Fast forward to the 1970’s, when the effort to produce a commercially viable optical communications technology was finally bearing fruit. And just as in Professor Bell’s day, the major focus was still on the invention of a transmission media capable of carrying light signals over long distances with acceptable levels of attenuation (power loss). During transit, light pulses lose some of their energy. Researchers had pegged the acceptable loss value as 20dB/km or less. That is, 1% of the light entering the media would remain after traveling 1 km.

In the early 1970’s, scientists at Corning Glass successfully developed a glass fiber with a loss of 20dB/km, and in the late 70’s DuPont introduced the first small diameter acrylic fiber with similar attenuation.

Commercial applications of the new optical fiber followed quickly. Telephone companies rapidly began replacing copper wire backbones with optical fiber lines in both regional phone systems as well as in long distance, inter-city telecommunication systems.

Cable television companies also began integrating fiber optics into cable systems as bandwidth upgrades to the trunk lines interconnecting central offices and regional hubs or nodes. Many colleges, universities, office buildings, and industrial plants also began to make use of fiber to interconnect mainframe and local area network computing systems - both to increase bandwidth, and also in an attempt to improve reliability of their increasingly complex computer networks.

The use of fiber optics in avionics and in other military aerospace applications also saw its debut around this same time. In 1976 the U.S. Air Force replaced a wiring harness of an A-7 aircraft with an all optical data link in its airborne light technology program (ALOFT): 302 electrical cables, over 1,200 meters in length and weighing over 40,000 grams were replaced with 12 fibers, 76 meters in length, weighing less than 1,700 grams.

Today, the use of fiber optic systems to carry digitized video, voice and data is universal. In business and industry, fiber optics have become the standard for terrestrial transmission of telecommunication information. In military and defense, the need to deliver ever larger amounts of information at faster speeds is the impetus behind a wide range of retrofit and new fiber optic programs. Although still in its infancy, fly-by-light flight control systems may someday replace fly-by-wire systems with cabling which is both lighter, smaller and safer.

Today, fiber optics, combined with satellite and other broadcast media, represents the new world order for both commercial telecommunications as well as specialized applications in avionics, robotics, weapon systems, sensors, transportation and other high performance environments.

Here’s What You Get

Optical fiber is particularly useful in airborne applications due to its electromagnetic immunity (see QwikConnect Vol. 6 No. 2). Since fiber optics use light to transmit signals, it is not subject to electromagnetic interference, radio frequency interference, or voltage surges. The total electrical isolation of fiber also makes it a safer, spark-free media for use in hazardous environments, such as aircraft fuel cells. This characteristic also provides for enhanced transmission security, as light pulses, unlike electrical signals, are almost impossible to intercept or monitor.

But the most important benefit of fiber as a transmission media is its huge bandwidth and low data loss. Fiber can transmit a mind-boggling quantity of data with extremely good transmission quality. Two strands of optical fiber, both no thicker than a human hair, can transmit the equivalent of 24,000 telephone calls simultaneously. By way of comparison, two strands of copper wire can transmit but a single phone conversation - in a much heavier and larger cable. Doing the math, the smaller and lighter fiber strand has over 150 times the data carrying capacity of the bulkier copper cable. Additionally, the data is transmitted digitally (the natural form for computerized equipment) rather than analogically, which reduces translation errors and bottlenecks. Simply put, fiber can transmit signals over the longest distance at the lowest cost.

Connect the Dots

The "encoding" side of an optical communication system is called the transmitter. This is the place of origin for all data entering the fiber optic system. The transmitter essentially converts coded electrical signals into equivalently coded light pulses. A light-emitting diode (LED) or an injection-laser diode (ILD) is typically the source of the actual light pulses. Using a lens, the light pulses are funneled into the fiber optic connector (or terminus), and transmitted down the line.

Light pulses move easily down the fiber optic line because of the principle of “total internal reflection,” which basically holds that whenever the angle of incidence exceeds a certain value, light will not emit through the reflective surface of the material, but will bounce back in. In the case of optical communications systems, this principle makes it possible to transmit light pulses down a twisting and turning fiber without losing the light out the sides of the strand.

At the opposite end of the line, the light pulses are channeled into the “decoding” element in the system, known as the optical receiver or detector. Again, the actual fiber to detector connection is accomplished with a specialized fiber optic connector/terminus. The purpose of an optical receiver is to detect the received light incident on it and to convert it to an electrical signal containing the information impressed on the light at the transmitting end. The information is then ready for input into electronic based devices, such as computers, navigation control systems, video monitors and so on.

A Little Fiber Goes a Long Way

Surrounding the cladding is a buffer material which acts as a shock absorber to protect the core and cladding from damage. A strength member, typically Aramid, surrounds the buffer adding critical tensile strength to the cable to prevent damage from pull forces during installation. The outer jacket protects against abrasion and environmental damage. The type of jacket used also defines the cable's duty and flammability rating.

Rays of light passing through a fiber do not travel randomly. Rather, they are channeled into modes - the thousands of possible paths a light ray may take as it travels down the fiber. A fiber can support as few as one mode and as many as tens of thousands. The number of modes in a fiber is significant because it helps determine the fiber's bandwidth. Multimode fiber has a much larger core than singlemode fiber, allowing hundreds of rays of light to propagate through the fiber simultaneously.

Singlemode fiber has a much smaller core, allowing only one mode of light to propagate through the core. Paradoxically, the higher the number of modes, the lower the bandwidth of the cable. The reason is dispersion.

"Modal" dispersion is caused by the different path lengths followed by light rays as they bounce down the fiber (some rays follow a more direct route down the middle of the fiber, and so arrive at their destination well before those rays which waste their time bouncing back and forth against the sides). “Material” dispersion occurs when different wavelengths of light travel at different speeds. By reducing the number of possible modes, you reduce modal dispersion. By limiting the number of wavelengths of light, you reduce material dispersion.

Singlemode fibers are manufactured with the smallest core size (approximately 8 - 10 um in diameter) and so they eliminate modal dispersion by forcing the light pulses to follow a single, direct path. The bandwidth of a singlemode fiber so far surpasses the capabilities of multimode fiber that its information-carrying capacity is essentially infinite. Singlemode fiber is thus the preferred medium for long distance and high bandwidth applications.

Multimode fiber is generally chosen for applications where bandwidth requirements fall below 600 MHz. Multimode fiber is also ideally suited for short distance applications such as interconnect assemblies used within a single premise or contained space. Because of its larger size, multimode fiber is easier to polish and clean than singlemode, a critical concern in interconnect applications which expose the polished ends of the fibers to debris during connector mating and unmating.

Stand and Deliver

In many such aerospace applications, fiber optics are being employed as replacements or upgrades to existing copper conductor cable harnesses servicing existing black box flight deck equipment, weapon systems, surveillance cameras, sensors, and so on. In all applications of this caliber, the new fiber optic system must adhere to the same rigorous qualification standards and performance requirements that applied to the legacy electrical systems.

For this reason, the design, configuration and packaging of fiber optic interconnects has closely mirrored existing Mil-Spec standards, such as those covering interconnect mateability, accessory interface dimensions, material finishes, and so on. The design of fiber optic termini, special purpose backshells and other accessories is similarly controlled by existing packaging requirements and aerospace industry dimensional standards.

Over the past two decades there have been dramatic tolerance improvements in terminus design to insure precise, repeatable, axial and angular alignment between pin and socket termini within the connector shell. Ferrule design, critical to the performance of the termini, has traditionally relied on a machined stainless steel ferrule incorporating a precision micro-drilled hole.

Glenair’s fiber optic termini for D38999 Series III connectors are designed to meet MIL-T-29504/4 and /5 requirements. Unique precision ceramic ferrules, with concentricity and diametric tolerances controlled within a micron (.00004 of an inch), meet the needs of high bandwidth and low allowable insertion loss applications. Glenair’s ferrules are approximately 10 times more accurate than alternative designs, and have reduced insertion loss values from 1.5dB to less than .5dB.

Glenair custom single and multichannel fiber optic connectors utilize the latest composite thermoplastic materials technology and are designed for use with Glenair’s family of fiber optic connector accessories.

Fully Accessorized

Glenair’s composite thermoplastic fiber optic accessories - including elbows, transitions and end-bells - are designed with smooth 45° or 90° bends to insure the non-abrupt routing of the cable. Composite quick clamps and heat shrinkable boots provide strain relief without applying severe compression to the cable. Glenair’s cable overmolding capability enables the integration of unique straight or angular shapes directly into the cable to insure the best possible fiber position and alignment.

Glenair’s "Fiber-Con" backshells are specifically designed to meet the unique requirements of the media. For both single fiber leads as well as multichannel applications, “Fiber-Cons” provide full support and vibration dampening while allowing the fiber to “float” as required to eliminate micro-bending. Fiber optic terminations differ from electrical in one critical way: during connector mating the fiber optic spring-loaded socket or pin retracts from .040 to .080 inches. It is critical that the backshell design accommodates this movement within the shell cavity to prevent data loss due to micro bending which leads to localized light refraction. The unique rubber support grommet utilized in Glenair’s design employs the same layout pattern as the connector shell - providing both necessary axial alignment, as well as strain relief and float.

May We Have the Bill Please

Taking the long view, investing in the conversion to fiber optics often makes good sense, especially given the performance benefits - EMI immunity, security, weight reduction, bandwidth, etc. - as well as cost of-ownership factors such as reduced cable maintenance costs and ease of installation. The ability to more easily accommodate future bandwidth requirements as well as the ability to incorporate redundant fibers for improved safety and reliability further reduces the long-term cost-of ownership. Glenair has worked closely with engineers on a broad range of programs - from the F-22 to the RAH-66 Helicopter - to analyze system requirements and to design high-reliability fiber optic solutions that meet both short and long term cost requirements.

Full Circle