This PDF file contains the front matter associated with SPIE Proceedings Volume 7960, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing

Digital coherent receivers with data-rates of 100 Gbit/s based on dual-polarization quaternary phase shift keying (DPQPSK) have become a reality. One research trend is now directing towards even higher bit-rates of 400 Gbit/s and 1 Tbit/s. However, it is also very desirable to improve the performance of the current basic 100 Gbit/s DP-QPSK. Algorithms have a huge improvement potential and exemplary recent advances will be introduced in this paper.

We review the recent development in real-time coherent optical OFDM (CO-OFDM) transmission for their algorithm and implementation. The unique challenge of real-time implementation of OFDM for high speed optical data transmission including relatively large phase noise, frequency offset and dynamically changing optical channels are discussed. The fundamental digital signal processing (DSP) architectures of transmitter and receiver are presented in the manner achievable in state of the art field-programmable gate arrays (FPGAs) or application-specific integrated circuits (ASICs). Primary DSP components' algorithms and implementations are presented and discussed. The successful demonstration of real-time CO-OFDM receivers includes a receiver with sampling rate of 2.5-Gsamples/s to receive a 3.55-Gb/s single channel and 53.3-Gb/s multi-band CO-OFDM signal.

High speed transmission systems (> 10 Gb/s) for cost-sensitive applications such as metropolitan network have attracted extensive interest due to the explosive data traffic growth in such applications. Optical orthogonal frequency division multiplexing (OFDM) based on direct modulation and direct detection for single-mode fiber (SMF) and multi-mode fiber (MMF) without optical amplification and chromatic dispersion (CD) compensation was proposed. Recent research has also shown that optical OFDM can be used with electronic dispersion compensation using direct detection in SMF. However, laser frequency chirp has been identified as a key limiting factor of capacity-versus-reach performance. In this paper, we present a novel concept of low cost optical OFDM with direct modulation of distributed feedback (DFB) lasers and coherent detection at 51.4 Gb/s and 64 QAM. A comprehensive theoretical model of the proposed system is developed. The proposed optical OFDM system concept and performance is based upon using electronic precompensation of laser frequency response, and electronic post compensation of DFB laser frequency chirp and CD. A numerical simulation of the transmission performance of the aforementioned system is conducted using different fiber lengths (40 km, 60 km, 120 km) and chirp parameters, which shows its attractiveness for access and metro applications.

40 and 100 Gb/s Ethernet services have been recently defined and 400G and 1 Tb/s services are anticipated in the future. Optical transport networks are capable of separating the services provided and the underlying optical transport; service bandwidths may be larger or smaller than the bandwidth carried on a single optical 'wave'. In this work, we compare different arrangements of optical transport systems using polarization division multiplexed (PDM) coherent modulation formats of 4 to 8 bits/symbol for baud rates spanning 8 - 91 GHz. A nonlinear threshold has been defined and nonlinear performances are compared over 20 uncompensated spans of SMF and NZDSF for various modulation formats. The finding is that lower baud rate equates to slightly more reach at equal capacity. For example, for 100G using single carrier vs. dual carrier on a 50 GHz BW, it is observed that for the EDFA-only case, using dual-carrier transmission yields a reach improvement of 5%, whereas in the Raman-assisted EDFA case, a reach improvement of 4% in favor of dual-carrier transmission. This shows that one can achieve the same or better optical performance without having to drive up the baud rate and the speed of associated electro-optics.

We review our recent progress toward 100 Gbps and beyond, focusing on integrated optical devices. Topics include our recently developed integrated optical front-ends for 100 Gbps PDM-QPSK based on multi-channel micro collimator optics and hermetically sealed O/E converters, and PLC-LiNbO3 hybrid optical modulators for 100 Gbps PDM-QPSK. We also describe our recent work on exceeding 100 Gbps, including 64 QAM modulators, modulation-level-selectable modulators, and high-speed digital-analog converter ICs for future multi-level transmissions.

Reliable simulations of high-speed fiber optic links are necessary to understand, design, and deploy fiber networks. Laboratory experiments cannot explore all possible component variations and fiber environments that are found in today's deployed systems. Simulations typically depict relative penalties compared to a reference link. However, absolute performance metrics are required to assess actual deployment configurations. Here we detail the efforts within the Georgia Tech 100G Consortium towards achieving high absolute accuracy between simulation and experimental performance with a goal of ±0.25 dB for back-to-back configuration, and ±0.5 dB for transmission over multiple spans with different dispersion maps. We measure all possible component parameters including fiber length, loss, and dispersion for use in simulation. We also validate experimental methods of performance evaluation including OSNR assessment and DSP-based demodulation. We investigate a wide range of parameters including modulator chirp, polarization state, polarization dependent loss, transmit spectrum, laser linewidth, and fiber nonlinearity. We evaluate 56 Gb/s (single-polarization) and 112 Gb/s (dual-polarization) DQPSK and coherent QPSK within a 50 GHz DWDM environment with 10 Gb/s OOK adjacent channels for worst-case XPM effects. We demonstrate good simulation accuracy within linear and some nonlinear regimes for a wide range of OSNR in both back-to-back configuration and up to eight spans, over a range of launch powers. This allows us to explore a wide range of environments not available in the lab, including different fiber types, ROADM passbands, and levels of crosstalk. Continued exploration is required to validate robustness over various demodulation algorithms.

Rate-adaptive optical transmission techniques adjust information bit rate based on transmission distance and other factors affecting signal quality. These techniques enable increased bit rates over shorter links, while enabling transmission over longer links when regeneration is not available. They are likely to become more important with increasing network traffic and a continuing evolution toward optically switched mesh networks, which make signal quality more variable. We propose a rate-adaptive scheme using variable-rate forward error correction (FEC) codes and variable constellations with a fixed symbol rate, quantifying how achievable bit rates vary with distance. The scheme uses serially concatenated Reed-Solomon codes and an inner repetition code to vary the code rate, combined with singlecarrier polarization-multiplexed M-ary quadrature amplitude modulation (PM-M-QAM) with variable M and digital coherent detection. A rate adaptation algorithm uses the signal-to-noise ratio (SNR) or the FEC decoder input bit-error ratio (BER) estimated by a receiver to determine the FEC code rate and constellation size that maximizes the information bit rate while satisfying a target FEC decoder output BER and an SNR margin, yielding a peak rate of 200 Gbit/s in a nominal 50-GHz channel bandwidth. We simulate single-channel transmission through a long-haul fiber system incorporating numerous optical switches, evaluating the impact of fiber nonlinearity and bandwidth narrowing. With zero SNR margin, we achieve bit rates of 200/100/50 Gbit/s over distances of 650/2000/3000 km. Compared to an ideal coding scheme, the proposed scheme exhibits a performance gap ranging from about 6.4 dB at 650 km to 7.5 dB at 5000 km.

Polarization multiplexing is an integral technique for generating spectrally efficient 100 Gb/s and higher optical links. Post coherent detection DSP-based polarization demultiplexing of QPSK links is commonly performed after timing recovery. We propose and demonstrate a method of asynchronous blind source separation using the constant modulus algorithm (CMA) on the asynchronously sampled signal to initially separate energy from arbitrarily aligned polarization states. This method lends well to implementation as it allows for an open-loop sampling frequency for analog-to-digital conversion at less than twice the symbol rate. We show that the performance of subsequent receiver functions is enhanced by the initial pol demux operation. CMA singularity behavior is avoided through tap settling constraints. The method is applicable to QPSK transmissions and many other modulation formats as well, including general QAM signals, offset-QPSK, and CPM, or a combination thereof. We present the architecture and its performance under several different formats and link conditions. Comparisons of complexity and performance are drawn between the proposed architecture and conventional receivers.

Interchannel nonlinear impairments are one of the major limitations to the channel capacity and transmission distance in WDM systems. It is shown that nonlinear impairments arising from cross-phase modulation between two independent WDM channels can be compensated. Advanced digital back propagation algorithms based on advanced split step method allow for compensating inter-channel nonlinear impairments with low complexity. With the new algorithm, the complexity of compensating inter-channel impairments is comparable to the complexity required for intra-channel impairments.

The nonlinear effect of Mach-Zehnder modulator (MZM) in optical fiber under the presence high peak power of optical orthogonal frequency division multiplexing (OFDM) is investigated. A full optical coherent communication system is presented and analyzed numerically. A method to mitigate the nonlinear effect by means of digital pre-distortion is proposed. Inclusive quantitative analysis of the effect of peak to average power (PAPR) reduction on the performance of the proposed Optical-OFDM system is presented.

Nonlinear refraction in fiber optic links is a capacity limiting mechanism, whereby the phase of each propagating signal is modulated by intensity variations of signals in nearby channels. The transition to coherent detection enables a wide variety of modulation formats to be considered. Indeed, the choice of modulation format plays a primary role in determining the degree of amplitude variation in the channel as well as the robustness to the phase noise impairment that nonlinearities induce. On one hand, constant envelope formats (or nearly-constant) avoid fluctuations in the signal and produce lower nonlinearity-based impairments. Alternatively, star-QAM modulation formats enhance the receiver's robustness to phase noise. Using simulated and experimental results we demonstrate the effectiveness of each format in avoiding fiber nonlinearity effects for both standard fiber (17ps/nm-km) and NZDF (5 ps/nm-km). We show sensitivity of several formats to nonlinear phase modulation from adjacent channels. We show the interaction between dispersion and constant envelope formats that guides the applications in which constant envelope formats, such as continuous phase modulation (CPM) provide gain over non-constant formats, such as QPSK. Consideration is made to scaling to 100 Gb/s and beyond in practical implementations.

Higher-order multi-level modulation formats are very attractive for achieving the high spectral efficiency and high speed channels needed to accommodate ultra-high speed client signals on the optical transport network (OTN). In particular, quadrature amplitude modulation (QAM) is a promising modulation technique to achieve the high spectral efficiency with PDM. However, required OSNR is increased and transmission distance is restricted as the number of signal point increase. Moreover, system requirements, such as laser line-width, ADC/DAC resolution, and circuit linearity, become severe. We recently demonstrated the 3000-km-class long-haul transmission of a single channel 160 Gb/s 16-QAM signal. We employed three key technologies; optical 16-QAM signal synthesis by superposing two optical QPSK signals, proposed pilot-less detection scheme with digital PLL-based frequency offset compensator and OSNR improvement by ultra low-loss fiber and EDFA/distributed Raman amplification. In this paper, we review system configurations for higher-order QAM, and then describe the single channel transmission performance of 16-QAM.

Polarization multiplexing and quadrature phase shift keying (QPSK) both double spectral efficiency. Combined with synchronous coherent polarization diverse intradyne receivers this modulation format is ultra-robust and cost-efficient. A feedforward carrier recovery is required in order to tolerate phase noise of normal DFB lasers. Signal processing in the digital domain permits compensation of at least chromatic and polarization mode dispersion. Some companies have products on the market, others are working on them. For 100 GbE transmission, 50 GHz channel spacing is sufficient. 16ary quadrature amplitude modulation (16-QAM) is attractive to double capacity once more, possibly in a modulation format flexible transponder which is switched down to QPSK only if system margin is too low. For 16-QAM the phase noise problem is sharply increased. However, also here a feedforward carrier recovery has been implemented. A number of carrier phase angles is tested in parallel, and the recovered data is selected for that phase angle where squared distance of recovered data to the nearest constellation point, averaged over a number of symbols, is minimum. An intradyne/selfhomodyne synchronous coherent 16-QAM experiment (2.5 Gb/s, 81 km) is presented.

The step-index polymer optical fiber (SI-POF) is an attractive transmission medium for high speed communication links in automotive infotainment networks, in industrial automation, and in home networks. Growing demands for quality of service, e.g., for IPTV distribution in homes and for Ethernet based industrial control networks will necessitate Gigabit speeds in the near future. We present an overview on recent advances in the design of spectrally efficient and robust Gigabit-over-SI-POF transmission systems.

In this paper we propose a novel architecture for the optical generation of multiple RF signals based on the beating of couples of modes from a mode-locked laser in a single photodiode. We generate simultaneously two carriers at 10GHz and 30GHz, with very low phase noise, limited amplitude fluctuations, negligible interference (<-30dB) between each other, and equal time jitter. The obtained results show that the proposed architecture is a promising scheme for the generation of single or multiple high purity carriers up to extremely high frequency (W band), far beyond the limits of electronic oscillators.

A fiber-coupled superconducting nanowire single photon detector is presented considering practical applications. The system performances, such as dark count rate, system efficiency and noise equivalent power, are discussed. A system efficiency of 3% (1550 nm) and a dark count rate of 10 Hz at 4.2 K were achieved in experiment.

Time resolved photon counting was used to separate the different photon states emitted from a strongly attenuated laser source. We first describe a method to quantify the efficiency of our gated avalanche photo-detector, by relying on known Poissonian statistics. The detector was then optimized under different temperature and bias voltage conditions using the noise equivalent power as a metric. Finally, coherent pulses are sent into a ring cavity, such that the tapped output from the cavity forms a series of time multiplexed pulses, which then yield the photon counting statistics. We observed good agreement between theoretical estimates and experimental observations, to as low as 0.01% probability of detection.