Fibre still coming to terms with copper

Fibre-optic links have been a mainstay media in the telecommunications industry for many years. Fibre offers a number of specific advantages over copper interfaces, including: much-reduced attenuation and signal distortion over long distances; greatly increased bandwidth; immunity from EMI/ESD; much-reduced cable bulk and weight; reduced latency; no electrical shock hazard or ground loop potential; and greater data security.

Fibre has become the media of choice in medium- to long-haul, high-speed applications across additional segments, including remote storage, networking, cable television and high-performance computing.

The compelling advantages of fibre prompted many predictions of the demise of copper with a fibre alternative. The limiting factor has largely been economic. Until optical computing becomes a reality, fibre links must include electro-optic conversion at both ends, a process that takes up valuable space, consumes power and adds cost. Although the economically practical range of fibre-optic cable assemblies continues to shorten as component costs go down, copper continues to dominate today. Continuing advances in high-speed signal conditioning together with improved cable and connector design have allowed engineers to push copper interconnects to 10+ Gbps with the next target shaping up to be 25 Gbps.

At some point, the laws of physics, including the effects of attenuation, skew, crosstalk and sensitivity to EMI, will begin to limit the practical length of high-speed copper links. All these negative effects increase with speed and length of the link. Many system designers today are choosing copper patch cables from the servers to top of rack, but going with fibre on longer rack-to-rack links to support the aggregated speed. Indeed, shorter cables have become the fastest growth segment for fibre-optic cable assemblies.

To ‘futureproof’ an installation, some systems have adopted fibre in all I/O ports, taking advantage of the reduced cable bulk now, while having the installed infrastructure capable of supporting the bandwidth requirements of next-generation equipment.

Many system designers are choosing to use pluggable I/O interconnects, including SFP+ and QSFP+, which provide both copper and fibre options. These modules offer high-speed I/O as well as increased port density.

Active optical cables, which mate with traditional copper I/O connectors, offer another alternative. Electrical signals are converted to optical pulses within the connector body, which are propagated through fibre cable. Active optical cables add a degree of configuration flexibility in existing installations. Active optical cable assemblies are available with standard SFP+, QSFP+, CXP and CX4 connectors.

At 25 Gbps, it may be difficult for copper backplane interconnects to reach 1 m preferred channel length target without the use of exotic PCB materials, advanced signal conditioning silicone and greatly tightened manufacturing tolerances, all of which add cost. These limitations will continue to open new opportunities for the use of fibre both inside and outside the box.

Efforts to determine how the advantages of FO links can be used inside the box have been under investigation for at least 15 years. A variety of backplane concepts that featured embedded light pipes have been proposed. Optical signals generated on daughtercards would be channelled down to the surface of the PCB, where they would be directed 90° into an optic plane.

Unique backplane connectors would conduct the optic signal across a pluggable interface. Optical links embedded in the backplane would direct high-speed signals to the appropriate daughtercards.

Other concepts embedded the electro-optic conversion into the backplane connector, which allows the use of conventional copper-based daughtercards. Again, added costs, additional risk due to the introduction of new technology and the lack of a compelling application have kept these concepts in the development lab.

Several connector manufacturers, including Amphenol, Molex and TE Connectivity, have introduced optical interconnect systems designed to facilitate the use of discrete optical transmitters on the daughtercard. A pigtail fibre from the transceiver is passed through a conventional backplane to a mating optical connector with an optic loop to another board slot. These physically large transceivers and proprietary connector systems have tended to be expensive and have seen limited commercial acceptance.

More recently, advances in miniaturised optical transmitters, as well as the science of silicon photonics, are presenting opportunities to bring cost-effective optical links inside the box.

About two years ago, Intel announced its LightPeak optical I/O interface, with a bidirectional bandwidth of over 10 Gbps. It remains in the lab, while its copper Thunderbolt sister product has been implemented into a number of Apple-related products.

A series of new optical interfaces designed for high-speed applications inside the box have entered the market.

Reflex Photonics introduced its PCB-mounted LightABLE optical engine that mates with an MT-terminated ribbon fibre connector. Each of these low-profile modules provides 12 transmit or receive channels at up to 11.2 Gbps. The module is surface mounted via BGA attachment. The light on board concept is designed to support optically enabled ASIC packaging by mounting active devices and LightABLE optical engines on the common substrate.

The PRIZM LightTurn connector from US Conec is a miniature separable ribbon fibre connector designed for applications that use high-speed parallel optical transceivers. The unique housing provides accurate prealignment as well as keying and latching. Suppliers, including Molex and Timbercon, now offer custom cable assemblies using the PRIZM LightTurn connector.

Avago has introduced its MicroPOD and MiniPOD parallel optical transmitters, which provide 12 channels, each transmitting at more than 10 Gbps over ribbonised fibre-optic cable. MicroPODs are only 8.2 x 7.8 mm and are designed for direct solder or LGA socket attachment to the PCB.

Altera, a manufacturer of embedded processors, ASICs and FPGAs, announced the first optical FPGA featuring the Avago MicroPOD optical interface. The result is a 12-channel link between devices with an aggregate bandwidth of 150 Gbps. Questions have been raised about these connectors interfering with heat sinks, but the concept of direct chip-to-chip optical links is appealing.

Samtec has entered this market with its FireFly micro flyover system that allows a designer to choose either copper or fibre interfacing to a common PCB connector. The copper option uses 38 AWG ribbon coax cable, while the fibre version includes a miniature optical engine with an integrated heat sink. Both provide up to 28 Gb, bidirectional, protocol-agnostic transmission.

The PCB connector can be mounted on a daughtercard or directly on a semiconductor package to minimise PCB trace losses. Applications include chip-to-chip, board-to-board and system- to-system interconnections.

High-speed communication between chips on the same board can become a performance bottleneck. Copper links experience dielectric attenuation, as well as via structure reflections, skew and frequency-dependent crosstalk. As system speeds continue to increase, this problem will only grow worse. Silicon photonics enables the integration of electrical and optical elements on the same chip. Building optical components on the common silicon substrate allows fabrication of advanced electro-optical chips using conventional chip manufacturing processes.

IBM and Intel have developed hybrid silicon optical devices that operate at up to 40 Gbps. The objective is to increase the bandwidth of chip-to-chip communication by transitioning from electrical to optical signalling on the same chip. These development projects are focused on hybrid photonic devices fabricated on silicon substrates, the base of most current integrated circuits. Except for the vertical-cavity surface-emitting laser (VCSEL) light source, all elements of the optical transmitter and receiver are integrated at the wafer level.

IBM recently announced the ability to fabricate optical and copper structures on the same die. This opens the possibility of powerful processors that feature optical I/O using conventional manufacturing processes. Research is being carried out with the aim of creating direct high-speed optical chip-to-chip communication channels. Cisco recently joined the ranks of photonic IC developers with a prototype chip that integrates a laser, an optical lens, multiplexer and a CMOS modulator. Additional optical communication specialists working on integrated photonic devices include Aurrion, Kotura, Luxtera and OneChip Photonics. Advances in silicon photonics may provide the technology to eventually usher in true photonic computing.

New applications for fibre-optic links inside the box continue to pop up. There have been several recent articles written about interest in implementing PCI Express Gen 3 via optical cabling.

Huge potential exists as networks evolve to 40 and 100+ Gbps data rates. Initial applications would likely appear in enterprise equipment, but could eventually expand into the vast market for consumer products.

Dow Corning and IBM recently announced the development of a new flexible polymer material that can be fabricated into optical waveguides using conventional printed circuit board manufacturing techniques. It is possible that we may start seeing optical backplanes in high-end systems within the next five years.

Broad adoption of on-chip optical interconnects may be the next disruptive technology to shake up the market, but is likely many years off. In the long haul, photon-based computing, if ever realised, could revolutionise the industry. In the short term, miniature active optical cable assemblies now entering the market may offer a cost-effective alternative to copper for interconnection within the box.