The ABCs of
Optoelectronic
Datalinks for
Mission-Critical
Applications
An Introductory Primer
for “The Rest of Us”
Glenair is in the business of making the interconnect components and wiring systems used to carry power and data from one point to another in applications such as aircraft avionics. The overwhelming majority of the technologies we produce are electrical and use copper cabling and contacts as a transmission “medium” to move electrons throughout systems comprised of sensors, controls, and other electronic devices. By contrast, fibre optic interconnect systems utilise photons (light energy) and glass fibre cable as their transmission medium. Fibre optic interconnect technologies are used when data rates, distance, or other considerations such as size, weight or EMI immunity are key requirements. But regardless of the medium (electrical or optical), the system devices that produce and consume the signals and power are always electronic. In other words, they require that the transmitted signal be delivered in a form that can be read and processed electronically. This is where “Optoelectronics” comes in. Optoelectronics (also known as Photonics) are board-level devices that convert received electromagnetic energy into optical energy (and vice versa). The big event takes place on a printed circuit board, via devices called “transceivers” that receive, convert and transmit optical and electrical signals. This special issue of QwikConnect is geared for systems engineers, PCB designers and others who would like to gain a complete understanding of Glenair’s optoelectronic capabilities, especially when it comes to the many devices we produce designed for rugged high-temperature, high vibration and space (radiation belt) environments. This short introduction is geared for “the rest of us” that need a basic understanding of optoelectronics and the applications they serve.
The conversion of electrical energy to visible optical energy is really not so grand of a thing. People have been doing it for years. A common light bulb, for example, performs this conversion with the simple flip of a switch. Incandescent light bulbs, use a thin filament of tungsten wire to manage the conversion. The current flowing through the wire generates heat, some of which is emitted in the form of visible light. The use of light energy (photons) for communication is also not particularly new. Signal lamps have been used in naval vessels for generations. Modern signal lamps deliver a focused pulse of light in direct line-of-sight or via reflection on clouds. The light pulse is achieved by opening and closing shutters enclosing the high-intensity light source. Signal operators use Morse code (essentially an on/off pattern of dots and dashes) to spell out the message. Despite the old school feel of this technology, it is the grandparent of modern fibre optic communications. The principal difference is that modern high-datarate systems use modulated light (a narrow band or linewidth of light) to propagate digital pulses of light (ones and zeros) down an optical fibre instead of through the open air. As mentioned before,conversion is a two way street. Photonic transceivers must convert electrical energy to optical energy but also optical to electrical. Photovoltaic (PV) cells, used in solar panels are commercial implementations of this technology and are able to convert sunlight directly into electricity for use as power. When sunlight strikes the PV cell, it is absorbed within specially formulated “semiconductor” material. In the process, the absorbed energy knocks electrons loose, allowing them to flow freely as electrical current. In fibre optic interconnect datalinks, the role of the photovoltaic cell is performed by a miniature photodiode, an optical detector that converts photons into electrons by a process called “quantum absorption,” originally documented by Albert Einstein. In optoelectronic interconnect systems, optical transmitters and receivers (or transceivers when the two functions are combined) are integrated circuit assemblies built from radiation-resistant and heat / vibration tolerant lasers, controllers, amplifiers and other components. The function of these key technologies are discussed in depth in the following pages. For now it’s enough to note that these units are the “optical engines” that convert electrical data to optical pulses —and vice versa—and are incorporated into a wide range of COTS and custom optoelectronic solutions.
