An evaluation of architectures incorporating wavelength divisionmultiplexing for broad-band fiber access

Wavelength division multiplexing (WDM) offers the potential to transport very high bit rates in access systems serving residential and small business customers. We describe the options for WDM access systems, and evaluate them on the basis of an international standard that is emerging for passive optical networks (PONs) that use wavelength-independent power splitting. The greatest impediment to meeting the high-bandwidth potential of WDM for downstream (i.e., from the central office to the home) transmission is the need for a moderately priced multiple-wavelength light source, implementing WDM in the upstream (home to central office) direction presents greater hurdles than in the downstream. The technical aspects of overcoming these hurdles can all be met. The real challenge will be in finding technical solutions that can meet the very stringent cost requirements of access systems. Systems that use WDM in the downstream direction only, while using wavelength-independent power-combining in a PON configuration for upstream, may prove economical before full WDM systems. WDM may also prove valuable if upgrades are needed in the future for power-splitting PON's     

New architectures for diversity in fiber loop networks

Active and passive multiplexing technologies and alternative architectural approaches to diverse routing for fiber-loop networks are described. It is shown on the basis of Markov models that the mean time to failure (MTTF) for a 1:1 protected system can be significantly improved by deploying fiber with alternate paths and duplicate central offices. Specific network examples are used to compare installed first costs of the active and passive multiplexing alternatives for various loop-diversity approaches. The results indicate the requirements for diverse routing can have a large impact on the choice of technologies and approaches for fiber loop networks. Specifically, a passive double-star architecture, using dense wavelength-division multiplexing (WDM) techniques, represents a promising technological approach to diverse routing, for it has the lowest installed first cost for diversity in the loop of the several approaches studied, both in suburban and urban areas     

Powering the fiber loop optically-a cost analysis

Recent successes in transporting optical power near the 1-W level via optical fiber suggest that it may be possible to operate conventional telephone station sets using electricity derived photovoltaically from light in a fiber. The authors investigate the constraints on optical powering in fiber-in-the-loop (FITL) applications and assess its applicability in terms of end-to-end efficiency, loop length, and system cost. To make this assessment, they look at several different optical-powering system architectures and components vis-a-vis their cost-versus-delivered-power capability. Related issues such as safety and reliability are discussed.<>     

Lightwave subscriber loop systems toward broad-band ISDN

The state-of-the-art of lightwave subscriber loop systems is reviewed. An approach is described for deriving an architecture for subscriber loop systems that can accommodate technological trends in broadband ISDN such as asynchronous transfer mode (ATM) and synchronous digital networks. Key component technology is discussed, namely, optical fibers and cables, splicing and connectors, and switching     

Electrically tunable efficient broad-band fiber filter

We demonstrate a new type of fiber-optic electrically tunable filter that enables a tuning range of 60 mm with an electrical power of 0.17 W, corresponding to a tuning efficiency of approximately 320 nm/W. The device is based on a metal coated long-period fiber grating written in a specially designed “air-clad” fiber that is filled with a polymer whose index of refraction is a sensitive function of temperature. Resistive heating, caused by current flowing through the metal coating, heats the polymer and induces changes in its index of refraction; these changes induce shifts in the position of the attenuation dip associated with the long-period grating. The air-clad structure has two benefits: (1) it enables the introduction of temperature sensitive materials into the fiber thus enabling power efficient operation of the device and (2) ensures that the cladding modes are insensitive to the surrounding environment and thus are unaffected by the metal coating on the surface of the fiber     

A broad-band single polarization optical fiber

The authors report on a single-mode optical fiber that transmits one and only one polarization state of the fundamental mode over a broad wavelength band (13%). The good broadband performance is attributed to the fiber's refractive-index profile, which mathematically allows only one polarization state to cutoff. The authors discuss general design principles and show that the mode field diameter is a qualitative indicator of the fiber's performance. The fiber's guided polarization state is similar to the fundamental mode of a step-index fiber in its low attenuation, 5.5 dB/km, and small mode diameter, 5.8 μm (both measurements at 840 nm). Most important, the fiber shows a broad single polarization band, 13%, a feature well described by the change in mode field diameter with wavelength of an ideal, circularly symmetry, W -profile fiber     

Polarizer requirements for fiber gyroscopes with high-birefringence fiber and broad-band sources

Polarizer requirements for fiber-optic gyroscopes with high-birefringence fiber and broad-band sources are investigated theoretically by employing a model with a single coupling center between polarization modes. The phase bias offset due to a finite polarizer extinction ratio is greatly reduced by incoherent effects, and may be further reduced by using a depolarized source or by appropriate orientation of the polarizer transmission axis. Some experimental data is presented which supports the theoretical model.     

A broad-band superfluorescent fiber laser using single-mode dopedsilica fiber combinations

We demonstrate the possibility of broadening the emissions spectra of superfluorescent fiber lasers by combinations of different kinds of single-mode doped silica fibers. We obtained an optical bandwidth of more than 70 nm (FWHM) between 1030 and 1100 nm using Nd:Cs, Nd:Ge, and Yb-doped fibers. The total output power measured from the single-mode fibers was more than 100 μW in a single pass configuration     

Flexible synchronous broad-band subscriber loop system: opticalshuttle bus

A flexible, synchronous, broadband subscriber loop system called the optical shuttle bus is discussed. The emphasis is on the proposed design architecture and performance of a 1.8-Gb/s prototype system. The optical shuttle bus is constructed of multigigabit/second intelligent optical shuttle nodes (OSNs). The software-defined OSNs allow the bus to be configured as a synchronous transmission network with any topology and provides flexible channel allocation by means of a drop/insert/cross-connect function. The OSN can be configured as a highly reliable network by virtue of its self-healing functions such as line switch, bypass, and loop back. The proposed bus architecture has been successfully applied in a 1.8-Gb/s prototype in which three OSNs are connected by one 10-km and two 5-dm single-mode fibers in a ring configuration. Very compact and inexpensive equipment is realized by using high-speed ICs. Experimental results show the proposed architecture to be a powerful one for the realization of synchronous broadband subscriber loop systems     

Real-time heterodyne fiber polarimetry with narrow- and broad-band sources

A fast heterodyne polarimeter for fiber characterization is presented. The measurement principle gives rise to two basic system versions, suitable for a wide range of experimental conditions. The first solution, useful in the case of narrow-band sources, is based on a self-heterodyne configuration. The second scheme, free from coherence requirements of the source, employs a Mach-Zehnder interferometric structure placed at the output of the fiber. Very good performances of both implementations are extensively demonstrated through different sets of results (in time and frequency domain) related to statical and dynamical fiber tests. Other relevant applications in telecommunication and sensor areas are discussed.     

Fiber design for broad-band gain-flattened Raman fiber amplifier

We report here a novel fiber design which has inherently flattened effective Raman gain spectrum, with a relative 3-dB bandwidth of /spl sim/90 nm. Gain-flattened broad-band amplification can be achieved in any wavelength band by suitably choosing the fiber parameters and the pump wavelength. Simulations show that the proposed fiber also has high negative dispersion coefficient /spl sim/(-300 to -600) ps/km /spl middot/ nm in the operating range of wavelength. Hence, the designed fiber serves the purpose of a gain-flattened broad-band amplifier and dispersion compensator.     

Low-power all-optical broad-band switching device using ytterbium-doped fiber

All-optical photonic switching has been demonstrated at 1310 nm and 1545 nm using an all-fiber Mach-Zehnder interferometer incorporating ytterbium-doped silica fiber. The switching was achieved with 980 nm excitation from a laser diode, with a power-length product of 3.7 mW/spl middot/m and 5.8 mW/spl middot/m at 1310 nm and 1545 mm respectively, thus giving low power all-optical switching at the preferred telecommunications wavelengths.     

Japanese subscriber loop network and fiber optic loop development

Japanese local-loop networks are addressed with a special focus on ISDN and fiberoptic systems deployment. Basic telecommunication services and local network evolution in Japan are reviewed, and the various types of loop transmission systems in existence are described. Future networks and systems development are discussed     

星网、环网结合的大用户接入网络

网络现状从目前运营商为大用户提供的光纤接入网状况看,对于特大型用户,运营商普遍以SDH环网的方式提供服务,比如银行、税务、公检法等大的集团总部或政府上级机关;对于众多的中小型企业等商业用户和住宅小区,包括上述大用户中的分支机构,基本上没有进行区分性服务,绝大多数都是以铜线(双绞线)接入的方式提供语音和数据的通信服务,和普通居民用户相比基本没有差别,对于个别有条件的用户使用点对点光端机提供业务,但往往缺乏统一的网络管理和规划。现有网络存在的问题据有关部门统计,全国目前有1000万家中小型企业,并且各类小型企业的数量…     

Novel loop architectures for enhancing linearity and resolution of analog-to-digital converters

This paper proposes three mixed (analog and digital) loop architectures which involve an analog-to-digital converter and enhance its linearity and its resolution. Their benefits are discussed with mathematical models and high-level simulations (the ADC inserted in the loops is then a passive sigma-delta structure). One of the loop topologies is particularly highlighted: it is ideally able to enhance resolution by 5 bits without damaging bandwidth. The only added analog element is an active differential low-pass filter. The other operators are fully digital: a predictor and some models of the analog parts. The effect of some defaults, such as mismatch and common mode, is illustrated by high-level simulations. The needed accuracy for the digital parameters is evaluated to 16 bits. The test of a prototype realized in a 0.35 μm CMOS technology validates the principle and demonstrates that the critical element of the structure is the active differential filter.     

A numerical study on realizable broad-band and equivalent admittances for dipole and loop antennas

Some realizable broad-band equivalent circuits for straight-wire and wire-loop structures are developed. The realizability of the circuits is explored in terms of the positive-real (PR) function properties of the circuit admittances. The postulate of PR "terminal eigenadmittances" as couched by Pearson and Wilton [8] is generally supported in the numerical examples, but a potential counterexample-that of the quarter-driven wire-occurs as well. A "dominant pole-pair" approach leads to a quate satisfactory approximate synthesis for the structures studied here. The conduct of this approximate synthesis is described in the context of the Bott-Duffin synthesis procedure. The circuits developed are tested by comparing their computed transient response with the response of electrodynamic models for the same structures.     

Performance evaluation of the time delay digital tanlock loop architectures

This article presents the architectures, theoretical analyses and testing results of modified time delay digital tanlock loop (TDTLs) system. The modifications to the original TDTL architecture were introduced to overcome some of the limitations of the original TDTL and to enhance the overall performance of the particular systems. The limitations addressed in this article include the non-linearity of the phase detector, the restricted width of the locking range and the overall system acquisition speed. Each of the modified architectures was tested by subjecting the system to sudden positive and negative frequency steps and comparing its response with that of the original TDTL. In addition, the performance of all the architectures was evaluated under noise-free as well as noisy environments. The extensive simulation results using MATLAB/SIMULINK demonstrate that the new architectures overcome the limitations they addressed and the overall results confirmed significant improvements in performance compared to the conventional TDTL system.     

TCP for high performance in hybrid fiber coaxial broad-band accessnetworks

Motivated by the phenomenal growth of the Internet in recent years, a number of cable operators are in the process of upgrading their cable networks to offer data services to residential subscribers, providing them direct access to a variety of community content as well as to the Internet. Using cable modems that implement sophisticated modulation-demodulation circuitry, these services promise to offer a several hundredfold increase in access speeds to the home compared to conventional telephone modems. Initial experiences indicate that cable networks are susceptible to a variety of radio-frequency (RF) impairments that can result in significant packet loss during data communication. In the face of such losses, the transmission control protocol (TCP) that is predominantly used by data applications degrades dramatically in performance. Consequently, subscribers of broad-band data services may not perceive the projected hundredfold increase in performance. We analyze the performance of TCP under different network conditions using simulations and propose simple modifications that can offer up to threefold increase in performance in access networks that are prone to losses. These modifications require only minor changes to TCP implementations at the local network servers alone (and not at subscribers' PCs)     

Purely phase-sampled fiber Bragg gratings for broad-band dispersion and dispersion slope compensation

We demonstrated numerically that both the chromatic dispersion and the dispersion slope can be compensated by using purely phase-sampled superstructure fiber Bragg gratings provided both the grating period and the sampling period are chirped linearly along the grating. Adjusting the refractive index modulation and the chirp of sampling function, they can be designed to compensate dispersion of a large number of wavelength-division-multiplexing channels.