Small-form factors herald the next generation of optical components

Small-form factors herald the next generation of optical components

Carsten Schwantes Siemens

Here`s a close look at the ongoing connector competition from the viewpoint of a transceiver and cable assembly manufacturer.

The increasing use of optical fibers in the data-communications and telecommunications sectors is going hand in hand with the miniaturization of the relevant components. The most important elements of a transmission route, namely the connectors and transceivers, must combine ever higher transmission speeds and reduced power dissipation (i.gif., lower space requirement and smaller grid spacing) with higher packing densities and unchanged functionality--all while placing no restrictions on ease of handling.

An important factor for ensuring the continued rapid growth in the market share of fiber-optic technology is the increasing competitiveness of the available fiber-optic systems versus existing copper solutions such as Category 5 and 6 cables.

Optical fibers have long since ousted copper for high-speed transmission applications in the network backbone. Surveys make clear that more than 50% of all network administrators would switch without hesitation to optical fibers in their entire network, assuming comparable costs. Another 42% would seriously consider this option.

These results mean that system costs are of critical importance. Although these costs refer initially to purchases, they also include the administration costs involved in operating the network. Estimates show that between 60% and 80% of all current transmission problems and system faults in networks are due to excessive data rates run via unsuitable copper connections.

This situation led to the development and marketing of competing fiber-optic systems based on different connector approaches. A tough and continuing struggle for market success has developed among them.

Component competitors

The connectors now battling for market dominance include the VF-45, MT-RJ, LC, and SC/DC. Other systems are also available. However, various suppliers have announced active and passive components for the systems presented here.

All of these systems externally adapt the RJ-45 latch principle widely used in copper applications. This external appearance, as well as the fact that they are as simple to use as the RJ-45, facilitate the user`s switch to fiber-optic systems and enhance the competitiveness of these systems in relation to copper.

These systems have already been accepted by the Committee for Fiber Optics 2/6 of the Telecommunications Industry Association (TIA). But the MT-based connectors, which include the MT-RJ, were postponed for several months in view of the discussion on dimensions and tolerances.

All of these systems are based on the transceiver concept; i.gif., they make use of light-emitting diodes (LEDs), laser diodes (LDs), photodiodes (PDs), as well as all necessary drivers and repeater integrated circuits (ICs). Their transceivers thus have the same functions as today`s usual 1ٻ SC duplex multistandard transceivers. Early this year, leading fiber-optic suppliers AMP, Hewlett-Packard, Lucent, Nortel, Siemens, and Sumitomo signed a small-form-factor multisourcing agreement (SFF MSA) that describes the most important dimensions and pin assignments of such future transceivers (see Lightwave, April 1998, page 1). Connectors were excluded intentionally. But the transceiver described in the SFF MSA will support all the connectors mentioned here.

Transceivers

Trends quite apart from the efforts of the MSA can be discerned in the development of transceivers. One of these is their continuing diversification into 1300- and 850-nm systems. In view of the auto-negotiation in Ethernet 10/100 systems widely used in copper solutions, a 10/100 industry standard based on 850 nm is also being considered. The decisive factor here will be the extent to which 1300 nm can remain competitive in terms of cost.

Another trend is the evolving range of functions. In cooperation with leading IC manufacturers and designers, several components manufacturers are working on a greatly simplified pin-preamp LED (LD) converter interface that shifts the remaining functions to the downstream silicon. The overall cost situation for the user still has not been completely clarified, but this solution is also likely to offer savings potential.

Finally, the leading components manufacturers are continuing their discussions on a detachable SFF module. This option is driven by the requirements of many customers for full system configurability, as well as by the components manufacturers themselves, who wish to bypass printed circuit board manufacturing processes. The savings potential in transceiver manufacture thus lies in simplifying the materials and processes involved.

VF-45

Among the systems compared here, the VF-45 connector (part of 3M`s Volition structured-cabling system) is unique in that it dispenses with conventional ferrule technology. This feature opens up the greatest cost-reduction potential of all four connector systems. The Volition system is basically geared to fiber-to-the-desktop applications, making it a direct competitor to copper.

Volition is equivalent to copper in terms of cost. The concept is based on the idea of simple V-grooves in which both multimode and singlemode fibers can be precisely guided and coupled (see "Test procedures measure fiber reliability in ferruleless interconnect design" on page 79). The tolerance situation is not critical, and the position of the fibers in the V-groove is determined essentially by the extremely small tolerances of the fiber diameter. This technology contributes to the cost-effective design of the active and passive components. Extensive investigations and diverse system installations have proved the ruggedness and reliability of this system. Initial reservations due to the bare-fiber concept have been dispelled.

The VF-45 copies the dimensions of the packing density and backplane section found in RJ-45 copper ports. As a result, replacement of copper by fiber is particularly simple. In parallel to this step, active components compatible with SFF MSA are expected on the market to allow the customer to switch from other connector systems at any time. Stronger demand and diverse design-ins from manufacturers such as Cisco, ACT, and Racore led to the formation of the VF-45 Action Group around 3M in July. Its members constitute leading suppliers of fiber-optic system components such as Honeywell, Siemens, Corning, Sumitomo, Methode, Racore, BATM, MicroCosm, and DaviCom.

MT-RJ

The MT-RJ system is based on the MT technology developed by Nippon Telegraph & Telephone (NTT--Tokyo). The MT connector was designed originally as a multi-fiber connector or array plug to couple, transmit, or receive diode arrays to multi-fiber ribbon cables. Transfer-mold ferrule technology was developed to achieve the necessary precision of the fiber array. High-precision metal pins, which are also inserted into the ferrule, are used to ensure exact positioning. The MT-RJ connector is a modification of its larger MT sibling and represents a further development of the Mini-MT connector. It has two or four fibers per ferrule as well as the RJ latch mechanism. Both the Mini-MT and MT-RJ enjoyed a good reputation at an early stage of its development thanks to the alliance between leading manufacturers (Hewlett-Packard, AMP, USConec, Siecor, Fujikura) that spearheaded the connector`s introduction. The MT-RJ is the smallest of all the connectors compared here.

In view of the closely adjacent fibers in transceivers (in contrast to purely transmitter or purely receiver arrays), use of this technology requires the fiber spacing between the two optical channels within the transceiver to be increased from 750 microns to more than 4.5 mm in order to exclude optical and electrical crosstalk.

The expenditure this spacing adjustment involves reduces the cost-reduction potential on the transceiver side considerably. Although the systems available on the market do offer space-saving potential, their costs do not differ significantly from those of a 1ٻ SC duplex system. This factor limits their use to mostly backbone applications, as they offer no significant potential for cost savings in fiber-to-the-desk. This limitation applies to both the active and passive components of these systems.

Cost aside, the standardization of this technology is associated with complex tolerance adjustments to ensure interoperability between equipment from different manufacturers. The core element, namely the ferrule, has come from a single manufacturer. The tolerance situation of the MT-RJ must be regarded as critical, especially for singlemode applications. The last of three redesigns so far has succeeded in reducing the connector`s initial susceptibility to mechanical stresses (such as side load). Concluding investigations are still under way.

LC

The design of the LC connector developed by Lucent Technologies is similar to today`s widely used connectors based on zirconia ceramic. This connector concept has already been tested and widely used in patch panels, and its suitability for both multimode and singlemode applications is generally regarded as proven. The package is equipped with a latch mechanism very similar to the RJ-45 latch. Ferrules with a diameter of 1.25 mm are used for guiding the fibers.

The LC connector combines high packing density and proven connector technology and is thus predestined for singlemode telecommunications applications. Active components also are being offered for it already. The fiber spacing of more than 6 mm allows a transceiver design comparable to current SC duplex transceivers--simpler than with the MT-RJ and SC/DC but more complex than for the VF-45. The connector itself creates no new cost-reduction opportunities compared with current concepts. Its obvious application is in the telecommunications sector.

SC/DC

The SC/DC system has a special position in this comparison because it dispenses with the latch of the RJ-type connectors but makes extensive use of the existing elements of the SC and SC-duplex latch. Instead of the ceramic ferrule found in conventional SC connectors, an extruded circular plastic ferrule with two or four fibers and a diameter of 2.5 mm is used. The fiber spacing corresponds to that of the MT-RJ--and also gives rise to similar problems.

The SC/DC dispenses with metallic guide pins, which give it a cost advantage over the MT-RJ but reduce its singlemode capability. The mutual alignment of the connectors is implemented via guide grooves in the plastic ferrule. This connector has hitherto been used in passive cabling systems. No active components are yet available for it.

The SFF for the job

The MT-RJ and SC/DC focus essentially on data-communications applications currently serviced by the SC duplex. In contrast, the LC connector concentrates on the telecommunications singlemode sector and offers an alternative to currently used connectors like the SC duplex, SC simplex, and FC. The VF-45 opens up new applications in the high-volume fiber-to-the-desk market but will also serve today`s backbone applications.

In the near future, the MT-RJ, SC/DC, and VF-45 will have to prove their singlemode capability. The LC is the only SFF connector that already has a secured application sector and will be used there at a correspondingly early stage.

The transition from today`s connectors to SFF concepts can be expected, but at differing rates depending on the application. The undisputed success of one or two of these connectors will essentially depend on the formation of further alliances in the components sector. The most interesting potential for users is in competition with copper solutions. In particular, it is now possible to dispense with the frequent re-installations and new wiring operations required for every new generation of copper systems and to invest in future-proof optical-fiber solutions for high-speed networks all the way to the desktop. u

Carsten Schwantes is small-form-factor program manager at the Fiber Optics business unit of Siemens AG Semiconductors (Berlin, Germany).