Opto-electronics -> High-density fiber-optic modules eye next-gen switching architecture

To meet the demand for increased capacity while keeping the overall size of the equipment reasonable, designers have turned to optical-component manufacturers for higher-density solutions. They are demanding smaller optical modules for traditional port-side (customer-accessible) applications. And with the advent of scalable terabit switching fabrics, the proprietary system interconnects, traditionally made with copper, are also transitioning to fiber optics.

Port-side optics need to provide links with very short reach (VSR; 300 meters), short reach (2 kilometers), intermediate reach (15 km) and long reach (40 km) at data rates of OC-3 (155.52 Mbits/second), OC-12 (622.08 Mbits/s), OC-48 (2.488 Gbits/s) and OC-192 (10 Gbits/s).

With the exception of the VSR links, the port-side optics are connected via single-mode fiber, while the 300-meter link distance requirements for OC-192 VSR connections open the door for lower-cost multimode solutions. In all cases the optic modules must be designed to run Sonet/SDH applications.

To let network equipment makers achieve the port-side density objectives of next-generation designs, optical-component manufactures have responded with small-form-factor (SFF) transceivers with LC connectors. The SFF LC transceiver offers equipment manufacturers the industry's highest-density solution for OC-48, OC-12 and OC-3 Sonet/SDH applications. The transceivers require only 0.54 inch of space on the card edge — a reduction of about 50 percent compared with SC connectors, resulting in a doubling of the data capacity of a line card.

Major advantages of the SFF multisource-agreement compliance include the following.

- Twice as many small-form-factor ports can fit as SC or ST ports, since SFF is half the size of an SC connector (on a duplex or two-fiber connection).

- Copper or fiber fit in the same real estate — 24 copper ports or 24 duplex LC ports occupy the same panel space, thanks to the higher-density SFF interface.

- It saves space on patch panels and communications outlets.

- Familiar RJ-style latching is used.

- Thanks to a common footprint, SFF transceivers from different vendors ca n be interchanged.

- More than 11 million LC connectors have been proven in the field.

- Multisource agreements exist for both pin-through-hole and hot-pluggable transceivers.

- IEEE-1394 Firewire and TIA/EIA-568B standards are met.

The doubling of the line card capacity, coupled with an architecture shift from copper to fiber, has driven the need for a very high-density optical module. In the newly emerging switch architectures these optical modules will be used to link line cards to switch fabric and switch fabric to switch fabric. The density requirements of proprietary-system interconnect modules are much greater than for port-side modules, since they will be used to provide redundancy and flexible partitioning of bandwidth.

In addition, the need for scalable switching architecture means that pieces of equipment that must be interconnected can be up to 300 meters apart. The interconnects have conventionally been made using copper cabling.

Copper's pra ctical length is limited by the data rates it carries. At higher data rates, copper suffers significant signal loss and electromagnetic interference. As telecommunications central offices expand, cabinets must be linked over greater distances, and copper connections at these high data rates are currently limited to a range of approximately 5 meters.

Copper exhibits an order-of-magnitude higher skew (the difference in the delay time for a signal to propagate along different wires or fibers) than fiber. Copper interconnects are much more difficult to scale to higher bandwidths than optical interconnects.

One solution offering an almost ideal combination of characteristics for this challenge is parallel optics. Designed as individual 12-channel transmitter and receiver modules, parallel optics offers tremendous density and low power dissipation. Using vertical-cavity surface-emitting lasers (VCSELs) as light sources, parallel-optic modules are designed to operate at speeds of 1 to 2.7 Gbits/s per channel. This provides a total aggregate data capacity of 30 Gbits/s per pair and requires only 1.5 inches of space on the card edge.

The modules are attached to the printed-circuit board with a surface-mount-capable ball-grid-array package and are designed to be installed with industry-standard pick-and-place equipment. The mechanical interfaces of the modules are industry-standard MPO connectors, which can be easily terminated on 12-wide ribbon cables. Using standard 500-MHz-km 50-µm ribbon fiber, the parallel modules are capable of achieving link distances of 300 meters.

Although Agilent is offering integrated 12-channel parallel-optic transmitters and receivers, several companies are providing portions, including ICs for parallel-to-serial and serial-to-parallel conversion and combinations of ICs with VCSEL emitters and photodetector arrays. Multisource agreements for these kinds of parallel-optic modules will certainly be published. For example, last spring, Mitel Corp. (Ottawa ) and Agilent signed a multisource agreement that defines a standard package and electrical and optical interfaces for four-channel parallel fiber-optic modules.

The OC-192 VSR application is a port-side connection that must achieve link distances ranging from 2 to 300 meters. A range of optical components, including serial and parallel modules, can be used in these very short-reach applications. Twelve-channel parallel optics offers a high-density solution.

Remarkably, leveraging the volume of proprietary system interconnects and low-cost VCSELs, the 12-channel parallel-optical line offers a solution that is half the cost of serial transceivers.