Ziad A. El-Sahn

Spectral-Amplitude-Coded OCDMA Optimized for a Realistic FBG Frequency Response

We develop a methodology for numerical optimization of fiber Bragg grating frequency response to maximize the achievable capacity of a spectral-amplitude-coded optical code-division multiple-access (SAC-OCDMA) system. The optimal encoders are realized, and we experimentally demonstrate an incoherent SAC-OCDMA system with seven simultaneous users. We report a bit error rate (BER) of 2.7times10-8 at 622 Mb/s for a fully loaded network (seven users) using a 9.6-nm optical band. We achieve error-free transmission (BER<1times10-9) for up to five simultaneous users

Experimental Demonstration of a SAC-OCDMA PON With Burst-Mode Reception: Local Versus Centralized Sources

In this paper, we demonstrate experimentally the uplink of a 7 times 622 Mb/s incoherent spectral amplitude coded optical code-division multiple access (SAC-OCDMA) passive optical network (PON) with burst-mode reception. We consider two network architectures: local sources (LS) at each optical network unit (ONU) versus a single source located at the central office. We examine both architectures over a 20-km optical link, as well as a reference back-to-back configuration. Our architectures can adopt two-feeder and single-feeder topologies; however, we only test the two-feeder topology and therefore the effect of Rayleigh backscattering is neglected. We also study the relative merits (cost and performance) of local sources versus centralized architectures. A penalty of less than 2 dB between the LS and the centralized light sources (CLS) architectures was measured at a bit error rate (BER) of 10-9 under certain assumptions on the relative power of the sources. The power budget in the CLS architectures is more critical than in the LS architectures; extra splitting and propagation losses exist as the uplink travels through the network back and forth. Doubling the number of users while maintaining the same distance and source power in LS architectures imposes 3-dB additional losses, whereas for CLS architectures, there are 6-dB extra losses. CLS architectures can overcome these penalties using amplification at the central office. Alternately, central office amplification can be used to more than double the number of users in LS SAC-OCDMA PONs. A standalone (no global clock) burst-mode receiver with clock and data recovery (CDR), clock and phase alignment (CPA), and Reed-Solomon RS(255,239) forward-error correction (FEC) decoder is demonstrated. A penalty of less than 0.25 dB due to the nonideal sampling of the CDR is reported. The receiver also provides an instantaneous phase acquisition time for any phase step between consecutive packets, and a good immunity to silence periods. A coding gain of more than 2.5 dB was reported for a single-user system, and BER floors were completely eliminated. Error-free transmission (BER < 10-9 ) for a fully loaded PON was achieved for the LS architecture as well as the CLS architecture. Continuous and bursty upstream traffic were tested. Due to the CPA algorithm, even with zero preamble bits we report a zero packet loss ratio (PLR) for up to four simultaneous users in case of bursty traffic, and more than two orders of magnitude improvement in the PLR for fully loaded PON systems.

Pilot-aided carrier phase recovery for M-QAM using superscalar parallelization based PLL.

In this paper, we present a carrier phase recovery (CPR) algorithm using a modified superscalar parallelization based phase locked loop (M-SSP-PLL) combined with a maximum-likelihood (ML) phase estimation. Compared to the original SSP-PLL, M-SSP-PLL + ML reduces the required buffer size using a novel superscalar structure. In addition, by removing the differential coding/decoding and employing ML phase recovery it also improves the performance. In simulation, we show that the laser linewidth tolerance of M-SSP-PLL + ML is comparable to blind phase search (BPS) algorithm, which is known to be one of the best CPR algorithms in terms of performance for arbitrary QAM formats. In 28 Gbaud QPSK (112 Gb/s) and 16-QAM (224 Gb/s), and 7 Gbaud 64-QAM (84 Gb/s) experiments, it is also demonstrated that M-SSP-PLL + ML can increase the transmission distance by at least 12% compared to BPS for each of them. Finally, the computational complexity is discussed and a significant reduction is shown for our algorithm with respect to BPS.

Experimental demonstration of a 10 Gb/s RSOA-based 16-QAM subcarrier multiplexed WDM PON.

We experimentally demonstrate the performance of a 10 Gb/s subcarrier multiplexed (SCM) wavelength-division multiplexed (WDM) passive optical network (PON) using 2.5 Gbaud 16-QAM transmission signals. Digital signal processing (DSP) and square-root raised cosine (SRRC) pulse shaping enable both the uplink and downlink channels to achieve net spectral efficiencies up to 2.8 bit/s/Hz per channel using reflective semiconductor optical amplifier (RSOA)-based optical network units (ONUs) and economical 10 GHz intensity modulation and direct detection transceivers. We characterize the systems bit error rate (BER) performance over a 20 km single feeder PON with both remote continuous-wave (CW) seeding and full-duplex transmission scenarios, assuming standard forward error correction (FEC) codes.

Dense SS-WDM Over Legacy PONs: Smooth Upgrade of Existing FTTH Networks

We propose a hybrid passive optical network (PON) architecture supporting time-division multiplexing (TDM) and dense spectrum-sliced wavelength-division multiplexing (SS-WDM) over the legacy PON infrastructure. We use a fiber Bragg grating (FBG)-based self-seeded reflective semiconductor optical amplifier (RSOA) transmitter in conjunction with a recently proposed balanced receiver (BR); identical transceiver pairs are placed at the central office and customer side. Self-seeded RSOAs obviate the need for centralized sources, providing a high power, directly modulated source. Intensity noise mitigation of this thermal source is investigated by operating the RSOA in saturation and employing the recently proposed BR. We study the optimal reflectivity for seeding that balances signal power and noise cleaning to achieve the best bit error rate (BER) possible; channel widths are comparable with dense WDM when using coherent sources.

Experimental investigation of the equalization-enhanced phase noise in long haul 56 Gbaud DP-QPSK systems

We experimentally demonstrate the impact of equalization-enhanced phase noise (EEPN) on the performance of 56 Gbaud dual-polarization (DP) QPSK long haul transmission systems. Although EEPN adds additional noise to the received symbols, we show that this reduces the phase variance introduced by the LO laser, and therefore should be considered when designing the carrier phase recovery (CPR) algorithms and estimating system performance. Further, we experimentally demonstrate the performance degradation caused by EEPN when a LO laser with a large linewidth is used at the receiver. When using a 2.6 MHz linewidth distributed feedback (DFB) laser instead of a ~100 kHz linewidth external-cavity laser (ECL) as a LO, the transmission distance is reduced from 4160 km to 2640 km due to EEPN. We also confirm the reduction of the phase variance of the received symbols for longer transmission distances showing its impact on the CPR algorithm optimization when a DFB laser is used at the receiver. Finally, the relationship between the EEPN-induced penalty versus the signal baud rate and the LO laser linewidth is experimentally evaluated, and numerically validated by simulations.

A Standalone Receiver With Multiple Access Interference Rejection, Clock and Data Recovery, and FEC for 2-D

We demonstrate a standalone (no global clock) receiver for two-dimensional wavelength-time optical code-division multiple-access. The receiver provides the following functions: quantization (to eliminate multiple access interference), clock and data recovery, return-to-zero to nonreturn-to-zero conversion (for optical code-division multiple-access compatibility with digital logic), framing (for byte synchronization), and forward-error correction (FEC) using a (255, 239) Reed-Solomon decoder. The receiver more than doubles the number of supported users at a bit-error rate <10-10. The receiver supports an information rate of 156.25 Mb/s. We performed the measurements at a bit rate of 167.4 Mb/s and a chip rate of 1.339 Gb/s (eight chips per bit) to account for FEC overhead

lambda -t

We experimentally demonstrate a 10 Gb/s bidirectional subcarrier multiplexed (SCM) wavelength-division multiplexed (WDM) passive optical network (PON). We use digital signal processing and square-root raised cosine pulse shaping techniques to generate M-ary quadrature amplitude modulation electrical signals for the downlink and uplink transmissions using simple intensity modulation and direct detection optoelectronics, and 10 GHz transceivers. The design includes spectral preemphasis and offset optical filtering to increase the effective bandwidth of a commercial 2.2 GHz reflective semiconductor optical amplifier based optical network unit. We realize symmetric 10 Gb/s transmission over a 20 km single feeder PON with optical line terminal launch powers from 1 to 9 dBm. We also demonstrate the ability of these SCM WDM PONs to withstand the effects of upstream impairments.

OCDMA

We propose a training symbol based channel estimation (TS-EST) algorithm that estimates the 2 × 2 Jones channel matrix. The estimated matrix entries are then used as the initial center taps of the 2 × 2 butterfly equalizer. Employing very few training symbols for TS-EST, ultrafast polarization tracking is achieved and tap update can be initially pursued using the decision-directed least mean squares (DD-LMS) algorithm to mitigate residual intersymbol interference (ISI). We experimentally verify the proposed TS-EST algorithm for 112 Gbps PDM-QPSK and 224 Gbps PDM-16QAM systems using 10 and 40 training symbols for TS-EST, respectively. Steady-state and transient bit error rates (BERs) achieved using the TS-EST algorithm are compared to those obtained using the constant modulus algorithm (CMA) and the training symbol least mean squares (TS-LMS) algorithm and results show that the proposed TS-EST algorithm provides the same steady-state BER with a superior convergence speed. Also, the tolerance of the proposed TS-EST algorithm to laser phase noise and fiber nonlinearity is experimentally verified. Finally, we show by simulation that the superior tracking speed of the TS-EST algorithm allows not only for initial polarization tracking but also for tracking fast polarization transients if four training symbols are periodically sent during steady-state operation with an overhead as low as 0.57%.

Experimental Demonstration of a 10 Gb/s Subcarrier Multiplexed WDM PON

We experimentally investigate the performance of a low-complexity non-iterative phase noise induced inter-carrier interference (ICI) compensation algorithm in reduced-guard-interval dual-polarization coherent-optical orthogonal-frequency-division-multiplexing (RGI-DP-CO-OFDM) transport systems. This interpolation-based ICI compensator estimates the time-domain phase noise samples by a linear interpolation between the CPE estimates of the consecutive OFDM symbols. We experimentally study the performance of this scheme for a 28 Gbaud QPSK RGI-DP-CO-OFDM employing a low cost distributed feedback (DFB) laser. Experimental results using a DFB laser with the linewidth of 2.6 MHz demonstrate 24% and 13% improvement in transmission reach with respect to the conventional equalizer (CE) in presence of weak and strong dispersion-enhanced-phase-noise (DEPN), respectively. A brief analysis of the computational complexity of this scheme in terms of the number of required complex multiplications is provided. This practical approach does not suffer from error propagation while enjoying low computational complexity.