Showing papers in "Journal of Lightwave Technology in 2012"
TL;DR: In this article, simultaneous transmission of six spatial and polarization modes, each carrying 40 Gb/s quadrature-phase-shift-keyed channels over 96 km of a low-differential group delay few-mode fiber, is reported.
Abstract: We report simultaneous transmission of six spatial and polarization modes, each carrying 40 Gb/s quadrature-phase-shift-keyed channels over 96 km of a low-differential group delay few-mode fiber. The channels are successfully recovered by offline DSP based on coherent detection and multiple-input multiple-output processing. A penalty of ;28 dB.
901 citations
TL;DR: This paper is devoted to an in-depth discussion of the Gaussian Noise model which describes non-linear propagation in uncompensated coherent transmission systems and its implications as to system and networks design and optimization are discussed.
Abstract: This paper is devoted to an in-depth discussion of the Gaussian Noise (GN) model which describes non-linear propagation in uncompensated coherent transmission systems. Similar models and validation efforts are reviewed. Then, the main equations of the GN model are introduced. An intuitive physical interpretation of the equations and their features is proposed. The main characteristics of the non-linear interference (NLI) noise spectra that the GN model produces are discussed. To ease model exploitation, a new formulation in hyperbolic coordinates is proposed, which makes numerical integration faster. New approximate closed-form solutions are also provided. An extension of the GN model to distributed-amplification scenarios is introduced. NLI noise accumulation versus distance and bandwidth are studied in depth. Finally, the GN model implications as to system and networks design and optimization are discussed.
690 citations
TL;DR: General OFDM principles, including orthogonality, cyclic prefix use, frequency-domain equalization, and multiuser OFDMA are summarized, followed by an overview of various optical OFDM(A) transceiver architectures for next-generation PON.
Abstract: In this tutorial overview, the principles, advantages, challenges, and practical requirements of optical orthogonal frequency division multiplexing (OFDM)-based optical access are presented, with an emphasis on orthogonal frequency division multiple access (OFDMA) for application in next-generation passive optical networks (PON). General OFDM principles, including orthogonality, cyclic prefix use, frequency-domain equalization, and multiuser OFDMA are summarized, followed by an overview of various optical OFDM(A) transceiver architectures for next-generation PON. Functional requirements are outlined for high-speed digital signal processors (DSP) and data converters in OFDMA-PON. A techno-economic outlook for such a “software-defined,” DSP-based optical access platform is also provided.
511 citations
TL;DR: In this paper, perturbative models for the impact of nonlinear propagation in uncompensated links were proposed and analyzed for a set of formats including PM-BPSK, PM-QPSK and PM-8QAM.
Abstract: We address perturbative models for the impact of nonlinear propagation in uncompensated links. We concentrate on a recently-proposed model which splits up the signal into spectral components and then resorts to a four-wave-mixing-like approach to assess the generation of nonlinear interference due to the beating of the signal spectral components. We put its founding assumptions on firmer ground and we provide a detailed derivation for its main analytical results. We then carry out an extensive simulative validation by addressing an ample and significant set of formats encompassing PM-BPSK, PM-QPSK, PM-8QAM, and PM-16QAM, all operating at 32 GBaud. We compare the model prediction of maximum system reach and optimum launch power versus simulation results, for all four formats, three different kinds of fibers (PSCF, SMF, and NZDSF) and for several values of WDM channel spacing, ranging from 50 GHz down to the symbol-rate. We found that, throughout all tests, the model delivers accurate predictions, potentially making it an effective general-purpose system design tool for coherent uncompensated transmission systems.
417 citations
IBM1
TL;DR: Optical printed circuit boards and silicon based integrated photonics are potential technologies to meet challenges of large scale high performance computing systems and to greatly improve in cost, power, areal density and reliability.
Abstract: High-performance computing systems are of steadily growing interest to provide new levels of computational capability for an increasing range of applications. The growing use of and dependence on optical interconnects to meet these system's scaling bandwidth demands has given rise to “computercom” as a distinct market segment, alongside the traditional datacom and telecom markets. This paper discusses the trends, requirements, tradeoffs, and potential technologies for this market.
408 citations
TL;DR: New technologies that facilitate multiple access beyond 10 Gb/s time division multiple access (TDMA)-PONs will be reviewed, with particular focus on the motivation, key technologies, and deployment challenges.
Abstract: This paper reviews the future directions of next generation passive optical networks. A discussion on standardized 10 Gb/s passive optical network (PON) systems is first presented. Next, new technologies that facilitate multiple access beyond 10 Gb/s time division multiple access (TDMA)-PONs will be reviewed, with particular focus on the motivation, key technologies, and deployment challenges. The wavelength division multiplexed (WDM) PON will be discussed and in combination with TDMA, the hybrid WDM/TDMA PON will be reviewed in the context of improving system reach, capacity, and user count. Next, discussions on complementary high-speed technologies that provide improved tolerance to system impairments, capacity, and spectral efficiency will be presented. These technologies include digital coherent detection, orthogonal frequency division multiple access (OFDMA), and optical code division multiple access (OCDMA).
361 citations
TL;DR: In this article, the authors review high-spectral-efficiency optical modulation formats for use in digital coherent systems and highlight important trade-offs pertaining to the design and performance of coherent higher-order QAM transponders.
Abstract: As 100-Gb/s coherent systems based on polarization- division multiplexed quadrature phase shift keying (PDM-QPSK), with aggregate wavelength-division multiplexed (WDM) capacities close to 10 Tb/s, are getting widely deployed, the use of high-spectral-efficiency quadrature amplitude modulation (QAM) to increase both per-channel interface rates and aggregate WDM capacities is the next evolutionary step. In this paper we review high-spectral-efficiency optical modulation formats for use in digital coherent systems. We look at fundamental as well as at technological scaling trends and highlight important trade-offs pertaining to the design and performance of coherent higher-order QAM transponders.
360 citations
TL;DR: In this article, a general perturbation theory of the propagation of a signal in an optical fiber in the presence of amplification and Kerr nonlinearity was developed, valid for arbitrary pulse shapes.
Abstract: In this paper, we develop a general first-order perturbation theory of the propagation of a signal in an optical fiber in the presence of amplification and Kerr nonlinearity, valid for arbitrary pulse shapes. We obtain a general expression of the sampled signal after optical filtering, coherent detection, and optimal sampling. We include intrachannel and as well as interchannel nonlinear effects. We obtain simplified expressions in the case in which the accumulated dispersion is high (equivalent to the far-field limit in paraxial optics). This general theory is applied in detail to the special case of spectral-efficient sinc pulses. This exercise shows that the characteristics of the neighboring wavelength-division multiplexed channels are essential in determining the nonlinear impairments.
357 citations
TL;DR: Staircase codes, a new class of forward-error-correction (FEC) codes suitable for high-speed optical communications, are introduced, and an ITU-T G.709-compatible staircase code with rate R = 239/255 is proposed, exhibiting a net coding gain and an error floor analysis technique.
Abstract: Staircase codes, a new class of forward-error-correction (FEC) codes suitable for high-speed optical communications, are introduced. An ITU-T G.709-compatible staircase code with rate R = 239/255 is proposed, and field-programmable-gate-array-based simulation results are presented, exhibiting a net coding gain of 9.41 dB at an output error rate of 10-15, an improvement of 0.42 dB relative to the best code from the ITU-T G.975.1 recommendation. An error floor analysis technique is presented, and the proposed code is shown to have an error floor at 4.0 × 10-21.
315 citations
TL;DR: In this paper, the authors reported the high-frequency modulation of individual pixels in 8 × 8 arrays of III-nitride-based micro-pixellated light-emitting diodes, where the pixels within the array range from 14 to 84 μ m in diameter.
Abstract: We report the high-frequency modulation of individual pixels in 8 × 8 arrays of III-nitride-based micro-pixellated light-emitting diodes, where the pixels within the array range from 14 to 84 μ m in diameter. The peak emission wavelengths of the devices are 370, 405, 450 and 520 nm, respectively. Smaller area micro-LED pixels generally exhibit higher modulation bandwidths than their larger area counterparts, which is attributed to their ability to be driven at higher current densities. The highest optical -3 dB modulation bandwidths from these devices are shown to be in excess of 400 MHz, which, to our knowledge, are the highest bandwidths yet reported for GaN LEDs. These devices are also integrated with a complementary metal-oxide-semiconductor (CMOS) driver array chip, allowing for simple computer control of individual micro-LED pixels. The bandwidth of the integrated micro-LED/CMOS pixels is shown to be up to 185 MHz; data transmission at bit rates up to 512 Mbit/s is demonstrated using on-off keying non return-to-zero modulation with a bit-error ratio of less than 1 × 10-10, using a 450 nm-emitting 24 μm diameter CMOS-controlled micro-LED. As the CMOS chip allows for up to 16 independent data inputs, this device demonstrates the potential for multi-Gigabit/s parallel data transmission using CMOS-controlled micro-LEDs.
295 citations
TL;DR: In this paper, a two-mode (de)multiplexer based on Si nanowire for mode-division multiplexing is designed, which is composed of two multimode interference (MMI) waveguides and a butterfly-shape tapered phase shifter between two MMI waveguiders.
Abstract: A compact two-mode (de)multiplexer (TM-MUX) based on Si nanowire for mode-division multiplexing is designed. The TM-MUX is composed of two multimode interference (MMI) waveguides and a butterfly-shape tapered phase shifter between two MMI waveguides. Numerical simulations show that the designed device is compact compared with conventional-mode (de)multiplexer and operates as a TM-MUX in the whole C-band. In addition, we show that the designed TM-MUX has relatively good fabrication tolerance.
TL;DR: One and two dimensional forms of carbon, Carbon nanotube and graphene, have interesting and useful not only electronic but also photonic properties as discussed by the authors, and we refer the reader to the tutorial.
Abstract: One and two dimensional forms of carbon, Carbon nanotube and graphene, have interesting and useful not only electronic but also photonic properties. This tutorial will review their photonic properties, linear and nonlinear, and applications of nonlinear photonic properties as laser mode lockers and nonlinear functional devices.
TL;DR: In this paper, the authors demonstrate one of the first experiments of optical transmission based on mode-division multiplexing over a few-mode optical fiber using a programmable liquid crystal on silicon panel.
Abstract: We demonstrate one of the first experiments of optical transmission based on mode-division multiplexing over a few-mode optical fiber. The mode multiplexer and demultiplexer are based on a programmable liquid crystal on silicon panel. Using this system, we transmit two 100 Gb/s polarization division multiplexed quaternary phase-shift keying data streams modulated on two different modes of the prototype few-mode fiber. At the receiver, a set of optical coherent detectors with DSP including multiple-input multiple-output algorithms recover the signal and permit to mitigate the crosstalk stemming from imperfect mode conversion.
TL;DR: The disaster protection scheme exploiting anycasting provides more protection, but uses less capacity than dedicated single-link failure protection, and it is shown that a reasonable number of datacenters and selective content replicas with intelligent network design can provide survivability to disasters while supporting user demands.
Abstract: Survivability against disasters-both natural and deliberate attacks, and spanning large geographical areas-is becoming a major challenge in communication networks. Cloud services delivered by datacenter networks yield new opportunities to provide protection against disasters. Cloud services require a network substrate with high capacity, low latency, high availability, and low cost, which can be delivered by optical networks. In such networks, path protection against network failures is generally ensured by providing a backup path to the same destination (i.e., a datacenter), which is link-disjoint to the primary path. This protection fails to protect against disasters covering an area which disrupts both primary and backup paths. Also, protection against destination (datacenter) node failure is not ensured by a generic protection scheme. Moreover, content/service protection is a fundamental problem in a datacenter network, as the failure of a datacenter should not cause the disappearance of a specific content/service from the network. So content placement, routing, and protection of paths and content should be addressed together. In this work, we propose an integrated Integer Linear Program (ILP) to design an optical datacenter network, which solves the above-mentioned problems simultaneously. We show that our disaster protection scheme exploiting anycasting provides more protection, but uses less capacity than dedicated single-link failure protection. We show that a reasonable number of datacenters and selective content replicas with intelligent network design can provide survivability to disasters while supporting user demands. We also propose ILP relaxations and heuristics to solve the problem for large networks.
TL;DR: A mixed integer linear programming model is developed to optimize the physical topology of IP over WDM networks with the objective of minimizing the network total power consumption and the results show that optimizing the physicalTopology increases the utilization of the renewable energy sources.
Abstract: The energy consumption of information and communications technology networks is increasing rapidly as a result of the Internet expansion in reach and capacity. In this paper, we investigate energy-efficient physical topologies for backbone IP over wavelength-division multiplexing (WDM) networks. We develop a mixed integer linear programming model to optimize the physical topology of IP over WDM networks with the objective of minimizing the network total power consumption. We consider the National Science Foundation network topology and compare its energy consumption with the energy consumption of optimized physical topologies under different IP over WDM approaches and nodal degree constraints. We study the physical topology optimization under a symmetric full-mesh connectivity traffic matrix and an asymmetric traffic demand, where data centers create a hot node scenario in the network. We also investigate the power savings obtained by deploying topologies that eliminate the need for IP routers, including a full-mesh topology and a star topology. Simulation results show that the full-mesh and star topologies result in significant power savings of 95% and 92%, respectively. Furthermore, the optimization of the physical topology is investigated considering the presence of renewable energy sources in the network. The results show that optimizing the physical topology increases the utilization of the renewable energy sources.
TL;DR: In this article, a transmission fiber for mode division multiplexing supporting LP01 and LP11 modes, with low differential group delay, low mode coupling, and low loss for both modes is presented.
Abstract: A transmission fiber for mode division multiplexing supporting LP01 and LP11 modes, with low differential group delay, low mode coupling, and low loss for both modes is presented. Spatially and spectrally resolved mode imaging (S2 imaging) is used for characterization.
TL;DR: In this paper, a detailed analysis of the modal properties, dispersive behavior, and nonlinear mode coupling in graded-index multimode fibers (GIMFs), and a simplified form of a generalized nonlinear Schrodinger equation, which can be used to explore the rich nonlinear dynamics related to the propagation and interaction of light pulses in GIMFs, is presented.
Abstract: We present a detailed analysis of the modal properties, dispersive behavior, and nonlinear mode coupling in graded-index multimode fibers (GIMFs), and lay out a simplified form of a generalized nonlinear Schrodinger equation, which can be used to explore the rich nonlinear dynamics related to the propagation and interaction of light pulses in GIMFs in a tractable manner. We also briefly discuss an application of the presented formalism in the study of four-wave mixing in these fibers. While the reported formalism is fairly general, our presentation is mainly targeted at device applications in which short segments of GIMFs are used.
TL;DR: In this paper, the authors experimentally investigate the response time of humidity sensors based on polymer optical fiber Bragg gratings and show that using a reduced diameter of 135 microns reduces the diffusion time of water into the polymer and hence speed up the relative wavelength change caused by humidity variations.
Abstract: In this work we experimentally investigate the response time of humidity sensors based on polymer optical fiber Bragg gratings. By the use of etching with acetone we can control the poly (methyl methacrylate) based fiber in order to reduce the diffusion time of water into the polymer and hence speed up the relative wavelength change caused by humidity variations. A much improved response time of 12 minutes for humidity decrease and 7 minutes for humidity increase, has been achieved by using a polymer optical fiber Bragg grating with a reduced diameter of 135 microns.
TL;DR: In this article, the first terabit PON based on hybrid WDM orthogonal frequency division multiple access (OFDMA) technology is proposed and experimentally verified, which enables high-speed, long-reach transmission with simplified optical network unit (ONU)-side digital signal processing, multiband OFDMA with ONU-side sub-band selectivity.
Abstract: Next-generation optical access networks are envisioned to evolve into a converged, high-speed, multiservice platform supporting residential, business, mobile backhaul, and special purpose applications. Moreover, bandwidth demand projections suggest that terabit aggregate capacity may need to be reached in such next-generation passive optical networks (PON). To satisfy these requirements while leveraging the large investments made in existing fiber plants, a wavelength division multiplexed (WDM)-based long-reach PON architecture combined with a multiple access technology that features a passive last-mile split, large per-λ speeds, and statistical bandwidth multiplexing can be exploited. In this paper, the first terabit PON based on hybrid WDM orthogonal frequency division multiple access (OFDMA) technology is proposed and experimentally verified. To enable high-speed, long-reach transmission with simplified optical network unit (ONU)-side digital signal processing, multiband OFDMA with ONU-side sub-band selectivity is proposed. Design challenges and tradeoffs between analog and digital domain sub-band combining and selection are also discussed. Finally, the experimental setup and results of the first 1.2 Tb/s (1 Tb/s after overhead) symmetric WDM-OFDMA-PON over 90 km straight single-mode fiber and 1:32 passive split, featuring multiband OFDMA, digitally selective ONUs, and a coherent-receiver OLT are presented and analyzed. By supporting up to 800 ONUs with 1.25/10 Gb/s guaranteed/peak rates and exhibiting a record rate-distance product achieved in long-reach PON, the demonstrated architecture may be viewed as promising for future converged terabit optical metro/access.
TL;DR: In this paper, a new electronic nonlinearity compensation scheme based on inverse Volterra series transfer function (VSTF) is proposed, which shows 1 dB improvement in Q-factor with a 256 Gb/s polarization-division-multiplexed 16-level quadratic amplitude modulation format.
Abstract: The Volterra series transfer function (VSTF), in which the input-output relationship of a nonlinear system is represented by a series of nonlinear kernel functions, is an elegant tool to model nonlinear systems. The inverse of a nonlinear system can be constructed by analyzing VSTF. We propose a new electronic nonlinearity compensation scheme based on inverse VSTF. We show 1 dB improvement in Q-factor with a 256 Gb/s polarization-division-multiplexed 16-level quadratic amplitude modulation format, and 50% reduction in complexity by lowering the processing rate.
TL;DR: In this paper, the authors demonstrated a wavelength scanning time division multiplexing of 1000 ultra-weak fiber Bragg gratings (FBG) for distributed temperature sensing, where the measurement accuracy of Bragg wavelengths for more than 80% FBGs is less than 20 pm, corresponding to 2 $^{\circ}$ C in temperature.
Abstract: We demonstrated a wavelength scanning time division multiplexing of 1000 ultra-weak fiber Bragg gratings (FBG) for distributed temperature sensing. The strong multiplexing capability and low crosstalk of the ultra-weak FBG sensors is investigated through both theoretical analysis and experiment. An automated FBG fabrication system was developed for fast 1000-FBG fabrication. The measurement accuracy of Bragg wavelengths for more than 80% FBGs is less than 20 pm, corresponding to 2 $^{\circ}$ C in temperature.
TL;DR: In this article, a reconfigurable mode demultiplexer based around a liquid crystal spatial light modulator is demonstrated to share a propagation constant as separate modal channels using a single shared phase mask per polarization for all channels.
Abstract: A reconfigurable mode demultiplexer based around a liquid crystal spatial light modulator is demonstrated to optically demultiplex modes that share a propagation constant as separate modal channels using a single shared phase mask per polarization for all channels.
TL;DR: In this article, a low-complexity coherent receiver solution is presented to improve spectral efficiency in WDM systems based on the receiver-side partial-response equalization and maximum-likelihood sequence detection.
Abstract: A novel low-complexity coherent receiver solution is presented to improve spectral efficiency in wavelength-division multiplexing (WDM) systems. It is based on the receiver-side partial-response equalization and maximum-likelihood sequence detection (MLSD) in prefiltered WDM systems. The partial-response equalization shapes the channel into an intermediate state with a known partial response which is finally recovered by MLSD without the need of channel estimation. In this scheme, the severe intersymbol interference induced by the prefiltering can be shared between the partial-response equalization and MLSD. Therefore, a tradeoff can be made between complexity and performance. The feasibility of receiver-side partial-response shaping relaxes the efforts and requirements on the transmitter-side prefiltering, which permits the mature WDM components to implement prefiltering. In addition, the partial-response equalization or shaping structure is also improved based on our prior art, which further simplifies the overall scheme. For near-baudrate-spacing optically prefiltered WDM systems, duobinary response is experimentally proved as a good intermediate response to shape. Due to the short memory of the duobinary response, the complexity of the receiver based on duobinary shaping has been reduced to a low level. As a whole, the proposed scheme provides a good alternative to Nyquist-WDM at comparable spectral efficiencies. With the proposed receiver-side duobinary shaping technique, three sets of experiments have been carried out to verify the improved duobinary shaping scheme and meanwhile demonstrate the main features, including 5 ×112-Gb/s polarization-multiplexed quadrature phase-shift keying (PM-QPSK) WDM transmission over a 25-GHz grid, single-channel 40-Gbaud PM-QPSK experiment, and 30-GHz-spaced 3 × 224-Gb/s PM 16-ary quadrature amplitude modulation transmission.
TL;DR: In this article, a method to measure inter-core crosstalk of single-mode multi-core fiber (MCF) statistically, and characteristics of the CRS of a previously fabricated MCF was described.
Abstract: We describe a newly-developed method to measure inter-core crosstalk of single-mode multi-core fiber (MCF) statistically, and characteristics of the crosstalk of a previously fabricated MCF. To measure the crosstalk lower than -60 dB, we suppressed power floor of the measurement system by employing trench-assisted fiber as launching and receiving fibers. We measured statistical crosstalk distributions by utilizing wavelength dependence of the crosstalk, and the mean crosstalk between the neighboring cores was observed to be about -70 dB after 17.4-km propagation at 1625 nm. The measurement results were in good agreement with the previously proposed model. Based on the measurement results and the model, the crosstalk after 10000-km propagation was estimated to be less than -30 dB. We also confirmed that the crosstalk was varied stochastically depending on polarization state, and on mechanical shock and stress on the MCF.
TL;DR: A 2-D (time and wavelength) discrete-time input-output model of physical impairments that can become the foundation of signal processing for optical communications is developed based on the Volterra series transfer function (VSTF) method.
Abstract: Dense wavelength-division multiplexing (DWDM) is a promising approach to design ultrahigh-capacity fiber-optic communication systems ( >; nn50 Tb/s). However, DWDM gives rise to severe physical impairments that adversely affect system performance. To mitigate various physical impairments in DWDM systems and exploit their system capacity, there is a need to develop a 2-D (time and wavelength) discrete-time input-output model of physical impairments that can become the foundation of signal processing for optical communications. This paper develops such a model based on the Volterra series transfer function (VSTF) method. We overcome the well-known triple integral problem associated with the VSTF method and reduce it to a simple integral. This model takes into account multiple channel effects, fiber losses, frequency chirp, optical filtering, and photo detection, which are ignored in the current literature. The model is in excellent agreement with results obtained by split-step Fourier simulation. Furthermore, with this model, we define coefficients that capture intersymbol interference, interchannel interference, self-phase modulation, intrachannel cross-phase modulation (XPM), intrachannel four-wave mixing (FWM), XPM, and FWM to characterize the impact of these effects individually on the system performance. We also apply this model to analyze the effects of varying system parameters and pulse shapes on the individual physical impairments.
TL;DR: In this paper, the theory of modal propagation through symmetric and asymmetric 2D weakly guiding Y-junctions is extended to cover few-mode, multimode, and multiarm Y-joints.
Abstract: The theory of modal propagation through symmetric and asymmetric 2-D weakly guiding Y-junctions is extended to cover few-mode, multimode, and multiarm Y-junctions. A conceptual approach based on the evolution of modal effective indexes and composite supermodes is used to determine the qualitative functionality of these devices, with quantification being determined numerically using the beam propagation method.
TL;DR: In this article, a high conversion efficiency microwave photonic frequency downconverter based on an integrated dual-parallel Mach Zehnder modulator (DPMZM) and optical phase shifter configuration is presented.
Abstract: A high conversion efficiency microwave photonic frequency downconverter based on an integrated dual-parallel Mach Zehnder modulator (DPMZM) and optical phase shifter configuration, is presented. This structure features the advantages of high conversion efficiency, robust operation, and ability to function over a very wide frequency range. The introduction of a novel phase shifter that is incorporated within the structure enables a high rejection of over 45 dB of the local oscillator (LO) to be achieved. The integrated DPMZM based photonic mixer also has a lower noise figure and a higher spurious free dynamic range compared to the conventional dual-series modulator based photonic mixer. Experimental results demonstrate a significant improvement of 23.7 dB in the conversion efficiency compared to the conventional dual-series modulator based photonic mixer for the same optical power into the photodetector.
TL;DR: This paper reviews recent advances in the generation, detection and transmission of orthogonal-frequency-division-multiplexing (OFDM) based superchannels, enabled by efficient and powerful digital signal processors.
Abstract: This paper reviews recent advances in the generation, detection and transmission of orthogonal-frequency-division-multiplexing (OFDM) based superchannels, enabled by efficient and powerful digital signal processors. The use of OFDM to form a superchannel can be (1) at the modulation stage by naturally realizing a square-like signal spectral shape to allow close packing of multiple modulated signals, and/or (2) at the optical multiplexing stage by seamlessly multiplexing these modulated signals. This paper reviews recent advances in this field. Several OFDM-based superchannel architectures are described and compared.
TL;DR: In this article, a short polarization beam splitter (PBS) is presented based on an asymmetrical evanescent coupling system, which consists of a narrow input waveguide, a narrow output waveguide and a wide middle optical waveguide between them.
Abstract: A short polarization beam splitter (PBS) is presented based on an asymmetrical evanescent coupling system, which consists of a narrow input waveguide, a narrow output waveguide, and a wide middle optical waveguide between them. The width of the waveguides is designed so that the phase-matching condition is satisfied for the TM fundamental (TM0) mode in the narrow input/output waveguide and the first higher order TM (TM1) mode in the wide middle waveguide. Meanwhile, there is a significant phase mismatch for the case with TE polarization. Therefore, for the launched TE polarized light, almost no coupling happens when it goes through the coupling region and finally the TE polarized light is output from the through port. For the launched TM0 mode in the narrow input waveguide, it is completely coupled to the TM1 mode in the wide middle waveguide by choosing the optimal length of the coupling region. Furthermore, the TM1 mode excited in the wide middle waveguide is then coupled to the TM0 mode in the narrow output waveguide through the evanescent coupling between them. A short (~25 μm long) PBS is designed based on silicon-on-insulator nanowires, while the gap width is chosen as large as 300 nm to make the fabrication easy. Numerical simulations show that the present PBS has a good fabrication tolerance for the variation of the waveguide width (more than ± 20 nm) and a broadband ( ~ 50 nm) for an extinction ratio of >; 15 dB.
TL;DR: An equivalent circuit model for the coplanar waveguide (CPW) which serves as the traveling wave electrode to drive carrier-depletion-based silicon modulators is proposed in this article.
Abstract: We propose an equivalent circuit model for the coplanar waveguide (CPW) which serves as the traveling wave electrode to drive carrier-depletion-based silicon modulators. Conformal mapping and partial capacitance techniques are employed to calculate each element of the circuit. The validity of the model is confirmed by the comparison with both finite-element simulation and experimental result. With the model, we calculate the modulation bandwidth for different CPW dimensions and termination impedances. A 3 dB modulation bandwidth of 15 GHz is demonstrated with a traveling wave electrode of 3 mm. The calculation indicates that, by utilizing a traveling wave electrode of 2 mm, we can obtain a 3 dB modulation bandwidth of 28 GHz.