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`
`PERFORMANCE OF A 576x576 OPTICAL CROSS CONNECT
`
`Herzel Laor
`
`Astarte Fiber Networks, Inc.
`2555 55* St. Boulder, CO 80301
`Tel: 303-641-0514
`
`herzel . laor@starswitch . com
`
`Edward Fontenot
`
`Astarte Fiber Networks, Inc.
`2555 55* St. Boulder, CO 80301
`Tel: 303-443-8778
`
`ed.fontenot@starswitch.com
`
`Alan Richards
`
`Astarte Fiber Networks, Inc.
`2555 55th St. Boulder, co 80301
`Tel: 303-443-8778
`
`alan.richards@starswitch.com
`
`Jack D’Entremont
`
`Texas Instruments, Inc.
`34 Forest Street
`
`Attleboro, MA 02703
`Tel: 508-236-1971
`
`jdentremont@ti.com
`
`Marshall Hudson
`
`Texas Instruments, Inc.
`34 Forest Street
`
`Attleboro, MA 02703
`Tel: 508-236-3317
`
`mhudson@ti.com
`
`David Krozier
`
`Texas Instruments, Inc.
`34 Forest Street
`Attleboro, MA 02703
`
`Tel: 508-23 6-21 17
`dkrozier@ti.com
`
`Abstract
`
`Fiber has been used in the transport of information for over two decades. Over this period the
`performance of optical transport equipment has skyrocketed while its cost has spiraled down. This has
`led to the proliferation of fiber in long haul, local, and private networks, and even the appearance of fiber
`connections to the home or desktop. The recent rise of Dense Wave Division Multiplexing (DWDM)
`technology added multiplexing and demultiplexing fi11’1CtlO1’1S to the use of photonics in communications.
`The next challenge for photonics as we move toward all optical networks is optical switching.
`
`Evolution towards all optical networks necessitates switching equipment that can operate on any rate and
`format of optical transmission without hardware changes. Such protocol transparency simplifies the
`network management task. Switching large numbers of fiber optic signals at the photonic level eliminates
`expensive conversion and switching electronics. This ability to create end-to-end optical channels fi1ClS
`wavelength routing applications. Until recently, optical switching technology with this capability and
`capacity was unavailable. Texas Instruments and Astarte Fiber Networks have developed high port count,
`transparent optical switching technology that is now in evaluation. This paper describes the architecture,
`components, and operation of this optical switching technology and provides initial test results.
`
`Introduction
`
`The typical central office has thousands of fibers to manage with each fiber capable of carrying millions
`of dollars worth of traffic each hour. SONET is the most prevalent optical protocol in use today, but
`other protocols such as optical IP are growing in popularity. Switching equipment used to manage these
`fibers must have the flexibility to effectively handle any traffic protocol in order to minimize the burden
`on network management systems. In addition, as transmission speeds increase optical switching
`
`

`
`equipment should continue to operate without the need to ‘forklift upgrade’ the switching equipment.
`Only transparent optical switching equipment has the ability to continue to function normally as rates and
`formats change or as DWDM migrates into a network.
`
`Network reconfiguration and service provisioning are two areas where a large transparent optical
`switching system can provide immense benefits in optical networks. For network reconfiguration
`applications a large optical cross connect (OXC) is located at each hub. Each cross connect is controlled
`by a management system in a coordinated fashion to create fiber or wavelength connections throughout
`the network. Since the network can carry multiple rates and formats of optical signals the cross connects
`must be able to switch any type of optical signal that may traverse the network.
`
`Service provisioning places the OXC in a central office to manage the fiber connections to the outside
`world. The OXC switches fiber connections to circuit switches, electronic cross connects, ATM switches,
`
`or even optical IP switches. In this situation the OXC is likely to see frequent configuration changes as
`customer services churn. Protocol transparency in this application provides the ability to carry various
`rates and formats as customer needs require.
`
`System configuration
`
`The OXC developed by TI and Astarte is a free space, single stage, strictly non-blocking, transparent,
`matrix optical switch capable of supporting 1152 fibers. Figure 1 illustrates the general architecture of
`this OXC. For illustration purposes Figure l is simplified to show only sixteen fibers. The illustration
`shows fibers (on the left) coming into an array of switch actuating modules. Each of these modules
`represents an electro-optical assembly that is capable of focusing, and accurately controlling the
`deflection of a light beam coming out of a fiber. All modules face a mirror which turns all optical beams
`back toward the array of modules. By varying in two axes the deflection angle of a beam coming out of
`any switch-actuating module, the beam can be aimed at any other module. If the second module is also
`aimed correctly an optical connection is created between the two modules.
`
`The figure shows three optical connections. Since the beams are projected through free space, a beam can
`freely cross any other without effect. New connections can be created without any impact on existing
`connections. Every connection involves only two switch actuating modules. This single stage switch
`architecture means signal attenuation is low and consistent. Once a connection is established between
`two modules, any optical signal on one fiber is carried through the switch to the other fiber,
`bidirectionally, without regard to the rate, format, or protocol of the signal. DWDM signals can be
`switched very cost effectively as a group or demultiplexed outside the switch and switched individually.
`With 1152 fibers, 576 connections can coexist on the OXC.
`
`

`
`
`
`Figure l — The OXC architecture showing three simultaneous fee-space optical connections
`
`A unique characteristic of this OXC architecture is its modularity. This modularity allows craft to perform
`board-level replacement of any failed module in-service, as well as the ability to partially equip an OXC
`and add additional modules as needed.
`
`Also unique to this OXC architecture is the redundancy built into the system. Redundant main processors
`handle all communications within the OXC, and to the network management system. Two redundant
`power supply units power the OXC. The processors and power supplies are hot-swappable. Connection
`commands are generated by the OXC main processors, and downloaded to a Digital Signal Processor
`(DSP) within each module over redundant communications channels. Each module is then autonomous in
`performing and maintaining the commanded connection. Rearranging any number of connections does
`not influence existing connections in the OXC.
`
`The Optical Path
`
`The optical path, representing a connection through two modules, is shown in Figure 2. Light from a
`fiber passes through a focusing lens. The light is reflected off a fixed mirror which folds the beam to
`keep device packaging small. It is then reflected off a movable mirror which precisely directs the beam in
`two axes.
`
`To make an optical connection between modules the moveable mirror directs the beam directly at the
`moveable mirror of a targeted second unit. At the same time the second unit controls its moveable mirror
`to deflect the beam toward its fixed mirror, into its lens, and then into its fiber completing the connection.
`It is the coordinated control of deflection angles by the two moveable mirrors that creates the optical
`connection between two fibers.
`
`Clearly, if the second mirror is not positioned to deflect the incoming beam to the precise angle where the
`beam will enter the fiber then no connection will be made. For this reason, a free space beam moving
`within the switch cavity, which traverses other units, will not induce a crosstalk signal with any other
`connections.
`
`It should also be noted that the optical path is the same from right to left and left to right. Therefore the
`optical path through any two modules in a connection is bidirectional. The optical path is the same for
`
`

`
`every possible connection in the OXC resulting in minimal and consistent optical attenuation in this
`single stage architecture.
`
`
`
` Folding mirror
`
`
`
`Figure 2 — The optical path through two modules in the OXC
`
`To precisely control the position of the beam with the moveable micromirror, a closed-loop servo control
`system is employed. This servo control system enables each module to locate and identify an opposing
`module for targeting. With this control system the moveable micromirror can precisely position the beam
`on the fiber core and hold it there. This active positioning system provides high optical coupling
`efficiency into the fiber and protects the optical signal against vibration and drift.
`
`Optical Module
`
`In its physical implementation each optical module supports two fibers, as illustrated in Figure 3. This
`allows a transmit/receive connection to be configured within the same module simplifying fiber
`management. A DSP on each module handles
`communication with the main processor and manages servo
`control operation for both micromirrors. Once the main
`processor has communicated connection commands to a
`module, that module is autonomous in control of the beam
`
`position and maintenance of an optical connection.
`Redundant power supplies and communication paths are
`provided to the module, and the module is designed to be
`hot-swappable.
`
`The switch module contains electronics to operate the two
`actuators that handle the optical signal. One of these
`actuators is illustrated in Figure 4. This optical switch
`actuator assembly contains all the optical components: the
`single mode fiber, a lens, folding mirror, moveable MEMS
`mirror and several detectors and emitters for the servo
`
`system.
`
`
`
`Figure 3 — Illustration of an optical module
`showing two fibers and redundant
`communications cables
`
`

`
`Fig“?
`‘ The Switch actuator assembly
`gggtglmng 311 the Opucal Components of the
`
`
`
`The moveable microrrnirror contained in the actuator
`assembly is fabricated using Micro Electro-
`Mechanical Systems (MEMS) technology. This
`micromirror is electro-magnetically controlled and
`capable of two-axes movement. Texas Instruments
`developed this MEMS device specifically for
`application in the OXC. It provides extremely low
`optical loss, and the high reliability and switching
`speed required for OXC applications. The MEMS
`device is in a sealed package. These devices have
`been tested for the equivalent of hundreds of millions
`of switching operations.
`
`Test results
`
`Texas Instruments and Astarte have built prototype hardware as a vehicle for evaluating the performance
`of the OXC architecture. The fiber in use in the OXC is singlemode. The design of the transparent,
`single stage, switch architecture means the ability to carry all optical signals with wavelengths from 1250
`nm to 1700 nm.
`
`With its closed loop servo control systems the demonstration hardware is able to control the beam within
`0.5 microns of the center of the fiber core. This precision results in a measured fiber-to-fiber optical
`connection with 4.8 dB of optical loss.
`
`The other optical performance parameters are consistent with expectations for a single stage, free space
`optical switch design. Back reflection typically is less than —40dB. Optical crosstalk typically is less than
`-80 dB. Polarization dependent loss (PDL) is measured at less than 0.5dB which is at the noise level of
`the measurement. Dispersion through a switch optical connection is so low that available test equipment
`cannot provide a reliable measurement.
`
`The worst case switch time with the architecture of this OXC occurs when a module is requested to
`switch from a connection at one comer to one at the diagonal comer. This worst case switch time is 10
`milliseconds from the time the switch command is received at the OXC. Because of the autonomous
`
`operation of each module in the OXC a complete switch configuration can occur in the same 10
`milliseconds.
`
`Conclusions
`
`This paper describes the architecture of a large, central office class, optical cross connect. This OXC is a
`free-space, single-stage, strictly non-blocking, transparent, matrix optical switch capable of supporting
`1152 fibers. Test results presented verify the excellent optical performance of this switch architecture.
`The unique redundancy and modularity of the OXC architecture is finther support for the suitability of
`this design in carrier networks.
`
`The OXC technology presented in this paper has been successfully demonstrated with engineering
`models. The initial implementation of this OXC architecture has been developed to produce a 576 x576
`OXC for network restoration and service provisioning applications. Other configurations can be achieved
`with the same basic technology. Smaller OXCs accommodating 72, 144 or 288 modules, providing
`72x72, 144x144 or 288x288 connectivity can be produced. All these smaller OXCs would retain the fi1ll
`
`

`
`modularity designed into the larger unit and will use identical modules to the ones used in the 576x5 76
`OXC. Larger OXCs of 2,000 X 2,000 to 10,000 X 10,000 can also be developed from the technology
`described in this paper. These immense OXCs would use the same basic technology with second-
`generation components to provide increased fiber switching density.
`
`The OXC presented in this paper will provide the final piece of photonic technology necessary for
`construction of effective all optical networks.
`
`References
`
`D. Krozier, A. Richards, “Optical switch demos in cross connect,” Electronic Engineering Times, May
`31,1999, p. 80