Broadband Communication Systems UNIT-III Multiband Systems

1. Broadband Communication SystemsUNIT-III Multiband Systems Dr. AchalaDeshmukh achala.deshmukh@gmail.com amdeshmukh.scoe@sinhgad.edu 9422319006

2. UNIT:III

3. WDM Concept and Components

4. Why WDM? • Capacity upgrade of existing fiber networks (without adding fibers) • Transparency: Each optical channel can carry any transmission format (different asynchronous bit rates, analog or digital) • Scalability– Buy and install equipment for additional demand as needed • Wavelength routing and switching: Wavelength is used as another dimension to time and space

5. Wavelength Division Multiplexing Each wavelength is like a separate channel (fiber)

6. Wavelength Division Multiplexing • Passive/active devices are needed to combine, distribute, isolate and amplify optical power at different wavelengths

7. 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

8. WDM and DWDM • First WDM networks used just two wavelengths, 1310 nm and1550 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

9. ITU-T Standard Transmission DWDM windows

10. 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 Principles of DWDM Ex. 10.1

11. Nortel OPTERA 640 System 64 wavelengths each carrying 10 Gb/s

12. Key Components for WDM Passive Optical Components • Wavelength Selective Splitters • Wavelength Selective Couplers Active Optical Components • Tunable Optical Filter • Tunable Source • Optical amplifier • Add-drop Multiplexer and De-multiplexer

13. Passive Devices • These operate completely in the optical domain (no O/E conversion) and does not need electrical power • Split/combine light stream Ex: N X N couplers, power splitters, power taps and star couplers • Technologies: - Fiber based or • Optical waveguides based • Micro (Nano) optics based • Fabricated using optical fiber or waveguide (with special material like InP, LiNbO3)

14. Filter, Multiplexer and Router

15. Basic Star Coupler May have N inputs and M outputs • Can be wavelength selective/nonselective • Up to N =M = 64, typically N, M < 10

16. Fused-Biconical coupler OR Directional coupler • P3, P4 extremely low ( -70 dB below Po) • Coupling / Splitting Ratio = P2/(P1+P2) • If P1=P2It is called 3-dB coupler

17. Definitions Try Ex. 10.2

18. Coupler Characteristics • power ratio between both output can be changed by adjusting the draw length of a simple fused fiber coupler • It can be made a WDM de-multiplexer: • Example, 1300 nm will appear output 2 (p2) and 1550 nm will appear at output 1 (P1) • However, suitable only for few wavelengths that are far apart, not good for DWDM

19. Wavelength Selective Devices These perform their operation on the incoming optical signal as a function of the wavelength Examples: • Wavelength add/drop multiplexers • Wavelength selective optical combiners/splitters • Wavelength selective switches and routers

20. Fused-Fiber Star Coupler Splitting Loss = -10 Log(1/N) dB = 10 Log (N) dB Excess Loss = 10 Log (Total Pin/Total Pout) Fused couplers have high excess loss

21. 8x8 bi-directional star coupler by cascading 3 stages of 3-dB Couplers 1, 2 1, 2 5, 6 1, 2 3, 4 7, 8 (12 = 4 X 3) Try Ex. 10.5

22. Fiber Bragg Grating

23. Fiber Brag Grating (FBG) • Basic FBG is an in-fiber passive optical band reject filter • FBG is created by imprinting a periodic perturbation in the fiber core • The spacing between two adjacent slits is called the pitch • Grating play an important role in: • Wavelength filtering • Dispersion compensation • Optical sensing • EDFA Gain flattening • Single mode lasers and many more areas

24. Bragg Grating formation

25. FBG Theory Exposure to the high intensity UV radiation changes the fiber core n(z) permanently as a periodic function of z z: Distance measured along fiber core axis : Pitch of the grating ncore: Core refractive index δn: Peak refractive index

26. Reflection at FBG

27. Simple De-multiplexing Function Peak ReflectivityRmax = tanh2(kL)

28. Wavelength Selective DEMUX

29. ADD/DROP MUX FBG Reflects in both directions; it is bidirectional

30. FBG Properties • Advantages • Easy to manufacture, low cost, ease of coupling • Minimal insertion losses – approx. 0.1 db or less • Passive devices • Disadvantages • Sensitive to temperature and strain. • Any change in temperature or strain in a FBG causes the grating period and/or the effective refractive index to change, which causes the Bragg wavelength to change.

34. OPTICAL AMPLIFIERS

35. ... ... l l l l l l l l l 4 5 7 8 1 2 3 6 16 Why Optical Amplifiers? • Increase transmission distance • by increasing optical power coupled to transmission fiber(power booster) • by compensating optical fiber losses(in-line amplifier, remote pump amplifier) • by improving receiver sensitivity (optical preamplifier) • Function : Amplification of optical signal without conversion to electrical signal • Ingredients : Pump energy, amplification medium Optical output Pumping of energy Optical input amplification medium l l l l l l l l l

36. General Application of Optical Amplification • In-line amplifier • Preamplifier • Power (booster) amplifier • LAN booster amplifier Amplifier Standard Fiber Pump Lasers

37. Erbium Doped Fiber Amplifiers (EDFA)

38. Input WDM Isolator 1480 or 980 nm Pump Laser Output Erbium Doped Fiber Basic EDF Amplifier Design • Erbium-doped fiber amplifier (EDFA) most common • Commercially available since the early 1990’s • Works best in the range 1530 to 1565 nm • Gain up to 30 dB

39. Operation of an EDFA Power interchange between pump and data signals Power level Power level 980 nm signal 1550 nm data signal 980 nm signal 1550 nm data signal Isolator Isolator Output Input WDM Erbium Doped Fibre Pump Source = Fusion Splice

40. 540 670 820 980 Metastable state 1480 Ground state Erbium Properties • Erbium: rare element with phosphorescent properties • Photons at 1480 or 980 nm activate electrons into a metastable state • Electrons falling back emit light in the 1550 nm range • Spontaneous emission • Occurs randomly (time constant ~1 ms) • Stimulated emission • By electromagnetic wave • Emitted wavelength & phase areidentical to incident one

41. Erbium-doped Fibre Amplifier When you actually look at an EDFA block diagram you see the input on the left the output on the right, photo diodes to measure input and output power levels, pump lasers which are used to excite this piece of erbium doped fiber to a higher energy state, and 1550 light is allowed to pass through. The isolators are used to keep the 980 and 1480 pump wavelengths contained in the fiber but allow 1550nm light to pass through.

42. Typical Packaged EDFA

43. EDFAs In DWDM Systems • Gain flatness requirements • Gain competition • Nonlinear effects in fibers Optical amplifiers in DWDM systems require special considerations because of:

44. Raman Amplifier • Raman Amplifier was demonstrated in the 1980s • Unavailability of high-power diode laser pump source • Why do you need it : amplify signals from 1270 to 1670 nm any optical fiber can serve as the amplifying medium • Raman process itself provides high-power laser • Disadvantage: Cross-talk

45. Raman Amplifiers • Based on Stimulated Raman Scattering occurs at high optical powers. • Gain mechanism can be achieved through a pumped or distributed amplifier. • Lumped Raman Amplifier: • A spool of about 80 m of small core fiber along with appropriate pump lasers • Distributed Raman Amplifier: • One or more Raman Pump lasers convert the final 20-40 km of transmission fiber into a preamplifier.

47. Semiconductor Optical Amplifier (SOA) • Basically a laser chip without any mirrors • Metastable state has nanoseconds lifetime(-> nonlinearity and crosstalk problems) • Potential for switches and wavelength converters

48. Semiconductor optical amplifiers, like their semiconductor-laser, consist of gain and passive regions. Layers of antireflective coatings prevent light from reflecting back into the circuit while the incoming signal stimulates electrons in the gain region

49. Semiconductor Optical Amplifier (SOA) • Semiconductor optical amplifiers are similar in construction to semiconductor lasers. • They consist of a gain (active) section and a passive section constructed of a semiconductor material such as indium phosphide. • The main difference is that SOAs are made with layers of antireflection coatings to prevent light from reflecting back into the circuit. • Optical gain occurs as excited electrons in the semiconductor material are stimulated by incoming light signals; when current is applied across the p-n junction the process causes the photons to replicate, producing signal gain. • The gain medium can be either a bulk or a multiple-quantum-well active layer.

50. Semiconductor Optical Amplifiers • Semiconductor Optical Amplifiers (SOA’s) • Laser diodes without end mirrors • Fiber attached to both ends • Amplified version of the optical signal is produced • Advantage: bidirectional • Drawbacks • High-coupling losses • High noise figure