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S-72.227 Digital Communication Systems
Overview into Fiber Optic Communications
Timo O. Korhonen, HUT Communication Laboratory
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
Timo O. Korhonen, HUT Communication Laboratory
Capacity of telecommunication networks
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)
1 GHz->
4 kHz
100 kHz
10 MHz
MESSAGEBANDWIDTH
Timo O. Korhonen, HUT Communication Laboratory
Summarizing advantages of optical systems
Enormous capacity: 1.3 m ... 1.55 m 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
Two windows available, namely at
– 1.3 m and 1.55 m The lower window is used
with 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 fibermanufacturer's page!
Timo O. Korhonen, HUT Communication Laboratory
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:
2 1(1 )n n 1 1.48n 0.01
multimode fibers single mode fibersproperty step index graded index step index graded indexconnectionof light ++ + - - -BW - - - ++ ++losses + - + - + - + -price + - + -
1n
2n1 1 2 2cos cosn n
1 1
21 2n n
core
Timo O. Korhonen, HUT Communication Laboratory
Single mode and multimode fibers
Timo O. Korhonen, HUT Communication Laboratory
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 as
where 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
2
2 2 2 21 2
2/ 2, where
aM V V n n
2 2 1 1
0
,
2 /
n k k k n k
k
2.405V
Timo O. Korhonen, HUT Communication Laboratory
Inter-modal (mode) dispersion
Multimode fibers exhibit modal dispersion that is caused by different propagation modes taking different paths:
mod max minT T
/
/
v s t
v c n
1 2 1( ) /n n n 2 1(1 )n n
1 1 2cos 1n n 1 2 1cos / 1 /n n L s
1 1Path 1
Path 2 core
cladding
cladding
L
1/ cos /(1 )s L L
max 1/ / (1 ) /T s v L c n
mod max min 1 1
1 1mod
/ (1 ) /
1
T T Ln c Ln c
Ln Ln
c c
min
1/
LT
c n
1 1 2 2cos cosn n
s
1n
Timo O. Korhonen, HUT Communication Laboratory
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% of0 , Laser BW about 0.1 % of0
Optical fibers have dispersion minimum at 1.3 m but their attenuation minimum is at 1.55 m. Therefore dispersion shifted fibers were developed.
Example: GaAlAs LED is used at 0=1 m. This source has spectral width of 40 nm and its material dispersion is Dmat(1 m)=40 ps/(nm x km). How much is its pulse spreading in 25 km distance?
ps40nm 40 25km=40ns
nm kmmat
Timo O. Korhonen, HUT Communication Laboratory
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:
2 2intermod intramodtot
waveguide+chromatic dispersionDispersion due to different mode velocities
Chromatic and waveguide dispersion cancel each other
Chromatic
Timo O. Korhonen, HUT Communication Laboratory
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 is
where tFWHM is the “full-width-half-maximum”-value
Relationship between fiber risetime and its bandwidth is (next slide)
2 2( ) exp / 2 / 2g t t 2 2( ) exp / 2 / 2G
1/ 21/ 2 (2ln 2) / 2FWHMt t
3 3
0.44dB dB
FWHM
f Bt
Timo O. Korhonen, HUT Communication Laboratory
Using MathCad to derive connection between fiber bandwidth and rise time
g t( )
expt2
2 2
2 G f( )
exp 2 2 f2 2
2 g 0( )
1
2
2
exp
t h2
2 2
2
1
4
2
0
t h 2 ln 2( )
2 ln 2( )
t h
2 ln 2( )
G 0( )1
2
2
exp 2 2 f 3dB2 2
2
1
4
2
0 f 3dB
1
2 ( )( )2 ln 2( )
1
2 ( )( )2 ln 2( )
1
2 ( )( )2 ln 2( ) substitute
t h
2 ln 2( ) yeilds
1
t h
ln 2( )
f 3dBln 2( )
t h
ln 2( )
0.221 t FWHM 2 t h
Timo O. Korhonen, HUT Communication Laboratory
System rise-time
Total system rise time can be expressed as
where 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
1/ 22 2
2 2 2 2mat
0
440 350q
sys tx
rx
Lt t D L
B B
transmitter rise-timeintra-modal dispersion
mode dispersionreceiver rise-time
0( )M q
BB L
L
( ) 1 exp 2 ( )LP rxg t B t u t
Timo O. Korhonen, HUT Communication Laboratory
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
400MHz km
1/ 22 2
2 2 2 2mat
0
440 350q
sys tx
rx
Lt t D L
B B
1/ 22 2 2 2(15ns) (21ns) (3.9ns) (14ns)
30ns
sys
sys
t
t
350 / [ ] 11.7 MHztotB ns
Timo O. Korhonen, HUT Communication Laboratory
Optical amplifiers
Direct amplification without conversion to electrical signals Three major types:
– Erbium-doped fiber amplifier at 1.55 m (EDFA and EDFFA)– Praseodymium-doped fiber amplifier at 1.3 m (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)
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
Pump
Isolator
Erbium fiberSignal in (1550 nm)
Signal out
Residual pump
980 or 1480 nm
Isolator
Timo O. Korhonen, HUT Communication Laboratory
EDFA - energy level diagram
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
Er3+ levels
E1
E2
E3
E4
1530 nm 1480 nm980 nm
980 nm
Fluoride class level(EDFFA)
32 1 s
21 10ms
excited state absorption
Timo O. Korhonen, HUT Communication Laboratory
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
Timo O. Korhonen, HUT Communication Laboratory
Modern GaAlAs light emitter
Timo O. Korhonen, HUT Communication Laboratory
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
Timo O. Korhonen, HUT Communication Laboratory
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
30 40 nm
60 80 nm
100 nm
FWHM width
Timo O. Korhonen, HUT Communication Laboratory
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
Timo O. Korhonen, HUT Communication Laboratory
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
2 1(1 )n n
2 1sin /C n n
1 1 2 2cos cosn n
2 2 1/ 20,min 1 1 2
1
sin sin ( )
NA 2
Cn n n n
n
/fibre sourceP P
Timo O. Korhonen, HUT Communication Laboratory
Modulating lasers
Timo O. Korhonen, HUT Communication Laboratory
Example: LD distortion coefficients
Let us assume that an LD transfer curve distortion can be described by
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
and
For the applied signal we assume and therefore
2 31 2 3( ) ( ) ( ) ( )y t a x t a x t a x t
10
1
20log nn
AH
A
0 1 2 3( ) cos cos2 cos3 ...y t A A t A t A t
( ) cosx t t
1 1
2 22 2
33 3
( ) cos
( ) cos ( ) ( / 2)(1 cos2 )
( ) ( / 4)(3cos cos3 )
a x t a t
a x t t a t
a x t a t t
2
1
3 32 21
3( ) cos cos2 cos3
2 4 2 4AA
a aa ay t a t t t
2 22 10 10
1 3 1
3 33 2 10
1 3 1
220log 20log
3 4
20log 20log3 4
A aH
A a a
A aH
A a a
Timo O. Korhonen, HUT Communication Laboratory
Optical photodetectors (PDs)
PDs work vice versato LEDs and LDs
Two photodiode types
– PIN
– APD For a photodiode
it is required that itis
– 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
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.)
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:
Timo O. Korhonen, HUT Communication Laboratory
The bitrate-transmission length grid1-10 m 10-100 m 100-1000 m 1-3 km 3-10 km 10-50 km 50-100 km >100 km
<10 Kb/s10-100 Kb/s100-1000 Kb/s1-10 Mb/s10-50 Mb/s50-500 Mb/s500-1000 Mb/s>1 Gb/s
I
II
III IV
V
V
VI
VII
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 ELE
D 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 LD with GI MMF
VI Region: 100 Mb/s BL
100 Gb/s LD with SMF
VII Region: 5 Mb/s BL 100 Mb/s LD with SI or GI MMF
SI: step index, GI: graded index, MMF: multimode fiber, SMF: single mode fiber