Advanced Modulation Formats:

 

bullet Yaman, F.; Guifang Li, "Nonlinear Impairment Compensation for Polarization-Division Multiplexed WDM Transmission Using Digital Backward Propagation," Photonics Journal, IEEE , vol.1, no.2, pp.144-152, Aug. 2009

  • It is shown experimentally that impairments induced by dispersion and Kerr nonlinearity can be compensated digitally for polarization-division multiplexed wavelength division multiplexing (WDM) transmission. The method of digital backward propagation based on solving the Manakov equation can be used to efficiently compensate for the nonlinear interactions between orthogonally polarized channels.  

 

bullet Gilad Goldfarb and Guifang Li, "Efficient backward-propagation using waveletbased filtering for fiber backward-propagation," Optics Express, vol. 17, no. 11, pp. 8815-8821, May 12, 2009

  • With the goal of reducing the number of operations required for digital backward-propagation used for fiber impairment compensation, wavelet-based filtering is presented. The wavelet-based design relies on signal decomposition using time-limited basis functions and hence is more compatible with the dispersion operator, which is also time-limited. This is in comparison with inverse-Fourier filter design which by definition is not time-limited due to the use of harmonic basis functions for signal decomposition. Artificial, after-the-fact windowing may be employed in this case; however only a limited amount of saving in the number of operations can be achieved, compared to the wavelets-base filter design. Waveletbased filter design procedure and numerical simulations which validate this approach are presented in this paper.  

 

bullet Eduardo F. Mateo and Guifang Li, "Compensation of interchannel nonlinearities using enhanced coupled equations for digital backward propagation," Applied Optics, vol.48, no.25, September 1, 2009
  • Enhanced coupled-nonlinear equations are introduced for full compensation of cross-phase modulationand partial compensation of four-wave mixing (FWM) via split-step digital backward propagation. Comparedto full FWM compensation, the new backward propagation equations provide a significant reductionin the number of steps required in the split-step method. This increased step size together with morerelaxed sampling requirements reduced the computational load by more than a factor of 20. 

 

bullet Xiaoxu Li, Guifang Li,  "Electrical Postcompensation of SOA Impairments for Fiber-Optic Transmission," IEEE Photonics Technology Letters, vol.21, no.9, pp.581-583, May 1, 2009
  • An electrical postcompensation scheme for semiconductor optical amplifier (SOA) impairments has been proposed. An on–off keying transmission over 100-km standard single-mode fiber and three SOAs was demonstrated experimentally with SOA impairment compensation. An 8-dB improvement in $Q$-factor has been achieved. 

 

bullet Xiaoxu Li, Guifang Li, "Static Gain, Optical Modulation Response, and Nonlinear Phase Noise in Saturated Quantum-Dot Semiconductor Optical Amplifiers," IEEE Journal of Quantum Electronics,  vol.45, no.5, pp.499-505, May 2009
  • A rate equation model preserving charge neutrality for quantum-dot semiconductor optical amplifiers (QD-SOAs) is established to investigate the nonlinear gain dynamics in the saturation regime. The static gain of QD-SOA is calculated assuming overall charge neutrality and compared with that without overall charge neutrality. Optical modulation response and nonlinear phase fluctuation through saturated QD-SOAs are calculated numerically based on a small-signal analysis. The gain dynamics of QD-SOAs are strongly dependent on the current injection level. The carrier reservoir in the wetting layer and continuum state is necessary for QD-SOAs to operate with high gain, high saturation power, and ultrafast gain recovery. 

 

bullet Xin Chen, Xiaobo Xie, Inwoong Kim, Guifang Li, Hanyi Zhang, Bingkun Zhou, "Coherent Detection Using Optical Time-Domain Sampling," IEEE Photonics Technology Letters, vol.21, no.5, pp.286-288, March1, 2009
  • Coherent optical time-domain sampling is proposed to relax the requirement for photodetector bandwidth as well as the speed of analog-to-digital converter (ADC) and digital signal processing. Transmission of 10-Gb/s binary phase-shift keying signals over 220-km standard single-mode fiber was demonstrated using 10-Gsa/s ADCs. 

 

bullet Likai Zhu, Xiaoxu Li, Eduardo Mateo, and Guifang Li, "Complementary FIR Filter Pair for Distributed Impairment Compensation of WDM Fiber Transmission", IEEE Photonics Technology Letters, Vol. 21, No. 5, pp. 292-294, March 1, 2009
  • A method of designing a complementary filter pair is proposed to reduce error accumulation in the split-step backward propagation for distributed impairment compensation of wavelength-division-multiplexing transmission. The complementary filter pair is designed so that the individual errors of the two filters cancel each other. A 12 $times$ 100 Gb/s 16-ary quadrature amplitude modulation transmission system in nonzero dispersion-shifted fiber is simulated using such a filter pair. The required filter length is halved by the complementary filter pair design for a transmission distance of 1200 km. 

 

bullet Guifang Li, "Recent advances in coherent optical communication," Advances in Optics and Photonics, Vol. 1, Issue 2, pp. 279-307, February, 2009

  • Recent progress in coherent optical communication, a field revived by advances in digital signal processing (DSP), is reviewed. DSP-based phase and polarization management techniques make coherent detection robust and practical.With coherent detection, the complex field of the received signal is fully recovered, allowing compensation of linear impairments including chromatic dispersion and polarization-mode dispersion using digital filters. In addition, fiber nonlinearities can also be compensated by using backward propagation in the digital domain. 

 

bullet Gilad Goldfarb, Michael G. Taylor, and Guifang Li, "Experimental Demonstration of Fiber Impairment Compensation Using the Split-Step Finite-Impulse-Response Filtering Method", Vol. 20, No. 22, pp. 1887-1889, November 15, 2008
  • Digital distributed impairment postcompensation for dispersion and nonlinear effects using backward propagation is demonstrated in a wavelength-division-multiplexing environment for the first time. The experimental results clearly show the benefit of employing distributed postcompensation, compared to dispersion compensation only or lumped dispersion and nonlinearity compensation.

 

bullet Eduardo Mateo, Likai Zhu, and Guifang Li, "Impact of XPM and FWM on the digital implementation of impairment compensation for WDM transmission using backward propagation," Opt. Express, Vol. 16, No. 20, pp.16124-16137, September 29, 2008

  • The impact of cross-phase modulation (XPM) and four-wave mixing (FWM) on electronic impairment compensation via backward propagation is analyzed. XPM and XPM+FWM compensation are compared by solving, respectively, the backward coupled Nonlinear Schr¨odinger Equation (NLSE) system and the total-field NLSE. The DSP implementations as well as the computational requirements are evaluated for each post-compensation system. A 12×100 Gb/s 16-QAM transmission system has been used to evaluate the efficiency of both approaches. The results show that XPM post-compensation removes most of the relevant source of nonlinear distortion. While DSP implementation of the total-field NLSE can ultimately lead to more precise compensation, DSP implementation using the coupled NLSE system can maintain high accuracy with better computation efficiency and low system latency.

 

bullet Gilad Goldfarb and Guifang Li, "Demonstration of fibre impairment compensation using split-step infinite-impulse-response filtering method", Vol. 44, No. 13, pp.814-816, June 19, 2008
  • Distributed impairment compensation for dispersion and nonlinear effects using backward propagation in a wavelength-division multiplexing (WDM) environment is demonstrated. Dispersion compensation is achieved using complex infinite-impulse-response filtering, which significantly reduces the computational load compared to using finite-impulse-response filtering.

 

bullet Xiaoxu Li, Xin Chen, Gilad Goldfarb, Eduardo Mateo, Inwoong Kim, Fatih Yaman, and Guifang Li, "Electronic post-compensation of WDM transmission impairments using coherent detection and digital signal processing", Optics Express, Vol. 16, Issue 2, pp. 880-888, January, 2008
  • A universal post-compensation scheme for fiber impairments in wavelength-division multiplexing (WDM) systems is proposed based on coherent detection and digital signal processing (DSP). Transmission of 10 × 10 Gbit/s binary-phase-shift-keying (BPSK) signals at a channel spacing of 20 GHz over 800 km dispersion shifted fiber (DSF) has been demonstrated numerically.

 

bullet Gilad Goldfarb, Guifang Li, and Michael G. Taylor, "Orthogonal Wavelength-Division Multiplexing Using Coherent Detection", IEEE Photonics Technology Letters, Vol. 19,  No. 24, Page(s): 2015-2017, December 15, 2007
  • Optical wavelength-division multiplexing (WDM) with channel spacing equal to the symbol rate is demonstrated. Coherent detection with subsequent digital signal processing is used for demultiplexing and demodulation. Experimental results for a 6-Gbaud binary phase-shift keying WDM transmission with a 6-GHz channel spacing achieve a -factor penalty of 2.8 dB compared to a single-channel transmission.

 

bullet Inwoong Kim, Gilad Goldfarb, and Guifang Li, "Electronic wavefront correction for PSK free-space optical communications", Electronic Letters, Vol. 43, No. 20, pp. 1108-1109, September 27, 2007.
  • Atmospheric turbulence-induced wavefront distortion in free-space optical communications can be corrected using coherent detection and digital signal processing, instead of adaptive optics. It was found that the performance of the coherent arrayed receivers can approach that of the system without any wavefront distortion. Wavefront correction for 10 Gbit=s binary phase-shift keying signals was demonstrated in experiment.

 

bullet Inwoong Kim, Kevin Croussore, Xiaoxu Li, and Guifang Li, "All-Optical Carrier Synchronization Using a Phase-Sensitive Oscillator", IEEE Photonics Technology Letters, Vol. 19,  No. 13, Page(s): 987-989, July 1, 2007
  • An all-optical carrier synchronization (carrier-phase and polarization recovery) scheme from binary phase-shift keying signals is proposed and demonstrated for the first time. The proposed scheme uses a degenerate optical parametric oscillator which has a phase-sensitive amplifier as a gain block.

 

bullet Gilad Goldfarb and Guifang Li, "Chromatic Dispersion Compensation Using Digital IIR Filtering With Coherent Detection", IEEE Photonics Technology Letters, Vol. 19,  No. 13, Page(s): 969-971, July 1, 2007
  • Digital infinite impulse response (IIR) filtering is proposed as a means for compensating chromatic dispersion in homodyne- detected optical transmission systems with subsequent digital signal processing. Compared to finite impulse response (FIR) filtering, IIR filtering achieves dispersion compensation (DC) using a significantly smaller number of taps. DC of 80 and 160 km in a 10-Gb/s binary phase-shift-keying is experimentally compared for the two filtering schemes. IIR filtering can achieve performance similar to the FIR filtering scheme.

 

bullet Xin Chen, Cheolhwan Kim, Guifang Li, Bingkun Zhou, "Numerical Study of Lumped Dispersion Compensation for 40-Gb/s Return-to-Zero Differential Phase-Shift Keying Transmission", IEEE Photonics Technology Letters, Vol. 19, No. 8, pp.568-570, April 15, 2007.
  • Lumped dispersion compensation is numerically investigated for 40-Gb/s return-to-zero differential phase-shift-keying transmission. Using pseudorandom binary sequence lengths up to 211-1, simulation results indicate that better long-distance transmission performance can be achieved using lumped dispersion compensation than using conventional periodic inline dispersion compensation. Improved performance is found to be a result of reduced nonlinear effects and the elimination of periodic accumulation of nonlinear effects. The lumped compensation scheme provides a simple, flexible, and potentially low-cost solution for transmission link design

 

bullet Xiaoxu Li and Guifang Li, "Comments on “Theoretical Analysis of Gain-Recovery Time and Chirp in QD-SOA”," Photonics Technology Letters, Vol. 18, No. 22, pp. 2434-2435, November 15, 2006.
  • (Not available)

 

bullet Gilad Goldfarb and Guifang Li, "BER estimation of QPSK homodyne detection with carrier phase estimation using digital signal processing," Optics Express, Vol. 14, No. 18, pp. 8043-8053, September 4, 2006.
  • An approximate analytical expression for the bit error rate of a QPSK homodyne receiver employing digital signal processing for carrier recovery is derived. BER estimated using the analytical expression is in excellent agreement with Monte-Carlo simulations. The analytical approximation leads to an intuitive understanding of the trade off in such systems and allows optimization of system parameters without resorting to Monte-Carlo simulations.

 
bullet Yan Han and Guifang Li, "Theoretical Sensitivity of Direct-Detection Multilevel Modulation Formats for High Spectral Efficiency Optical Communications," IEEE Journal of Selected Topics in Quantum Electronics, Vol. 12, No. 4, pp. 571-580, July/August 2006.
  • Sensitivities of several direct-detection multilevel modulation formats, including M-ary differential phase-shift keying (M-DPSK), M-ary differential amplitude-phase-shift keying (M-DAPSK), and M-ary differential polarization-phase-shift keying (M-DPolPSK) are systematically calculated. The theory is based on a unified mathematical framework for direct-detection receivers using optical delay interferometers and balanced detectors. Results quantitatively elucidate the tradeoffs between these direct-detection multilevel modulation formats as well as the classic multilevel modulation format, M-ary amplitude-shift keying (MASK). Although the conventional bit error rate estimation method based on the eye Q-factor with Gaussian approximation is not accurate enough for DPSK and DQPSK, it is found to be reasonably accurate for M-DPSK with M > 8 and in M-DPolPSK.

 
bullet Zhihong Li and Guifang Li, "Linear and Nonlinear Crosstalk Suppression for DWDM RZ-DPSK Transmitter Using a Saturated SOA as Power Booster Amplifier," Photonics Technology Letters, Vol. 18, No. 7, pp. 811-813, April 1, 2006.
  • Saturated semiconductor optical amplifiers (SOAs) can be used as booster amplifiers for dense wavelength-division-multiplexing (DWDM) return-to-zero differential phase-shift keying (RZ-DPSK) transmitters. By introducing time interleaving (TI), both linear crosstalk induced by WDM components and nonlinear crosstalk induced by SOA nonlinearities are suppressed. Receiver sensitivities for four-channel DWDM RZ-DPSK signals with 100- and 50-GHz channel spacing were improved 2.2 and 4.2 dB, respectively, by applying proper TI between adjacent channels. A 1-dB gain enhancement was also achieved.

 
bullet Gilad Goldfarb, Cheolhwan Kim, and Guifang Li, "Improved Chromatic Dispersion Tolerance for Optical Duobinary Transmission Using Coherent Detection," Photonics Technology Letters, Vol. 18, No. 3, pp. 517-519, February 1, 2006.
  • The impact of chromatic dispersion on the complex electric field of an optical phase-coded duobinary signal is investigated through numerical simulation. Dispersion causes distortion and rotation of the optical constellation with increasing transmission distance, leading to eye closure of the received signal. When direct detection is employed, rapid eye closure starts after approximately 213 km. Employing coherent detection increases this dispersion limit by 70 km.

 
bullet Yan Han and Guifang Li, "Experimental demonstration of direct-detection quaternary differential polarisation-phase-shift-keying with electrical multilevel decision," Electronic Letters, Vol. 42, No. 2, pp. 109-110, January 19, 2006.
  • Direct-detection quaternary differential polarisation-phase-shift keying (DPolPSK) has been experimentally demonstrated for optical communications. DPolPSK encodes information on both the polarisation and phase of lightwave. The polarisation-phase symbols are decoded using optical interferometric demodulation and electrical multilevel decision; no dynamic optical polarisation control is required at the receiver. In the experiment, 20 Gbit/s quaternary DPolPSK was transmitted through 3 x 80 km standard singlemode fibre with a measured BER of less than 10-11.

 
bullet Yan Han and Guifang Li, "Coherent optical communication using polarization multiple-input-multiple-output," Optics Express, Vol. 13, No. 19, pp. 7527-7534, September 19, 2005.
  • Polarization-division multiplexed (PDM) optical signals can potentially be demultiplexed by coherent detection and digital signal processing without using optical dynamic polarization control at the receiver. In this paper, we show that optical communications using PDM is analogous to wireless communications using multiple-input-multiple-output (MIMO) antennae and thus algorithms for channel estimation in wireless MIMO can be ready applied to optical polarization MIMO (PMIMO). Combined with frequency offset and phase estimation algorithms, simulations show that PDM quadrature phase-shift keying signals can be coherently detected by the proposed scheme using commercial semiconductor lasers while no optical phase locking and polarization control are required. This analogy further suggests the potential application of space-time coding in wireless communications to optical polarization MIMO systems and relates the problem of polarization-mode dispersion in fiber transmission to the multi-path propagation in wireless communications.

 
bullet Yan Han and Guifang Li, "Nonlinear effects in high-spectral-efficiency transmission using differential polarization-phase-shift keying," Optics Letters, Vol. 30, No. 16, pp. 2077-2079, August 15, 2005.
  • The effect of optical nonlinearities on the fiber transmission of differential polarization-phase-shift keying (DPolPSK) signals is analyzed. For single-channel transmission, the effect of self-phase modulation depends on the order of the DPolPSK. In dense wavelength-division multiplexing, the dominant nonlinear effect is cross-phase-modulation-induced polarization scattering, which exists even if there is no intensity noise. It is found that differential detection greatly reduces the effect of polarization scattering in DPolPSK. Unlike in conventional polarization-division-multiplexed intensity-modulated direct-detection systems, the effect of polarization scattering strongly depends on the channel spacing and bit rate.

 
bullet Cheolhwan Kim, Gilad Goldfarb, Yan Han, Zhihong Li, Xiaoxu Li, and Guifang Li, "WDM Transmission over 320 km EDFA-Amplified SSMF Using 30 Gb/s Return-to-Zero Optical Differential 8-Level Phase-Shift Keying (OD8PSK)," Optics Express, Vol. 13, No. 11, pp. 4044-4049, May 30, 2005.
  • Fiber transmission of optical differential 8-level phase-shift keying (OD8PSK) signals is demonstrated for the first time. Co-polarized 8 WDM channels of 10 Giga-symbol/s or 30 Gb/s return-to-zero (RZ) OD8PSK signals with a channel spacing of 50 GHz were transmitted over 320 km of standard single mode fiber (SSMF) with an EDFA spacing of 80 km. The BER of the worst WDM channel after transmission of 320 km was 2.3×10-5.

 
bullet Yan Han and Guifang Li, "Noise statistics of and BER estimation using demodulated signals for direct detection differential polarization-phase-shift keying," Optics Express, Vol. 13, No. 7, pp. 2761-2766, April 4, 2005.
  • The noise statistics of optically demodulated signal of the recently proposed modulation format, namely differential polarization-phase-shift keying, are analyzed. It is found that, in the linear regime, the Gaussian approximation is reasonably accurate in estimating the BER. In the nonlinear regime, the noise of the demodulated signal is dominated by the effect of nonlinear phase noise and obeys chi-squared statistics with a degree of freedom of one. Gaussian approximation underestimates the BER in the nonlinear regime. A two-step BER estimation method accounting for both linear beat noise and nonlinear phase noise is proposed. The effectiveness of this two-step BER estimation method at all power levels has been established by comparison with direct error counting.

 
bullet Yan Han and Guifang Li, "Sensitivity Limits and Degradations in OD8PSK," Photonics Technology Letters, 17 (3), 720-722, March 3, 2005.
  • A fundamental sensitivity limit is calculated for optical differential eight-level phase-shift keying (OD8PSK). Performance degradations in practical implementations due to polarization and spectral filtering, single-ended receiving, phase offset error, and finite laser linewidth are investigated. From these results, critical OD8PSK transmitter and receiver design parameters are extracted.

 
bullet Yan Han and Guifang Li, "Direct detection differential polarization-phase-shift keying based on Jones vector," Optics Express, 12 (24), 5821-5826, November 29, 2004.
  • In this paper, differential polarization-phase-shift keying (DPolPSK) is proposed for digital lightwave communication, in which information is encoded in both polarization and phase. The DPolPSK symbols are demodulated by optical delayed interference and electrical multilevel detection, without using any polarization control or polarization selection at the receiver. Simulation results show that single-channel error-free transmission at 20 Gb/s (10 Gsymbol/s) through 25 spans of 100 km standard single-mode fiber is possible. The constellation diagram of DPolPSK can be represented by a set of Jones vectors. Hence DPolPSK can also be called differential Jones-vector shift keying (DJSK).

 
bullet Y. Han, C. Kim and G. Li, "Simplified receiver implementation for optical differential 8-level phase-shift keying," Electronics Letters 40 (21), 1372-1373, October 14, 2004.
  • A simplified receiver implementation for optical differential 8-level phase-shift keying using direct detection is proposed. This implementation reduces the number of optical components in the receiver to half. The impact of non-ideal receiver parameters is analysed and quantified for a 1 dB OSNR penalty.

 
bullet Cheolhwan Kim and Guifang Li, “Direct-detection optical differential 8-level phase-shift keying (OD8PSK) for spectrally efficient transmission,” Optics Express, Vol. 12, No. 15, 3415-3421, July 26, 2004.
  • An implementation of optical differential 8-level phase-shift keying (OD8PSK) is proposed for spectrally efficient high capacity long-haul optical fiber transmission systems. Interferometric demodulation and direct detection at the receiver yield three output binary sequences identical to the three input binary sequences. This is accomplished by proper design of electrical encoding and optical encoding at the transmitter. Three optical encoding schemes are proposed with corresponding differential electrical encoding schemes. Numerical simulations are performed for a single channel transmission to evaluate the transmission performances of OD8PSK systems.

 
bullet Inwoong Kim, Cheolhwan Kim and Guifang Li, “Requirements for the Sampling Source in Coherent Linear Sampling,” Optics Express, 12(12), 2723-2730, June 2004.
  • Complex envelope measurement using coherent linear optical sampling with mode-locked sources is investigated. It is shown that reliable measurement of the phase requires that one of the optical modes of the mode-locked laser be locked to the optical carrier of the data signal to be measured. Carrier-envelope offset (CEO) is found to have negligible effect on the measurement. Measurement errors of the intensity profile and phase depend on the pulsewidth and chirp of the sampling pulses as well as the detuning between the carrier frequencies of the data signal and the center frequency of sampling source.