Optical Transport Network Technology

Optical Transport Network Technology

Optical Transmission Network is about converting an electrical signal into an optical signal to send into the optical fiber. It needs amplifiers equipment and  and an optical receiver to recover the signal as an electrical signal. So it containing of part as the following.

Transmitters

Normally used  light-emitting diodes (LEDs) and laser diodes for optical communications but it must be efficient and reliable for operating in an optimal wavelength range and directly modulated at high frequencies.

LEDs are suitable primarily for local-area-network applications with bit rates of 10-100 Mbit/s and transmission distances of a few kilometers. Today, LEDs have been largely superseded by VCSEL (Vertical Cavity Surface Emitting Laser) devices, which offer improved speed, power and spectral properties, at a similar cost. Common VCSEL devices couple well to multi mode fiber. 

Laser diodes are need to be modulated, it is the light output is controlled by a current applied directly to the device. For very high data rates or very long distance links, a laser source may be operated continuous wave, and the light modulated. 

Receivers

Commonly used the photodetector to convert light into electricity. The photodetector is a semiconductor-based photodiode. The photodiodes  used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.

Fiber cable types

An optical fiber cable consists of a core, cladding, and a buffer. The core and cladding are made of high-quality silica glass, although they can both be made of plastic as well. 

There are two types of optic communication:

1. Single-mode optical fibers:

fiber is smaller (<10 micrometers) and requires more expensive components and interconnection methods, but allows much longer, higher-performance links. These modes define the way the wave travels through space, how the wave is distributed in space. Waves can have the same mode but have different frequencies. This mode is mostly used for communication over short distances between node by node of the service provider. 

2. Multi-mode optical fibers: 

has a larger core (≥ 50 micrometers), allowing less precise, cheaper transmitters and receivers to connect to it as well as cheaper connectors. However, a multi-mode fiber introduces multimode distortion, which often limits the bandwidth and length of the link. Furthermore, because of its higher dopant content, multi-mode fibers are usually expensive and exhibit higher attenuation. 

This mode is mostly used for communication over short distances, such as within a building or on a campus. Typical multimode links have data rates of 10 Mbit/s to 10 Gbit/s over link lengths of up to 600 meters (2000 feet) — more than sufficient for the majority of premises applications.

Amplifier

Amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. Normally the fiber optic is limited by fiber attenuation distortion then they use the amplifier for doping a length of fiber and pumping it with light from a laser with a shorter wavelength than the communications signal (typically 980 nm). Amplifiers have largely replaced repeaters in new installations.

Wavelength-division multiplexing

Wavelength-division multiplexing (WDM) is multiplexing and de-multiplexing the fiber optic signal. 

The multiplexing is required in the transmitting equipment to transmit the signal out and then de-multiplexing at the receiving equipment.