This technology is cost effective and more flexible for upgrading channel capacity, adding / removing channels, rerouting and distributing traffic, supporting all types of network topology and protection and synchronization systems. Here are the main components -
- TP (transponder)
- VOA (variable optical attenuator)
- MUX (multiplexer)
- DEMUX (demultiplexer)
- BA (booster amplifier)
- Line (OFC media)
- LA (aline amplifier)
- PA (Preamplifier)
- OSC (Optical Supervisory Channel)
Transponder
This device is an interface between STM-n wide pulse optical signal and MUX / DEMUX equipment. This optical signal can be co-located or come from different physical media, different protocols and types of traffic. It converts the wide pulse signal into a narrow wavelength (spot or colored frequency) of the order of nanometer (nm) with a spacing of 1.6 nm; send to MUX.
In the reverse direction, the colored output of the DEMUX is converted to a wide pulse optical signal. output power is between +1 and –3 dBm in both directions. The conversion is optical to electrical and electrical to optical (O to E and E to O) in 2R or 3R method.
In 2R, regeneration and re-shaping are performed, while in 3R, regeneration, re-shaping and re-timing are efcarried out. TP can be the color of the wavelength and bit rate dependent or tunable for both (expensive and not used). However, in 2R, any bit rate, PDH, STM-4 or STM-16 can be the channel rate. The unit has limitation with receiver sensitivity and overload point.
Although Intermediate electrical stage is inaccessible, STN-n overload bytes are used for supervision. This device also supports Optical Safety Operation (ALS) on ITU-T Recommendation G.957.
Variable Optical Attenuator (VOA)
This is a passive network as the pre-emphasis needed to adjust for even distribution of signal level over the EDFA band so that the optical output power of each channel of the Mux unit remains the same regardless of the number of channels loaded in the system.
The optical attenuator is similar to a simpthe potentiometer or circuit used to reduce a signal level. The attenuator is used whenever a performance test needs to be run, for example, to see how the bit error is affected by the change in signal level in the link. One way is to have a precise mechanical configuration in which the optical signal passes through a glass plate with a different amount of darkness and then returns to the optical fiber as shown in the figure.
The glass plate is gray density ranging from 0% at one end to 100% at the other end. As the plate is moved through space, more or less light energy is allowed to pass. This type of attenuator is very precise, and can handle any light wavelength (since the plate attenuates all light energy by the same amount, regardless of the wavelength), but it is mechanically expensive.
Multiplexer (MUX) and demultiplexeur (De-MUX)
Since DWDM systems send signals from multiple stations over a single fiber, they must include some means to combine the incoming signals. This is done with the help of a multiplexer, which takes the optical wavelengths of several fibers and converts them into a bundle. On reception, the system must be able to separate the transmitted wavelengths of the light beam so that they can be detected discreetly.
Demultiplexers perform this function by separating the received beam into its wavelength components and coupling them into individual fibers.
Multiplexers and demultiplexers can be passive or active in design. Passive design uses prism, arrays of diffraction or filters while the active design combines passive devices with tunable filters.
The main challenges of these devices aret minimize crosstalk and maximize channel separation (the wavelength difference between Crosstalk is a measure of how well the channels are separated, while channel separation refers to the ability to distinguish each wavelength.
Types of multiplexer / demultiplexer
Type of prism
A simple form of multiplexing or demultiplexing of wavelengths can be carried out using a prism.
A parallel beam of polychromati Light c touches a prism surface and each component wavelength is refracted differently. That is the rainbow effect . In the output light, each wavelength is separated from the next by an angle. A lens then focuses each wavelength at the point where it should go into a fiber. Components can be used in reverse to multiplex different wavelengths onto a fiber.
Typediffraction grating
Another technology is based on the principle of diffraction and optical interference. When a polychromatic light source strikes a diffraction grating, each wavelength is diffracted at a different angle and therefore at a different point in space. By using a lens, these wavelengths can be focused on individual fibers, as shown in the following figure. The Bragg grating is a simple passive component, which can be used as a wavelength-selective mirror and is widely used to add and remove channels in DWDM systems.
Braggs gratings are made using an ultraviolet laser beam to illuminate the core of a single modal fiber through a phase mask. The fiber is doped with phosphorus, germanium or boron to make it photo-sensitive. Once the light has passed through the mask, a pattern of fringes is produced,which is "printed" in the fiber. This creates a permanent periodic modulation of the refractive index of the fiberglass core. The finished grating reflects light at the Bragg wavelength (equal to twice the optical spacing between the high and low index regions) and transmits all other wavelengths.
Tunable Bragg grating
A Bragg fiber grating can be bonded to a piezoelectric element. By applying a voltage to the element, the element stretches so that the grating is stretched and the Bragg wavelength changes to a longer wavelength. Current devices can provide an adjustment range of 2nm for a 150V input.
Arrayed Waveguide Array
Matrix waveguide arrays (AWG) are also based on diffraction principles. An AWG device, sometimes referred to as an optical waveguide router or waveguide network router, consists of a networkof curved channel waveguides with a fixed difference in path length between adjacent channels. The waveguides are connected to inlet and outlet cavities.
Optical multiplexer
When light enters the entrance cavity, it is diffracted and enters the waveguide array. Thus, the difference in optical length of each waveguide introduces phase delays in the output cavity, where an array of fibers is coupled. The process results in different wavelengths having maximum interference in different places, which corresponds to the output ports.
Multilayer interference filters
A different technology uses interference filters in devices called thin film filters or multilayer interference filters. By positioning the filters, made up of thin films in the ical opt path, the wavelength can be demultiplexed. The property ofeach filter is such that it transmits one wavelength, while reflecting the others. By cascading these devices, many wavelengths can be demultiplexed.
Filters provide good stability and isolation between channels at a moderate cost, but with high insertion loss (AWGs have a flat spectral response and low insertion loss ). The main disadvantage of the filter is that it is temperature sensitive and may not be used in virtually all environments. However, their big advantage is that they can be designed to perform multiplexing and demultiplexing operations simultaneously.
OM coupling type
OM coupling is an interactive surface with two or more fibers welded together. Generally, it is used for OM, and its working principles are shown in the following figure.
The coupling OM cannot effectCtuer multiplexing function at low manufacturing cost. Its disadvantage is high insertion loss. Currently, the OM used in ZTWE's DWDM equipment uses OM coupling. The OD adopts AWG components.
Amplifiers amplifiers (optical amplifiers)
Due to attenuation, there are limits to the length of time a fiber segment can propagate a signal with integrity, before to regenerate. Before the arrival of optical amplifiers (OA), a repeater was needed for each transmitted signal. OA had made it possible to amplify all wavelengths at the same time and without optical-electrical-optical (OEO) conversion. In addition to being used in optical links (as a repeater), optical amplifiers can also be used to increase signal power after multiplexing or before demultiplexing.
Types of optical amplifiers
In each optical channel, the amplifiersOptical ators were used as repeaters in simplex mode. One fiber was used in the outgoing path and the second fiber was used in the return path. The latest optical amplifiers will work in two directions at the same time. We can even use the same wavelength in two directions, as long as we use two different bit rates. Only one fiber can therefore be used for duplex operation.
Optical amplifiers also need to have sufficient bandwidth to pass a range of signals operating at different wavelengths. For example, an SLA with a spectral bandwidth of 40 nm, for example, can handle ten optical signals.
In a 565 mb / s system, for an optical link of 500 km, five SLA optical amplifiers are needed, spaced at an interval of 83 km. Each amplifier provides a gain of about 12 dB, but also introduces du noise in the system (BER of 10-9.)
SLA amplifiers have the following disadvantages -
- Sensitive to temperature changes
- Sensitive to changes in supply voltage
- Sensitive to mechanical vibrations
- Unreliable
- Subject to crosstalk
Erbium Doped Fiber Amplifier (EDFA)
In DWDM systems, EDFAs are used. Erbium is a rare earth element which, when excited, emits light around 1.54 micrometers, which is the low loss wavelength for optical fibers used in DWDM. A weak signal enters the erbium-doped fiber, into which light at 980nm or 1480nm is injected using a pump laser.
This injected light stimulates the erbium atoms to release their stored energy in the form of additional light of 1550 nm. The signal is getting strong. Spontaneous emissionss in EDFAs also add the noise figure of an EDFA. EDFAs have a typical bandwidth of 100nm and are required at an interval of 80 to 120 km along the optical route.
EDFAs also suffer from an effect called four-wave mixing due to the non-linear interaction between adjacent channels. Therefore, increasing the power of the amplifier to increase the distance between repeaters leads to more crosstalk.
Raman Amplifier
The use of SLA and EDFA amplifiers in WDM is limited as already described and, moderator n WDM systems look to Raman amplification , which has a bandwidth of around 300nm. Here, the pump laser is at the receiving end of the fiber. Crosstalk and noise are greatly reduced. However, Raman amplification requires the use of a high pump laser.
The dispersion in the fiber actually helps to minimize the"four-wave mixing" effect. Unfortunately, early optical links often used zero dispersion fibers in an attempt to minimize dispersion over long distances, when those same fibers are upgraded to carry WDM signals; they are not the ideal medium for broadband optical signals.
Special fibers in mono mode are under development for WDM use. These have alternating segments of positively and negatively dispersed fibers, hence the total dispersion amounts to zero. However, the individual segments provide dispersion to prevent four-wave mixing.
Line amplifiers
This is a two-stage EDFA amplifier consisting of a preamplifier (PA) and a booster amplifier (BA). Without the two stages, it is not possible to amplify the signal up to 33 dB on the EDFA principle (to avoid the noise generated by emission spontaneous). The Line Amplifier (LA) compensates for line loss by 22dB or 33dB respectively for long and very long haul systems. This is entirely an optical stage device.
Online Media (OFC)
This is the fiber-optic media over which DWDM signals travel. Attenuation and dispersion are the main limiting factors determining transmission distance, bit rate capacity, etc. Normally 22dB and 33dB are considered line loss for long haul and super long haul systems, respectively.
The wavelength of the very long distance line can be 120 km without repeater (LA). However, with a number of cascade repeaters, the length can be up to 600 km, which can be further increased to 1200 km using the dispersion compensation module. After such a distance it needs regenerationin the electrical stage instead of the repeater in the optical stage only.
Preamp proud (PA)
This amplifier alone is used at the terminal level to interface the DEMUX and the signal reception line from the remote station. Therefore, the attenuated line signal is amplified to a level of +3 dBm to 10 dBm before entering the DEMUX unit.
Optical supervision channel
The function of transmitting additional data (2 mbps: EOW, user specific data, etc. via the interface) at a length Separate wave (1480 nm according to ITU-T Recommendation G-692) of lower optical level without any optical security provision, accompanied and independent of the main optical traffic STM-n signal, is carried out by the OSC. EOW (0.3 to 3.4 KHz) for selective and omnibus channel is 64 kbps in 8-bit PCM code.
The optical supervision channel (OSC) allows control and monitoring of devicesoptical line tifs as well as fault location management, configuration, performance and security achieved using LCT.
