Devices

1. Devices • Couplers • Isolators and Circulators • Multiplexers and Filters • Lasers and LEDs • Detectors • Amplifiers • Switches Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

2. Couplers • When two fibers are placed in proximity to each other, the signal are coupled from one fiber to another. • The coupler are treated mathematically in exactly the same manner as a electrical RF-coupler with scattering parameters. • Bild s84 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

3. Isolators and Circulators • Isolators couples the signals in one direction and block the transmission in the other direction • A circulator couples the signal to another port in a circular manner • Insertion loss is the power loss in the coupled direction (As low as possible) • Isolation is the loss in the blocked direction (As high as possible) Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

4. Isolator • A isolator is using a combination of polarization filters and polarization rotators. • A single polarization isolator is simple. • A polarization independent isolator are using polarization splitters, a rotator and a /2-plate. • Bild s88 • Bild s 89:1 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

5. Circulator • A circulator have similar operation as an isolator with more than two ports. • The signal are coupled from port 1 to 2, 2 to 3 and so on. • Bild s 89:2 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

6. Multiplexers and Filters • In the optical domain there exist • Filters • Multiplexers and Demultiplexers • Wavelength Routers • Bild s 91 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

7. Filters • Filters are usually designed with Bragg grating • Any periodic perturbation in the propagating medium serve as a Bragg grating (Usually refractive index). • The wavelength corresponding to the Bragg grating frequency are reflected while all other are transmitted. • The side lobes can be reduced by having smaller refractive index changes near the edges of the filter. • Bild s 98 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

8. Fabry-Perot Filters • Fabry-Perot Filter (Etalon) is fabricated by a cavity surrounded by two mirrors • The transfer function of an etalon filter is periodical due to the multiple of standing waves in the cavity. • Etalon is very simple and cheap. • Bild s 103,105 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

9. Multi Layer Filters • A single Etalon filter is very narrow banded. • The bandwidth can be increased by using multiple cavities. • Bild s 106,107:1 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

10. Add/Drop elements • Add/Drop elements can be realized by a fiber gratings (isolator), a circulator and a coupler. • Bild s 100 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

11. Multiplexers and Demultiplexers • Static multiplexers can be realized using one multi-layer filters for each wavelength. • The same device can be used as a multiplexer as well. • Bild s 107:2 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

12. Wavelength Routers • A Static wave-length router can be realized by using a few multiplexers and demultiplexers • Bild s 92 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

13. Planar Lasers • An amplifying medium is surrounded by two mirrors (one semi-transparent) • The amplifying medium is mostly a quantum well • Bild Agrawal s 96, 98 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

14. Planar Lasers • The Laser must Be confined in one direction • Gain Guided Laser • Oxide Strip • Junction Strip • Index Guided Laser • Ridge wave guide structure • Etched mesa structure • Bild Agrawal s 99,100 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

15. Lasers • The semiconductor amplifier is wide-banded • The monochromatic lasing wavelength is determined by the cavity surrounding the amplifier • For low power, high reflectivity is required for lasing operation. • Cavity Laser (Fabry Perot Cavity) • Distributed Feedback Reflector (DFR) • Distributed Bragg Reflector (DBR) • Bild Agrawal s 105, 107 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

16. Vertical Cavity Surface Emitting Lasers (VCSEL) • The cavity is made of epitaxial layers. • Possible to make very small devices • Devices can be tested before assembly • Bild Pessa 22:2 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

17. Pulse lasers • For fast communication it is necessary to modulate the laser signal quickly. • Bild Oleg 1-2 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

18. Saturable Absorber • A saturable absorber change the absorption very quickly. • By population inversion the absorption decreases. • For short decay time the life-time of the absorber must be reduced • Introduce a trap level in the band-gap • Bild Oleg 3-4 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

19. Pulse AmplifierMode Lock Laser • Short Pulse amplifier • The gain in the amplifier is constant • At the pulse the absorber bleaches, giving a net gain • Prior and after the pulse the absorber giving a net loss. • Colliding Pulse Mode-Lock Laser • Two pulses together have enough energy to saturate the absorber. • Bild Oleg 5-6 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

20. Light Emitting Diodes (LEDs) • Much Simpler and cheaper compared with lasers • For many applications with short distances and low data rates LEDs are sufficient. • A LED is a forward biased pn-junction, where the injected minority carriers recombine by spontaneous emission of light Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

21. LEDs • Telecommunication LEDs can either be surface or edge emitting. Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

22. Detectors • Telecommunication detectors are traditional pn-detectors • PIN diodes are used to increase the efficiency • Avalanche photodiodes are also used to increase the signal • To high amplification reduces noise performance Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

23. Semiconductor Optical Amplifiers (SOAs) • A semiconductor amplifiers is realized as a semiconductor laser without mirrors • Very short compared with fiber amplifiers • Bild Reale Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

24. SOAs • For high amplification and large bandwidth a very good antireflective coatings are necessary. • Difficult to achieve. • The antireflectivity can be improved by: • Tilted stripe structure • Window faced structure • Bild s 370, 369 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

25. SOAs • SOAs are polarization dependent • Multiple SOAs can be used to realize a polarization independent amplifier. • Bild s 372 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

26. Electro-Optic Switch • Electro-optic directional coupler switch • Semiconductor Optical Amplifier switch • An optical amplifier where the amplifier bias switches the signal • Bild s 155 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

27. All-Optical Regeneration • Nonlinear Loop Mirror • Without control pulse: Reshaping • With control pulse: Retiming and reshaping • A saturable absorber can be used as an optical gate for retiming and reshaping. • Bild Oleg 7-8 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

28. All Optic Switch • Loop Mirror • If the two signals are equal the signal are coupled to the input. • If the two signals experiences different absorption or index the signal are fully coupled to the output. • The control signal saturates the SOA for a short moment. • The two pulses reaches the SOA with a time difference, one where the amplified is saturated. • The control pulse are filtered away • The two other are based on Mach-Zehnder Interferometers. • Bild Toliver Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

29. All Optic Switch • Each data-pulse induces an non-linear refractive index change in the SOAs. • Each clock pulse is split in two parts and passing the SOAs. • The two pulses are interfering either destructively or constructively depending on the clock pulse arrival. • Bild Nakamura, 2xUeno, Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

30. All-Optical Pulse Regeneration • Electro optical timing and reshaping • All Optical signal path • Electrical signal for timing signal. • For all Optical Pulse Regeneration only the clock recovery is missing • Bild Oleg 5-6 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

31. Optical Clock Recovery • For Optical Clock recovery a lasers which are able to create short pulses are required • Self Pulsating Laser • Mode Lock Laser • Optical Clock recovery • up to 40 Gbit/s have • been achieved Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

32. Large Switches • A multi-port switches can easily be realized using several 2:2 switches in a matrix. • Bild s158 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

33. Wavelength converters • Opto-electric approach • Cross Gain Modulation in a SOA • Bild s162, 164 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

34. Solitons • Introduction • Robustness • Sliding-Frequency Filter • Tapered Fibers • Dispersion Managed Fibers • Pulse-to-Pulse Interaction Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

35. Introduction • Narrow pulses with high peak power and special shape. • A soliton is not affected by dispersion. • The dispersion is exactly compensated by nonlinear effects in the fiber. Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

36. Robustness • A soliton is, when once created very robust. • A pulse where nonlinear effects not exactly compensate the dispersion is shifted towards this case. • Solitons ”feels” only average parameters (fiber dispersion, fiber mode area, pulse energy) as long as the variations are faster than the soliton dispersion length. • Stable in systems with lumped amplifiers (Lamp< zdisp). • Slow variation can be used for pulse reshaping (compression and broadening) Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

37. Sliding-Frequency Filter • Etalon Filter can be used due to the narrow bandwidth of the soliton • Cheap and simple, compared with Gaussian filters • Same filter can be used for multiple channels • Bild 17:1 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

38. Sliding-Frequency Filter • The soliton adapts to successive slowly sliding-frequency shifting filters and moves in frequency • The noise cannot be moved in frequency, which is removed efficiently • Bild 19:2 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

39. Sliding-Frequency Filter • Noise Reduction • Bild 20:2 • Bild 21:1 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

40. Sliding-Frequency Filter • Transfer Function • In a long transmission line the transfer function looks like a step function. • Removing small signals and noise • Large signals are all given the same energy • 24:2 • 25:1 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

41. Wavelength Division Multiplexing (WDM) • In WDM, Solitons of different channels overtake and pass thought (Collide with) each other, which results in. Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

42. Tapered Fibers • Some of the problems can be corrected by using a tapered fibers, where the dispersion decays exactly as the intensity. • Behaves like loss less, constant D-fiber • The exponential dispersion can be replaced with an 3-step approximation. • Bild 35:2 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

43. Gain Flatness • Optical amplifiers have not a flat gain over several channels. • After multiple amplifiers the signal power differs very much between channels. • In Soliton transmission with filters the amplitude can be kept relative constant between the different channels. • Bild 39:1 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

44. Gain Flatness • Bild 40:2 • Bild 39:2 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

45. Dispersion Managed Fibers • Fibers with alternating dispersion where the pulses are true solitons only in a few points along the fiber. • All advantages of ’classical solitons’ • Power enhancement • Inexpensive and flexible design • High stability range • WDM Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

46. Pulse-to-Pulse Interaction • The pulse-to-pulse interaction depends on the pulse width-spacing ratio. • The interaction increases as the pulses starts overlap • At large overlap the interaction vanishes • Bild 60:1 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

47. Pulse overlapped dispersion managed Fibers • Use the non-pulse interaction regime just at the transmitter and receiver • Move quickly across the partially overlapped regime • Spend most of the time in /t >10 regime • Bild 61:2 Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se

48. State of The Art • 320 Gbit/s Transmission over 200 km. • 10 GHz Clock Recovery from a 160 Gbit/s stream. • 168 Gbit/s Demultiplexing • 84 Gbit/s All Optical 3R Regeneration (No clock recovery) • 40 Gbit/s All Optical Clock Recovery Department of Information Technology and Media Electronic Simulation Group Kent.Bertilsson@mh.se