The idea of fiber optics communication system is basically sending information through light. Optical fiber was first developed in 1970 as a basic communication purpose with a very low attenuation as transmitting light through fiber optics cable for long distance communication. In 1975, the first commercial fiber optics communication system was developed using semiconductor laser and operated at 0.8 µm wavelength and a bit-rate of 45 Mbps (Mega bits per second) up to 10 km (Elion & Elion 1978) (Sullivan & Curt 2003).
In long beach California was the first live telephone traffic sent at about 6 Mbps and it was in 1977. After that, generations of fiber optics system technologies were developed, improved, and upgraded to achieve the first transatlantic operation in 1988.
All fiber optics systems are limited by something called dispersion. The initial thought of fiber optics was an experiment involving a bucket of water and sunlight. It demonstrated the sun’s reflection within the bucket with a hole and water pouring out illuminating the water and the sunlight can be seen in the stream of water (Elion & Elion 1978). Then it moved on to optical voice transmission known as photo phone. Further, it went to fiberscope that was used to inspect welds within reactor vessels, combustion chambers of the jet engines, and then to the medical field utilized in laparoscopic surgery. Researches and improvements continued through the fiber optics generations to overcome the dispersion phenomena by using dispersion-shifted fibers to minimize the dispersion at 1.55 µm or by limiting the laser spectrum to a single longitudinal mode (Alwayn 2004) (Sullivan & Curt 2003). The idea of using fiber optical amplification came with the development of the fifth generation. The amplification development reduced the need of using repeaters and wavelength division multiplexing (WDM); which will be described in details later; and increases the data capacity. By these developments, a bit rate of 10 Tb/s was achieved in 2001. The developments of fiber optics generations are a continuous operation to especially for the huge market of the internet communications which requires an increase in communications bandwidth such as video on demand. These growing in using internet protocol data traffic are increasing side by side with faster rate integrated systems complexity. (Elion & Elion 1978) Fiber optics manufacturers had reduced the cost by the huge request of communication companies such as AT&T to take the advantage of delivering the technology of internet and telephone through higher data broadband services to customers’ homes (Sullivan & Curt 2003) (Snell 1996).
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Fiber optics is already being used nowadays in military and commercial aircraft, and some of the areas it altogether replaced the Digital Flight Data Recorder with the newer Distributed Flight Data Acquisition Unit that performs the same thing but evaluates much more information. Indeed, future aircrafts will see fiber optics technology in the flight controls. Fiber optics has evolved practically from ideas to a thing of the future (Alwayn 2004) (Elion & Elion 1978).
Fiber optics Application
The demand and usage of optical fiber has grown rapidly and optical fiber applications are numerous. Ranging from global networks to desktop workstation, telecommunication applications are widespread. These involve the transmission of data, voice, or video across distances of less than a meter to thousands of kilometers by utilizing one of a few standard fiber designs within one of several cable designs. Optical fibers are used by carriers to carry plain old telephone service (POTS) over their nationwide networks. Furthermore, local exchange carriers (LECs) employ fiber to carry the same service across central office switches at local levels and often as far as the individual home (fiber to the home, FTTH) or neighborhood (Alwayn 2004) (Elion & Elion 1978).
Moreover, optical fiber has a widespread use in transmission of data. Multinational firms require reliable and secure systems for transferring data and financial information among buildings to the computers of desktop terminals and around the world. Fiber is also used by cable televising companies in order to deliver digital video and data services. Due to the high bandwidth offered by fiber, it is the ideal choice for transmitting broadband signals like the high-definition television telecasts. Furthermore, intelligent transportations systems including smart highways equipped with intelligent traffic signals, changeable message signs, and automated tollbooths, also utilize telemetry systems based on fiber-optics (Alwayn 2004) (Fiber-Optics.Info 2010).
Biomedical industry is another significant application of optical fiber. Fiber-optic systems are used in almost all modern telemedicine systems and devices for transmission of digital diagnostic images. Additionally, other applications for optical fiber also include military, space, industrial and the automotive sector (Elion & Elion 1978) (Snell 1996).
Fiber optics communication technology is used by today’s telecommunications companies such as AT&T in the United States and BT in the UK. Also it is used by internet providers and cable television signal providers such as Sky. For the huge expense of fiber optics system, the technology was first used for long-distance communication only. But, now day’s developments of the cities infrastructure had to take place to install the fiber optics communication system regardless of the cost and time consuming. The challenge of fiber optics technology companies was reflected positively on the market and the cost of fiber optics communications dropped considerably (Alwayn 2004) (Elion & Elion 1978) (Fiber-Optics.Info 2010).
By the developments of optical amplification system, an intercontinental network of 250,000 km of submarine-communication-cable was developed with a capacity of about 2.5 Tb/s was achieved.
Also, the optical communication system was installed onboard aircrafts for data, video, and radio signals communication. It was first introduced to aviation industry by NASA researches on military aircrafts such as the first F/A-18 hornet through its RTDP; radar tracking and detecting system processor, missile video tracing system, and with the FLIR system; forward looking infra red sensor, and the integrated radar system with increased speed and memory capacity. Military aircrafts are always in-need to reduce weight when it is in slick phase to improve its capability of maneuvering and delivering various types of ordnance in a very precise targeting and accurate guidance. Lately, this technology was used on the F-22 airforce raptor on its high speed data bus and fiber optics transceiver (Alwayn 2004). This idea was reflected on the civil aircrafts later on as they always in-need to reduce weight to overcome the fuel usage and deliver larger number of passengers which means saving money and increasing profits. Besides that, the new avionics systems technology and complexity required new communication system other than the copper wires such as the normal ARINCs (Fiber-Optics.Info 2010). Boeing and Airbus developed very complex integrated systems that control the aircraft performance at an altitude of 30,000 ft and above which required bigger and more complicated communication systems which means:
More chance of shorting the wires.
More EMI (electro-magnetic-interference) which can cause a distortion for the signals
More chance of shorting and cause sparks and fire on-board and aircraft
The first official usage of fiber optics technology usage was on-board the Boeing 777 after the development of its AIMS (Aircraft Information Management System) which have more than 2 million of computer codes. Also Boeing 777 was the first aircraft to install an optical LAN (local area network) for on-board data communication and on its cabin systems communications (Sullivan & Curt 2003) (Snell 1996).
After that, fiber optics system was installed on the Boeing 757 flown by Air Mexico who did not experience a single failure. So here comes the safety, reliability, durability and stability of signals communications on-board an aircraft which can make the affordability part negligible. We have to mention that the cost of optical fiber systems are dropping for the establishments and marketing competition of huge number of manufacturers and suppliers (Alwayn 2004) (Fiber-Optics.Info 2010).
Fiber Optics vs. Electrical Copper Wires:
Fundamentally, there are three types of transmission media: copper wires, waveguides and free space. Copper wire, such as coaxial cable is broadly used. A signal is transmitted across the wire in either digital or analog form to a receiver placed at the end of the wire. Free-space transmission is also widely used through which radio, television and other across-the-air signals are carried. Waveguides describe the fiber-optic transmission. Significantly, a waveguide such as a optical fiber restrains the electromagnetic radiation, light (Bidgoli 2010).
Fiber optic transmission provides the best elements of both free-space and coaxial transmission. It is capable of carrying a signal from point A to point B in the absence of any limited electromagnetic spectrum. Nonetheless, it does not suffer from limited data rate and bandwidth is the same way as the coaxial cables do.
The advantages of fiber optics system over the copper wire’s are:
No Electro-Magnetic-Interference (EMI).
No radio-Frequency-interference (EMF).
Immunity from electromagnetic noise
High Signal quality
Lighter weight cables
Longer distance capability (Bidgoli 2010)
Smaller diameter cables which mean saving more space.
Greater bandwidth for data transfer
Safety against shorting and sparking
High bandwidth and greater information capacity
Easy upgrade: Can be upgraded easier without ripping and replacing cable harnesses.
Easier maintenance and handling, proved by Boeing and Lockheed martin engineers
Lower cost (Bidgoli 2010)
Lower signal loss for long distance communication
Higher resistance against stress, temperature and vibration and higher lightning strikes incidence.
Doesn’t require repeaters for long distance communication
Can operate for up to 100 km without passive or active processing
But, we have to say that copper wires have some specifications that fiber optics doesn’t:
Can carry electrical power beside the signals
Lower material costs
Doesn’t have minimum bending radius
Can easily be installed between boxes and chips (Bidgoli 2010)
Cable technology is used for connecting networks together; however, as optical fiber technology is moving forward, it is gradually replacing copper wires as an excellent medium of communication signal transmission. The main reason for this is that fiber optics offers much more benefits than conventional copper wires and cables, as stated above (Sullivan & Curt 2003).
Moreover, these benefits can be elaborated as follows:
Resistance to Interference: Fiber optics do not conduct electricity as it is obtained from glass, which eliminates activities like grounding and makes it potentially immune to electromagnetic interruption. Working of Fiber optics, unlike copper cables, is based on light pulses that make it usable outdoors and in close proximity to electrical cables (Bidgoli 2010).
Low Maintenance: This entails that optical fiber is not sensitive to elements like water and chemicals because it is produced from glass. Additionally, Fiber optics cannot be damaged by harsh elements. This makes the overall cost of maintenance and service lesser than its counterpart.
Efficiency and Security: Information can be transmitted with greater fidelity with the help of fiber optics’ unlimited bandwidth. It can offer nearly 1,000 times as much bandwidth across distances approximately 100 times farther than copper cables. This provides a super-fast connection running in circles around the bandwidth assigned by cable connections. Moreover, since fiber optics is harder to tap than regular copper wires, it can offer additional data security (Bidgoli 2010).
Picture Quality: In comparison to copper wires, the high quality technology embedded in fiber optics is much more powerful. One can obtain high-definition picture quality from fiber technology as there is no external interference.
Safety: Fiber optics poses no threat of physical injuries during breakage of fiber optic cables. Instead of transmitting through electricity, it transmits data via light. Users face no risk of injury from dangers such as sparking, electrocution, fire, etc. (Bidgoli 2010).
Interestingly, the benefits of converting into fiber technology such as Ethernet converters show proven advantages, considering that internet infrastructure is steadily making this transition rather than conventional copper cabling. Furthermore, applications of fiber optic include manufacturing and process control, supervisory control as well as data acquisition. Using transceiver modules provide a cutting edge and most importantly the highest quality data transmission for user’s television, home phone and internet (Sullivan & Curt 2003).
Basic components of fiber optics system:
It is consisted of:
Core: it is basically a cylinder of glass or plastic material.
Cladding: a layer causing the light signal to be confined to the core by using the total internal reflection method.
Buffer: a layer used to capsulate one or more core and cladding providing mechanical isolation and a protection from physical damage.
Jacket: a further isolation and protection.
The major characteristics of optical fiber transmission lines are:
Attenuation and its variation with transmission input wavelength, cable temperature and modal distribution.
Radiation: and its variation with fiber temperature and bend radius
Distortion and its variation with bandwidth, amplitudes, wavelength and modal distribution of the input light, length of the fiber, and lastly, environmental temperature (Snell 1996)
Physical Parameters: This includes weight, size, ease of installation, total volume, coupling and splicing.
Environmental parameters: This includes resistance to stress, water and chemical corrosion, temperatures and mechanical stresses (Alwayn 2004).
Types: single mode and multi-mode fibers.
A single-mode optical fiber
Multi-mode optical fiber
There are two categories of optical fibers, namely, single-mode fiber optical cable and multi-mode fiber optic cable. In essence, these types of fiber optic cables are comprised of numerous layers of glass, each having refractive index lower than the one next to moving from the center outwards. Since light is faster in lower glass refractive index, the wavelengths of light are broken outside the fiber, capable of traveling to the middle (Snell 1996).
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Multi-mode Fiber Optic Cable:
Optical fiber with a base diameter greater than 10 microns can be analyzed through geometrical optics, and is called a multi-mode fiber. In this type of optic cable, the rays of light along the core of the fiber are led by total reflection. Moreover, rays meeting the core-mantle border at a high angle over the critical angle for this limit are fully considered. The critical angle is said to be the difference in refractive index between the mantle and core materials. Rays hitting the border at a shallow angle form the base breaks into the mantle and does not transmit the light and information along the fiber. Moreover, the acceptance of the fiber is determined by the critical angle, and is often referred to as a numerical aperture (Snell 1996). A greater numerical aperture allows light to spread into two distinct beams at different angles near the axis, for the effective coupling of light into the fiber.
Single-mode Fiber Optic Fiber
Fiber core diameter of nearly ten times the wavelength of light propagation is impossible to model with geometric optics. Rather, they can be studied as an electromagnetic structure by solving Maxwell’s equations that are reduced to the electromagnetic wave equation. It acts as an optical waveguide and supports one or more confined transverse modes which allows light to propagate through the fiber. Fiber that supports only one mode is known as mono-mode or single mode fiber. It is an extremely focused source of light which limits beams to a smaller range or angles closed to the horizontal.
A fiber optic data cable has three primary functions. It converts an electrical input signal into an optical signal, transmit the optical signal across an optical fiber, and lastly, convert the optical signal back to an electrical signal (Green 2006).
Transmitter: It is a semiconductor device and can be an LED (light emitting diode) or a laser diode. It converts the electrical input signal into an optical signal, and its drive circuit changes the current flow across the light source, which in turn changes the irradiance of the source. This process of changing the irradiance of the source as a function of time is known as modulation.
Receiver: It is a photo-detector and the main component of the receiver which converts light signals into electrical using the photoelectric effects method. It is basically a semiconductor-based photodiode.
Amplifier: it is used instead of the complex repeaters. It amplifies the light and optical signals without converting it to electrical.
Fiber-optic telecommunication systems operate on pulse-code modulation in which information is sent across as a series of pulses. Moreover, the digital pulse-code modulation is coupled into a fiber and the fiber end set up by a connector in order to maximize the input power. In fiber-optic communication systems, semi-conductor lasers are best suited. Their shape and size enables efficient coupling of light within the small-diameter core of an optical fiber (Sullivan & Curt 2003). The fiber then carries the light towards the receiver which detects the light and recovers the digital signal. As scattering, dispersion and absorption in the fiber degrade the signal, optical amplifiers must be used to regenerate the signal (Snell 1996).
The U.S. military quickly turned towards fiber optics for enhanced and improved communications as well as tactical systems. During the early 1970’s, a fiber optic telephone link was installed by the U.S. Navy aboard the U.S.S. Little Rock. Toward that end, The Air Force developed its Airborne Light Optical Fiber Technology (ALOFT) program in 1976. Greatly motivated by the success of these practical applications, military R & D programs developed stronger fibers, ruggedized, high-performance components, tactical cables, and several demonstration systems that ranged from aircraft to undersea applications. Soon after, commercial application followed that included both AT&T and GTE installing fiber optic telephone systems in 1997 in Chicago and Boston respectively (Green 2006) (Keller 2010). Therefore, these successful applications resulted in the increase of fiber optic telephone networks. In the early 1980’s, single-mode fiber running in the 1310 nm and then in 1550 nm wavelength ranges were installed for these networks. Earlier, information networks, computers and data communications were far slower to employ fiber; however, today they are embracing the transmission system that has lighter weight cable, carries more data faster and across long distances, and resists lightning strikes (Elion & Elion 1978) (Snell 1996).
An average aircraft consists of over hundred miles of electrical wires and controls almost everything from landing gear to calls from flight-attendants. These insulated copper wires have proven to be a big bottleneck: it is heavy in weight, vulnerable to electromagnetic interference and if not accurately maintained, can cause system failures or fires. Some of these wires can be replaced with fiber-optic technology that is lighter than copper wire, immune to electrical shocks, and less sensitive to electromagnetic interference (Green 2006). Researchers have developed a new optical switch to be incorporated into the cockpit controls in order to manage operations that involve turning on and off, for instance, displays, landing gear, manual switching between fuel tanks, etc. Presently, on-off switches in a cockpit are connected to separate wires spread throughout a plane, controlling several functions. In case a switch fails to work due to wiring problems at the inside of the plane, detecting the offending line can consume a lot of time and effort, since the wires are usually bundled together. Therefore, this switch is capable of sensing whether a button has been pressed fro “off” to “on” (Keller 2010). Then, the information from the fiber-based device can be directed toward a main fiber artery that carries hundreds of signals simultaneously. Through this, the bulk of wires, and cost is eliminated, and maintenance is simplified. Although engineers have been working to replace aircraft wiring with fiber for several years, unfortunately, they have had only moderate success. Built in mid-1990’s, Boeing’s 777 uses a fiber-optic communication network, but the design and implementation being a part of an experiment. Moreover, the network was not a critical system and it was over-designed with more possibility of error than greater cost-effectiveness if in case it was broadly implemented in the industry. Nevertheless, Boeing’s 787 aircrafts, is equipped with a more cost-effective optical fiber communication network (Green 2006) (Keller 2010) (Sullivan & Curt 2003).
The first major variation in avionics databusing on military as well as commercial aircraft came during 1970’s and 1980’s. It was deployed as a 100 kilobit-per-second ARINC 429 databus which is a multiplex databus standard of ARNIC Inc. The databus is omnipresent on commercial airliners and is one of the most common avionics equipment used in flying today. The ARNIC 429 is equipped by all commercial aircrafts for legacy connections and for securing backup for flight critical controls. The ARNIC 429, apart from being the de-facto standard, is used on aircrafts for digital electronics, navigation and air data computers, engine control systems, and radios which are fully computerized and need to interact with each other. Furthermore, ARINC 429 is greatly responsible for setting up the digital era in commercial and military avionics. However, it is a comparatively slow twisted-pair databus and unidirectional like 1553. Only one terminal on the bus can broadcast and as much as 20 terminals can listen (Keller 2010). The ARINC 429 represents a tightly legalized architecture equipped with standard ARINC connectors. Before the introduction of ARINC 429, avionics designers utilized hard-wired point-to-point connectors with analog signals suitable for every sensor type, like the navigation gyros. Many sensors had multiple wires and multiple signals. In typical applications deployed on today’s jetliners, systems designers extensively utilize ARINC 429 in order to connect avionic subsystem boxes and components digitally. One master subsystem on the bus sends information to at least 20 slave subsystems, but the slaves are unable to send information back to the master. Therefore, to enable a slave subsystem to transmit data back to the master, another ARINC 429 bus is used with its direction reversed, thereby enabling the original slave to send and the original master to listen. More frequently, designers make use of two ARINC 429 buses, amounting to point-to-point bi-directional interconnections between avionics subsystems: one bus transmitting in one direction, and another bus transmitting in the reverse direction (Green 2006) (Keller 2010).
One such example includes jetliner’s flight-management computer that accepts and processes inbound signals from several different sensors and other subsystems, and estimates the paths taken by flights, their time of arrival, fuel burn, etc. In this approach, ARINC 429 busses send information from the sensors and allow the flight-management computer to listen to that data. Whenever the flight-management computer needs to transmit information tack to a particular sensor, designers employ another 429 bus that runs in the opposite direction. Regardless of its low speed, the digital ARINC 429 databus aids in increasing the efficiency, speed, and facilitates maintenance since it can move several digital data packages across the same twisted pair, where analog approaches required a different wire for each signal (Keller 2010). The digital ARINC 429 bus allows for multiplexing of the information on two wires. Since it is capable of sending data across only in one direction, it is greatly reliable due to its lower possibility of data corruption or data conflicts. Furthermore, this reliability of the databus makes it usable and well-known for flight-critical data involving avionics activities such as flight navigation and engine control (Keller 2010).
The sheer size of ARINC 429 installed base drives its popularity; it is included in almost every commercial jetliner recently manufactured. ARINC 429 has limited addressing capacity and limited bandwidth; still it is proved as a highly robust physical bus and has served the industry well enough for several years. Moreover, it is a viable bus and protocol where installations require limited bandwidth and limited address space on the bus. However, modern avionics architecture demands more bandwidth, more address space, and greater flexibility than what the ARINC 429 can deliver (Keller 2010).
ARINC 629 is a 2-megabit-per-second databus that is borne from the MIL-STD-1553 technology. The conversion into an ARINC 629-type of bus topology strongly supported the move in the aviation industry to greater degrees of integration within the full systems throughout the avionics and the aircraft systems.
The launch of ARINC 629 was set up on the highly-advanced architecture of the Boeing 777 double-engine jumbo jetliner. After its launch, the databus started as a Boeing invention known as Digital Autonomous Terminal Access Communications (DATAC). In comparison to ARINC 429, 629 is a completely different model altogether. It is bi-directional and does not need the master, which has been a potential single point of failure in 429. Although it is a sporty technology, it is still expensive. Additionally, ARINC 629 is far more cognate to 1553 than the ARINC 429, which has bi-directional flow of data (Keller 2010).
One of the most attractive aspects of ARINC 629 was the bus efficiency, or rather the ratio of actual data transmitted by the bus to the routing overhead code. ARINC 429 has an efficiency of nearly 45 percent; however, with ARINC 629 efficiency can reach up to 85 percent if the bus is architected right.
Furthermore, as compared to 429, ARINC 629 is a bus without connectors. In the ARINC 629 a twisted pair of wires is fed across the bus coupler that behaves like a transformer.
Nonetheless, it was the price of the bus that got designers in a dilemma about the usage of ARINC 629. Although, ARINC 629 was more reliable, but it was much costlier than what the designers had actually thought (Keller 2010).
Cost concerns, a lack of large new commercial aircraft projects when Boeing developed the 777, and the increasing popularity of Ethernet networking technology blended together to halt implementation of the ARINC 629 after the 777. ARINC 629 is heavy and expensive to implement.
Fiber optics is anticipated to have a better future in military aviation industry. It exclusively provides high bandwidth, immunity against electromagnetic interference, and is light weight. Optical fiber was deployed on the AV-8B several years ago; it is used across the F/A-18E/F including several other aircrafts, and is likely to be fielded at a higher degree in coming years. However, before fiber-based applications in intense aviation environment can proliferate, a couple of standards are required in component, training, testing and other areas. Techniques such as WDM, short for wavelength division multiplexing, might dramatically increase the throughput and lower the footprint needed today for analog and digital data communications, beyond the immediate horizon (Keller 2010) (Sullivan & Curt 2003).
Fiber optic standardization within the aerospace sector leaves out something to be desired. Unfortunately, there are no standards for:
The method for computing link loss power budgets
The geometrics of critical parts like the end faces of optical fiber cable terminations, and
Training military aviation personnel and technicians in the handling of optical fiber
There has been a consensus for standardization. Operating with the Society of Automotive Engineers (SAE), NAVAIR, short for Naval Air Systems Command, avionic companies, airframers and components suppliers have put extreme efforts in developing training standards and components to enhance the supportability of current systems, and to cover various emerging technologies. In essence, fiber optics has been a corporate initiative within NAVAIR, holding its base in the command’s Avionics Division instead of a particular program office (Adams 2005).
Fiber optic technology has evolved at the major contractor or airframer level, in military aviation, as stated by researchers, engineers, personnel, and F/A-18 fiber optic experts with PMA 265 at NAVAIR. Fiber optics avionics components’ standardization will greatly assist in eliminating ambiguities and allowing test and inspection of equipments in order to sufficiently cover the technology fielded today (Adams 2005) (Sullivan & Curt 2003).
JELLI is an SAE group that is developing performance standards for the initial test and inspection processes of avionics fiber optic assemblies. JELLI is short for jumpers, endfaces, link loss and inspection; i.e. the processes and components used in optical networks. They are supported by the following definitions:
Jumpers: The cables that are utilized in testing the overall optical performance of the fiber optic cables after installation.
Endface: This is the polished end of the high-precision ceramic cap of the fiber optic termination that enables optical coupling.
Link Loss: It is the attenuation of the signal, primarily from connector loss, and
Inspection: It involves the examination of a cable installation in order to verify its performance.
Different polish standards are required to apply to the ‘physical vs. non-contact’ connections. However, within those areas, everyone should satisfy a termination endface range for that kind of polish. The cleanness of the endface is another important parameter. Test jumpers have endfaces and their polish requirements must match the component being installed within the aircraft. The test jumpers fail work on the airplane without such as standard (Adams 2005).
Furthermore, standards for components like endfaces are essential to eliminate subjectivity on the engineering side. Additionally, it is mainly important since aircrafts with fiber optic systems have been launched in order to set up a baseline for the avionics technicians, and to instruct them on what exactly a good connector endface. In essence, work on inspection involves the magnification and several other criteria required of the equipment that is used for examining the termination endfaces and detect damages. For instance, tiny char
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