UNIT. 6.WDM CONCEPTS AND COMPONENTS-1.pptx

To knowledge gaining in wdm

1. OFC Systems Unit 6.WDM CONCEPTS AND COMPONENTS By Dr. Venkateswara Rao kolli Assistant Professor ECE,MCE.

2. Syllabus 1.WDM concepts, 2.Overview of WDM operation principles, 3.WDM standards, 4.Mach-Zehender Interferometer 5.Multiplexer, 6.Isolators and circulators, 7.Direct thin film filters, 8.Active optical components

3. Evolution of the Technology 1.WDM concepts

4. WDM concepts( contd …) Features of WDM Important advantages or features of WDM are as mentioned below – 1. Capacity upgrade : Since each wavelength supports independent data rate in Gbps . 2. Transparency : WDM can carry fast asynchronous, slow synchronous, synchronous analog and digital data. 3. Wavelength routing : Link capacity and flexibility can be increased by using multiple wavelength. 4. Wavelength switching : WDM can add or drop multiplexers, cross connects and wavelength converters

5. WDM concepts( contd …) Each wavelength is like a separate channel (fiber) Wavelength Division Multiplexing

6. WDM concepts( contd …) WDM, CWDM and DWDM WDM technology uses multiple wavelengths to transmit information over a single fiber Coarse WDM (CWDM) has wider channel spacing (20 nm) – low cost Dense WDM (DWDM) has dense channel spacing (0.8 nm) which allows simultaneous transmission of 16+ wavelengths – high capacity

7. WDM concepts( contd …) WDM and DWDM First WDM networks used just 2 wavelengths , 1310 nm and 1550 nm Today's DWDM systems utilize 16, 32,64,128 or more wavelengths in the 1550 nm window Each of these wavelength provide an independent channel (Ex: each may transmit 10 Gb /s digital or SCMA analog) The range of standardized channel grids includes 50, 100, 200 and 1000 GHz spacing Wavelength spacing practically depends on: laser linewidth optical filter bandwidth

8. Optical signals of different wavelength (1300-1600 nm) can propagate without interfering with each other. The scheme of combining a number of wavelengths over a single fiber is called wavelength division multiplexing (WDM). Each input is generated by a separate optical source with a unique wavelength. An optical multiplexer couples light from individual sources to the transmitting fiber. At the receiving station, an optical demultiplexer is required to separate the different carriers before photodetection of individual signals. Fig. 7.1.1 shows simple SDM scheme. To prevent spurious signals to enter into receiving channel, the demultiplexer must have narrow spectral operation with sharp wavelength cut-offs. 2.Overview of WDM operation principles

9. Principles of WDM BW of a modulated laser: 10-50 MHz  0.001 nm Typical Guard band: 0.4 – 1.6 nm 80 nm or 14 THz @1300 nm band 120 nm or 15 THz @ 1550 nm Discrete wavelengths form individual channels that can be modulated, routed and switched individually These operations require variety of passive and active devices Ex. 10.1

10. DWDM Limitations Theoretically large number of channels can be packed in a fiber For physical realization of DWDM networks we need precise wavelength selective devices Optical amplifiers are imperative to provide long transmission distances without repeaters

11. 3.ITU-T Standard Transmission DWDM windows

12. 4.Mach-Zehnder Interferometer Phase shift of the propagating wave increases with L, Constructive or destructive interference depending on L

13. Mach- Zehnder Interferometer( contd …) Initially a 3 dB directional coupler is used to split input signals. The middle stage, in which one of waveguide is longer by ΔL to given a wavelength dependent phase shift between the two arms. The third stage is a dB coupler which recombines the signals at output. Thus input beam is splitted an phase shift it introduced in one of the paths, the recombined signals will be in phase at one output and out of phase at other output. The output will be available in only one port

14. Mach- Zehnder Interferometer( contd …)

15. Mach- Zehnder Interferometer( contd …)

16. Mach- Zehnder Interferometer( contd …) Phase shift at the output due to the propagation path length difference: If the power from both inputs (at different wavelengths) to be added at output port 2, then,

17. Four-Channel Wavelength Multiplexer By appropriately selecting Δ L, wavelength multiplexing/de-multiplexing can be achieved Mach- Zehnder Interferometer( contd …)

18. MZI- Demux Example Mach- Zehnder Interferometer( contd …)

19. 5.Optical Add/Drop Multiplexer As add/drop multiplexer is essentially a form of a wavelength router with one input port and one output port with an additional local port where wavelengths are added to/dropped from incoming light signal. It is an application of optical filter in optical networks. Fiber grating devices are used for add/drop functions. Many variations of add/drop element can be realized by using gratings in combination with couplers and circulators.

20. 6.Isolator An isolator is a passive non-reciprocal device. It allows transmission in one direction through it and blocks all transmission in other direction. Isolator are used in systems before optical amplifiers and lasers mainly to prevent reflections from entering these devices otherwise performance will degrade. Important parameters of an isolator are its insertion loss (in forward direction) and isolation (in reverse direction). The insertion loss should be as small as possible while isolation should be as large as possible. The typical insertion loss is around 1 dB and isolation is around 40 to 50 dB.

21. Principle of operation Isolator works on the principle of state of polarization (SOP) of light in a single mode fibers. The state of polarization (SOP) refers to the orientation of its electric field vector on a plane that is orthogonal to its direction of propagation. The electric field can be expressed as linear combination of two orthogonal linear polarization supported by fiber. These two polarization modes are horizontal and vertical modes. The principle of operation is illustrated in Fig. 7.4.1.

22. Principle of operation( contd …) Let input light signal has vertical state of polarization (SOP) and blocks energy in horizontal SOP. The polarizer is followed by Faraday rotator. Faraday rotator is an asymmetric device which rotates the SOP clockwise by 45 o in both direction of propagation. The polarizer after Faraday rotator passes only SOPs with 45 o orientation. In this way light signal from left to right is passed through the device without any loss. Light entering the device from right due to reflection, with same 45 o SOP orientation, is rotated another 45 o by the Faraday and blocked by the next polarizer.

23. 7.Circulator A three part circulator is shown in Fig. 7.5.1. Signals of different wavelengths are entered at a port and sends them out at next port. All the wavelengths are passed to port-2. If port-2 absorbs any specific wavelength then remaining wavelengths are reflected and sends them to next port-3. 

24. Circulator( contd …) Circulators are used to implement demultiplexer using ∂ fiber Bragg grating for extracting a desired wavelength. The wavelength satisfying the Bragg condition of grating gets reflected and exits at next port. Fig. 7.5.2 illustrates the concept of demultiplexer function using a fiber grating and an optical circulator. Here, from all the wavelengths only λ3 is to be extracted. The circulator takes four wavelengths λ1, λ2, λ3 and λ4 from input port-1 tunable filter operates on similar principle as passive devices. It operates over a range of frequencies and can be tuned at only one optical frequency to pass through it.

25. Circulator( contd …) Fig. 7.6.1 illustrates concept of tunable filter. The system parameters for tunable optical filters are – 1) Tuning rage ( Δ v) 2) Channel spacing ( δ v) 3) Maximum number of channels(N) 4) Tuning speed.

26. Circulator( contd …)

27. Circulator( contd …)

28. 8.Dielectric Thin-Film Filter (TFF)

29. Dielectric Thin-Film Filter (TFF)( contd …)

30. References TEXT BOOKS: Optical Fiber Communication – Gerd Keiser, 4th Ed., MGH, 2008. 2. Optical Fiber Communications– – John M. Senior, Pearson Education. 3rd Impression, 2007.