Transcript S.72-227 Digital Communication Systems

S-72.227 Digital Communication Systems
Overview into Fiber Optic Communications
Overview into Fiber Optic Communications
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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
Timo O. Korhonen, HUT Communication Laboratory
Capacity of telecommunication networks
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Telecommunications systems
– tend to increase in capacity
– have increasingly higher
rates
Increase in capacity and rate
requires higher carriers
Optical system offers
– very high bandwidths
– repeater spacing up to
hundreds of km
– versatile modulation
methods
Optical communications
is especially applicable in
– ATM links
– Local area networks
(high rates/demanding environments)
Timo O. Korhonen, HUT Communication Laboratory
MESSAGE
BANDWIDTH
1 GHz->
10 MHz
100 kHz
4 kHz
Summarizing advantages of optical systems
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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
Timo O. Korhonen, HUT Communication Laboratory
Optical fibers
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Two windows available, namely at
– 1.3 mm and 1.55 mm
The lower window is used
with Si and GaAlAs
and the upper window
with InGaAsP compounds
There are single and monomode
fibers that have step or graded
refraction index profile
Propagation in optical fibers
is influenced by
– attenuation
– scattering
– absorption
to a fiber
– dispersion Link
manufacturer's page!
Timo O. Korhonen, HUT Communication Laboratory
Characterizing optical fibers
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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
n1  1.48   0.01 n2  n1 (1  )
n1 cos1  n2 cos 2
core n1 1
1
n2
2
n1  n2
Fibers can be divided into four classes:
property
connection
of light
BW
losses
price
Timo O. Korhonen, HUT Communication Laboratory
multimode fibers
single mode fibers
step index graded index step index graded index
++
-++
+
+-
++
++
-++
+-
Single mode and multimode fibers
Timo O. Korhonen, HUT Communication Laboratory
Fiber modes
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Electromagnetic field propagating in fiber can be described by
Maxwell’s equations whose solution yields number of modes M for step
index profile as
2
2

a

 2
2
M  V 2 / 2, where V 2  
  n1  n2 
  
where a is the core radius and V is the mode parameter, or normalized
frequency of the fiber
n2 k  k2    k1  n1k ,
Depending on fiber
k  2 / 0
parameters, number of
different propagating
modes appear
For single mode fibers
V  2.405
Single mode fibers do not
have mode dispersion
Timo O. Korhonen, HUT Communication Laboratory
Inter-modal (mode) dispersion
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Multimode fibers exhibit modal dispersion that is caused by different
propagation modes taking
cladding
different paths:
n
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
 mod  Tmax  Tmin
v  s / t

v  c / n
  (n1  n2 ) / n1
n1 cos1  n2 1
Path 1
1
Path 2
s
L
n2  n1 (1  ) n1 cos1  n2 cos 2
cos1  n2 / n1  1    L / s
s  L / cos1  L /(1  )
L
Tmin 
Tmax  s / v  L / (1  )c / n1 
c / n1
 mod  Tmax  Tmin  Ln1 / c(1  )  Ln1 / c
 mod 
Timo O. Korhonen, HUT Communication Laboratory
1
Ln1    Ln1



c 1   c
1
core
cladding
Chromatic dispersion
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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 0 , Laser BW about 0.1 % of 0
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 0=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?
 mat  40 nm  40
Timo O. Korhonen, HUT Communication Laboratory
ps
 25km=40 ns
nm  km
Chromatic and waveguide dispersion
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In addition to chromatic dispersion, there exist also waveguide
dispersion that is significant for single mode fibers in long wavelengths
Chromatic and waveguide
Chromatic and waveguide
dispersion cancel each other
dispersion are denoted as
Chromatic
intra- modal dispersion
and their effects cancel
each other at a certain
wavelength
This cancellation
is used in dispersion
shifted fibers
Fiber total dispersion is determined as the geometric sum effect of intramodal and inter-modal (or mode) dispersion with net pulse spreading:
2
2
 tot   intermod
  intramod
Timo O. Korhonen, HUT Communication Laboratory
Dispersion due to different mode velocities
waveguide+chromatic dispersion
Fiber dispersion, bit rate and bandwidth
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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
2 2
g (t )  exp  t 2 / 2 2  / 2  G( )  exp    / 2  / 2
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 is
t1/ 2  (2ln 2)1/ 2   tFWHM / 2
where tFWHM is the “full-width-half-maximum”-value
Relationship between fiber risetime and its bandwidth is (next slide)
0.44
f3dB  B3dB 
tFWHM
Timo O. Korhonen, HUT Communication Laboratory

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Using MathCad to derive connection between fiber
bandwidth and rise time
exp
g( t )
t
2
2
2
G( f )
2 2 2
exp 2    f  
2 
g( 0)
2
1
2
2
 
th
exp
2
2
2 
  2  l n( 2 )
th
th

  2  l n( 2 )
2  l n( 2 )
1
G( 0 )
1 2
2
1
( 2(   ) )
f 3 dB
exp 2   f 3 dB 

2
2 2
4
2
 2  l n( 2 ) su bst it ut e

l n( 2 )
 t h
Timo O. Korhonen, HUT Communication Laboratory
1 2

th
0
f 3 dB
y eil ds
2  l n( 2 )
l n( 2 )

0 .22 1
t FWH M 2  t h
( 2(   ) )
 2  l n( 2 )
1
 2  l n( 2 )
( 2(   ) )
1
 t
 l n( 2 )
h
2
1
2
4
 
0
System rise-time
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Total system rise time can be expressed as
1/ 2
tsys
2
q 2
2





440
L
350
2
2 2
 ttx  Dmat  L  
  B  
B
 0   rx  

mode dispersion
transmitter rise-time
intra-modal dispersion
receiver rise-time
where L is the fiber length [km] and q is the exponent characterizing
bandwidth. Fiber bandwidth is therefore also
B
BM ( L)  q0
L
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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
g LP (t )  1  exp  2 Brxt  u (t )
Timo O. Korhonen, HUT Communication Laboratory
Example
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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 400 MHz  km
bandwidth distance product with q=0.7. Therefore
1/ 2
tsys
tsys
2
q 2
2





440
L
350
2
2 2
 ttx  Dmat  L  
  B  
B
 0   rx  

1/ 2
 (15ns) 2  (21ns) 2  (3.9ns) 2  (14ns) 2 
tsys  30ns
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Note that this means that the electrical signal bandwidth is
B  350/  tot [ns]  11.7 MHz
For raised cosine shaped pulses thus over 20Mb/signaling rate can be
achieved
Timo O. Korhonen, HUT Communication Laboratory
Optical amplifiers
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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
Timo O. Korhonen, HUT Communication Laboratory
Erbium-doped fiber amplifier (EDFA)
Erbium fiber
Signal in
(1550 nm)
Isolator
Isolator
Pump
Signal out
Residual pump
980 or 1480 nm
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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
Timo O. Korhonen, HUT Communication Laboratory
EDFA - energy level diagram
Fluoride class level
(EDFFA)
E4
excited state absorption 980 nm
E3
 32  1m s
Er3+ levels
E2
1530 nm
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980 nm 1480 nm  21  10ms
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) -> a wide band
of amplified wavelengths
Practical amplification range 1525 nm - 1570 nm, peak around 1530 nm
Timo O. Korhonen, HUT Communication Laboratory
LEDs and LASER-diodes
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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
Timo O. Korhonen, HUT Communication Laboratory
Modern GaAlAs light emitter
Timo O. Korhonen, HUT Communication Laboratory
Light generating structures
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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
Timo O. Korhonen, HUT Communication Laboratory
LED types
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Surface emitting LEDs: (SLED)   100 nm
– light collected from the other surface, other attached to a heat sink
– no waveguiding
– easy connection into multimode fibers
Edge emitting LEDs: (ELED)   60  80 nm
– like stripe geometry lasers but no optical feedback
– easy coupling into multimode and single mode fibers
Superluminescent LEDs: (SLD)   30  40 nm
– 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
Timo O. Korhonen, HUT Communication Laboratory
  FWHM width
Lasers
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Timo O. Korhonen, HUT Communication Laboratory
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
Connecting optical power
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Numerical aperture (NA):
n2  n1 (1  )
n1 cos1  n2 cos 2
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Minimum (critical) angle
supporting internal reflection
 sin C  n2 / n1
n sin  0,min  n1 sin  C  ( n12  n22 )1/ 2
 NA  n1 2
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Connection efficiency is defined by   Pfibre / Psource
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
Timo O. Korhonen, HUT Communication Laboratory
Modulating lasers
Timo O. Korhonen, HUT Communication Laboratory
Example: LD distortion coefficients
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Let us assume that an LD transfer curve distortion can be described by
y(t )  a1 x(t )  a2 x2 (t )  a3 x3 (t )
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where x(t) is the modulation current and y(t) is the optical power
n:the order harmonic distortion is described by the distortion
coefficient
A
H n  20log10 n
A1
and
y(t )  A0  A1 cost  A2 cos 2t  A3 cos3t...
x(t )  cos  t

 2 a2 
the
applied
signal we assume
and therefore
A2
aFor
x
(
t
)

a
cos

t
1
1
 H 2  20log10 A  20log10  3a  4a 
 3
1
1 
a2 x 2 (t )  cos 2 (t )  ( a2 / 2)(1  cos 2 t ) 

a3
 H  20log A3  20log 
a3 x 3 (t )  (a3 / 4)(3cos  t  cos3 t )
2
10 

 3
A
3
a

4
a


1
3
1

a
a  3a
a

y (t )  2   3  a1  cos  t  2 cos 2 t  3 cos3 t
2  4
2
4

Timo O. Korhonen, HUT Communication Laboratory
A1
A2
Optical photodetectors (PDs)
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PDs work vice versa
to LEDs and LDs
Two photodiode types
– PIN
– APD
For a photodiode
it is required that it
is
– sensitive at the used 
– small noise
– long life span
– small rise-time (large BW,
small capacitance)
– low temperature sensitivity
– quality/price ratio
Timo O. Korhonen, HUT Communication Laboratory
Optical communication link
Timo O. Korhonen, HUT Communication Laboratory
Link calculations
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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
Timo O. Korhonen, HUT Communication Laboratory
Link calculations (cont.)
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Specifications: transmission distance, data rate (BW), BER
Objectives is then to select
FIBER:
– Multimode or single mode fiber: core size, refractive index profile,
bandwidth or dispersion, attenuation, numerical aperture or modefield diameter
SOURCE:
– LED or laser diode optical source: emission wavelength, spectral
line width, output power, effective radiating area, emission pattern,
number of emitting modes
DETECTOR/RECEIVER:
– PIN or avalanche photodiode: responsivity, operating wavelength,
rise time, sensitivity
Timo O. Korhonen, HUT Communication Laboratory
The bitrate-transmission length grid
1-10 m
<10 Kb/s
10-100 Kb/s
100-1000 Kb/s
1-10 Mb/s
10-50 Mb/s
50-500 Mb/s
500-1000 Mb/s
>1 Gb/s
10-100 m 100-1000 m 1-3 km
3-10 km
10-50 km 50-100 km >100 km
VII
V
I
V
II
III
IV
VI
I Region:
BL  100 Mb/s SLED
with SI
MMF
II Region: 100 Mb/s  BL  5 Gb/s
LED or LD with SI or GI MMF
III Region:
BL  100 Mb/s ELED or LD with SI
MMF
IV Region: 5 Mb/s  BL  4 Gb/s
ELED or LD with
GI MMF
V Region: 10 Mb/s  BL  1 Gb/s
VI Region: 100 Mb/s  BL  100 Gb/s
VII Region: 5 Mb/s  BL  100 Mb/s
LD with
GI MMF
LD with
SMF
LD with SI or GI MMF
SI: step index, GI: graded index, MMF: multimode fiber, SMF: single mode fiber
Timo O. Korhonen, HUT Communication Laboratory