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The State Of The Art

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VoicenData Bureau
New Update

Alastair M Glass, COLOR="#00553b" size="3"> director of Photonics Research at Lucent Technologies’ Bell

Laboratories.

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The huge bandwidth of optical fibre and the

potential of multiterabit communication was recognized back in the Seventies during the

early development of fibre optic technology. For the last two decades, the capacity of

experimental and deployed systems has been increasing at a rate of 100-fold each

decade–a rate exceeding the increase of integrated circuit speeds. By 1996, the first

terabit systems were demonstrated in a number of research laboratories leading to the

expectation, based on historical trends, that commercial terabit systems will be deployed

at the turn of the next century. Because of the electronic bottleneck, these systems will

be achieved by wavelength multiplexing on the optical fibre. Greater functionality will be

introduced at the optical layer, transforming conventional point-to-point transmission

systems into flexible optical networks. Great demands will be placed on new photonic

components to meet the needs with such systems. The development of new photonic devices is

growing at a tremendous rate, and the integration of photonics components and photonics

with electronics will continue to drive down the costs and increase the functionality of

photonic devices.

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The first-generation Dense Wavelength Division Multiplexed (DWDM)

systems developed in the early 1990s were point-to-point systems designed to increase the

capacity of installed fibre. Eight-wavelength systems have been deployed since 1995; 16-

and 32-channel systems and higher are now entering the market. In the research lab,

concepts and components for 100 channels and beyond are under development.

With the evolution of such high-capacity, long-haul systems, came the

concept of the wavelength add-drop in which individual wavelength channels are dropped and

added at amplifier nodes while allowing express traffic to pass through the node. This

minimizes the need for high-speed electronics at the nodes. The next step is to full

optical networking, including reconfigurable add-drop nodes and optical cross-connects in

which data traffic enters the network at any node and leaves the network at any other

node. Such networks will permit rapid restoration at the optical layer, flexible bandwidth

management in a multivendor environment, and cost savings.

Around the world, a number of operational testbeds are examining the

potential of optical networking. One of these, the Multiwavelength Optical NETwork

(MONET), consists of a fully functional testbed with three parts: a 2000-km long-haul

testbed, an optical cross-connect testbed, and a local exchange network. This testbed was

designed with multiple access points, spans with multiple originations, and terminations,

interoperability, scaleability, and transparency. This testbed has been fully operational

for the last 10 months and met performance targets for SONET, ATM, and analog FM

transmission. The economic study performed under this programme showed evident cost

savings of up to 20 percent offered by the optical network architecture.

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Demand for DWDM systems has been from the long-distance operator.

However, this market will soon expand to include Metropolitan Area Networks (MANs) and

eventually Local Area Networks (LANs) and residential access. In these markets, the full

benefits of optical networking may become more evident. For penetration in LAN and access

markets, the price of photonic components must fall considerably from the present values

and new manufacturing technologies must be introduced for high-volume, low-cost component

fabrication.

Opportunities presented in the components area by the evolution of

optical networking are dramatic. New fibre designs have been developed to transport

multiple wavelengths without cross-channel interference. Demands on optical amplifier

performance include increasing spectrum and gain flatness across the entire bandwidth.

Routers are required to handle more wavelengths with low cross talk, low loss, and flat

transmission. Wavelength selectable lasers will be required both because of inventory

issues and for maximum networking flexibility (for instance, selecting a wavelength at the

edge of the network or interchanging wavelengths with the network). Reconfigurable

add-drop filters and high-count cross-connect fabrics will be essential, and network

management devices such as in-line wavelength monitors will be needed for network

management and control.

From the early development of low-loss optical fibre and reliable

high-performance lasers, the evolution of photonic systems has been paced by the

development of photonic components. System advances have been enabled by advances in

components. The invention of the optical amplifier revolutionized network design and

enabled the concept of DWDM and optical networking. In this paper, the major focus is on

the current research in network elements which enable further progress towards the vision

of terabit optical networking.

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https://img-cdn.thepublive.com/filters:format(webp)/vnd/media/post_attachments/6cd2a3017430e992bedc6be9759652751ca466cf34dbb75933c2a4525bb8fe6d.jpg (9985 bytes) align="right" hspace="3" vspace="3">Optical Fibre

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Prior to the invention of the optical amplifier, the primary optical

fibre parameters of importance were optical attenuation (loss) and chromatic dispersion.

Installed fibre was optimized for use at 1.3 mm and 1.55 mm (dispersion-shifted fibre)

with low dispersion and losses approaching theoretical limits set by Rayleigh scattering.

The widespread introduction of erbium-doped fibre amplifiers and the large transmission

distance attainable before regeneration changed the focus on the fibre properties to

optical nonlinearity. Multiple wavelength channels propagating with the same velocity in

zero-dispersion fibre are seriously degraded by four-wave mixing of nearby channels. New

fibre designs which incorporate a small amount of dispersion at the signal wavelength

(i.e. Truewave fibre) eliminated this problem, allowing long-haul transmission of multiple

wavelength channels. At high frequencies (OC 192 and above) and long distances, dispersion

compensation is necessary. For longer distances or high bit rates, compensation,

etc., can be accomplished by appropriate cable design (dispersion managed cables) or by

dispersion compensation at amplifier modules.

Thus the three existing fibre alternatives are now: (1) dispersion

shifted fibre which supports high-bit-rate transmission but does not support DWDM;

polarization mode dispersion in early installed fibre may hinder upgrading some embedded

fibre to OC 192; (2) conventional unshifted fibre which can be used for DWDM but requires

too much dispersion compensation at 1.55 mm; and (3) Truewave fibre which is optimized for

WDM 1550 nm and permits high bit rates without dispersion compensation at distances up to

400 km at OC 192.

Beyond four-wave mixing, other optical fibre nonlinearities include

Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS), Self Phase

Modulation (SPM), Cross Phase Modulation (CPM), and Modulation Instability (MI). The

majority of these effects can be suppressed by techniques such as frequency dithering,

broadening, unequal channel spacing, or dispersion management and do not present a major

limitation in the foreseeable future. However, in future generation, ultra high capacity

systems utilizing a large number of wavelength channels and high total optical power, the

effects of these nonlinearities will have to be considered in systems design.

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https://img-cdn.thepublive.com/filters:format(webp)/vnd/media/post_attachments/57ff184daea95eed88695de61a82f6962c097f1e453f827c347e2f968f85b2f4.jpg (8737 bytes) align="left" hspace="3" vspace="3">Optical

Amplifiers

The first-generation Erbium Doped Fibre Amplifiers (EDFA) had a

wavelength-dependent gain profile determined by the erbium ion emission spectrum. When

multiple amplifiers are cascaded in a long transmission span, the gain slope of individual

amplifiers leads to large gain variations across the span. By incorporating fibre grating

filters with carefully tailored transmission profiles which compensate for the gain slope

into silica-based EDFA, flat-gain bandwidths of 40 nm with a gain variation ±1 dB have

been achieved with low noise amplifiers. Alternative approaches based on novel silicate

glass composition or fluoride-based glasses have been used to broaden amplifier gain

spectra, but for maximum bandwidth gain equalizing filters will be required. SIZE="2">

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The importance of these gain flattened amplifiers was demonstrated in a

system experiment consisting of a concatenated chain of eight EDFAs each separated by 80

km of Truewave fibre and carrying 32 wavelength channels separated by 100 GHz at 10 Gbps.

The received optical power after 640 km showed a variation of only 4 to 9 dB across all 32

channels when gain equalization is employed. This compares with a variation of over 30 dB

without gain equalization. In a recent breakthrough, a new amplifier design having a

bandwidth of 80 nm was demonstrated. Such an amplifier is capable of supporting 100

wavelength channels having 100 GHz spacing. This amplifier is based on silica EDFA with

two sections: one optimized for long wavelength channels beyond 1565 nm (L band) and the

other optimized for conventional channels 1525-1565 nm (C band), as previously

demonstrated. At this point, the gain spectrum in the L-band varies by several dB, but

simple addition of a second gain equalizing fibre grating filter in this section will

provide uniform gain over the entire 80-nm spectrum.

 

https://img-cdn.thepublive.com/filters:format(webp)/vnd/media/post_attachments/b9d690369005de4cdafd8a33240cce0e63458c75a3a7757192c33c839fbd12ac.jpg (9991 bytes) align="right" hspace="3" vspace="3">Passive Optical

Components

Perhaps the greatest diversity of network element options is in the

area of passive components. For instance, a broad array of optical filters are available

from multiple vendors based on various technologies such as UV-induced optical fibre

gratings, thin dielectric films, Fabry Perot device, and bulk optic gratings. Many of

these meet the specifications of current WDM systems, but the future acceptance will

depend on scaleability of cost and performance as the number of wavelengths increases. The

gain flattening filters described earlier for optical amplifiers were based on

temperature-independent, long-period fibre gratings in which the transmission spectrum can

be precisely tailored by careful control of UV illumination of a specially designed fibre.

This same technology has been used to fabricate a number of other devices including

optical taps, add-drop filters, dispersion compensators, and reflectors. SIZE="2">

A route to low-cost integration of photonic components uses planar

silica-on-silicon technology. Complex waveguide structures are readily fabricated by

photolithography in silicon oxides deposited on the silicon wafer. Waveguide grating

routers capable of routing up to 64 wavelengths are readily fabricated in this manner.

This technology is well suited to the low-cost integration of active photonic devices

(lasers, switches, and detectors), electronic chips and passive components on silicon

substrate, and the interconnection with optical fibre using techniques developed in the

silicon industry.

 

https://img-cdn.thepublive.com/filters:format(webp)/vnd/media/post_attachments/4bfc02ec6171e031740153ab1af308ce0afe89fea90131e810a679d54667c0ae.jpg (4560 bytes) align="left" hspace="3" vspace="3">Wavelength

Selectable Lasers

Commercial lasers are currently designed to operate at a single

precisely controlled wavelength, at frequencies up to 10 Gbps. High-performance and

low-frequency chirp is achieved by monolithic integration of a continuously operating

distributed feedback laser and an external cavity electro absorption modulator (EML

laser). This device represents the beginning of photonic integration of semiconductor

devices and incorporates many of the incredible advances in compound semiconductor

technology: multiple quantum well structure, chemical vapour deposition with atomic layer

control, and selective area growth. An alternative approach to EML is to use a

continuously working laser and an external Lithium Niobate modulator. Currently modulators

are commercially available for OC 384 systems. The next generation source for optical

networking presents even greater challenges. While it is feasible to construct

multichannel WDM systems with fixed frequency lasers, wavelength programmable sources will

be desirable for greatest flexibility and wavelength management in

future systems.

The simplest extension of the EML concept is the integration of eight

individual DFB lasers monolithically integrated with a combiner and electro absorption

modulator (or LiNbO3 modular). While this device has been used successfully in

experimental testbeds, it is not yet commercially available. Such an approach will meet

the needs of a modest number of wavelengths, but what about sources for 32, 64, and more

wavelengths?

For such applications, other approaches may be appropriate. A

completely novel approach to multiwavelength and wavelength programmable sources is the

chirped pulse laser. In this case, all the wavelength channels are derived from a single

ultrashort pulse laser which has broad spectral content. These pulses are dispersed in a

dispersive element (such as an optical fibre) to spread the spectrum in time. By using a

single high-speed modulator, individual time slots representing individual wavelengths can

be modulated. Experimental demonstration of 300 wavelength channels each carrying 37 Mbps

was achieved with a single modulator. By interleaving pulse trains from a modulator array

(TMD), 128 wavelengths were transmitted over 80 km fibre, each modulated at 2.4

Gbps–a total throughput of 0.3 Tbps. This device is still in the research stage and

its ultimate utility remains to be proven, but the attractive feature of wavelength

control (via modulator phase control) and the small number of components make such a

source an interesting option for many channel applications.

 

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Cross-Connect

Reconfigurable add-drop multiplexers and optical cross-connects require

first, an optical filter for wavelength selection and second, an optical switch to

dynamically interconnect optical fibres. A number of mechanical switches are commercially

available which may be appropriate for slow (>ms) bulk-optic 1 x n demonstrations.

Integrated thermo-optic switches based on silica-on-silicon technology are commercially

available, but these too are relatively slow and are in their infancy from the point of

view of systems demonstrations.

The first 32 x 32 cross-connect incorporated in a WDM system was based

on Lithium Niobate technology. LiNbO3 switch technology uses a manufacturing platform

similar to that used to fabricate optical modulators. Four optical fibres, each carrying

eight wavelength channels, were demultiplexed and coupled to an array of 352 electro-optic

switches capable of directing any input fibre into any output fibre. The entire switch

array is reconfigurable at sub-microsecond speeds suitable for restoration at the optical

layer. At 2.5 Gbps per channel, this cross-connect throughput was 80 Gbps in its first

implementation—a rate comparable to current-day large ATM switches. Clearly, this

device can be scaled to higher capacity, but the eventual practical limitations are

unclear.

 

https://img-cdn.thepublive.com/filters:format(webp)/vnd/media/post_attachments/b20ccca4a2899dbcb331b6a0919b626ca7dc8781d5b12c776b52fc7409240d11.jpg (13817 bytes) align="left" hspace="3" vspace="3">Optical Spectrum

Tap

Techniques already exist for management of DWDM systems, but as optical

networking evolves, optical monitoring is desirable to augment network management systems

and control systems, fault protections, etc. For instance, the gain of an optical

amplifier depends on the optical power in the signal channel. To control the gain and gain

transients, it is necessary to know how many wavelength channels are present at the node

and the signal levels. Local intelligence and feedback reduces the complexity of software

control.

A simple in-line optical spectrum analysis provides a simple technique

for in-line monitoring of multiple wavelengths which can be scaled to be arbitrarily

suitable to a large number of wavelength channels. Using a chirped and blazed fibre

grating, a small fraction of the signal is tapped out of the fibre and directed onto a

detector array for real-time monitoring of the spectrum and signal levels. These signals

in turn can be used to control the gain module locally.

This paper has outlined some of the progress made on network elements

which will be necessary to realize this vision, but the eventual choice of architecture

will be determined by the costs of the elements and the complexity of the software

required to manage the networks. For terabit systems, these issues remain major

challenges. As the development of network elements proceeds at an accelerating rate around

the world, similar rapid progress is being made on network architectures which meet

diverse custom and standardized interfaces requirements. As fibre continues to penetrate

LANs and moves closer to the home, the concept of optical networking discussed here for

WANs will eventually apply to these short-reach networks. The rate at which this proceeds

will depend on the development of optical networks elements which meet the criteria of

both performance and cost. 

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