Through Equivalent Phase Modulation

A novel approach to generate phase-coded microwave signals using a dual-polarization quadrature phase shift-keying (DP-QPSK) modulator is proposed and experimentally demonstrated. The key component of the proposed scheme is a DP-QPSK modulator, in which, the upper dual parallel Mach-Zehnder modulator (DP-MZM) modulated by a coding signal is properly biased to implement equivalent phase modulation, while the lower DP-MZM modulated by an RF signal functions as an optical wavelength shifter. A phase-coded microwave signal is generated by beating the phase-modulated optical carrier and the wavelength-shifted optical sideband at a photodetector. The proposed approach has a compact structure and great reconfigurability. By applying different coding signals, binary or quaternary phase-coded signals can be generated. A proof-of-concept experiment is carried out. The generation of binary and quaternary phase-coded signals with a microwave carrier frequency at 8 and 13 GHz is demonstrated.

Coherent UDWDM-PON With Dual-Polarization Transceivers in Real-Time

We report a real-time experimental demonstration of a long-reach coherent ultra-dense wavelength-division multiplexing (WDM)-passive optical network (PON) system with quadratic-amplitude modulation (QAM)-based dual-polarization optical transceivers. Using field-programmable gate array-based digital signal processing, the transmission of 20-WDM channels in 2.5-GHz grid, employing both 2.5-Gb/s dual-polarization quadrature phase-shift keying (DP-QPSK) and 3.75-Gb/s DP-8PSK signals, is successfully demonstrated with bit-error rate in real-time. The performance is evaluated over 100-km standard single mode fiber in terms of receiver sensitivity and ODN power budget for both modulation formats, showing the feasibility of coherent PON scenarios using flexible dual-polarization strategies supported by software-defined transceivers.

4 m/100 Gb/s Optical Wireless Communication Based on Far L–Band Injection Locked Quantum-Dash Laser

An indoor 100 Gb/s free space optical communication link using an injection locked ∼1621 nm quantum-dash laser has been experimentally demonstrated. The results are achieved using dual-polarization quadrature phase shift keying (DP-QPSK) and coherent detection over a 4 m indoor free space link. The system exhibited a receiver sensitivity of –17.5 dBm to achieve the bit error rate forward error correction (BER FEC) threshold of 3.8 × 10–3. This displays the potential of tunable injection locked far L -band quantum-dash lasers, with less scattering loss compared to C -band counterpart, as promising light sources in both indoor and outdoor future high speed free space optical information and communication technology.

Analysis of chromatic dispersion compensation and carrier phase recovery in long-haul optical transmission system influenced by equalization enhanced phase noise

The performance of long-haul coherent optical fiber transmission system is significantly affected by the equalization enhanced phase noise (EEPN), due to the interaction between the electronic dispersion compensation (EDC) and the laser phase noise. In this paper, we present a comprehensive study on different chromatic dispersion (CD) compensation and carrier phase recovery (CPR) approaches, in the n-level phase shift keying (n-PSK) and the n-level quadrature amplitude modulation (n-QAM) coherent optical transmission systems, considering the impacts of EEPN. Four CD compensation methods are considered: the time-domain equalization (TDE), the frequency-domain equalization (FDE), the least mean square (LMS) adaptive equalization are applied for EDC, and the dispersion compensating fiber (DCF) is employed for optical dispersion compensation (ODC). Meanwhile, three carrier phase recovery methods are also involved: a one-tap normalized least mean square (NLMS) algorithm, a block-wise average (BWA) algorithm, and a Viterbi-Viterbi (VV) algorithm. Numerical simulations have been carried out in a 28-Gbaud dual-polarization quadrature phase shift keying (DP-QPSK) coherent transmission system, and the results indicate that the origin of EEPN depends on the choice of chromatic dispersion compensation methods, and the effects of EEPN also behave moderately different in accordance to different carrier phase recovery scenarios.

An improved CMA for dispersion compensation in 100 Gbps DP-QPSK optical signal transmission system

An improved constant modulus algorithm (CMA) for dispersion compensation in 100Gbps Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) optical signal transmission system is proposed to achieve better reliability receiving performances. In the receiver, we combine variable step size and Mean-Square Error (MSE) to improve the conventional CMA. Compared to conventional CMA, the proposed algorithm has a better performance in adapting the time varying channel and the strong noise. Meanwhile, the proposed algorithm has a faster convergence rate than the previous one. Simulation results show that the improved CMA can achieve the dispersion and polarization demultiplexing of 100Gbps DP-QPSK optical signal transmission system efficiently.

Proposal and experimental demonstration of SDM node enabling path assignment to arbitrary wavelengths, cores, and directions.

We propose a novel simple space division multiplexing (SDM) node which is rearrangeble nonblocking, and effectively utilizes enhanced network resources through SDM. The proposed node can reduce a number of ports of wavelength selective switches (WSSs) and a number of WSS modules by modifying conventional multi-stage switches and employing integrated multiple arrayed WSSs. We experimentally actualized the newly proposed node, and demonstrate wavelength, core, and direction switching functions based on 127-Gbps Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) signals. We also confirm the feasibility of the proposed SDM node through SDM transmission experiments using a 40-km multicore fiber and a multicore amplifier.

Generation of a frequency-quadrupled phase-coded signal using optical carrier phase shifting and balanced detection.

A novel approach for photonic generation of a frequency-quadrupled phase-coded signal using optical carrier shifting and balanced detection is proposed and demonstrated. The key component of the scheme is an integrated dual-polarization quadrature phase shift-keying (DP-QPSK) modulator. In the modulator, an RF signal is applied to the upper QPSK modulator to generate high-order optical sidebands, while an electrical coding signal is applied to the bottom QPSK modulator to perform optical carrier phase shifting. After that, a frequency-quadrupled phase-coded signal with an exact π-phase shift is generated through balanced detection. The proposed scheme has a simple, compact structure and good tunability. Besides, a phase-coded pulse can be directly obtained when a three-level rectangular coding signal is applied. A proof-of-concept experiment is carried out. The generation of a 2-Gbit/s phase-coded signal with a frequency tuning from 12.12 to 28 GHz is experimentally demonstrated, and the generation of a phase-coded microwave pulse is also verified.

Self-seeded quantum-dash laser based 5 m-128 Gb/s indoor free-space optical communication: erratum

We demonstrate an indoor 5 m free-space optical wireless coherent communication in mid L-band (1606.7 nm) by employing a tunable self-seeded InAs/InGaAlAs/InP quantum-dash (Qdash) laser as a subcarrier generator for 128 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) modulation signal. The bare Qdash laser diode displays ∼6 nm self-locked Fabry–Perot mode tunability with ∼30 dB side mode suppression ratio (SMSR) and ∼10 dBm mode power across the tuning range, thus encompassing ∼10 modes with an achievable capacity of 1.28 Tb/s (10×128 Gb/s) and potentially qualifying the source requirements for future access networks.

Photonic Generation of Frequency and Bandwidth Multiplying Dual-Chirp Microwave Waveform

A photonic approach to generate a frequency and bandwidth multiplying dual-chirp microwave waveform using a single dual-polarization quadrature phase shift keying (DP-QPSK) modulator is proposed and demonstrated. In the proposed scheme, an RF carrier is applied to one QPSK modulator and a baseband single-chirp waveform is applied to another. By setting the driving signals applied to the modulator and adjusting the dc bias phases in the modulator, high-order optical sidebands can be obtained with the optical carrier suppressed. After optical to electrical conversion, a frequency-doubling and bandwidth-quadrupling, or frequency-quadrupling and bandwidth-octupling dual-chirp microwave can be generated. The approach is verified by simulation. Dual-chirp microwave waveforms with 16-GHz central frequency, 2048-MHz bandwidth and 32-GHz central frequency, 4096-MHz bandwidth are generated through an 8-GHz RF carrier and 512-MHz bandwidth baseband signal. The generated dual-chirp microwave waveform can be used in a radar system to improve its range-Doppler resolution.

Generation of Flat Optical Frequency Comb Based on a DP-QPSK Modulator

An approach to generate flat optical frequency comb (OFC) is proposed and experimentally demonstrated using a single integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator. By adjusting six dc bias points and two modulation indices of the modulator, the OFC with adjustable comb number and exactly the same intensity can be theoretically generated according to the equations derived from the DP-QPSK modulation model. Experimental results show that the proposed OFC generator can produce 7, 9, 11, and 13 lines with a spectral flatness of 0.96, 1.13, 2.01, and 1.17 dB, respectively. The unwanted-mode suppression ratio of the generated 7, 9, 11, and 13 lines OFCs are 10.33, 9.54, 6.79, and 10.45 dB, respectively. In addition, the generated OFC exhibits good frequency tunability. The nine-lines OFCs with frequency spacing of 6.02, 7.02, 8.02, and 9.02 GHz are experimentally generated.

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.

Probabilistic Design of Optical Transmission Systems

Traditionally, optical fiber nonlinearity is considered a limiting factor for transmission systems. Nevertheless, from a system design perspective this nonlinearity can be exploited to minimize the impact of uncertainty on the system performance. A consequence of this is that it becomes beneficial to consider the uncertainty at the design stage, resulting in a probabilistic design, rather than conventional design approaches whereby uncertainty is added by way of system margins to a deterministic design. In this paper, we conduct extensive experimental measurements to quantify the impact of uncertainty for a multispan wavelength division multiplexed system transmitting 100 GbE or 200 GbE as dual polarization quadrature phase-shift keying (QPSK) or 16 quadrature amplitude modulation (16QAM), respectively. The impact of uncertainty in the power launched into a span is assessed for a 10 × 80 km link. For dual polarization (DP)-QPSK, the intralink power deviation with the probabilistic design with 100% reliability is $\pm$ 1.3 dB falling to 99% reliability with $\pm$ 1.6 dB. In contrast, for DP-16QAM maximum deviation for 100% reliability is $\pm$ 0.5 dB falling to 99% for $\pm$ 0.6 dB. Following this, we consider the interplay between polarization dependent loss (PDL) and fiber nonlinearity over an 8 $\times$ 80 km system again for both DP-QPSK and DP-16QAM. A system Q variation of less than 0.15 dB due to the interaction between PDL and fiber nonlinearity is observed for 99.9% of examined PDL values for DP-QPSK and DP-16QAM, thereby allowing the two effects to be considered separately.

An Optical Frequency Shifter Based on High-Order Optical Single-Sideband Modulation and Polarization Multiplexing

We propose and experimentally demonstrate a novel electro-optic frequency shifter with frequency multiplication operation based on high-order optical single-sideband (SSB) modulation and polarization multiplexing using an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator. By changing the dc bias phases of the modulator and the phase difference between the input RF signals, two-high-order SSB modulation signals are generated from the two QPSK modulators. After that, the two optical signals are polarization multiplexed and then projected to one direction to remove the low-order sidebands while only the high-order sideband is reserved. The scheme is numerically analyzed with the modulation indices of the modulator for optimal operation. A distinct advantage of this approach is the capability to generate a frequency shifting with a continuous and ultrawide tunable range (more than ±100 GHz), and the tuning operation is accurate, simple, and flexible. An experiment is carried out to achieve a frequency-doubled optical frequency shifter. A continuous frequency shifting ranged from −36 to 36 GHz is experimentally verified.

Photonic Generation of Equivalent Single Sideband Vector Signals for RoF Systems

A novel approach for generating RF vector signals with equivalent single sideband (SSB) modulation based on a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator is proposed and experimentally demonstrated. One QPSK modulator in the DP-QPSK modulator is driven by a single-frequency RF signal. By properly setting its biases, a SSB carrier suppressed (SSB-CS) signal is generated. The other QPSK modulator in the DP-QPSK modulator is modulated by I/Q data streams, resulting in a baseband vector signal carried by the optical carrier. A polarizer is placed after the DP-QPSK modulator to combine the two signals with orthogonal polarization, leading to the generation of an equivalent SSB signal, which is free from the dispersion-induced power fading effect. By adjusting the angle of the polarizer, the carrier-to-sideband ratio can also be flexible tuned to maximize the transmission performance. The proposed system is verified by an experiment. The generation of a 1.25-GBd vector signal with QPSK modulation format at 10.5 GHz and the transmission of the signal over 25-km single-mode fiber are evaluated. An error-free transmission is achieved at a received optical power of -12 dBm and the power penalty is <;1 dB. The proposed approach features all-optical and compact configuration.

Elastic Optical Networking by Dynamic Optical Arbitrary Waveform Generation and Measurement

This paper discusses elastic optical networking (EON) by dynamic optical arbitrary waveform generation and measurement (DOAWG/DOAWM) technologies for flexible and agile bandwidth-variable transponders. The ability to generate and measure flexible bandwidth signals with arbitrary modulation formats and subwave-length granularity, as well as superchannels, makes DOAWG/DOAWM an attractive technology for EON. We introduce the DOAWG/DOAWM concept and its application to EON, and we discuss the use and advantages of DOAWG/DOAWM as the technology for a sliceable bandwidth variable transponder (SBV-T). Then we report our most recent experimental demonstration of a two-spectral-slice OAWG-SBV-T experiment to generate a single-carrier 60 Gbaud channel with dual-polarization quadrature phase-shift keying and dual-polarization 16 quadrature amplitude modulation formats. Finally, we discuss our current progress on the development of a chip-scale DOAWG on a heterogeneous Si 3 N 4 -InP integration platform.