Differential Quadrature Phase Shift Keying Research Articles

Electronic Predistortion by Polar Coordinate Transformation Using the CORDIC Algorithm

In this paper, we propose a polar coordinate transformation in real time using the COordinate Rotation DIgital Computer (CORDIC) algorithm in order to realize an electronic predistorting transmitter with a practical circuit implementation. The proposal enables a dual-drive Mach-Zehnder modulator (DDMZM) to use vector in-phase/quadrature-phase (I/Q) modulation to generate predistorted complex electronic fields. The basic theory of coordinate transformation and the CORDIC algorithm are explained in detail. The expected dispersion compensation performance for 40-Gb/s differential quadrature phase-shift keying (DQPSK) is evaluated by numerical simulations. The required number of iterations of the CORDIC algorithm and its impact on signal quality are also clarified. Simulation shows that only seven iterations are required to achieve signal quality sufficient for predistortion transmission. A practical size of 6.9-M gates is estimated to be sufficient for the large scale integration (LSI) implementation for 40-Gb/s DQPSK systems.

Applications of Artificial Neural Networks in Optical Performance Monitoring

Applications using artificial neural networks (ANNs) for optical performance monitoring (OPM) are proposed and demonstrated. Simultaneous identification of optical signal-to-noise-ratio (OSNR), chromatic dispersion (CD), and polarization-mode-dispersion (PMD) from eye-diagram parameters is shown via simulation in both 40 Gb/s on-off keying (OOK) and differential phase-shift-keying (DPSK) systems. Experimental verification is performed to simultaneously identify OSNR and CD. We then extend this technique to simultaneously identify accumulated fiber nonlinearity, OSNR, CD, and PMD from eye-diagram and eye-histogram parameters in a 3-channel 40 Gb/s DPSK wavelength-division multiplexing (WDM) system. Furthermore, we propose using this ANN approach to monitor impairment causing changes from a baseline. Simultaneous identification of accumulated fiber nonlinearity, OSNR, CD, and PMD causing changes from a baseline by use of the eye-diagram and eye-histogram parameters is obtained and high correlation coefficients are achieved with various baselines. Finally, the ANNs are also shown for simultaneous identification of in-phase/quadrature (I/Q) data misalignment and data/carver misalignment in return-to-zero differential quadrature phase shift keying (RZ-DQPSK) transmitters.

A Comparison of Electrical and Optical Dispersion Compensation for 111-Gb/s POLMUX–RZ–DQPSK

In this paper, we experimentally investigate the tolerance of 111-Gb/s polarization multiplexed return-to-zero differential quadrature phase-shift keying (POLMUX-RZ-DQPSK) signals with respect to nonlinear impairments in a link with either (inline) optical or (receiver-side) electrical dispersion compensation. After transmission of more than 2000 km of LEAF, a comparable nonlinear tolerance is observed for both dispersion maps. However, we observe that in the presence of differential group delay (DGD) the nonlinear tolerance of the transmission link with in-line optical dispersion compensation is reduced significantly, whereas the nonlinear tolerance of the configuration with receiver-side dispersion compensation is virtually unaffected.

1500-km SSMF Transmission of Mixed 40-Gb/s CS-RZ Duobinary and 100-Gb/s CS-RZ DQPSK Signals

We demonstrated transmission of eight channels of 200-GHz channel spacing and 100-Gb/s carrier-suppressed return-to-zero (CS-RZ) differential quadrature phase-shift keying (DQPSK) signals together with eight channels of 40-Gb/s CS-RZ duobinary (DRZ) signals also with 200-GHz channel spacing in order to improve the optical spectral efficiency of the wavelength-division-multiplexing system. Each DRZ channel is inserted in the middle of two adjacent CS-RZ DQPSK channels. A bit-error rate (BER) of less than 5E-4 is achieved for the 40-Gb/s DRZ channel after 1500-km SSMF transmission while a BER of 1E-3 is achieved for the 100-Gb/s CS-RZ DQPSK signals.

Optical phase erasure based on FWM in HNLF enabling format conversion from 320-Gb/s RZDQPSK to 160-Gb/s RZ-DPSK

Through four-wave mixing (FWM) process in highly-nonlinear fiber (HNLF) or semiconductor device, the phase-modulation depth of one converted FWM component could be doubled compared with that of the original input signal. Therefore, with a multilevel phase-modulated signal and a CW light as inputs, after FWM process in a nonlinear media, phase pattern (0, pi) carried in the input multilevel phase-modulated signal will not be transferred to one converted FWM component, which could be referred to as an optical phase erasure process. We experimentally demonstrated format conversion from 320-Gb/s return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) to 160-Gb/s return-to-zero differential phase-shift keying (RZ-DPSK) through the proposed all-optical phase erasure scheme. The phase information carried in the converted binary RZ-DPSK is logically equal to the input Q-component, or a logical XOR operation result between I and Q components of the input RZ-DQPSK, which correspond to a serial or parallel DQPSK transmitter for the input RZ-DQPSK signal. It can be applied to erase a binary tributary from a multilevel modulation format.

Low Power Consumption DQPSK Demodulator Using Silica-Based Planar Lightwave Circuit

We demonstrate a low power consumption differential quadrature phase-shift keying (DQPSK) demodulator using silica-based planar lightwave circuit (PLC) technology. A reduction of the power consumption is achieved by suppressing the temperature dependence of the circuit characteristic and eliminating a thermo-electric cooler (TEC) device. The maximum power consumption of the fabricated demodulator with an operating temperature range of -5degC to 70degC is 1.4 W, which is 60% lower than that of a conventional PLC demodulator with TEC. We report experimental results that demonstrate its good demodulation performance for a 43-Gb/s DQPSK system.

Stratified Sampling Monte Carlo Algorithm for Efficient BER Estimation in Long-Haul Optical Transmission Systems

We propose an efficient stratified sampling (SS) algorithm for estimating the bit error rate (BER) of a digital communication system. Our algorithm efficiently exploits the observations of an approximate, but usually fast, model of the system under investigation to drive a clever Monte Carlo (MC) estimation based on SS. The proposed method is faster than standard MC even at BER in the range 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> . Moreover, it is possible to evaluate the estimated standard deviation of the measured BER, such as in an MC simulation, so that it is possible to associate a confidence to the results. We test the algorithm both in a simple optical system distorted by group velocity dispersion (GVD) and in more complex differential quadrature phase shift keying (DQPSK) systems. In the last case, we measured computational savings up to 70% compared with standard MC.

Analytical Performance Evaluation of Optical DQPSK Systems With Postdetection Filtering

A novel analytical method for evaluating the performance of optical transmission systems using differential quadrature phase-shift keying is presented. It is shown that the bit-error probability (BEP) can be evaluated through the statistics of the received signal phase difference directly filtered by the postdetection filter. The accuracy of the method is checked against the results obtained through a simulator based on the multicanonical Monte Carlo algorithm. Some results in terms of BEP evaluation and nonlinear phase noise issues are also provided.

Scheme for differential quadrature phase-shift keying/quadrature phase-shift keying signal all-optical regeneration based on phase-sensitive amplification

The performance of a scheme for all-optical regeneration of differential quadrature phase-shift keying (DQPSK) and quadrature phase-shift keying signals using phase-sensitive amplifiers (PSAs) is investigated through numerical simulation. The results show that both phase and amplitude noises of 40 Gb/s RZ-DQPSK signals are significantly suppressed after regeneration by leveraging the ability of PSAs to provide phase and amplitude regenerative amplification. The phase Q factor is also defined to evaluate the performance of regenerated signals, and it is found that the phase Q factor after regeneration is improved by about 3 dB.

Tolerance of laser frequency offset in optical minimum-shift keying transmission systems

A mismatch between laser frequency and delay interferometer (DI) phase is found to be the most critical impairment for the receiver performance in a practical phase-modulated (PM) system. This paper investigates the receiver performance degradation caused by frequency offset between optical signal and DI in a 10 Gb/s minimum-shift keying (MSK) system, and compares it with the conventional PM formats, optical differential-phase-shift keying (DPSK) and differential-quadrature-phase-shift keying (DQPSK), which are nonreturn-to-zero (NRZ) and 50% duty cycle return-to-zero (RZ). Results show that the MSK system is about double times and six times more robust to frequency offset than the DPSK and DQPSK systems operating at the same bit rate, respectively.

XPM reduction in hybrid 10G/40G transmission using 10-Gb/s narrow-filtered DPSK modulation

We propose a possible improvement for optical hybrid transmission systems, based on 40G differential quadrature shift Keying (DQPSK) and 10G differential phase shift keying (DPSK) received by means of a Gaussian narrow filter. We report a reduced cross phase modulation (XPM), at the cost of a slightly more complex setup. This technique is especially effective for systems with low inline dispersion and offers similar performances on different types of fiber.

Optical Phase Add–Drop for Format Conversion Between DQPSK and DPSK and its Application in Optical Label Switching Systems

We propose and experimentally demonstrate an optical phase ldquoadd-droprdquo scheme to enable the format conversion between a 20-Gb/s differential quadrature phase-shift keying (DQPSK) and 10-Gb/s differential phase-shift keying. Meanwhile, the incoming DQPSK could be directly forwarded to another wavelength through the proposed optical phase ldquodroprdquo scheme, which is implemented by four-wave-mixing effect in highly nonlinear fiber. By orthogonally encoding the label and payload of the optical packet on the in-phase ( I) and quadrature ( Q) components of DQPSK, the proposed scheme could be applied in the optical label switching networks. The label erasing and rewriting of optical packets can be achieved through the proposed optical phase ldquodroprdquo and ldquoaddrdquo schemes, respectively.

Measurement of Differential Phasor Diagram of Multilevel DPSK Signals by Using an Adjustment-Free Delay Interferometer Composed of a 3$\,\times\,$3 Optical Coupler

We develop an adjustment-free differential-phase demodulator based on a delay-interferometer (DI) made of a 3 times 3 optical coupler, which is used as a 120-degree optical hybrid, and demonstrate the possibility of using it as a phasor monitor for the multilevel differential phase-shift keyed (DxPSK) signal. The key features of the proposed demodulator are twofold: (a) in-phase and quadrature (I-Q) components and the phasor diagram can be obtained only by using a single DI, and (b) the phase delay of the DI can be derived from the output power of the DI without the knowledge of the signal under test. These features enable us to demodulate the I-Q components of DxPSK signals without any adjustments. We formulate a theoretical model for deriving the differential phasor from the output signals of the 3 times 3 coupler regardless of unbalances in the phase retardations and the splitting ratios of the 3 times 3 coupler. Thus, the discrepancy from the ideal 120-degree optical hybrid is not a problem, and hence the requirements of the 3 times 3 couplers are greatly relaxed. For a demonstration, we implement the proposed demodulator by using a fiber Michelson interferometer with Faraday rotator mirrors and use it as a differential phasor monitor capable of the plug-and-play (wavelength- and polarization-independent) operation. We show that this phasor monitor can be used for diagnosis of differential phase-shift keyed (DPSK) and differential quadrature phase-shift keyed (DQPSK) transmitters by identifying the sources of impairments from the measured constellation diagram and phasor trajectories. The proposed phasor monitor can also be used for monitoring the optical signal-to-noise ratio (OSNR) of DPSK signal.

Tunable 105 ns optical delay for 80 Gb/s RZ-DQPSK, 40 Gb/s RZ-DPSK, and 40 Gb/s RZ-OOK signals using wavelength conversion and chromatic dispersion

We demonstrate a variable, optical-delay element using tunable wavelength conversion in a periodically poled lithium niobate waveguide, dispersion-compensating fiber and intrachannel dispersion compensation. A delay of up to 105 ns is demonstrated using 80 Gb/s return-to-zero differential-quadrature phase-shift keying, 40 Gb/s return-to-zero differential phase-shift keying, and 40 Gb/s return-to-zero on-off keying modulation formats. Bit-error rates <10(-9) are demonstrated for each waveform at various delay settings.

差分正交相移键控格式的产生及其抗偏振模色散的性能研究

We propose a novel transmitter and receiver for differential quadrature phase-shift keying (DQPSK) format. The impact of the first-order polarization mode dispersion (PMD) on systems using DQPSK with 100-Gb/s non-return-to-zero (NRZ) or return-to-zero (RZ) format is investigated. Through computing the eye openings of DQPSK formats under different PMD conditions, it is found that NRZ DQPSK, as compared with RZ DQPSK, incurs smaller eye opening due to PMD. Carrier-suppressed return-to-zero (CSRZ) DQPSK format has better tolerance than RZ DQPSK format to PMD for a given bit rate.

基于相位敏感放大器的DPSK/DQPSK信号相位再生技术分析

Amplification and phase regeneration can be realized using a phase-sensitive amplifier (PSA). The phase regeneration of differential phase-shift keying (DPSK) signals based on PSA is analyzed theoretically. We realize the phase regeneration of differential quadrature phase-shift keying (DQPSK) signals based on a structure using two balanced PSAs. Simulations show that nearly ideal phase regeneration can be achieved for the DPSK/DQPSK signals.

Monte Carlo Estimation of PDM-QPSK/OOK and DQPSK/OOK Hybrid Systems Tolerance Against Nonlinear Effects

We compare through Monte Carlo simulations the performance of polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK)/on-off keying (OOK) and differential QPSK (DQPSK)/OOK hybrid systems, taking into account nonlinear phase noise. DQPSK turns out to be better with optimal dispersion-management, while PDM-QPSK proves to be a good candidate for bit rates higher than 40 Gb/s.

Rayleigh Backscattering Noise-Eliminated 115-km Long-Reach Bidirectional Centralized WDM-PON With 10-Gb/s DPSK Downstream and Remodulated 2.5-Gb/s OCS-SCM Upstream Signal

We propose a novel scheme of Rayleigh backscattering noise-eliminated, long-reach, single-fiber, full-duplex, centralized wavelength-division multiplexed passive optical network with differential quadrature phase-shift keying (DPSK) downstream and remodulated upstream using an optical carrier-suppressed subcarrier-modulation (OCS-SCM) technique and optical interleaver. The error-free transmissions of 10-Gb/s downstream and 2.5-Gb/s upstream signals are experimentally demonstrated over 115-km single-fiber bidirectional SMF-28 with less than 0.5 and 1.9 dB power penalties, respectively.

100-Gb/s DQPSK Transmission: From Laboratory Experiments to Field Trials

We discuss the generation, detection, and long-haul transmission of single-polarization differential quadrature phase shift keying (DQPSK) signals at a line rate of 53.5 Gbaud to support a net information bit rate of 100 Gb/s. In the laboratory, we demonstrate 10-channel wavelength-division multiplexed (WDM) point-to-point transmission over 2000 km on a 150-GHz WDM grid, and 1200-km optically routed networking including 6 reconfigurable optical add/drop multiplexers (ROADMs) on a 100-GHz grid. We then report transmission over the commercial, 50-GHz spaced long-haul optical transport platform LambdaXtremereg. In a straight-line laboratory testbed, we demonstrate single-channel 700-km transmission, including an intermediate ROADM. On a field-deployed, live traffic bearing Verizon installation between Tampa and Miami, Florida, we achieve 500-km transmission, with no changes to the commercial system hardware or software and with 6 dB system margin. On the same operational system, we finally demonstrate 100-Gb/s DQPSK encoding on a field-programmable gate array (FPGA) and the transmission of real-time video traffic.

All-Optical Chromatic Dispersion Monitoring for Phase-Modulated Signals Utilizing Cross-Phase Modulation in a Highly Nonlinear Fiber

We propose and experimentally demonstrate an all-optical chromatic dispersion (CD) monitoring technique for phase-modulated signals utilizing the cross-phase-modulation effect between the input signal and the inserted continuous-wave probe. The probe's optical spectrum changes with the accumulated CD on the input signal, indicating that the optical power variations can be measured for monitoring. The experimental results show that this technique can monitor up to 120 ps/nm of CD for a 40-Gb/s return-to-zero differential phase-shift keying (RZ-DPSK) transmission system, with the maximum measured optical power increment of 16.5 dB. The applicability of this monitoring technique to higher bit-rate phase-modulated signals, such as 80-Gb/s RZ differential quadrature phase-shift keying and 80-Gb/s polarization-multiplexed RZ-DPSK, is also investigated via simulation.