OPTICAL FIBERS www.final-yearprojects.co.cc

What are Fiber Optics Arranged in bundles called optical cables Long thin strands of very pure glass about the size of human hair Arranged in bundles called optical cables Used to transmit light signals over long distances Hundreds of thousands arranged in bundles to form optical cables

What is an Optical Fiber? An optical fiber is a waveguide for light consists of : core inner part where wave propagates cladding outer part used to keep wave in core buffer protective coating jacket outer protective shield

Passage of light from a material with a high index of refraction(n1) to a material with a lower index of refraction(n2) At the critical angle light will not go into n2 but instead travel along the surface between the two media

What are Optical Fibres ? Optical Fibres are fibres of glass, usually about 120 micrometres in diameter, which are used to carry signals in the form of pulses of light over distances up to 50 km without the need for repeaters. These signals may be coded voice communications or computer data

The optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. The light transmitted through the fiber is confined due to total internal reflection within the material. In telecommunications applications, the light used is typically infrared light Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction Fibers, like waveguides, can have various transmission modes. The fibers used for long-distance communication are known as single mode fibers, as they have only one strong propagation mode.

Multi-mode fibers, where light transmitted in the different modes arrives at different times, resulting in dispersion of the transmitted signal. single mode equipment is generally more expensive than multi-mode equipment. single-mode optical fiber, data rates of up to 40 Gbit/s are possible in real-world use on a single wavelength. Wavelength division multiplexing can then be used to allow many wavelengths to be used at once on a single fiber

Types of optical fibers Single mode only one signal can be transmitted use of single frequency Multi mode Several signals can be transmitted Several frequencies used to modulate the signal

Types of Fibres GRIN nc nf nc nc nc nc nf nc Multi-mode step index Single-mode step index nc nc multi-mode graded index nc GRIN nf nc

Typical core and cladding diameters Type Core (mm) Cladding (mm) Single mode 8 125 Multimode 50 125 62.5 125 100 140

Launching the Light Factors that effect the Launching of Light Intensity Area Acceptance Angle Fresnell Loss

Signal Production Convert electrical input to modulated light 2 Basic Schemes On/Off Linear Variation 2 Common Devices used Light Emitting Diode (LED) Laser Diode (LD)

Through the Wire Light Propagates through the wire due to total internal reflection

Fibre can be bent!! Illustration of total internal reflection

Total internal reflection Trapping light in the fiber

Total Internal Reflection

Types of fiber ends beam patterns can be: spherical cylindrical

Fibers carry modes of light a mode is : a solution to the wave equation a given path/distribution of light higher # modes gives more light, which is not always desirable

Controlling the # of Modes From the V parameter, we see that we can reduce the number of modes in a fiber by reducing: (1) NA (2) diameter (wrt ) This is exactly the case in single mode fibers.

The V Parameter a = fiber radius o = incident wavelength known as the “V-parameter” or the fiber parameter an important parameter that governs the number of modes parameters that relates yucky EM wave solutions for both core and cladding

How Fibers Work The classical understanding of fiber optics comes once again from out longtime friend, Snell’s Law! Step index fibers: Total Internal Reflection

Optical Fiber Bandwidth Bandwidth Limitation Light entering at different angles reach the end of the cable at different times Smearing is produced: uncertainty of beginning and end of signal less smearing higher the bandwidth smearing can be reduced by reducing the size of the fiber core

Areas of Application Telecommunication's Local Area Networks (LAN's) Optical fibres are now the standard point to point cable link between telephone substations. Local Area Networks (LAN's) Multimode fibre is commonly used as the "backbone" to carry signals between the hubs of LAN's from where copper coaxial cable takes the data to the desktop. Fibre links to the desktop, however, are also common.

Optical Fibre Sensors Cable TV CCTV As mentioned above domestic cable TV networks use optical fibre because of its very low power consumption. CCTV Closed circuit television security systems use optical fibre because of its inherent security, as well as the other advantages mentioned above. Optical Fibre Sensors

Long-haul trunks common in telephone networks Metropolitan trunks to join phone exchanges in metro areas Rural exchange trunks connect exchanges of different phone companies

Subscriber loops central exchange to subscriber LANs Can support hundreds of stations on a campus

Other Applications Endoscope X-ray Imaging Night Vision

Advantages of optical Fibres Can carry much more information Much higher data rates Much longer distances than co-axial cables Immune to electromagnetic noise Light in weight Unaffected by atmospheric agents

Disadvantages of optical Fibres expensive need to convert electrical signal into optical signal when transmitting and convert it back to electrical signal when receiving

The Optical Transmitter:

The source of the optical signal can be either a light emitting diode, or a solid state laser diode. The transmitter converts an electrical analog or digital signal into a corresponding optical signal. The most popular wavelengths of operation for optical transmitters are 850, 1300, or 1550 nanometers.

Optical Receivers Converts modulated light from the cable into the original signal Photodiode: Pin or Avalanche type High gain internal amplifiers Large sensitive detecting area several microns thick

The Optical Receiver: The receiver converts the optical signal back into a replica of the original electrical signal. The detector of the optical signal is either a PIN-type photodiode or avalanche-type photodiode.

Degradation of the Signal Glass must be extremely pure Most general purpose optical fiber Signal losses per km traveled 850nm = 60-75% 1300nm = 50-60% 1550nm = 40% Excessive bending

Signal Regeneration Optical regenerators spliced along the cable to boost weakened signals Optical Regenerator Optical fibers with specially doped coating Doped portion is pumped with a laser When signals enters energy from the laser allows doped material to imitate lasers Doped molecules now emit a stronger signal with the same initial characteristics

Optical Fiber - Transmission Characteristics Act as wave guide for 1014 to 1015 Hz Portions of infrared and visible spectrum Light Emitting Diode (LED) Cheaper Wider operating temp range Last longer Injection Laser Diode (ILD) More efficient Greater data rate Wavelength Division Multiplexing - Multiple beams of light at different frequencies can be transmitted simultaneously

Global crossing fibre networks

Atlantic crossing networks

Thank You