We have already discussed Optical fiber, Power meter and OTDR in our previous videos. Today we will focus on fiber losses only.
Fiber Optic Comm
Fiber optic cable is lighter, smaller and flexible than copper cable. It can transmit optical signal with faster speed over longer distance. Many factors can influence the performance of fiber optic. When light travels through the core of fiber optic, the strength of the light decreases gradually.This loss of light power is called attenuation in fiber cable
Attenuation
Attenuation is the reduction in power of the light signal as it is transmitted.
Attenuation is the loss of optical signal due to absorption, scattering, bending, and other parameters of optical fiber over a long distance. when light is transmitted through OFC, light intensity through the fiber optic decreases between transmitter and receiver. Optical losses are measured by using Optical measuring instruments like OTDR and Optical power meter.loss in fiber is measured in decibels per km (db/Km). Some factors are discussed below.
Absorption Loss
Light absorption is a major cause of loss in optical fiber during optical communication. Fiber cable is manufactured by using ultra pure glass. It is impossible to manufacture materials that are total pure. During fabrication metal impurities are introduced in the fiber material. Absorption loss is due to the presence of metal particle in the glass components of a fiber optic cable. These impurities absorb the light travelling through the material.Modern fabrication techniques can reduce impurity levels.
Scattering Loss
Scattering losses occur when light ray interacts with a particle with different density level and sent in all direction without any absorption. scattering loss is due to the interaction of light with more dense or light region within a fiber. These density differences are produced when optical fibers are manufactured. Scattering is a wavelength dependent loss.
Dispersion / Chromatic dispersion
Dispersion is the spreading of the signal over time.When light emit from the source, it contains multiple wavelength and speed. Spreading of the signal is due to the different speeds of light rays. These dispersed rays do not arrive at same time and result in chromatic distortion. Dispersion loss can be minimized by using certain wavelengths say 870nm, 1300 nm and 1550 nm.
Modal dispersion
Spreading of the signal over time resulting from the different propagation modes in the fiber
Inter modal dispersion occur due to multiple mode of propagation in multi mode fiber. Since, many modes are propagating, different wavelengths & different velocities will take different time to propagate through the fiber, which leads to inter modal dispersion.Modal dispersion limits the maximum bit rate and link length. It causes the distorted pulse.For single mode transmission, MD is not a factor. However, at higher bit rates and over longer distances, chromatic dispersion rather than modal dispersion limits maximum communication link length.
Bend / Radiation Loss
Bend loss is generated by improper handling of fiber optic cable.It is caused by fiber optic bend. There are two basic types of bend loss.The following causes introduces the bend loss:
- Defects of manufacturing.
- local mechanical mechanical on the fiber.
Micro bending: Micro bending are small microscopic bends that occur mainly in the core-cladding interface. Micro bend change the path of light wave up to a few microns..
Macro bending: Happens when the fiber is bent into a large radius . It is found in a splice tray. when fibers are bent below a certain radius, radiation causes big light power loss.
Loss Margin
When measuring the total losses in optical fiber, the above-mentioned losses should be considered. System designer add a loss margin of up-to 10 db during system design.
How to reduce optical losses
In order to meet the power requirement of the receiver, it is an issue to reduce the losses in optical fiber. Here are some useful practices in fiber link installation.
- Purchase high-quality cables as much as possible.
- Minimize tight bends and keep the coil radius as large as possible.
- Avoid any stress on the fiber, during installation.
- Push is recommended when you needs pulling.
