by Michael Tryson, Sr. Fiber Optic Engineering Manager, Erin Byrne, Director of Fiber Optics Engineering, TE Connectivity Ltd., Harrisburg, Pa.

The need for super-speedy data connections has brought advances in fiber optic connection technology that could spill over into automotive and industrial uses.

Big data calls for big data flows. That is one of the reasons for the intense development of fiber optics technology as a means of handling data and network traffic. One outcome of this interest in fiber optics is the advent of advanced fiber connections called active optical cables (AOCs) and transmission techniques that simplify the process of hooking up optical data paths.

The most immediate benefits of these better fiber optic connections will accrue to data centers. Fiber optics are the only practical way of handling the data streams measured in the tens of gigabits-per-second that characterize modern server farms.

Energy efficiency is also an issue. Data centers can use 80 times the energy of traditional office space and a single rack can dissipate 20 kW. New AOC technology consumes less power than older-generation equipment providing the same bandwidth. Moreover, AOCs are easy for installers to handle and could eventually find their way into applications far removed from data centers. One candidate is that of rugged industrial uses associated with machine control. Even automotive systems could eventually benefit from new fiber technology as costs decline with rising part volume. Here, fiber optics could soon help support the trend toward data-intensive infotainment systems.

User-friendly fiber optics
To better understand AOC connections, it can be useful to briefly review the traditional methods of making connections between fiber optic cables. Most optical fiber connectors have been spring-loaded with the fiber faces pressed together when the connectors mate. An example of this style of connector is the MPO, for multi-fiber push-on. There is physical glass-to-glass or plastic-to-plastic contact with this method. The contact eliminates signal losses that would otherwise arise from an air gap between the joined fibers. Typical connectors are rated to handle up to about 1,000 such mating cycles.

There are several different types of connectors that make contact between the fibers this way. The main differences among types of connectors are dimensions and methods of mechanical coupling. Some connectors use a fiber with a polished end and a ferrule end such that there is a slightly convex surface with the apex of the curve accurately centered on the fiber. Then when the connectors mate, the fiber cores touch each other.

Some manufacturers have several grades of polish quality, with higher grades of polish giving less insertion loss and lower back reflection. Many connectors have the fiber end face polished at an angle to prevent light that reflects from the interface from traveling back up the fiber. Because of the angle, the reflected light does not stay in the fiber core, but instead leaks out into the cladding. Thus, mating an angle-polished connector to a non-angle version causes a high insertion loss.

A point to note about these connection schemes is that they are all sensitive to dust, damage to the end face geometry, and need a high mating force to make a reliable connection.

The AOC approach to fiber optic connections eliminates the possibility of fiber-end damage and the need for high mating forces. The idea is to expand the light beam before it enters the physical connection, pass the expanded light beam to the other connector, then recondense the beam as it travels out of the recipient connector. In this design, a collimating lens takes the light traveling through the fiber and expands it to encompass about four times its original diameter. As a result, a speck of dust at the connector interface blocks a significantly smaller percentage of the light passed through the connector than would otherwise be the case.

The use of collimated beams at the connector interface has only become practical in recent years with the development of optical transceiver electronics small enough to fit in a connector shell. The transceiver on the sending end converts electrical signals to light with enough optical power to ensure light passing through the collimating lens can make it through the interface without losing information. The transceiver on the receiving end converts the light back to electrical impulses. Thus, the connector embeds optics and transceivers in the connector. The design approach makes it possible to devise high-speed data links much less expensively than if transceivers and fibers were separate entities.

The electronics that serves as the transceiver is called an optical engine. The optical engine typically consists of a vertical-cavity surface-emitting laser (VCSEL) for generating light, a PIN photodiode detector array and control logic circuitry. The PIN photodiode converts light to electrical signals for the control logic. (The VCSEL gets its name from its emission of a laser beam from the top of the chip surface rather than from the edge, as with conventional edge-emitting or in-plane lasers.) A VCSEL in this category is the Coolbit optical engine made by TE Connectivity.

There is no physical contact at the end face of an AOC, so expanded-beam interconnects have slightly more insertion loss than than an MPO device, 0.6 dB compared to 0.3 to 0.4 dB. But expanded-beam interconnects tolerate more misalignment than conventional connectors. And they take less mating force as well, about 3 to 4 N compared to about 20 N for an MPO device. Because there is no end-face fiber polishing and end-face interferometry involved in fashioning an AOC, the manufacturing process is simpler than for an MPO and is more easily automated.

The first application for AOCs will be in fiber connections on server racks as found in data centers. The first such connector is called the MXC and is made by U.S. Conec Ltd., Corning Inc., Molex Inc. and TE Connectivity Ltd. are all partners in its development. The MXC was announced in 2013 and now is in the final phases of being released.

The key component in the MXC is the optical engine. It consists of the Coolbit VCSEL and ancillary control logic. The Coolbit chip is a relatively efficient version of a VCSEL device. For example, a Coolbit chip dissipates about 1.5 W/transceiver compared to about 2.5 W for comparable devices when working at 25 Gbit/sec.

The mechanism that limits any VCSEL is the speed with which the laser can be modulated on and off. Coolbit VCSELs today work at 25 Gbit/sec, and research indicates the technology can be extended to hit speeds above 50 Gbit/sec.

Finally, the primary focus of AOCs today is in making connections between data center servers in the style of the classic trunk cable. It is likely the technology will be extended in the future to handle card-to-front panel connections and eventually card-to-card signals.

Reprint info >>

U.S. Conec Ltd.
www.usconec.com

Corning Inc.
www.corning.com/opcomm/nafta/en/markets_applications/lanscape/unicam.aspx

Molex Inc.
www.molex.com

TE Connectivity Ltd.
www.te.com