1

System Performance

2

Performance Limitations

• Two primary performance limitations

• Attenuation limited length

– Optical signal is too low to receive the correct data

– Depends on:

• Fiber attenuation

• Receiver sensitivity

• Transmitted power

• Dispersion limited length

– Pulse are spread too much

– Depends on:

• Fiber dispersion

• Linewidth of the optical source

3

Receiver Sensitivity• Signal needs to be higher than the noise

• The bit error rate (BER) depends on the received optical energy per bit

– Energy per bit Eb= h f N

• h is planks constant

• f is the optical frequency

• N is the number of photons

– The required value for Eb depends on the noise of the detector and supporting electronics

• The required optical power is then given by

– Pmin = Eb B (where B is the data rate)

• The required power scales with data rate

– Pmin(B) = Pmin(Bo) + 10log10 (B/Bo) dBm

4

Attenuation Limited Length

• Determined by the power budget

– Received power > Pmin

• Contributors to power loss

– Connections between components and fiber

– Connections between fibers

– Fiber attenuation

– Margin of error

• The power budget equation is

• Resulting in an attenuation limited length of

• The length varies with bit rate

oomcs B

BBPLPPP 10min log10

oomcs B

BBPPPPL 10min log10

1

BLL o 10log10

5

Power Budget

6

Attenuation Limited Length

7

Dispersion Limited Length

• To eliminate inter-symbol interference (ISI) the change in pulse width must be

• The dispersion limited length becomes

• Where Dtot is a combination of intermodal and intramodal dispersion

• Usually this breaks up into two distinct cases

– Multi-mode (Dinter » Dintra

– Single mode (Dinter = 0)

totDBL

4

1

2intra

2intertot DDD

B

TLDtot 4

1

4

intertot DD

intratot DD

8

Standard Optical Fiber Dispersion

• Standard optical fiber

– Step index ≈0.004

– Graded index ≈0.02

• Dispersion

– Step index multi-mode optical fiber (Dtot~10ns/km)

– Graded index multi-mode optical fiber (Dtot~0.5ns/km)

– Single mode optical fiber (Dintra~18ps/km nm)

4

21

intertot

c

nDD g

intratot DD

21

inter

c

nDD g

tot

9

What is the laser linewidth?

• Wavelength linewidth is a combination of inherent laser linewidth and linewidth change caused by modulation

– Single mode FP laser laser~2nm

– Multimode FP laser or LED laser~30nm

– DFB laser laser~0.01nm

• Laser linewidth due to modulation

– laser ~mod when B=600Mb/s

nmB

E

EB

cB 12

22

101683

655.122

2mod

2laser

10

Total Dispersion

• Total dispersion of multimode optical fiber

• Total dispersion of SMF28 single mode optical fiber

– With single mode FP laser

– With DFB laser

(0.69 ps/km @2.4Gb/s) [2.86 ps/km @10Gb/s]

s/km5.0

4

2.0

103

491.1

4

2

5

21

intertotal nc

nDD g

kmps7.53nm3nmkm

ps9.17intratotal

DD

kmpsB10286.0nm1016nmkm

ps9.17 912intratotal

BDD

11

Dispersion Length

• Multimode fiber

– 0.2km @ 2.4Gb/s

• Single mode fiber with FP laser

– 1.9km @ 2.4Gb/s

• Single mode fiber with DFB laser

– 152km @ 2.4 Gb/s

BL9max 105.04

1

BL9max 10054.04

1

221max 10286.04

1

BL

BL

12max 10179.04

1

12

Length Limitations

• Solid lines: attenuation limit

• Dashed lines: dispersion limit

13

Length Limitations

• Low bit rates: attenuation limit

• High bit rates: dispersion limit

14

System Performance Examples

• Configurations

– Single mode Vs. graded index multimode fiber

– FP laser Vs. DFB laser

– Common laser wavelengths in order of increasing cost

• 850nm, 1310nm, 1550nm

• Low cost system

– Short distance (LAN)

– Data rates around 100Mb/s

• Mid cost system

– Moderate distances (WAN)

– Data rates 622Mb/s or 2.4 Gb/s

• High cost system

– As far as possible

– Data rates 2.4 Gb/s or 10 Gb/s

15

Example: Low Cost System

• Data Rate: B=100 Mbps

• Use standard multimode graded index optical fiber (=0.02)

– Easier alignment and connection (lower cost)

• With multimode fiber the length will probably be dispersion limited

– Intermodal dispersion dominates

– Wavelength linewidth doesn’t matter

– FP and DFB laser have the same performance so use FP laser

• Laser (FP-LD)

– Pt=10mW = 10 log10(10)= 10 dBm

• Photodetector sensitivity

– Pmin=-22 dBm @2.5 Gbps

– Scaled to B=100 Mbps: Pmin=-36 dBm

16

Example: Low Cost System (cont.)

• Total dispersion

• The attenuation limit only needs to greater than 5km

– Pmin=Pt - L - extra losses

– -36 dBm = 10 dBm – dB/km * L – 6dB

– L = 1/ (40)

– Use a laser with a wavelength of =850nm

• =2.72 dB/km

– L=14.7km

• Dispersion limited

– L = 5 km

• Summary: Graded index MM fiber, 850nm FP laser

kmnsEc

nD g 5.0

4

02.0

53

49.1

4

221

tot

kmskms

L 510100105.04

1169max

17

Example: Mid Cost System

• Data Rate: B=622 Mbps

• Use a FP laser =1550nm for lower cost

– Pt=10mW = 10 log10(10)= 10 dBm

– =2 nm

• Photodetector Sensitivity

– Pmin=-22 dBm @2.4 Gbps

– Scaled to B=622 Mbps: Pmin=-27 dBm

• Standard single mode optical fiber (smf28)

– = 0.22 dB/km

)/(9.17)/(1550

13101550

4

092.03

4

kmnmpskmnmpsD

18

Example: Mid Cost System (cont.)

• Attenuation Limit

– Pmin=Pt - L – extra losses

– -27 dBm = 10 dBm - 0.22 dB/km * L – 6dB

– L = 140 km

• Dispersion

• Dispersion limited

– L = 22.5 km

• Faster data rate and longer distance than low cost system

kmpsnmnmkmpsD /8.352)/(9.17tot

kmskms

L 5.2210622108.354

11612max

19

Example: High Cost System

• Data Rate: B=2.4 Gbps

• Laser (DFB laser)

– =1550 nm

– Pt=10mW = 10 log10(10)= 10 dBm

– =0.01 nm

• Photodetector Sensitivity

– Pmin=-22 dBm @2.4 Gbps

• Standard single mode optical fiber (smf28)

– = 0.22 dB/km

)/(9.17)/(1550

13101550

4

092.03

4

kmnmpskmnmpsD

20

Example: High Cost System (cont.)

• Attenuation Limit

– Pmin=Pt - L – extra losses

– -22 dBm = 10 dBm - 0.22 dB/km * L – 6dB

– L = 118 km

• Laser linewidth (dominated by modulation)

• Total dispersion

• Attenuation limited

– L = 118 km

nmB121016

kmpsnmnmkmpsD /72.004.0)/(9.17tot

kmskms

L 145104.21072.04

11912max

nm04.0

21

Increasing Link Length

• Signal regeneration

– Before the attenuation or dispersion length

– Convert signal to electrical signal

– Demodulate

– Retransmit the signal optically

• Correcting attenuation

– Amplify the optical signal

– Use Erbium Doped Fiber Amplifiers (EDFA)

• Correcting dispersion

– Intermodal dispersion (in multimode fiber)

• Cannot be easily corrected

– Intramodal dominated by chromatic dispersion

• Can be corrected using dispersing elements