2nd article, on fiber optic circuits

2nd article, on fiber optic circuits Article 2 : Management of Fiber Optic Circuits
by
S.Mugunthan
paired with Shahrin Imran.

Information Systems Engineering
Department of Computing and
Electrical and Electronic Engineering
Imperial College, London

Contents:

5. Processing

Introduction

As optical components begin to increase for link applications within hardware, serious problems begin to arise in the management of fiber cables. A number of optical connections strategies have been brought up by designers. Channel waveguiding media like glass and polymers are some of the chosen medium for this board scale. Even holographic or beam stering methods using mirrors and lenses to distribute the optical signal among transmitters and receivers. This however, creates additional problems in terms of the non-standard packaging and the physical design required. Optical fiber seems to be the most common transport and widely accepted for transmitting light in a very limited or controlled way. The problem of handling a very large number of cumbersome cables in confined areas becomes very hard to solve and increasingly expensive. A large number of optical connections can take up a great deal of space and would require the time and expense of factory operations. Manual wiring has been replaced by automated wiring or printed multilayer technology. For this reason, the developement of flexible optical circuits with electronic hardware together with automated discrete optical wiring has been pursued.

Background

Now, conventional optical fiber apparatus that is used for application involving central markets are not really compatible for use in hardware. Standard single fiber connectors require a large area and use bulky bend limiters and strain relievers. Single fiber jumper cordage is jacketed for handling and protection. In wiring closets within buildings, large cross connects are used for splicing, patching and storage in pull out shelves and drawers.However, switches and other types of system equipment represent situations where many of the similar functions are required, but the interconnect remains permanent after an installation. The use of conventional hardware for fiber management at major computing level will defeat the purpose of using fiber optics. Optical fabric can be broken up into 3 different parts, as in the figure below.

Figure 1: Optical Interconnect Elements
The distribution fabric is the major interconnecting part that spans the physical dimensions of the architecture. The functional components add special capabilities to the system such as splitting, combining and filtering of signals. Finally the termination points provide the interface between the fabric and the transceiver components. Here, the main focus will be on distribution and the termination factors. The concept of flexible optics requires complex fabric capable of transporting optical signals between termination points with negligible loss. This solutions also has other physical requirements such as:

1. Low loss distribution.
2. Large size. ( about 30 inches )
3. Compact and flexible.
4. Customised design and fit.
5. Protection of fiber.
6. Management of connections.
7. Low cost.

The circuit must distribute optical signals over a distance of at least one standard shelf width without much loss. The need to bridge gap and attach to boards suggested the need of an add on structure that could mount and exist alongside typical hardware. A rugged and flexible circuit is attractive because it may be attached in a variety of ways to panels, using thin tabs that bend and twist the optical leads into connectors. Constraint on the length of an optical fiber lead is reduced because of this flexibility. A physically engineeered circuit also provides the necessary protection by avoiding undue kinking, crushing and macro bending that could result during the system installation and use. The biggest advantage of this circuit approach is that of the compactness obtained when using array or multi-fiber termination. The emerging array technologies can greatly reduce the space required for optical termination. Due to this, the circuit capabilities are even more advantageous in providing break out and cross connect functions. Finally, the optical fabric must be of low cost. This is achieved through the use of design and automation methods suited for the handling fragile glass. The method used to design this is further illustrated in the next section.

Design

The design of the optical circuit must anticipate the manufacturing prcess and its capabilities. The process begins with a tabulation of the required connections and the specified physical boundaries of the circuit. This would include the body of the circuit, tabs used to lead into the connectors and any other obstacle. Then, a scaled computer drawing is made of the fiber pathways in the optical circuit. For this purpose, software packages such as AutoCAD(computer aided design) and VersaCAD on a PC platform has to be used. However, the design has to be followed by a few rules which provide the laying out of the fiber arrangement. These rules provide the proper limits of bend radius, spacing capability, intersection angles, crossovers and other circuit parameters. Proper measures will also insure the fault of the optical fibers with the minimum of loss.

When the layout is complete, a custom developed conversion program is used to translate the CAD drawing file into a data file suitable for driving the wooorkstation used for routing. The software reads the drawing elements contained in the CAD file, locates the starting point and extracts the unique routing order required to continuously lay the optical fiber in the specified design. The software also recognises various path elements such as straightsn bends and arcs, and must determine the correct direction of movement. All curved paths are circularly interpolated to match the translation capabilities of the workstations. A typical design that shows this complexity is shown in the figure below.

Figure 2: Cross Connection Circuit Design

This layout contains a total of 8 tabs that form a shuffle type of fan-out suitable for switching application. The entire circuit is constructed such that it may be fabricated in a continuous path, always looping around to form the next connection line in the opposite direction. With this method of looping, the loop portions of the circuit may be ultimately cut away to expose the individual leads.

Routing

When the design process is completed, the circuit data is moved to a computer driven workstation where optical fiber is bonded onto a substrate. The manner in which the routing process is performed is shown in the figure below.

Figure 3: Autmated Laying of Optical Fiber

A multi axis workstation is is used to move the routing head over a workarea supporting the substrate board. This substrate consists of a rigid plate holding the flexible substrate material and an adhesive layer. The plate is then registered to the work surface using a clamp arrangement. The routing head was built to dispence optical fiber from a storage spool and then bond the fiber into the adhesive surface during translation. The head functions by guiding fiber from the spool to an alignment tool placed at the contact point with the surface. As the board surface moves relative to the head, glass fiber is precisely places and bonded in place. The entire head is able to rotate during the routing process and thus enables the laying of fiber along curved paths. The resolution of the translation movement is important in determining the ability to place optical fibers in an array. Equipment that is used have a step size of 0.0001 inches and have achieved excellent results in forming close packed fiber arrays. The repeatability obtained has also been sufficient to form circuits of approximately 100 metres total length without any loss in placement consistancy. The workstatio is controlled by a PC through the programmable robotic language known as MML(Modular Manufacturing Language). This language allows rapid construction of program routines for axis control.

Processing

The flex substrate provides a dimensionally stable base which must be further processed to provide protection during handling and environmental conditions. After the optical fiber is routed onto the substrate board, the next step is to laminate the circuit into a composite. The circuit is encapsulated using a thermoplastic material which has a high melting point and is compatible with fiber coating. The good things about thermoplastic are that it remains soft and pliable(bend easily) when used in thickness of several layers. This thickness of encapsulant is necessary to completely cover and seal the circuit high points created at fiber crossovers. Hot melting materials are also able to form and flow into the voids around intersecting optical fibers. The thermoplastic also develops sufficient adhesion to the substrate for sealing of the circuit. This seal, however, should not be permanent because the composite must be separated in order to gain access to the fibers. Stiff materials such as epoxies were avoided because of brittleness that may cause cracking during flexture of the circuit.

The lamination step has to do with the removal of the circuit board from the workstations and the placement of labels. The circuit is laid over with a sheet of the thermoplastic material before placement in a platen press. The pressing occurs during a schedule that can be as hot as 150 degrees Celcius. The equipment used is a convcentional press. Programmed controlled by a computer is critical in maintaining the parameters of time, pressure and heat during the lamination. This process remoives most of the trapped air and tightly conforms the encapsulant around the fibers. This is also optimized to avoid stress and heat on the fibers which may cause dislocation of the placed fiber.

Terminating and Mounting

After the lamination process, the circuit must be terminated with optical connectors. This requires trimming of the border of the circuit to produce very precise ribbon like tabs. In this process the circuit is returned to the workstation where a cutting head is mounted on the manipulator. The cut operation is programmed to automatically trim the border of the circuit. The termination fiber array leads is done by multifiber connectors. The multifiber connector is built around precision etched silicon chips used for fiber allignment. The connector is easily suited for field termination because it uses an insertion mode of assembly. For this reason, the end of the ribbon is stripped of all material and the fiber array is inserted into the connector piece. This piece contains features necessary to guide the fibers accurately into the grooves between 2 silicon chips. For easy insertion, an automatic cutting procedure is required. After insertion of the fiber array, the connector is pollished to produce a fully connectorized array. The array could also be spliced with several available mechanical splicing systems. If the lead emanating from the circuit does not contain multiple fibers, a single conductor connector can be used.

Once the circuit has been connectorized, it can be mounted in the system hardware. The connector system includes a variety of circuit pack and backplane housings fully compatible with the packaging hardware. In this case, the flexible optical circuitry is terminated in backplane housings situated on the backpanel board and in bulkhead fittings on an I/O shelf. The backplane leads would mate to circuit board-mounted connectors inserted into the shelf. The optical circuit is held on a support fixture away from the backpanel, allowing the flexible leads to bend into the connector housings. The connector allows the mating of about 18 fibers and consumes very little length at the board edge. This arrangement leaves a lot of space for electrical wiring, pins and connectors that may still be necessary for power and lower bandwidth interconnection.

Skew Control

The backplane circuit shown in Figure 2 is an example where the length of each optical connection was closely controlled during design and manufacture. In certain applications, clock and data signals must experience a minimum amount of timing skew when transmitted between connectors. No problems arise during the usage of ribbon cable alone but some situations require arrays to be fanned out and spatially distributed across several boards. The circuits have been engineered in such a way that each fiber has the same physical length. This is done using the Computer Aided Design(CAD) tools to program the exact fiber path needed. Each optical trace in the design is composed of a series of vector lengths with arc segments spaced between. These arcs are all identical in radius and angle but may be either clockwise or anticlockwise. With the trace endpoints specified by the fiber location within each tab array, simple formulas are derived to specify the vector lengths required to keep the total trace length constant. These lengths are then propotioned to prevent congested areas in the circuit while maintaining the overall dimensions required of the fabric.

Optical Time Domain Reflectrometry (OTDR) was used to measure and compare the length of several optical fibers. For this particular experiment, which was conducted in the AT&T Bell Labs, a sample circuit of the similar design including several fiber groups routed between the same tab endpoints were fabricated. The leads were stripped and the finer arrays at each end were cleaved. The circuit was fabricated using single mode fiber to minimize pulse broadening. Light pulses from single mode diode laser operating at a repetition rate of 30kHz were launched into the cleaved ends of each fiber. Reflected pulses were oberved, using the sampling oscilloscope, from both the input and output faces of all fibers within the circuit. The total round trip time of flight was measured for several fiber lines, about 23 inches in length. The delay of each fiber was measured to within 3 ps and the variation in the average delay across several fibers was also within this range. The conclusion to this was that the length skew of the routed fibers is controlled to within 0.05 inches or 0.2% or targeted length.

Conclusion

In summary, there is a new and manufacturable solution to the problem of fiber optic management in large system packaging. The flexible optical fiber fabric is a compact interconnect platform that has many advantages over the use of conventional lines and cables. The computer aided fabrication process enables the physical designer to concurrently lay out optical pathways with the capability to add cross connections. The automated routing of optical fiber results in a low cost and a consistent that is easily installed. However, I personally feel that optical fiber circuits nowadays represent the most convenient means of merging dense optical connections into exixting hardware.

References