S-72.227 Digital Communication Systems

1. S-72.227 Digital Communication Systems Overview into Fiber Optic Communications

2. Overview into Fiber Optic Communications • Capacity of telecommunication networks • Advantages of optical systems • Optical fibers • single mode • multimode • Modules of fiber optic link • Optical repeaters - EDFA • Dispersion in fibers • inter-modal and intra-modal dispersion • Fiber bandwidth and bitrate • Optical sources: LEDs and lasers • Optical sinks: PIN and APD photodiodes • Design of optical links

3. Capacity of telecommunication networks MESSAGEBANDWIDTH • Telecommunications systems • tend to increase in capacity • have increasingly higher rates • Increase in capacity and raterequires higher carriers • Optical system offers • very high bandwidths • repeater spacing up tohundreds of km • versatile modulationmethods • Optical communications is especially applicable in • ATM links • Local area networks (high rates/demanding environments) 1 GHz-> 10 MHz 100 kHz 4 kHz

4. Summarizing advantages of optical systems • Enormous capacity: 1.3 mm ... 1.55 mm allocates bandwidth of 37 THz!! • Low transmission loss • Optical fiber loss 0.2 dB/km, Coaxial cable loss 10 … 300 dB/km ! • Cables and equipment have small size and weight • aircrafts, satellites, ships • Immunity to interference • nuclear power plants, hospitals, EMP (Electromagnetic pulse) resistive systems (installations for defense) • Electrical isolation • electrical hazardous environments • negligible crosstalk • Signal security • banking, computer networks, military systems • Fibers have abundant raw material

5. Optical fibers • Two windows available, namely at • 1.3 mm and 1.55 mm • The lower window is usedwith Si and GaAlAs and the upper window with InGaAsP compounds • There are single and monomodefibers that have step or graded refraction index profile • Propagation in optical fibersis influenced by • attenuation • scattering • absorption • dispersion Link to a fiber manufacturer's page!

6. Characterizing optical fibers • Optical fiber consist of (a) core, (b) cladding, (c) mechanical protection layer • Refraction index of core n1 is slightly larger causing total internal refraction at the interface of core and cladding • Fibers can be divided into four classes: core

7. Single mode and multimode fibers

8. Fiber modes • Electromagnetic field propagating in fiber can be described by Maxwell’s equations whose solution yields number of modes M for step index profile aswhere a is the core radius and V is the mode parameter, or normalized frequency of the fiber • Depending on fiberparameters, number ofdifferent propagating modes appear • For single mode fibers • Single mode fibers do nothave mode dispersion

9. Inter-modal (mode) dispersion • Multimode fibers exhibit modal dispersion that is caused by different propagation modes taking different paths: cladding Path 1 core Path 2 cladding

10. Chromatic dispersion • Chromatic dispersion (or material dispersion) is produced when different frequencies of light propagate using different velocities in fiber • Therefore chromatic dispersion is larger the wider source bandwidth is. Thus it is largest for LEDs (Light Emitting Diode) and smallest for LASERs (Light Amplification by Stimulated Emission of Radiation) diodes • LED BW about 5% of l0 , Laser BW about 0.1 % of l0 • Optical fibers have dispersion minimum at 1.3 mm but their attenuation minimum is at 1.55 mm. Therefore dispersion shifted fibers were developed. Example: GaAlAs LED is used at l0=1 mm. This source has spectral width of 40 nm and its material dispersion is Dmat(1 mm)=40 ps/(nm x km). How much is its pulse spreading in 25 km distance?

11. Chromatic and waveguide dispersion • In addition to chromatic dispersion, there exist also waveguide dispersion that is significant for single mode fibers in long wavelengths • Chromatic and waveguide dispersion are denoted as intra- modal dispersion and their effects cancel each other at a certain wavelength • This cancellationis used in dispersion shifted fibers • Fiber total dispersion is determined as the geometric sum effect of intra-modal and inter-modal (or mode) dispersion with net pulse spreading: Chromatic and waveguide dispersion cancel each other Chromatic Dispersion due to different mode velocities waveguide+chromatic dispersion

12. Fiber dispersion, bit rate and bandwidth • Usually fiber systems apply amplitude modulation by pulses whose width is determined by • linewidth of the optical source • rise time of the optical source • dispersion properties of the fiber • rise time of the detector unit • Assume optical power emerging from the fiber has Gaussian shape • From time-domain expression the time required for pulse to reach its half-maximum, e.g the time to have g(t1/2)=g(0)/2 iswhere tFWHM is the “full-width-half-maximum”-value • Relationship between fiber risetime and its bandwidth is (next slide)

13. Using MathCad to derive connection between fiber bandwidth and rise time

14. System rise-time • Total system rise time can be expressed aswhere L is the fiber length [km] and q is the exponent characterizing bandwidth. Fiber bandwidth is therefore also • Bandwidths are expressed here in [MHz] and wavelengths in [nm] • Here the receiver rise time (10-to-90-% BW) is derived based 1. order lowpass filter amplitude from gLP(t)=0.1 to gLP(t)= 0.9 where

15. Example • Calculate the total rise time for a system using LED and a driver causing transmitter rise time of 15 ns. Assume that the led bandwidth is 40 nm. The receiver has 25 MHz bandwidth. The fiber has bandwidth distance product with q=0.7. Therefore • Note that this means that the electrical signal bandwidth is • For raised cosine shaped pulses thus over 20Mb/signaling rate can beachieved

16. Optical amplifiers • Direct amplification without conversion to electrical signals • Three major types: • Erbium-doped fiber amplifier at 1.55 mm (EDFA and EDFFA) • Praseodymium-doped fiber amplifier at 1.3 mm (PDFA) • semiconductor optical amplifier - switches and wavelength converters (SOA) • Optical amplifiers versus opto-electrical regenerators: • large bandwidth and gain • easy usage with wavelength division multiplexing (WDM) • easy upgrading • insensitivity to bitrate and signal formats • All based on stimulated emission of radiation - as lasers (in contrast to spontaneous emission) • Stimulated emission yields coherent radiation - emitted photons are perfect clones

17. Erbium-doped fiber amplifier (EDFA) Erbium fiber • Amplification (stimulated emission) happens in fiber • Isolators and couplers prevent resonance in fiber (prevents device to become a laser) • Popularity due to • availability of compact high-power pump lasers • all-fiber device: polarization independent • amplifies all WDM signals simultaneusly Signal in (1550 nm) Signal out Isolator Isolator Pump Residual pump 980 or 1480 nm

18. EDFA - energy level diagram Fluoride class level(EDFFA) • Pump power injected at 980 nm causes spontaneous emission from E1 to E3 and there back to E2 • Due to the indicated spontaneous emission lifetimes population inversion (PI) obtained between E1 and E2 • The higher the PI to lower the amplified spontaneous emission (ASE) • Thermalization (distribution of Er3+ atoms) and Stark splitting cause each level to be splitted in class (not a crystal substance) -> a wide band of amplified wavelengths • Practical amplification range 1525 nm - 1570 nm, peak around 1530 nm E4 980 nm excited state absorption E3 Er3+ levels E2 1530 nm 980 nm 1480 nm E1

19. LEDs and LASER-diodes • Light Emitting Diode (LED) is a simple pn-structure where recombining electron-hole pairs convert current into light • In fiber-optic communications light source should meet the following requirements: • Physical compatibility with fiber • Sufficient power output • Capability of various types of modulation • Fast rise-time • High efficiency • Long life-time • Reasonably low cost

20. Modern GaAlAs light emitter

21. Light generating structures • In LEDs light is generated by spontaneous emission • In LDs light is generated by stimulated emission • Efficient LD and LED structures • guide the light in recombination area • guide the electrons and holes in recombination area • guide the generated light out of the structure

22. LED types • Surface emitting LEDs: (SLED) • light collected from the other surface, other attached to a heat sink • no waveguiding • easy connection into multimode fibers • Edge emitting LEDs: (ELED) • like stripe geometry lasers but no optical feedback • easy coupling into multimode and single mode fibers • Superluminescent LEDs: (SLD) • spectra formed partially by stimulated emission • higher optical output than with ELEDs or SLEDs • For modulation ELEDs provides the best linearity but SLD provides the highest light output

23. Lasers • Lasing effect means that stimulated emission is the major for of producing light in the structure. This requires • intense charge density • direct band-gap material->enough light produced • stimulated emission

24. Connecting optical power • Numerical aperture (NA): • Minimum (critical) angle supporting internal reflection • Connection efficiency is defined by • Additional factors of connection efficiency: fiber refraction index profile and core radius, source intensity, radiation pattern, how precisely fiber is aligned to the source, surface quality

25. Modulating lasers

26. Example: LD distortion coefficients • Let us assume that an LD transfer curve distortion can be described bywhere x(t) is the modulation current and y(t) is the optical power • n:the order harmonic distortion is described by the distortion coefficientandFor the applied signal we assume and therefore

27. Optical photodetectors (PDs) • PDs work vice versato LEDs and LDs • Two photodiode types • PIN • APD • For a photodiodeit is required that itis • sensitive at the usedl • small noise • long life span • small rise-time (large BW, small capacitance) • low temperature sensitivity • quality/price ratio

28. Optical communication link

29. Link calculations • In order to determine repeater spacing on should calculate • power budget • rise-time budget • Optical power loss due to junctions, connectors and fiber • One should also estimate required margins with respect of temperature, aging and stability • For rise-time budget one should take into account all the rise times in the link (tx, fiber, rx) • If the link does not fit into specifications • more repeaters • change components • change specifications • Often several design iteration turns are required

30. Link calculations (cont.) • Specifications: transmission distance, data rate (BW), BER • Objectives is then to select • Multimode or single mode fiber: core size, refractive index profile, bandwidth or dispersion, attenuation, numerical aperture or mode-field diameter • LED or laser diode optical source: emission wavelength, spectral line width, output power, effective radiating area, emission pattern, number of emitting modes • PIN or avalanche photodiode: responsivity, operating wavelength, rise time, sensitivity FIBER: SOURCE: DETECTOR/RECEIVER:

31. The bitrate-transmission length grid SI: step index, GI: graded index, MMF: multimode fiber, SMF: single mode fiber