IS IT NECESSARY?

The computer systems' point of view

Nowadays with data rates in the region of Gbits/sec, electrical connections have become a bottleneck because of cable dimensions and the space needed for the sheilding parts which have to avoid the signal degradation. A solution to overcome these restrictions is the use of fiber optics. For board to board or frame to frame connections space saving solutions with very high transmission capacity can be established with fiber optic ribbon cables and special designed array connectors.

...electronic applications on the other hand....

As sophistication of functions performed by electronics continue to increase throughout the 90's, data communications will play an increasing role. Accordingly, electromagnetic compatibility will become increasingly difficult to attain both functionally and economically with all-conductor based data transmission. The need for alternatives such as fiber optic connections become apparent.

Content | Introduction | Is It Necessary? | A Parallel Solution | An Electro-Optic Connection System | Conclusion

A PARALLEL SOLUTION

In the development of computer systems the number of processors are rising and the clockrate and even the number of interactions are growing rapidly for each processor. Therefore the capacity of the information transmission has to grow in a similar manner. This is done by spreading the bus lines and rising the transmission speed of each line e.g. to 64 or 128 bit parallel with 100Mbaud each or more. With rising frequencies electrical connections of this type show severe problems due to different electrical effects like crosstalk, reflections and sensitivity to electro magnetic interference. With parallel optical fiber interconnections these restrictions can be solved.

• First there is practically no bandwidth limitation up to some Gbaud per line and no electrical signal erosion.

• Secondly, the optical high density backplane connection is a space saving solution which needs a very small back panel area only even for massive parallel connections.

Figure 1 below shows an exploded view of the high density optical backplane connector. The combination with parallel optical module will provide an optical solution for bus transfer rates up to several 10Gbaud.

Figure 1

A further application is the construction of high capacity switching nodes like ATM-switch networks with switching capabilities of several 100 Gbaud. The switching is done electrically on special boards. Between these boards a large number of signal lines with data rates of about 800 Mbaud for STM4 has to be managed. Great care has to be taken with the electrical crosstalk between the individual lines. For the electrical board connections via the backpanel large space is needed which causes the bottleneck for the system.

With the use of optical fibers these problems can be avoided. The individual signal lines show no electrical crosstalk. For the backpanel a very high density feed through can be achieved with the connector which is demonstrated here.

The Connector System

The connector system consists of three main parts: (refer Figure 1)

• The board side mounted connector.
  This consists of two parts. The first is the system frame which has the same outside dimensions as a usual female connector and has the same foot print for the printed circuit board. The second part is designed as a small drawer and is snapped into the first frame. Ribbon cables leave the drawer between the snap arms and can be directed to the desired modules on the board. This drawer has four single subrooms to pick up four MT-ferrules including the accompanying four spring.

• Backpanel parts for the optical feed through.
  Four tubes feed and center the pairs of MT-ferrules meeting in the middle of the backplane. This centre part is metallised and electronically connected with the sheilding parts of the backplane.

• The rear backpanel connecter.
  Up to four MT terminated ribon fiber cables can be inserted (Each fiber contains 12 fibers). Any type of multi fiber arrangement can be inserted into this back pannel connector housing.

This high dense array connector can be arranged with a wide range of cable types, fibercounts and connectors. For each application an optimal configuration can be found.

Content | Introduction | Is It Necessary? | A Parallel Solution | An Electro-Optic Connection System | Conclusion

AN ELECTRO-OPTIC CONNECTION SYSTEM

As electronic content and sophistication continue to increase the need for electromagnetic compatibility becomes pronounced. In addition, the costs of routing high data rate signals around systems are prompting designers to look for alternatives to metallic wires. Here we look at three designs designed not by computer or telecommunication engineers but automotive engineers. In comparison to the above (A Parallel Solution), the focus here is mainly on connections between circuit boards. We will look at optimum ways of packaging connection devices used for connecting the fiber optics with the circuit board.

Harness Connector Packaging - mating electrically.

Figure 2

The mated connection is electrical rather than optical. The electrical lead length would be longer than the other options and hence results in increased electromagnetic radiation from the LED because of the relatively high current.

On The PC Board (Pigtailed) - mating fiber to fiber

Figure 3

Here is an improvement towards the previous design where the electro-optic device is mounted to the PCB with a fiber pigtail to connect to the module connector. The electromagnetic compatibility is also improved. But there are two main disadvantages. First, there is an added optical interface involved. The module connection would be fiber-to-fiber connection but the device requires a pigtail connection which increases the cost. Secondly, the fiber pigtail would require special attention to minimise the opportunities for damage.

Header Connector Packaging - mating fiber to device

Figure 4

This design offers all the advantages of the pigtailed version without the disadvantages. Here device lead length is minimised, reducing electromagnetic interference. The devices are not pigtailed, so the device cost is reduced and since the devices are in the header connector, occupied PC board space is reduced.

The figure below shows the components of the electro-optic connection system.

Figure 5

Content | Introduction | Is It Necessary? | A Parallel Solution | An Electro-Optic Connection System | Conclusion

CONCLUSION

Here a new high density optical backplane system was demonstrated (A Parallel Solution). Up to 48 fibers can be connected through the backpannel and hence improves interconnections between multi boards and frames. The introduction of optical fibers not just improves bandwidth but also saves space in the computer systems.

The electro-optic connection system on the other hand can meet performance requirements in many electronic applications and offers a low-cost, high reliability connection system. Field terminations can be made easily and the connection system is robust, by design, to withstand abusive handling.

Use of these connection system in provides means of increasing the volume usage, which in turn results in reduced cost and spurs increased development activity of products which can capitalize on the combined applications of products which can capitalise on the combined applications of computer systems and non-computer system markets.

Content | Introduction | Is It Necessary? | A Parallel Solution | An Electro-Optic Connection System | Conclusion

2. Siemens AG: Products, Solutions & Services
http://www.siemens.de/products/frames_e.htm

3. Photonic Systems Group, IIT Canada
http://alpha.ps.iit.nrc.ca/english/photonics.html