1. Optical Amplifiers An Important Element of WDM Systems Xavier Fernando ADROIT Group Ryerson University

2. Nortel OPTERA System 64 wavelengths each carrying 10 Gb/s

3. Optical Amplifiers Conventional Repeaters in N-WDM systems are very inefficient: –Wavelength de-multiplexing  –O/E conversion  electrical amplification  retiming  pulse shaping  E/O conversion} –wavelength multiplexing Optical Amplifiers: A single device that amplify multiple format signals that are carried by multiple wavelengths N - times

4. Basic Concepts Most optical amplifiers use stimulated emission An optical amplifier is basically a laser without feedback Optical gain is realized when the amplifier is pumped optically (or electrically) to achieve population inversion Gain depends on wavelength, internal light intensity and amplifier medium Two types: semiconductor optical amplifiers and fiber doped amplifiers

5. Applications Power Amp Configurations

6. Similar to Laser diodes but the emission is triggered by input optical signal Work in any wavelength (+) Have high integration, compact and low power consumption (+) Gain fluctuation with signal bit rate (-) Cross talk between different wavelengths (-) Two types: Fabry-Perot or Traveling Wave Amp. Semiconductor Optical Amplifiers

7. Generic optical amplifier Continuous Wave (Constant) Energy is transferred from the pump to signal

8. Solid State Amplifier Gain versus Power

9. Distributed Fiber Amplifiers The active medium is created by lightly doping silica fiber core by rare earth element Ex: Erbium (Er) Long fiber length (10-30 m) Low coupling loss (+) Transparent to signal format and bit rate No cross talk Broad output spectrum (1530 – 1560 nm) Works only in specific Wavelengths

10. ~1550 nm 980 nm Radiationless Decay ~1550 nm Pump Signal Output Optical Pumping to Higher Energy levels Rapid Relaxation to "metastable" State Stimulated Emission and Amplification N 1 N 2 N 3 N 1 N 2 N 3 N 1 N 2 N 3 Amplification Process of EDFA

11. Fig. 11-4: Erbium energy-level diagram

12. EDFA configurations Co-Directional Pumping Counter Directional Dual Pumping

13. Gain versus EDFA length There is an optimum length that gives the highest gain Negative gain if too long

14. Gain versus pump level Gain decreases at large signal levels Signal dependant gain This increases with the pump power

15. Amplified Spontaneous Emission (ASE) Noise

16. EDFA Noise Figure = (Input SNR)/(Output SNR)

17. SNR degradation due to amplification

18. Fig. 11-12a: Gain-flattened EDFA-A Research Topic: Unequal gain across the spectrum. How do we flatten the gain ?

19. Fig. 11-12b: Gain-flattened EDFA-B

20. Fig. 11-13: Passive gain control Operating the DFA in the gain saturation region

21. Fig. 11-14: Wavelength conversion