1. System Performance
Stephen Schultz
Fiber Optics
Fall 2005

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
Fiber dispersion
Linewidth of the optical source
Stephen Schultz
Fiber Optics
Fall 2005

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
Stephen Schultz
Fiber Optics
Fall 2005

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
Stephen Schultz
Fiber Optics
Fall 2005

5. Power Budget
Stephen Schultz
Fiber Optics
Fall 2005

6. Attenuation Limited Length
Stephen Schultz
Fiber Optics
Fall 2005

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 Dl)
Single mode (Dinter = 0)
Stephen Schultz
Fiber Optics
Fall 2005

8. Standard Optical Fiber Dispersion
Step index D≈0.004
Graded index D≈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)
Stephen Schultz
Fiber Optics
Fall 2005

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 Dllaser~2nm
Multimode FP laser or LED Dllaser~30nm
DFB laser Dllaser~0.01nm
Laser linewidth due to modulation
Dllaser ~Dlmod when B=600Mb/s
Stephen Schultz
Fiber Optics
Fall 2005

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 [2.86
Stephen Schultz
Fiber Optics
Fall 2005

11. Dispersion Length Multimode fiber 0.2km @ 2.4Gb/s
Single mode fiber with FP laser
2.4Gb/s
Single mode fiber with DFB laser
2.4 Gb/s
Stephen Schultz
Fiber Optics
Fall 2005

12. Length Limitations Solid lines: attenuation limit
Dashed lines: dispersion limit
Stephen Schultz
Fiber Optics
Fall 2005

13. Length Limitations Low bit rates: attenuation limit
High bit rates: dispersion limit
Stephen Schultz
Fiber Optics
Fall 2005

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
Stephen Schultz
Fiber Optics
Fall 2005

15. Example: Low Cost System
Data Rate: B=100 Mbps
Use standard multimode graded index optical fiber (D=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 Gbps
Scaled to B=100 Mbps: Pmin=-36 dBm
Stephen Schultz
Fiber Optics
Fall 2005

16. Example: Low Cost System (cont.)
Total dispersion
The attenuation limit only needs to greater than 5km
Pmin=Pt - a L - extra losses
-36 dBm = 10 dBm – a dB/km * L – 6dB
L = 1/a (40)
Use a laser with a wavelength of l=850nm
a =2.72 dB/km
L=14.7km
Dispersion limited
L = 5 km
Summary: Graded index MM fiber, 850nm FP laser
Stephen Schultz
Fiber Optics
Fall 2005

17. Example: Mid Cost System
Data Rate: B=622 Mbps
Use a FP laser l=1550nm for lower cost
Pt=10mW = 10 log10(10)= 10 dBm
Dl=2 nm
Photodetector Sensitivity
Pmin=-22 Gbps
Scaled to B=622 Mbps: Pmin=-27 dBm
Standard single mode optical fiber (smf28)
a = 0.22 dB/km
Stephen Schultz
Fiber Optics
Fall 2005

18. Example: Mid Cost System (cont.)
Attenuation Limit
Pmin=Pt - a L – extra losses
-27 dBm = 10 dBm dB/km * L – 6dB
L = 140 km
Dispersion
Dispersion limited
L = 22.5 km
Faster data rate and longer distance than low cost system
Stephen Schultz
Fiber Optics
Fall 2005

19. Example: High Cost System
Data Rate: B=2.4 Gbps
Laser (DFB laser)
l=1550 nm
Pt=10mW = 10 log10(10)= 10 dBm
Dl=0.01 nm
Photodetector Sensitivity
Pmin=-22 Gbps
Standard single mode optical fiber (smf28)
a = 0.22 dB/km
Stephen Schultz
Fiber Optics
Fall 2005

20. Example: High Cost System (cont.)
Attenuation Limit
Pmin=Pt - a L – extra losses
-22 dBm = 10 dBm dB/km * L – 6dB
L = 118 km
Laser linewidth (dominated by modulation)
Total dispersion
Attenuation limited
Stephen Schultz
Fiber Optics
Fall 2005

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
Stephen Schultz
Fiber Optics
Fall 2005