See the Light: Keep your eyes on fiber optics

See the Light

Keep your eyes on fiber optics

Fred WorkleyBy Fred Workley

November 2001

The fiber optics industry has developed rapidly with many new innovations. Those of us that are involved in aircraft maintenance now have to add knowledge of basic photonics to our list of skills. We are aware of fly-by-wire and now we are finding more and more applications of fly-by-light.

Fiber optics
Optical fibers are a key component in photonics. They are long, thin strands of treated glass — about the diameter of a human hair and are arranged in bundles that form cables. The light bounces off the cladded surface of the cable similar to that of a flashlight on multiple mirrors.
When we transmit signals over a wire using electricity, electrons move over the wire. Photonics sends light through glass and is cheaper and faster than wire. Optical cable costs far less than copper wire. Furthermore, optical fibers are thinner and more flexible than copper. This permits more lines or channels in a smaller space thus weighing far less than copper. Photonics use less energy while carrying a higher volume of information over a longer distance without any loss. Also, unlike electricity in copper wires, light signals do not generate heat or cause a fire hazard. There is no interference with other wires, which eliminates shielding costs and improves quality of signals. Light signals do not degrade as much as electrical signals and thus require less power to be added to maintain the original signal strength.

The more bandwidth, the more capacity is available to pass higher volumes of data, audio and video. Recent innovations are primarily directed to increasing bandwidth. The new ways to increase volume and speed thus reduce costs are the so-called "next generation" of equipment for fiber optics.

Two approaches: WDM and DWDM
With Wave Division Multiplexing (WDM), multiple wavelengths can be sent down a single fiber at the same time, multiplying the amount of data that can be sent by using different wavelengths of light. Dense Wavelength Division Multiplexing (DWDM) refers to spacing between the various wavelengths. The most advanced systems can stream up to 160 waves of light down a single fiber.
Replacement of every electronic function in data collection systems can be done with all-optical networks. The latest technology creates optical switches and systems that keep the signals traveling as photons at every part of the system rather than ever converting them back to electrons passed around wires. The industry offers an extensive line of optical fibers and accessories, including patch cords, bifurcated assemblies and splitters, for a variety of UV-VIS and VIS-NIR applications. All optical fibers couple easily via SMA terminations.

A fly-by-light aircraft system is a closed–loop system. It can reduce take-off weight by eliminating pneumatic, hydraulic, and mechanical systems. Ground equipment is also reduced. Along with weight savings is EMI (Electro Magnetic Field) immunity. Some of the challenges that have already been met are fiber optic position sensor development, decoding of electronics into optics, optical power management, and cable design and manufacturing for aircraft applications meeting vibration standards.
The wireless flight control system (WFCS) is a reality using a Jam Resistant Composite Actuator (JRCA). This is done through Feed Forward Remote Units with an Interface Converter through Dual Redundant Actuators. The major components are the Flight Control Computer, an interface box, aircraft 400Hz power source, Power Control Monitor Electronics and an actuator. There are essentially three types of actuators: EHA (Electro-Hydraulic Actuator), EPAD (Electrically Powered Actuator) and EMA (Electro-Mechanical Actuator).

Light Emitting Diodes
Since the incandescent lamp is seeing fewer aviation design applications, we are seeing more applications of fiber optics and high-efficiency light emitting diodes (LEDs). Fiber optics is a source of light that doesn’t produce any heat. The applications include general illumination, status illumination, panel overlays, back lighting, and illuminating push buttons. LEDs are used in liquid crystal displays. Both fiber optics and LED brightness is measured by the number of photons streaming from a point.

Photoacoustics can be used to detect water in oil. Water contamination in lubrication and hydraulic oil is one of the primary causes of destructive wear and corrosion. The system detects moisture with Fourier transform infrared spectroscopy that can detect water in oil with concentrations as low as 100 ppm by using a layered-prism cell. Photoacoustics spectroscopy measures the acoustic wave that is generated in a substance returning to the ground state after optical stimulation.

Looking for leaks
Fiber optic sensors can be used to detect gas leaks. This application incorporates optrodes coated with a hydrogen-sensing substance. Light travels via an optical fiber cable to the optrodes (the optical-based equivalent of the glass pH electrode) and the color and intensity of the returning light indicates the presence of and the concentration of hydrogen. A similar application detects the presence of nitrogen and carbon dioxide. For applications where high pressures are present, sapphire windows are used. This window holds out pressure while permitting light to reach the fiber optics.

Inerting systems using fiber optics
An interesting new system has recently been developed to
1. Monitor environments in cargo holds, other inaccessible areas, and fuel tanks of aircraft based on the measurement of the level of oxygen in the environment under surveillance.
2. To develop a new fire suppression system based on miniaturized cryogenic technology and liquid nitrogen and any other agent that displaces oxygen. It uses a patent pending application that incorporates miniature fiber optic spectrometers, sensors, light sources and sampling optics.
The system monitors oxygen levels. The inerting system integrates a series of fiber optic sensors into a microprocessor-based controller that will provides data to the cockpit, fire suppression controller, and the aircraft data highway. The product will be designed in a building block configuration allowing economic selection of components for different size aircraft while insuring commonality of maintenance procedures and spare parts. The oxygen monitoring system will have a very low preventative maintenance requirement and will incorporate self-diagnostics and modularity to ensure ease of corrective maintenance.
Accurate measurement of the oxygen levels permits the required release of an inert gas into the monitored environment. This starves the fire of oxygen, thus prohibiting further combustion.

Gas spectrometry
A spectrometer-based gas sensor is a critical component of the system. Gas spectrometry has historically relied on large, delicate instruments used in a laboratory environment for batch sample gas and liquid analysis. Recent developments in the science have resulted in miniaturization of the optical bench and simplification of the associated electronics. The spectrometer used in the aircraft fuel tank monitoring-system relies on an optical bench that is small enough to fit in the palm of the hand. The bench is mounted directly onto a printed circuit board (PCB) where the electrical components are soldered in place and the optics are creatively designed to allow beam focusing in three dimensions. Once adjusted, the optics become part of a very rugged, low cost assembly that can be assembled into a cage of multiple spectrometers.
Although the unique aspect of the approach is to use miniaturized spectroscopy, a further development was required to enable monitoring for oxygen. Scientists determined that blue light from an LED could be quenched by diffusion of oxygen into a chemical thin film coating. The resulting signal attenuation is dramatic and clearly recognizable by state of the art electronics and an embedded microprocessor.
The Fiber Optic Oxygen Sensor uses a fluorescence method to measure the absolute concentration of oxygen. Optical fiber carries excitation light produced by a Blue LED to the thin-film, proprietary coating at the probe tip Fluorescence generated at the tip is collected by the probe and carried by the optical fiber to the high-sensitivity spectrometer. When oxygen in the gas or liquid sample diffuses into the thin-film coating, it quenches the fluorescence. The degree of quenching correlates to the level of oxygen concentration.

Lasers have long been used as a basis for very accurate measuring. New applications include lasers for welding and can be used to join miniature components. This is done by bonding fiber grade glass to other elements in passive components — one of many examples of technology transfer. Advances have been made in developing ceramic lasers that offer benefits in design and fabrication because the laser rods do not require iridium crucibles and a lengthy growth process. The most common lasers are CO2 and UV (ultra-violet).
As we has seen there are many applications where optics are utilized for aircraft systems and applications. As time goes on, we expect to see more applications using light optics and photonics. However, you have already seen the light.
Keep your eyes on fiber optics to "Keep ’em Flying.