Outline: Fibre and fibercharacteristics Transmitters Modulation Receivers Passive couplers

1. Outline: Fibre and fibercharacteristics Transmitters Modulation Receivers Passive couplers Filters Transmission systems and optical networks

2. Optical fibre, characteristic • Large bandwidth (theoretical 50 THZ) • Low attenuation (0,2 dB/km at 1550nm). • Physical size beneficial, light and thin, simplifies installation • Splicing and mounting connectors more complex • Immune to electromagnetic interference • Environmentally friendly material (sand!).

3. Propagation through fibre • Lightpulses are reflected in the core when hitting the cladding => approximately zero loss Andreas Kimsås, Optiske Nett

4. Snells law • Snells law: • θkappe= 90° (for total refraction) • Refractive index: • Critical angle for total reflection:

5. Multi-Mode vs. Single Mode Fibre • Multi mode • Core > 50 um. • Light being reflected with different angle travels different distances • Pulse spreading • Single Mode • Core < 10 um => single mode • Less pulse spreading Andreas Kimsås, Optiske Nett

6. Fibermodes • Multimode: • Core diameter typical 50-100μm. • NA = Numerical Aperture • Number of modes (m) depends on normalized frequency (V), a = core-diameter, NA: • Singlemode • Core-diameter typically 10μm. • Criteria for single-mode is V < 2.4048 • No mode-dispersion gives better transmission properties than multimode more difficult to couple to the lightsource.

7. Coupling light into the fibre • Single modus • Coupling into the tiny 10 micrometer core is demanding • Lining up the light-source is a significant part of the production cost • Laser is preferred light-source • LED has too large beam • Multimode • Larger core diameter simplifies coupling

8. Attenuation in the fibre • Rayleigh-scattering: • Dominant • Inhomogenities in the fibre and the structure of the glass. • Occurs when the lightbeam hits the inhomogenities in the glass • Sets the theoretical lower limit of fibre attenuation L≈1/λ^4 • Absorption: • Metal-ions, especially hydroksyliones (OH¯) at approx. 1400nm. • Pollution from production, or doped material for achieving the optical properties desired. • Radiation loss: • E.g variations in core-diameter and inhomogenities between the core and the cladding, e.g. Microbends or airbubbles.

9. Attenuation in the fibre

10. Transmission window and applied wavelength bands Figur fra “Fiber Optic Communication Systems”, G. Agrawal, Wiley

11. Dispersion • Pulse spreading when propagating through the fibre. • To much spreading results in intersymbol- interference • Limits the maximum transmissionrate through the fibre. • Three types of dispersion: • Modi-dispersion: Light travelling in different modi undergoes different delays through the fibre. Not present in SM! • Material-dispersion (chromatic): Refractive index is function of wavelength • Waveguide-dispersion: Propagation of different wavelengths depends on the characteristic of the waveguide, e.g. Index, geometry of core and cladding.

12. Zero dispersion • At 1300 nm in standard fibre • Material (chromatic) dispersion is close to zero at 1300 nm • Not minimum loss • ~ 1500 nm in dispersion shifted fibre • Manufactured for zero dispersion in 1500 nm region • Design core and cladding to give negative waveguide dispersion • At a specific wavelength, material and waveguide dispersion will result in zero total dispersion.

13. Chromatic dispersion Figure: S. Bigo, Alcatel: Talk at Norwegian electro-optics meeting 2004

14. Pulse Spreading Transmitter (Laser+ modulator) Receiver (fotodiode + amplifier) Time Time Fibre Important limitation: Attenuation: Some light being absorbed in fibre Dispersion: Speed of light depends on wavelength Illustration: Lucent Technologies Point to point fibre-optical system

15. Optical transmitters - LASER Constructive interference 1. Active laser medium 2. Laser pumping energy 3. Mirror (100%) 4. Mirror (99%) 5. Laser beam

16. Semiconductor laser • Most common transmitter in optical communication • Compact design • Material give frequency ranges (Fermi-Dirac distr.) • Population inversion: Electrons in n-region and holes in P-region • Electrons in n-region (conduction band) combine with holes (valence band) in p-region • Cavity length decides frequency • Forward biasing create • population Inversion • 2) Electrons combine • with holes, releases photons • 3) Stimulated emission

17. Stimulated emission Ei Ef

18. Stimulated emission A chain reaction!

19. Optical transmitters - LED

20. Light Emitting Diodes (LEDs) • Not sufficient in long distance fibre transmission • Wideband source => dispersion • Power is lower than for a laser • Employed at shorter distances • Maximum a few hundred meters, depends on bitrate

21. Optical receivers Photodiode: Avalanche diode = Higher sensitivity

22. Modulation • OOK modulation (on-off-keying) • NRZ (No Return Zero) most often used • RZ (Return Zero), some use • More advanced modulation formats being launched for 40 and 100 Gb/s pr. Channel systems. • Employ phase and/or polarisation • Phase and polarisation modulation not employed in systems for < = 10 Gb/s bitrate. • External modulation, e.g. Employing external modulator: MZ interferometer

23. Modulation II • Direct modulation of laser • Switch laser on and off • Difficult to fabric laser that can be switched at high speed, simultaneously having proper transmission characteristics. • Undesirable frequency variations (chirp) and Limited extinction ratio • External modulation • Mach-Zehnder interferometer • External component being fed electrically • May be Integrated with laser • High extinction ratio prolongs transmission distance

24. Pulse Spreading Transmitter (Laser+ modulator) Receiver (photodiode + aplifier Time Time Fibre Must be compensated: Attenuation: Some light being absorbed Dispersion: Light of speed wavelength dependent Illustration: Lucent Technologies fibre-optical transmission at longer distances

25. What is a long distance? • 100 m? • 10 Km? • 1000 Km?

26. What is a long distance? • 100 m? • LAN • 10 Km? • Access network • 1000 Km? • Transport network

27. Long distance optical system • Attenuation must be compensated • Regeneration • Attenuation • Dispersion must be compensated • Dispersion compensation employing fibre • Electronic compensation

28. Regeneration • 1R regeneration = Amplification (Reamplification) • Usually an optical amplifier • Amplifies the signal without conversion to electrical • Typically transparent for signal (shape, format and modulation) • 2R Reamplification & Reshaping: • Reshapes the flanks of the pulse as well as the floor and roof of the pulse, removes noise. • Usually electronic • Optical solutions still subject to research • 3R Reamplification & Reshaping & Retiming: • Synchronisation to original bit-timing. (regeneration of clock) • Usually involves electro-optic conversion • Optical techniques in the research lab.

29. Optical amplifier characteristics • Amplifier parameters: • Gain • Bandwidth of gain • Saturation level • Polarisation sensitivity • Amplifier noise

30. Optical fibre amplifier • Doped-fiber amplifier: • Doping = Inserting small amounts of one material into a second material • An Erbium doped silica fibre is fed with a pump-signal together with the original signal. • Doped atoms are being excited to a higher energy level • The pumping signal is a high power signal with a wavelength lower than the wavelength to be amplified (typically 980 nm or 1480 nm fore EDFA).

31. Erbium Doped Fiber Amplifier (EDFA) • Widely deployed in optical networks

32. Optical amplifiers overview • Semiconductor-laser amplifier: • Signal is sendt through the active region of the semiconductor • Stimulated emission results in a stronger signal • May be integrated with other components (e.g. Output of a switch or a transmitting laser) • Widely employed in research projects on all-optical switches. • Recently employed in commercially available compact tunable laser-modules

33. Available wavelength range depends on amplifier technology PDFA 1300 nm EDFA C - band 1530-1562 EDFA L - band 1570-1600 ALTERNATIVE AMPLIFIER TECHNLOGIES: RAMAN AND SOA Commercially available Still subject to research

34. Pulse Spreading Time Time Long distance fibre-optical transmission Receiver (photodiode + amplifier) Transmitter (Laser+ modulator) EDFA Fibre To be compensated: Dispersion: Speed of light is wavelength dependent Illustration: Lucent Technologies

35. Dispersion in transmission fibre • Dispersion depends on fibretype • G652, “Standard fibre” -17 Ps/nm*km @ 1550 nm • Dispersion shifted fibre: 0 dispersion @ 1550 nm • Non – Zero (NZ) dispersion shifted fibre: -3 to -6 Ps/nm*km

36. Dispersion Compensating Fibre (DCF) • Negative dispersion compared to transmission fibre • Much higher dispersion/km => Shorter fibre than transmission fibre required for achieving zero dispersion

37. Long distance fibre-optical transmission Transmitter (Laser+ modulator) Receiver (fotodiode + amplifier) EDFA DCF Long Fibre Compensation of amplitude and dispersion

38. Noise from optical amplifiers • Amplified Spontaneous Emission (ASE) • Photons are being emitted without stimulation • Noise distributed through the entire amplification band • May be limited through filtering out the wavelengths where amplification is desirable • Optical filter needed

39. Interference between two light sources • Constructive • Light in phase results in addition and increased intensity • Destructive • Light out of phase (180 degrees) results in extinguished pulse

40. Mach-Zehnder interferometer • At given frequencies the delay equals duration of a wavelength => constructive interference • At given frequencies the delay equals duration of half a wavelength => destructive interference

41. Mach-Zehnder based modulator • Modulates phase of one or both paths • E.g voltage on => phase being changed => extinguished pulse Electronic modulation

42. Series of Mach-Zehnder • Applicable as an optical filter • Adjustable delay enables adjustable frequency • A chain of filters helps sharpening up the filter characteristic • Very fast adjustment-time: As low as 100 ns • High attenuation (multiple stages)

43. Etalon based adjustable filter • Cavity with parallell mirrors in each end • Free spectral range (FSR) • Periode between repetition of pass-band • Finesse • FSR/width of channel • Fabry-Perot • Mechanical, large range adjustable, slow adjustment - 10 ms. Adjustable to n wavelengths

44. Acusto-optical filter • RF waves converted to sound-waves in a piezo electrical crystal (transducer) • Soundwaves results in mechanical movements • Mechanical movements in crystal alters refractive index • The crystal then works as a grating • Adjustment within 10 Micro-seconds • Possible to filter out several frequencies simultaneously by sending several RF waves with different frequency to a transducer

45. Filters with fixed wavelength • Gratingbased filters e.g. Diffraction gratings • Flat layer of transparent material, constructive interference in bumps for a given wavelength, destructive for other wavelengths • Arrayed Waveguide Grating (explained later)

46. Optical couplers • One or more fibers in, several fibres out • Divides the optical signal on several fibres. • Signal power is divided on the output-fibres • Splitting ratio is varying • 50/50, 50 % on each of two fibres • 10/90, 10 % in one, 90 % in a second. • Attenuation from input to output depends on splitting ratio • 50/50 splitter results in 3 dB attenuation (halving the power) Combiner Splitter

47. Optical couplers • Coupler employed as splitter: • One input divided on two or more outputs • Splitting ratio (α) indicates share of power to each output • 1x2 splitter is typical 50:50, however some power is being reflected (40-50 dB weaker than payload signal). This is called return-loss. • Connection-loss between fibre and coupler also attenuates the signal • Coupler employed as combiner: • Opposite use as a splitter; several inputs, single output. • Returnloss and connection-loss as for the splitter

48. Arrayed waveguide Grating • 1 X N or N X N coupler divides the light on N waveguides of different length • Waveguides is then coupled together, resulting in interference • On each of the N outputs, constructive interference is achieved for a specific wavelength and destructive interference for the other wavelengths

49. Multiplexing/Demultiplexing • Optical multiplexing: Couple several waveguides together into a fibre. • Optical demultiplexing: Separate wavelengths from an input fibre into several output fibres with a single wavelength in each. • Is this useful?

50. Transmission systems and aspects for optical networks By: Steinar Bjørnstad As part of the training course ”optical networks”