Losses in Optical Fiber

Signal Degradation in Optical Fiber :

• Attenuation and dispersion determine the maximum distance an optical signal can be transmitted before the receiver is unable to detect it.
• The attenuation and dispersion of a fiber are wavelength dependant.

Transmission Losses in Fibers:

Transmission loss or attenuation of the signal in an optical fiber is measured in dB/km.

Types of Losses

–Material Absorption

–Rayleigh scattering

–Waveguide imperfections

–Radiative loss

Material Absorption

•Intrinsic Absorption:

–Intrinsic absorption losses correspond to absorption by fused silica.

–intrinsic material absorption for silica in the wavelength range 0.8~1.6um is below 0.1dB/km.

•Extrinsic Absorption:

–Extrinsic absorption is related to losses caused by impurities within silica.

–The main source of extrinsic absorption silica fibers is the presence of water vapors.

Rayleigh Scattering

•Silica molecules move randomly in the molten state and freeze in place during fiber fabrication.

•Density fluctuation lead to random fluctuations of the refractive index.

•Light scattering in such a medium is known as Rayleigh scattering.

Waveguide Imperfections

•Imperfections at the core-cladding interface, such as random core-radius variations, can lead to losses.

•This has been taken good care of in optical fiber manufacturing and the core radius is made sure not to vary significantly along the fiber length.

Radiative Losses

•Radiative losses occur whenever an optical fiber undergoes a bend of finite radius of curvature.

•Fibers can be subjected to two types of bends.

–Macroscopic bending: 

Bends having diameter smaller than the specified by manufacturer e.g. when a fiber cable turns a corner.

–Microscopic bending: 

Random microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables, e.g. deformation of axis.

Gap- Loss :

•Gap-loss happens when there is a space, breakage, between fiber connection.

•Light can cross this gap, but spreads out and is weakened and diffused

Types of Fiber

Optical fiber is classified based on material used, mode & refractive index.

Step Index Optical Fiber:

• In the step index fiber, the refractive index of the core is uniform throughout.
• Different light modes in a step-index multimode fiber follow different lengths along the fiber in zigzag way
• The arrival of different modes of the light at different times causes Inter Modal Dispersion.
• 50 mm core for step index.

Graded Index Optical Fiber:

• Graded-index fiber’s refractive index decreases gradually away from its center.
• Light rays follow sinusoidal paths.
• Reduces inter modal dispersion.
• 62.5 mm core for graded index.

Single Mode Fiber:

• In a single mode fiber only one mode can propagate through the fiber.
• Core of SM fiber is smaller (8.3 to 10 microns). Common size is 9um.
• Laser is used as light source for single mode fiber.
• Used in long distance communication upto100 Gbit/s data in DWDM.
• 1310 or 1550 nm wavelength is used in SM fiber with higher data rate.

Multimode Fiber:

• Multimode fibers allow a large number of modes.
• A multimode optical fiber has larger core (50 to 100um). Most common size is 50um or 62.5um.
• DWDM is not normally used on multi-mode fiber.
• 850 or 1300 nm wavelength is used in multimode fiber.
• LED is used as a light source in multimode fibers .

Practically used Optical Fibers:

• MM Step index: 62.5/125 µm
• MM Graded index: 50/125 µm
• SM Step index: 9/125 µm
• Single mode graded index is not practically used

Fiber Design

Construction of Fiber

An optical fiber consists of a core, cladding and buffer (a protective outer coating). The cladding guides the light along the core by using the method of total internal reflection. In the glass fiber core and cladding are made of high quality silica glass.

Principle of Optical Communication

Optical fiber communication works on the principle of Total Internal Reflection.

Total Internal Reflection

– For all angles of incidence greater than the critical angle, the incident ray will get reflected back into the denser medium itself. This phenomenon is called total internal reflection.

Wavelengths in Optical Fiber

Normally we use light wavelengths around 850, 1310 and 1550 nm for fiber optics with glass fibers. These wavelengths are falling in the infrared region and having less attenuation. 

Optical Fiber as Telecommunication Medium

Physical path between transmitter and receiver is called transmission medium, it may be wireline or wireless.

Wireless (unguided) media

–Microwave radio.

–Communication satellites

•Wireline (guided) medium

–Copper wires

–Fiber-optic cables.

Fiber Medium Vs Microwave medium

Optical Fiber Communication

Transmitting information from one place to another by sending pulses of light (wavelength) through an optical fiber. Modulation used: Light Intensity Modulation

Advantages of Optical Fibers

• Very high information carrying capacity
• Less attenuation (order of 0.2 db/km)
• Small in diameter and size & light weight
• Greater safety and immune to EMI & RFI, moisture & corrosion
• It is dielectric in nature so can be laid in electrically sensitive surroundings
• Difficult to tap fibers, so secure
• No cross talk and disturbances

Disadvantages of Optical Fibers

• The terminating equipment is still costly as compared to copper equipment.
• It is delicate so has to be handled carefully
• High Cost
• Last mile is still not totally fiberised due to costly subscriber premises equipment
• Optical fiber splicing is a specialized technique and needs expertly trained manpower.
• The splicing and testing equipments are very expensive as compared to copper equipments.

Applications of Optical Fiber

• Long distance communication: Backbones
• Broadband services
• Computer data communication (LAN, WAN etc..)
• Medical Industry
• Military application