s-72.227 digital communication systems

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 Advantageous of optical systems Optical fibers

– single mode

– multimode Modules of fiber optic link Dispersion in fibers

– inter-modal and intra-modal dispersion Fiber bandwidth and bitrate Optical sources: LEDs and lasers Optical sinks: PIN and APD photodiodes Factors in design of optical links


Capacity of telecommunication networks

Telecommunications systems

– tend to increase in capacity

– have higher rates Increase in capacity and rate

requires 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->

4 kHz

100 kHz

10 MHz

MESSAGEBANDWIDTH


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


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!


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


Single mode and multimode fibers


Fiber optic link


Inter-modal dispersion

Multimode fibers are characterized by the modal dispersion that is caused by different propagation 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


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 ofpropagating modes ischanged

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


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


Chromatic and waveguide dispersion

In addition to chromatic dispersion, there exist also waveguide dispersion that is significant for single mode fibers in long wavelengths

Both chromatic and waveguide dispersion are denoted as intra-modal dispersion and their effect can cancels each other

This cancellationis used in dispersion shifted fibers

Fiber total dispersion is determined as the geometric sum effect of intra-modal and inter-modal dispersion resulting net pulse spreading

2 2intermod intramodtot

waveguide+chromatic dispersionDispersion due to different mode velocities

Mode and chromatic dispersion


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


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


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

inter-modal dispersionreceiver rise-time

0( )M q

BB L

L

( ) 1 exp 2 ( )LP rxg t B t u t


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


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


Modern GaAlAs light emitter


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


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


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


Connecting optical power

Numerical aperture (NA):

Minimum 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


Coupling efficiencies

Power connected to the fiber is evaluated by:

Efficiency for step and graded index fibers:2

2, (NA)LED step s

s

aP P

r

2, 1

22 1

2s

LED graded s

rP P n

a

NA( ) NA(0) 1 ( / ) ( )r r a u a r

0,max2 2

0 0 0 0

( , )

[ ( , )sin ]

f f

m

s s sA

r

s

P d B A

B d d d rdr

2 1(1 )n n 2 2 1/ 21 2NA=( )n n


Modulating LDs


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 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

21

3 32 21

3( ) cos cos cos3

2 4 2 4ÀA

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


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


Optical links

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, senstivity

FIBER:

SOURCE:

DETECTOR/RECEIVER:


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 marginal 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


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

s-72.227 digital communication systems

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