Week 3 (23-29.03) Fiber-optics. Revolution?

Internet. We all use it, we all need it. But do we see difference, or maybe I should ask do we need to know the difference in the way it’s provided? Recently one of the network and TV providers damaged one of the main roads leading to my place. I asked the workers what they were doing and they answered me that they were putting fiber-optic cables for Network and TV. I have mixed feelings about this. On the one hand I can have really fast internet, but on the other hand the road is still a mess (though it's been 2 weeks since they “finished” their job). But that’s not what I wanted to talk about. Below I present some information about fiber-optic communication.






Source:
http://media.cleveland.com/strongsville/photo/11947794-large.jpg


Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information.

Optical Communication using (incoherent) visible light waves ( 380-780 nm) has been known for a long time. In the 60s there was conducted research that allowed to modulate laser light by using a digital signal. It was the first step to transfer information through a transparent medium. In 1980 at Bell Labs there were designed and tested first fiber optics (their length was around 1000 km). Soon after the company started to develop a corporate telecommunications system based on fiber-optics.





Source:
http://cityofwatsonville.org/wp-content/uploads/2013/09/FiberOptic-719x349.jpg


Modern fiber-optic communication systems base on:
· Information source
· Coder
· Optical transmitter
· Cable
· Decoder
· Optical receiver
· Sink








Transmitters


The most commonly-used optical transmitters are semiconductor devices such as light-emitting diodes (LEDs) and laser diodes.

The emitted light is incoherent , it has relatively wide spectral width - 30-60 nm. LEDs are used mainly for local-area-network applications with bit rates of 10-100 Mbit/s and transmission distance of a few kilometers.

Laser diodes are often directly modulated. It means that the light output is controlled by current directly applied to the device. For very high data rates or very long distance links, a laser source may be operated with a continuous wave, and the light can be modulated by an external device. External modulation increases the achievable link distance by eliminating laser chirp, which broadens the linewidth of directly-modulated lasers, increasing the chromatic dispersion in the fiber.


Fiber Optic Cable


A fiber optic cable is made of one or more glass fibers conducting impulses of light (in telecommunications usually it is infrared light). Such cables, formed of glass fibers, are resistant to electromagnetic interference and have large bandwidth. It is possible to achieve transfer rates up to 100 Gb / s (about 12.5 GB / s) when using them. The fastest fiber-optic systems can transfer data even with a speed of several Tb / s.



Wavelength-division multiplexing (WDM) is the practice of multiplying the available capacity of an optical fiber by adding new channels, each channel on a new wavelength of light. Using WDM technology now commercially available, the bandwidth of a fiber can be divided into as many as 160 channels to support a combined bit rate into the range of terabits per second.

A fiber transmission system is characterized by the so called bandwidth-distance product. It is expressed in units of MHz×km and it is a trade-off between the bandwidth of the signal and the distance it can be carried on. A commonly use multi-mode fiber has a bandwidth-distance product of 500 MHz×km and it can carry a 500 MHz signal for 1 km or a 1000 MHz signal for 0.5 km.




Source:

http://techbeat.com/wp-content/uploads/2013/07/space1-1024x637.jpg

Receiver


The receiver has to fulfill two requirements: It must sense or detect the light coupled out of the fiber-optic cable and convert the light into an electrical signal, and it must demodulate this light to determine the identity of the binary data it represents.

Optical receiver is based on a photodetector, which converts light into electricity using the photoelectric effect. Typically it is a semiconductor-based photodiode.

The main advantages of fiber optics over traditional wire cables:
· Less Expensive - many miles of optical cable are easier and less expensive to install than the same number of copper wire cables.
· Thinner - Fiber optics are thinner than copper wire cables. It is important for the underground cable systems.
· Higher Carrying Capacity - More information can be sent over fiber optic systems
· Less signal degradation - A characteristic feature of optical links is low level of noise (lower than the one in traditional telecommunication systems). The basic difference is that photons moving in a fiber do not interact with each other. Therefore they do not generate any noise. However, the optical transmission system has few extra noise sources ( so called multiplicative) .It is associated with fluctuations in attenuation transmitting and receiving stations. Multiplicative noise is different from the additive thermal noise in conventional receivers. It disappears when the broadcast signal is turned off.
· Use Light Signals - fiber optics use light signals instead of electricity, the signals don’t interfere with each other and are resistant to electromagnetic interference. This makes the signals clearer and easier to understand.
· Low Power - Optical fiber systems are based on low-power transmitters because the signal degrades less (instead of high-power electric transmitters used for copper wires).
· Flexible - fiber optics are more flexible, they can go around corners and into smaller places than traditional cable. This characteristics is extremely important when it comes to computer device and very big office networks.



Questions:
· Are You familiar with the topic?
· Do You think that fiber-optics are the future of network/communication?
· Do You know any other fields/ usage of fiber-optics?
· Will the X company fix my road?:(





Source:
http://computer.howstuffworks.com/fiber-optic.htm
http://en.wikipedia.org/wiki/Optical_fiber