Overview of Wavelength Division Multiplexing (WDM) Technology
Learn about the concepts and components of Wavelength Division Multiplexing (WDM) in optical fiber communication. Understand how WDM combines multiple wavelengths on the same fiber to increase capacity efficiently. Discover the operational principles, spectral bands, standards, and benefits of WDM technology.
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Subject Name: Optical Fiber Communication Subject Code: 10EC72 Prepared By: Sreepriya Kurup, Pallavi Adke, Sowmya L(TE) Department: Electronics and Communication Engineering Date: 10/11/2014 8/12/2014
Chapter 7 WDM Concepts and Components
Unit 7 Overview of WDM WDM Standards Mach-Zehnder Interferometer Isolators and Circulators Dielectric Thin-Film Filters Active Optical Components Tunable Light Sources 25/8/2014
Overview of WDM The technology of combining a number of independent information carrying wavelengths onto the same fiber is known as Wavelength Division Multiplexing . The original optical fiber links contained a single fiber with one light source at the transmitting end and one photo detector at the receiving end . These simplex systems greatly underutilize the large BW capacity of fiber. The WDM allows a dramatic increase in the capacity of an optical fiber compared to original simple point to point link. 25/8/2014
Operational Principles of WDM (1) WDM uses multiple wavelengths as carriers which can can be separated, routed, and switched without interfering with each other. WDM networks require a variety of passive and active devices to combine, distribute, isolate, and amplify optical power at different wavelengths. 25/8/2014
Operational Principles of WDM (2) Multiplexer is needed to combine these optical outputs into continuous spectrum of signals and couple them onto a single fiber. At the receiving end de-multiplexer is required to separate the optical signals into appropriate detection channels. A typical WDM link have various types of optical amplifiers Pre-amplifier, In-Line amplifier, Post- amplifier 25/8/2014
Spectral Bands for WDM Fig shows there are many independent operating across the spectrum ranging from O- band through L-bands in which narrow line width optical sources can be used simultaneously. These regions are can be seen either in terms of spectral width or optical bandwidth. Fundamental relationship between frequency and wavelength is c = . Differentiating this we have for << 2 25/8/2014
WDM Standards WDM standards developed by the International Telecommunication Union (ITU) specify channel spacing in terms of frequency. ITU-T Recommendation G.692 was the first ITU-T specification for WDM. The term dense WDM (DWDM) generally referred to small wavelength separations. ITU-T Recommendation G.694.1 specifies DWDM frequency for 100 and 50 GHz spacing's in the S-, C-, and L-bands. 25/8/2014
Mach-Zehnder Interferometer (MZI) Multiplexers TheMach-Zehnder interferometer (MZI) consists of three stages: a 3-dB directional coupler which splits the input signals, a central section where one of the waveguides is longer by L to give a wavelength-dependent phase shift between the two arms, and another 3-dB coupler which recombines the signals at the output. 25/8/2014
(MZI) Multiplexers (1) Figure gives an example for a 4 x 4 multiplexer. The inputs to MZI1 are and +2 (which we will call 1 and 3, respectively), and the inputs to MZI2 are + and +3 (which are called 2 and 4, respectively). Since the signals in both interferometers of the 1st stage are separated by 2 , the path differences satisfy the condition L1 = L2 = c/2neff(2 ) 25/8/2014
Cascaded MZIs Using basic 2 2 MZIs, any size N N multiplexer (with N = 2n) can be constructed. Each module i has a different Li in order to have all wavelengths exit at port C 25/8/2014
Optical Isolators Optical isolators allow light to pass through them in only one direction. It prevents scattered or reflected light from traveling in the reverse direction. Optical isolator can keep backward-traveling light from entering a laser diode and possibly causing instabilities in the optical output. Polarization-independent isolator made of three miniature optical components. The core of device consists of a 450 Faraday rotator which is placed between two wedge shaped birefrigent plates. 25/8/2014
Optical Isolators (1) Light travelling in the forward direction is separated into ordinary and extraordinary rays by first briefrigent plate. The faraday rotator then rotates the polarization plane of each ray by 450 . After faraday rotator, the two rays pass through the second briefrigent plate. The axis of this polarizer plane is adjusted such that relationship between two types of rays is maintained. Thus, when they exit the polarizer they both are refracted in identical parallel direction. 25/8/2014
Optical Isolators (2) In reverse direction (from right to left), the relationship of ordinary and extra ordinary rays is reversed when exiting the Faraday rotator due to the non-reciprocity of the Faraday rotation. 25/8/2014
Optical Circulators An optical circulator is a nonreciprocal multiport passive device that directs light sequentially from port to port in only one direction. The operation of circulators is similar to isolator except that its construction is more complex. For the 3 port circulator, an input on port 1 is sent out on port 2, an input on port 2 is sent out on port 3, and an input on port 3 is sent out on port 1. 25/8/2014
Isolator and Circulator Parameters 25/8/2014
Dielectric Thin-Film Filters A dielectric thin-film filter (TFF) is used as an optical bandpass flter. It allows a very narrow wavelength band to pass straight through it and reflects all other wavelengths. The basis of these devices is a reflective mirror surfaces called a Fabry- Perot interferometer or an etalon. 25/8/2014
Etalon Theory The transmission T of an ideal etalon in which there is no light absorption by the mirrors is an Airy function The periodicity of the device is called the free spectral range or FSR 25/8/2014
Dielectric Thin-Film Filter Filter transmission (dB) Dielectric reflector stacks 0 Steep rolloff 3 cavities Throughput k -10 Input: 1 N 2 cavities -20 1 cavity Reflection: 1 k-1 , k+1 N -30 Glass substrate Dielectric cavity layers -40 0.996 0.998 1 1.002 1.004 Relative wavelength c/ A thin-film dielectric resonant cavity filter is a Fabry-Perot interferometer Mirrors surrounding cavity are multiple reflective dielectric thin-film layers Cavity length determines a particular wavelength to pass & reflects all others Flat passbands with steep rolloffs Low insertion loss: 0.5 dB at peak and < 3.5 dB at center frequency 10 GHz High optical return loss (> 45 dB) Thin-film filters with a 50-GHz passband are commercially available 25/8/2014
Active Optical Components Active components require some type of external energy either to perform their functions or to be used over a wider operating range than a passive device, thereby offering greater application flexibility. Various active devices are: MEMS Technology Variable optical attenuators Tunable optical filters Dynamic gain equalizers Optical add-drop multiplexers Polarization controllers Chromatic dispersion compensators 25/8/2014
MEMS Technology MEMS is acronym for micro electro mechanical systems. These are devices that can combine mechanical, electrical, and optical components to provide sensing and actuation functions. A simple example of a MEMS actuation method. 25/8/2014
Tunable optical filter A tunable optical filter can be varied to select a specific narrow spectral band within a much wider optical band. Tunable optical filters based on fiber Bragg grating involve a stretching and relaxation process of the spacing in the fiber grating, that is , in the periodic variation in the refractive index along the core. 25/8/2014
A dynamic gain equalizer (DGE) A dynamic gain equalizer (DGE) is used to reduce the attenuation of the individual wavelengths within a spectral band. DGE is equivalent to filtering out individual wavelengths equalizing them on a channel by channel basis. It equalizes the gain profile of an erbium-doped fiber amplifier (EDFA) 25/8/2014
Optical add/drop multiplexer An optical add/drop multiplexer (OADM) inserts (adds) or extracts (drops) wavelengths at a designated point in an optical network. As shown in fig, when no mirrors are activated, each incoming channel passes through the switch to the output port. Incoming signals can be dropped from traffic flow by activating the appropriate mirror pair. When an optical signal is dropped, another path is established simultaneously allowing a new signal to be added from port 2A to the traffic flow. 25/8/2014
Chromatic Dispersion Compensators In this, the grating spacing varies linearly over a length of the grating, which creates chirped grating. The spacing decreases with distance along the grating length. The shorter wavelength components of a pulse travel farther into the fiber before being reflected. Thereby they experience more delay in going through the grating than the longer wavelength components. This results in dispersion compensation, since it compresses the pulse. 25/8/2014
Tunable Light Sources The fundamental concept to making such a laser is to change the cavity length in which the lasing occurs in order to have the device emit at different wavelengths. The tuning range tune can be estimated by tune = eff eff Where, eff is the change in effective refractive index. 25/8/2014
Tunable Light Sources(1) Fig shows the relationship between tuning range, channel spacing and source spectral width. To avoid crosstalk between adjacent channels , a channel spacing of 10 times the source spectral channel is specified. That is, channel = 10 channel 25/8/2014
Tunable Light Sources(2) Maximum number of channels N that can be placed in the tuning range channel is N = tune channel 25/8/2014
