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