Optical fiber Communication

Fiber optics deals with the study of the propagation of light through transparent dielectric waveguides. The fiber optics are used for transmission of data from point to point location. Fiber-optic systems currently used most extensively as the transmission line between terrestrial hardwired systems

The carrier frequency used in conventional systems had the limitations in handling the volume and rate of data transmission. The greater the carrier frequency larger the available bandwidth and information-carrying capacity.

First-generation

The first generation of light-wave systems uses GaAs semiconductor laser and the operating region was near 0.8 µm. Other specifications of this generation are as under:

• Bit rate                  : 45 Mb/s
• Repeater spacing : 10 km

Second generation

• Bit rate                    : 100 Mb/s to 1.7 Gb/s
• Repeater spacing       : 50 km
• Operating wavelength     : 13 µm
• Semiconductor                   : In GaAsP

Third generation

• Bit rate                                    : 10 Gb/s
• Repeater spacing : 100 km
• Operating wavelength : 1.55 pm

Fourth generation

Fourth-generation uses the WDM technique.

• Bit rate                                    : 10 Tb/s
• Repeater spacing       : > 10,000 km
• Operating wavelength         : 1.45 to 1.62 µm

Fifth-generation

The fifth-generation uses the Roman amplification technique and optical solicitors.

• Bit rate                                   : 40 – 160 Gb/s
• Repeater spacing : 24000 km – 35000 km
• Operating wavelength : 1.53 to 1.57 µm

Need of Optical Fiber Communication

The fiber-optic communication system has emerged as the most important communication system. Compared to the traditional system because of the following requirements

1. In the long haul transmission system, there is a need for the low loss transmission medium.
2. There is a need for compact and least weight transmitters and receivers
3. There is a need for increased span of transmission.
4. There is a need for an increased bit of rate-distance products.

• A fiber optic communication system fulfills these requirements, hence most widely accepted.

Electromagnetic Spectrum

The radio waves and light are electromagnetic waves: The rate at which they alternate in polarity is called their frequency (f) measured in hertz (Hz). The speed of the electromagnetic wave (c) in free space is approximately 3×10⁸ m/sec. The distance traveled during each cycle is called wavelength (λ).

Wavelength (\lambda )=\frac{Speed of light}{Frequency}=\frac{c}{f}

In fiber optics, it is more convenient to use the wavelength of light instead of the frequency with light frequencies, the wavelength is often stated in microns or nanometers.

 1 micron (µ) = 1 micro-metre (1×10^-6)

 1 nano (n) = 10^-9 metre

The figure shown below shows the electromagnetic frequency spectrum.

• Fiber optics uses visible and infrared light. Infrared light covers a fairly wide range of wavelengths and is generally used for all fiber optic communications. Visible light is normally used for very short range transmission using a plastic fiber.

Optical Spectral Bands

The ITU has designated six spectral bands for use in optical communication within 1260 to 1675 nm region.

 The Table summarizes optical spectral band and their designation.

Spectrum (nm)	Designation	Name	Description
1260 to 1360	O-band	Original Band	The first region used for signal mode fiber links.
1360 to 1460	E-band	Extended Band	It can be used for fibers with low water content.
1460 to 1530	S-band	Short Band	Wavelengths are shorter than C-band but higher than E-band.
1530 to 1565	C-band	Conventional Band	The region used by EDFA.
1565 to 1625	L-band	Long Band	Gain reduces steadily to 1 at 1625 nm.
1625 to 1675	U-band	Ultralong Band	The region beyond the response capability of EDFA.

Elements of Fiber Optic Communication System

• The basic block diagram of an optical fiber communication system consists of following important blocks.

1. Transmitter
2. Information channel
3. Receiver

Message origin

Generally, message origin is from a transducer that converts a non-electrical massage into an electrical signal. Common examples include microphones for converting sound waves into currents and video (TV) cameras for converting images to current, for data transfer between computers the message is ready in electrical form.

Modulator

The modulator has two main functions.

1. It converts the electrical message into the proper format.
2. It impresses this signal onto the wave generated by the carrier source.

Two distinct categories of modulation are used i.e. analog modulation and digital modulation.

Carrier source

The carrier source generates the wave on which the information is transmitted. This wave is called the carrier. For the fiber-optic system, a Laser Diode (LD) or a Light Emitting Diode (LED) is used. They can be called as optic oscillators; they provide stable, single-frequency waves with sufficient power for long-distance propagation.

Channel coupler

Coupler feeds the power into the information channel. For an atmospheric optic system, the channel coupler is a lens used for collimating the light emitted by the source and directing this light towards the receiver. The coupler must efficiently transfer the modulated light beam from the source to the optic fiber. The channel coupler design is an important part of the fiber system because of the possibility of high losses.

 Information channel

The information channel is the path between the transmitter and the receiver. In fiber-optic communications, a glass or plastic fiber is the channel. Desirable characteristics of the information channel include low attenuation and a large light acceptance cone angle. Optical amplifiers boost the power levels of weak signals. Amplifiers are needed in very long links to provide sufficient power to the receiver. Repeaters can be used only for digital systems. They convert weak and distorted optical signals to electrical ones and then regenerate the original digital pulse trains for further transmission.

Another important property of the information channel is the propagation time of the waves traveling along with it. Signal propagation along a fiber normally contains a range of optic frequencies and divides its power along several ray paths. This results in a distortion of the propagating signal. In a digital system, this distortion appears as a spreading and deforming of the pulses. The spreading is so great that adjacent pulses begin to overlap and become unrecognizable as separate bits of information.

Optical detector

The information being transmitted is detected by a detector. In the fiber system optic wave is converted into an electric current by a photo-detector. The current developed by the detector is proportional to the power in the incident optic wave. Detector output current contains the transmitted information. This detector output is then filtered to remove the constant bias and then amplified

The important properties of photodetectors are small size, economy, long life, low power consumption, high sensitivity to optic signals and fast response to quick variations in the optic power.

Signal processing

Signal processing includes filtering amplification. Proper filtering maximizes the ratio of signal to unwanted power. For a digital system decision circuit is an additional block. The Bit Error Rate (BER) should be very small for quality communications

Message output

The electrical form of the message emerging from the signal processor is transformed into a sound wave or visual image. Sometimes these signals are directly usable when computers or other machines are connected through a fiber system.

Advantages of Optical Fiber Communication System

1. Wide bandwidth: The lightwave occupies the frequency range between 2×10¹² Hz to 3.7×10¹² Hz. Thus the information-carrying capability of fiber optic cables is much higher.
2. Low losses: Fiber optic cables offers very little signal attenuation over long distances. Typically It is less than 1 dB/km. This enables a longer distance between repeaters.
3. Immune to cross-talk: Fiber optic cables have very high immunity to the electrical and magnetic fields. Since fiber optic cables are non-conductors of electricity hence they do not produce the magnetic field. Thus fiber optic cables are immune to cross-talk between cables caused by magnetic induction.
4. Interference Immune: Fiber optic cables are immune to conductive and radiative interferences caused by electrical noise sources such as lighting, electric motors, fluorescent lights.
5. Lightweight: As fiber cables are made of silica glass or plastic which is much lighter than copper or aluminum cables. Lightweight fiber cables are cheaper to transport.
6. Small size: The diameter of the fiber is much smaller compared to other cables, therefore file cable is small in size, requires less storage space.
7. More strength: Fiber cables are stronger and rugged hence can support more weight.
8. Security: Fiber cables are more secure than other cables. It is almost impossible to tap into a fiber cable as they do not radiate a signal. No ground loops exist between optical fibers hence they are more secure.
9. Long-distance transmission: Because of less attenuation transmission at a longer distance possible.
10. Environment immune: Fiber cables are more immune to environmental extremes. They can operate over a large temperature variation. Also, they are not affected by corrosive liquids and gases.
11. Safe and easy installation: Fiber cables are safer and easier to install and maintain. They are non-conductors hence there are no shock hazards as no current or voltage is associated with them. Their small size and lightweight feature make installation easier.
12. Less cost: The cost of the fiber optic system is less compared to any other system.

Disadvantages of Optical Fiber Communication System

1. High initial cost: The initial cost of installation or set up costs is very high compared to all other systems.
2. Maintenance and repairing cost: The maintenance and repairing of fiber optic systems are not only difficult but expensive also.
3. Joining and test procedures: Since optical fibers are of very small size. The fiber joining process is very costly and requires skilled manpower.
4. Tensile stress: Optical fibers are more susceptible to buckling, bending and tensile stress than copper cables. This leads to restricted practice to use optical fiber technology to premises and floor backbones with a few interfaces to the copper cables.
5. Short links: Even though optical fiber cables are inexpensive, it is still not cost-effective to replace every small conventional connector (e.g. between computers and peripherals), as the price of optoelectronic transducers is very high.
6. Fiber losses: The amount of optical fiber available to the photodetector at the end of fiber length depends on various fiber losses such as scattering, dispersion, attenuation, and reflections.

 Applications of Optical Fiber Communication System

 Applications of optical fiber communications include telecommunications, data communications, video control, and protection switching, sensors and power applications.

Telephone networks

Optical waveguide has low attenuation, high transmission bandwidth compared to copper lines, therefore numbers of long haul co-axial trunks links between telephone exchanges are being replaced by optical fiber links.

Urban broadband service networks

Optical waveguide provides much larger bandwidth than co-axial cable, also the number of repeaters required is reduced considerably.

 Modem suburban communications involve videotext, videoconferencing videotelephony, switched broadband communication network. All these can be supplied over a single fiber optic link. Fiber optic cable is the solution to many of today’s high speed, high bandwidth data communication problems and will continue to play a large role in future telecom and data-com networks.