What is DWDM Technology?

online_learn_earn February 23, 2020

What is DWDM (Dense Wavelength Division Multiplexing)

DWDM stands for Dense wavelength division multiplexing, an optical technology used to increase the bandwidth of a single fiber optics. It allows many discrete transports channels by combining and transmitting multiple light signals simultaneously at different wavelengths on single or same fiber so one fiber is viewed into multiple virtual fibers. 

DWDM is a crucial component of optical networks that allows the transmission of e-mail,video,multimedia,data and voice-carried in different formats.

 WDM (Wavelength Division Multiplex)

WDM (Wavelength Division Multiplex) Technology is a very effective means of increasing the transmission of Fiber ,as it demands neither the installation of new links, nor any increasing in transmission speed. The data are transmitted along the fiber at different wavelengths each waves-length  or channel transmitting a signal. The Channels are defined according to the G-692 recommendations of the ITU-T.
This Technology demands new measurements to check the parameters. The most important item of equipment for carrying out these tests on WDM system is the Optical Spectrum Analyzer(OSA). It can be connected at critical measurement points in the WDM system to the ends of the links or to the amplifier location.

In Optical Network Measurements Made by the Three Types:-


  1. Optical Time Domain Reflectometer (OTDR).
  2. Optical Spectrum Analyzer (OSA)
  3. Polarization Mode Dispersion (PMD)

 OTDR Measurements

Optical Time Domain Reflectometer consists in injecting a light pulse into one end of the optical fiber to be analyzed and observing , at the same end ,the optical intensity passing through the fiber in the opposite direction to the propagation of the pulse. The signal detected  is exponentially in form.typical of the phenomenon of back scattering  with superimposed peaks due to reflections from the ends of the fiber or other variations in the refractive index.

 From a backscatter trace it is possible  , to determine the position of a section of fiber within a fiber link.

The Measurement result must reveal ;-

  • The Attenuation.
  • The Location of fault by their distance from a point of origin.
  • Attenuation with respect to distance(dB/km)
  • The reflectance of a reflective event or a link.

Reflectance:

Reflectance is a value with which the coefficient of reflection of a reflecting optical system may be quantified. It is defined as the ratio of the power reflected by the element over the incident power in optical link. These reflections are due to variations in refractive index all along with  the optical link in certain telecommunications applications or systems. If they are not controlled ,they may degrade the performance of the system by perturbing the operation of the emitting laser or may generate interference noise in the receiver by multiple reflections.

 OSA Measurements


The Optical Spectrum Analyzer is used in ROADM,DWDM,TML System . It can be connected at critical measurement points in the WDM system , to the ends of the links  ports or to the amplifier locations points.
Measurement result  of Optical Spectrum Analyzer displays a spectrum representing all the Channels. The OSA performs automatic detection and measurements on each channel

 PMD(Polarization  Mode  Dispersion )

The Transmission rate and range are two of the most important parameters of Fiber optics  paths  and must therefore be optimized and since more and more paths are being used for transmitting Wavelength Division Multiplex signals or for bit rates of 10 Gbps,It is becoming all the more important to determine the Polarization Mode Dispersion.
PMD Which is the basic property of single-mode Fiber, in particular affecting  the  transmission rate with respect to magnitude.
The PMD is also described to as mean value of all Differential Group Delays in pico seconds. The mean DGD factor the transmission pulse to broaden when transmitted along the fiber,generating distortion ,which in turns increases the bit-error-rate (BER) of the optical transmission medium .
The Consequence is that the PMD limits the transmission bit rate on a link. It is then important to know the PMD value to calculate what are the bit rate limits of the links.

Optical Power and attenuation measurements:

Power Meter

A Power meter ,is an optical device that is used to measure emitted or received power .

  • To measure emitted power the system ports ,connect the power meter directly to the output of the optical emitter.
  • To measure the power at the input ports of an optical receiver , the power meter is connected to the end of the fiber ,at the point where the optical receiver is connected.

Attenuation measurement


For measurement of the attenuation of the power in a fiber  link or in elements such as sections of fiber ,connections or optical components , a light source and power meter are required for testing also.

EDFA Measurement

The result analysis of an EDFA consists in performing two spectrum analysis: one before the signal is amplified and another one after the signal is amplified .both traces are further compared,providing the result in  power gain and noise figure.

I-OSNR Measurement

In ROADM networks every channel may traverse different route ,optical amplifiers , and add-drop filters,resulting OSNR for every channel. Conventional OSA measurements are respectable, as they indicate OSNR values that are too high:up to 10dB above the true OSNR. Using the In-band OSNR method will provide the true OSNR value in ROADM based networks

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