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-gt
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 Maxwells 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 l01 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
waveguidechromatic dispersion
Dispersion due to different mode velocities
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(t 1/2)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 q0.7. 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
Signal in (1550 nm)
Signal out
Isolator
Isolator
Pump
Residual pump
980 or 1480 nm

• 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

18
EDFA - energy level diagram
Fluoride class level(EDFFA)
E4
980 nm
excited state absorption
E3
Er3 levels
E2
1530 nm
980 nm
1480 nm
E1

• 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) -gt a wide band
  of amplified wavelengths
• Practical amplification range 1525 nm - 1570 nm,
  peak around 1530 nm

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-gtenough 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
• nthe 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 used l
• 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