Front Matter: Volume 8647

Successful joint experiments with Deutsche Telecom (DT) on long-haul transmission of 100G and beyond are demonstrated over standard single mode fiber (SSMF) and inline EDFA-only amplification. The transmission link consists of 8 nodes and 950-km installed SSMF in DT’s optical infrastructure with the addition of lab SSMF for extended optical reach. The first field transmission of 8×216.4-Gb/s Nyquist-WDM signals is reported over 1750- km distance with 21.6-dB average loss per span. Each channel modulated by 54.2-Gbaud PDM-CSRZ-QPSK signal is on 50-GHz grid, achieving a net spectral efficiency (SE) of 4 bit/s/Hz. We also demonstrate mixed data-rate transmission coexisting with 1T, 400G, and 100G channels. The 400G uses four independent subcarriers modulated by 28-Gbaud PDM-QPSK signals, yielding the net SE of 4 bit/s/Hz while 13 optically generated subcarriers from single optical source are employed in 1T channel with 25-Gbaud PDM-QPSK modulation. The 100G signal uses real-time coherent PDM-QPSK transponder with 15% overhead of soft-decision forward-error correction (SD-FEC). The digital post filter and 1-bit maximum likelihood sequence estimation (MLSE) are introduced at the receiver DSP to suppress noise, linear crosstalk and filtering effects. Our results show the future 400G and 1T channels utilizing Nyquist WDM technique can transmit long-haul distance with higher SE using the same QPSK format.

We will show the capabilities of optical modes known as optical vortices in order to perform mode-division multiplexing (MDM) propagation with a different approach. In particular, we will propose optical vortices not only as an alternative way to increase the fiber capacity, but also as a cost-effective and “green” solution able to reduce power dissipation in high capacity systems with respect to solutions based on coherent detection and computationally complex digital MIMO processing, employed in MDM with usual LP modes. Two different proposals based on the all-optical spatial mode demultiplexing and on a non-linear MIMO receiver can allow to employ direct detection and to exploit optical vortices in very simple systems useful for short and medium distances links, where the cost reduction and energy sustainability are mandatory.

In this paper, we present the measurement results of a spectrally efficient 2.56Tb/s free-space data link using orbital angular momentum (OAM) beams. This link includes 32 independent 20 Gbaud/s 16-quadrature-amplitude-modulation data streams, each encoded on a different OAM beam, which are mode-, polarization- and space-division multiplexed as one collocated beam. We measured the bit-error rate (BER) curves of all 32 channels, all of which can achieve a BER of <2×10-3. The performance degradation due to the spatial multiplexing using concentric ring scheme is analyzed. Additionally, the effect of the pre-filtering is investigated, and negligible penalty is observed.

We demonstrate wavelength-division multiplexed (WDM) and mode-division multiplexed (MDM) transmission over a fiber recirculating loop comprising of a 25-km span of low differential mode group dispersion (DMGD) few-mode fiber carrying the LP01 and LP11 mode groups, and an inline few-mode erbium-doped fiber amplifier (FM-EDFA) providing low mode-dependent gain (MDG) per span. We successfully transmitted a 10λ × 6 × 28-Gbaud QPSK signal over a distance of 700 km.

This paper introduces our recent results on mode-division multiplexing transmission with MIMO processing. We have been studying coherent optical MIMO transmission systems and developing few-mode fibers to reduce the complexity of MIMO processing, for example, by using multi-step index fibers to control the differential mode delay (DMD) of the fibers and to compensate for the total DMD. We also investigated a transmission system using reduced-complexity MIMO processing. Finally, we review our latest 2×2 WDM-MIMO transmission experiments with low MIMO processing complexity.

We propose a volume holographic mode demultiplexer incorporating a dual-wavelength method, which enables us to easily and flexibly set up the receiving system in mode division multiplexing for a large number of mode multiplexing. This demultiplexer can separate a lot of multiplexed modes with different wavelengths through angularly hologram multiplexing by appropriate angular difference between the two holographic writing beams. Thereby, the proposed method can be applicable to wavelength division multiplexing systems. In this study, we demonstrated the mode separation using the proposed demultiplexer with dual-wavelength. The results showed that the separation ratios of three LP modes reached around 90%.

We discuss the modeling of linear and nonlinear propagation in multi-mode optical fibers in the context of optical communications. A generalized Stokes space representation is introduced for handling multi-mode fiber propagation in the presence of mode coupling. Using this formalism, we define the modal dispersion vector and characterize its statistics. We also show that nonlinear propagation in the presence of random mode coupling is described by coupled multi-component Manakov equations, giving rise to interesting new physical phenomena.

The characteristics of a multicore fiber with one-ring structure are reviewed. The one-ring structure, which has no center core, can overcome issues on the hexagonal close-pack structure that is the most popular multicore structure. The onering structure has flexibility in the number of cores and is unrelated to the core pitch limitation due to cutoff wavelength lengthening thank to no center core structure. The one-ring structure is effective to suppress the worst case crosstalk that is crosstalk assuming all cores carry equal signal power. In the case of hexagonal close-pack structure, the worst case crosstalk of an inner core is 7.8 dB larger than that between two cores. The different worst crosstalk is observed depending on the number of nearest neighbor cores. The one-ring structure can limit the degradation to 3.0 dB for all cores. Fabricated 12-core fiber with the one-ring structure based on the simulation realized effective core area of 80 m2 and very low crosstalk less than -40 dB after 100-km propagation.

Today and next generation optical coherent systems rely more and more in DSP algorithms to improve capacity, spectral efficiency and fiber impairments mitigation. The amount of signal processing is remarkable, and because of that ASICs are preferable in order to comply with cost, power consumption and size, required in OIF 100G optical module standards. One important step in the ASIC development process is the validation of the DSP algorithms mathematical models in a high level language that consider HW characteristics and constrains. In this work we present, compare and evaluate in experimental data the mathematical model developed in Matlab and the SystemC model developed in C++. The DSP algorithms functionalities implemented were orthonormalization, CD equalizer, clock recovery, dynamic equalizer, frequency offset and phase estimation. The SystemC model considers clock signals, reset/enable structures, parallelization, finite fixed-point operations and structures that are closer to the ASIC HW implementation; due to these restrictions the performance is not as good as the mathematical modeling. The DSP algorithms models are evaluated in two 112 Gbit/s DP-QPSK experimental scenarios. In the first scenario the models are evaluated in back-to-back with ASE noise loading; in the second scenario the models are compared in a 226km optical fiber recirculation loop, with 80x112 Gbit/s DP-QPSK channels (8.96 Tbit/s). In the back-to-back experiment the OSNR penalty from the mathematical model to the SystemC model is only 1,0dB and in the recirculation loop the maximum reach is 2,600 km and 2,200 km for the Matlab and SystemC models respectively.

This paper reports on the design, fabrication and testing of quasi-phase-matched (QPM) lithium niobate electro-optic modulators optimized for the 40-60 GHz frequency range. The device used a single-drive, coplanar-waveguide (cpw) electrode structure that provided a good balance between impedance and RF loss, and a DC Vπ.L product of approximately 10 V.cm. Ferroelectric domain engineering enabled push-pull operation with a single drive, while achieving low chirp. A custom developed pulsed poling process was used to fabricate periodic domain QPM structures in lithium niobate. QPM periods were in the range of 3 mm to 4.5 mm, depending on the design frequency. The pulse method enabled precise domain definition with a minimum of overpoling. Low-loss diffused optical waveguides were fabricated by an annealed proton exchange (APE) process. By operating in both co-propagating and counter-propagating modes, the QPM devices can be used to implement dual band RF bandpass filters simultaneously covering both 10-20 GHz and 40-60 GHz frequency bands. Arrays of QPM device structures demonstrated in this work form the basis for a reconfigurable RF photonic filter. The RF photonic QPM technology enables efficient concurrent antenna remoting and filtering functionality. Applications of the technology include fiber radio for cellular access and finite impulse response filters for wideband electronic warfare receivers.

Optical propagation modeling is being pushed to the limits as the usage of optical fiber bandwidth is taken to the limits. Also, the widespread deployment of Passive Optical Networks (PON) requires extra power budgets, which are normally achieved by increased laser optical power or amplification. In these conditions the nonlinear effects become an extra impairment factor, which has to be brought to attention. Furthermore, while compensating their impacts into propagation by means of back-propagation the precise definition of their impact and magnitude is required. In this work we will observe the potential and validity of the Volterra series when applied to both high powers, high channel densities and back-propagation conditions.

We introduce a highly efficient high performance architecture for the digital signal processing of high speed coherent optical receivers. Our ASIC signal processing architecture ports for the first time to optical reception efficient filter bank signal processing structures. The resulting optical receiver ASICs are applicable to long-haul and metro photonic communication and provide substantial energy efficiency saving 30%-50% in power consumption. We aim to develop an ultra-high-speed optical receiver ASIC for transporting 160 Gb/s in a 25 GHz optical band - seven such channels will together carry 1Tb/s (plus overhead) in our 'TeraSanta' project of TeraBitPerSecond efficient transponders.

The high complexity of conventional intra-channel nonlinearity compensation algorithms, such as back-propagation, is considered as the major obstacle for the implementation. To reduce the complexity, perturbation analysis is applied because it considers multi-span transmission as one stage. In those perturbation based algorithms, such as perturbation back-propagation (PBP) and perturbation pre-distortion, the number of required compensation stage is much less than that of conventional back-propagation. To reduce the complexity further, the multi-tap finite impulse response filter (FIR) in PBP is replaced with one-tap infinite impulse response (IIR) filter. The number of required compensation stage of IIR PBP is only 15% of conventional back-propagation, whereas the complexity of each stage is almost same. In perturbation pre-distortion, the proposed perturbation combination reduces the number of terms from 19732 to 41, whereas no performance degradation is observed.

The digitally modulated signals spectral density depends directly on pulse format used for information symbols transmission. The modulated signal spectral occupancy can be modified according to the channel frequency response to facilitate information retrieval at the receiver. New generation of coherent optical transmission systems operating at high rates are subject to various bandwidth restrictions aspects, such as electronic components limitations and optical filtering via ROADMs deployed on networks. As noted in technical literature, the RZ pulse formats have some advantages compared to traditional NRZ pulses in optical fiber transmissions. In particular, RZ pulses have a better performance in situations where nonlinear effects of the fiber severely impact the quality of transmission. Among other situations, this occurs in systems that employ modulation formats for high order QAM (16QAM, 64QAM, etc.). Moreover, since RZ pulses have shorter duty cycle, temporal spread of the transmitted symbols causes less performance degradation due to ISI compared with NRZ pulses. This report presents results of experiments carried out in a 226 km recirculation loop, to evaluate the performance of NRZ, RZ 67%, 50% RZ and RZ 33% pulse shapes in a transmission of DP-16QAM (or PDM-16QAM). As application it is proposed and experimentally demonstrated a transmission system that employ 28 GBaud dual carrier PDM-16QAM channels operating with a total line rate of 448 Gb/s each, utilizing RZ pulse format and carrier narrow pre-filtering to increase spectral efficiency of transmission, aggregating a 400G channel in a 75 GHz WDM grid.

In this paper we numerically study the coherence properties of the supercontinuum generated in a lead-silicate microstructured fiber taper, with an increasing core radius along the propagation distance which tailors the dispersion property. Simulations are conducted by adding quantum noise into the input pulse at 1.55 μm, and the complex degree of first-order coherence function and the overall spectral coherence degree are both calculated. Although the spectral broadening is comparable, the coherence degree is shown to vary with different pumping conditions. It decreases with higher peak power and longer duration due to the significant competition between the soliton-fission process and the noise-seeded modulation instability. By controlling the input pulse parameters, it is possible to generate perfectly coherent supercontinuum with a flat broadened spectrum extending to ~5μm in this fiber taper.

A scheme named “spoof” four wave mixing (SFWM) is proposed, where a dynamic refractive index grating induced by the beating of the co-propagating pump and signal is able to modulate a Bragg grating (BG) to create additional reflective peaks (ARPs) at either side of the unperturbed BG bandgap. When a probe wave located at the wavelength of ARPs is counter-propagating, it is reflected from the induced ARPS while tracking the signal data information but at the new wavelength. In contrast to the well-known FWM, where the induced dynamic refractive index grating modulates photons to create a wave at a new frequency, the SFWM is different in that the dynamic refractive index grating is generated in a nonlinear BG to excite ARPS at either side of the original BG bandgap in reflection spectrum. This fundamental difference enable the SFWM to avoid the intrinsic shortcoming of stringent phase matching required in the conventional FWM, and allows novel all-optical wavelength conversion with modulation format transparency and ultrabroad conversion range, which represents a major advantage for next generation of all-optical networks.