s.72-227 digital communication systems - tkk … · ppt file · web view2016-01-28 · s-72.227...

S-72.227 Digital Communication Systems

Fiber-optic Communications - Supplementary

Timo O. Korhonen, HUT Communication LaboratoryG. Keiser: Optical Fiber Communications, McGraw-Hill, 2nd Ed.

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


Fundamental limits of silica fibers

C-band: supports early EDFA C+L-band: support for EDFA’s of today Raman amplifiers can be used over all bands - new

(medium loss) bands are now applicable (as S & U bands)

New fibers can reduce loss at E & S bands (however, EDFA does not work here & Raman gain small)

O-band Original 1260-1360E-band Extended 1360-1460S-band Short 1460-1530C-band Conventional 1530-1565L-band Long 1565-1625U-band Ultra-long 1625-1675

Band Description Wavelength (nm)

0.8 1.0 1.2 1.4 1.6 1.8Wavelength (mm)

Water spike

Rayleigh scattering

Infrared absorption

Loss

(dB

/km

)

Inter- and Intra-modal dispersion Attenuation (Loss) Non-linear effects

– Four-wave mixing (FWM)– Stimulated Raman & Brillouin scattering

(SRS,SBS)– Cross-phase & self-phase modulation

(SPM,XPM) Polarization fluctuations

100

50

10

1

0.5

0.1

5


Modulation of lasers


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

101

20log nn

AHA

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 cos32 4 2 4

AA

a aa ay t a t t t

2 22 10 10

1 3 1

3 33 2 10

1 3 1

220log 20log3 4

20log 20log3 4

A aHA a a

A aHA a a


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 be able to 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


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:


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

VV

VI

VII

I Region: BL 100 Mb/s SLED with SI MMFII Region: 100 Mb/s BL 5 Gb/s LED or LD with SI or GI MMFIII Region: BL 100 Mb/s ELE

D or LD with SI MMFIV Region: 5 Mb/s BL 4 Gb/s ELED or LD with GI MMFV Region: 10 Mb/s BL 1 Gb/s LD with GI MMFVI 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


Using Mathcad to derive connection between fiber bandwidth and rise time

g t( )

exp t2

2 2

2 G f( ) exp 2 2 f2 2

2 g 0( ) 1

22

exp

t h2

2 2

2

14

2

0

t h 2 ln 2( )

2 ln 2( )

t h

2 ln 2( )

G 0( ) 12

2

exp 2 2 f 3dB2 2

2

14

2

0 f 3dB

12 ( )( )

2 ln 2( )

1

2 ( )( )2 ln 2( )

1

2 ( )( )2 ln 2( ) substitute

t h

2 ln 2( ) yeilds 1

t hln 2( )

f 3dBln 2( )

t hln 2( )

0.221 t FWHM 2 t h

Timo O. Korhonen, HUT Communication Laboratory Ref: A.B.Carlson: Communication Systems, 3rd ed

s.72-227 digital communication systems - tkk … · ppt file · web view2016-01-28 · s-72.227...

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