1. Optical Amplifier

2. Generic optical amplifier

3. Optical Amplifiers Important Parameters: Gain Saturation Output Power
Erbium Doped Fiber Amplifier
Semiconductor Optical Amplifier
980 nm Pump
Signal
Amplified Signal
~10 m Erbium Doped Fiber
Important Parameters:
Gain
Saturation Output Power
Noise Figure

4. Optical Amplifier Applications
* B. Verbeek, JDSU

5. Applications of optical amplifiers

6. Gain bandwidth of optical amplifiers

7. Amplifier Comparison

8. Gain Saturation
Output saturation power is defined as the output power when gain drops by 3db
Power amplifiers usually operate at saturation.
Saturation gain is lower than the unsaturated one.

9. Amplifier gain versus power

10. Gain versus Amplifier length

11. Gain versus pump level

13. Noise Sources

15. Noise Figure
Noise Figure definition is similar as for electrical amplifiers. Essentially a degradation of signal.
However, we do not use the optical SNR, but rather the SNR that would be measured with an “ideal” square-law detector at the input and output of the amplifier.
Where E=Electric Field, I=Detector Current,
A=Signal amplitude, x,y=Amplifier Spontaneous Emission

16. Noise Figure
NF definition assumes shot-noise limited source. Laser noise is ignored.
Detector thermal noise is ignored/negligible.
Noise Figure 3
2.5 2
1.5 1
0.5 5 10 15 20 25 30
Gain (dB)
3 dB NF limit, for complete inversion, high gain

18. EDFA EDFA has revolutionized optical communications
All optical and fiber compatible
Wide bandwidth, 20~70 nm
High gain, 20~40 dB
High output power, >200mW
Bit rate, modulation format, power and wavelength insensitive
Low distortion and low noise (NF<5dB)

20. Pump Source 980 nm 1480 nm low ASE, low noise amplifier
higher power pump laser
high output power
not as efficient
degree of population inversion is lower

21. Gain Spectrum
Amorphous nature of silica and the codopants inside the fiber affects the spectrum considerably.

22. Gain Spectrum
Population at different levels are different resulting gain dependence on wavelength
Different pumping level has different spectrum

23. EDFA configurations

24. EDFA Gain Transient
Channel turn-on, re-routing, network reconfiguration, link failure….

25. Gain Transient
Power may become too high (nonlinearity) or too low (degrade SNR) when add/drop channels
transient happens in us to ms
transient penalty depends on data rate, number of EDFAs and number of channels.
power increase degrades performance due to SPM

26. EDFA Transient Dynamics

28. Semiconductor Optical Amplifier

29. Operating Principle: device physics same as EEL.
difference is that R< 10-5: AR, angled stripe, window region
SOA can be operated in saturation, or unsaturated. gain clamping
single pass chip gain: G=exp (g_modal * L)
packaging: TEC, high coupling efficiency, isolators

30. Gain vs. Wavelength Single SOA
40-80 nm, InGaAs/InGaAsP. Spanning from nm

31. Gain vs. Output Power
An SOA has a Saturation Output Power

32. Output Power
SOAs are linear for small input powers.

33. Gain Dynamics

34. Saturation Output Power
Saturation Output Power decreases for higher energy photons.

35. Noise Figure

36. Noise Figure
SOAs are noisier than EDFAs because the coupling efficiency is lower. Otherwise, they have the same theoretical limitations.
Thus, integrated SOAs should be less noisy.

37. Cross Gain Modulation
Saturating the SOA with a signal affects the overall gain spectrum. Thus, all wavelengths will be slightly modulated.

38. Cross Gain Modulation: solutions
low input power (linear regime). SOA not in saturation. 8x20 Gbs 160 km. Spiekman et al, 1999
reservoir channel, SOAs in saturation. 32x2.5 gbs km. Sun et al 1999
Gain-Clamped SOA
Solution: Fixed Gain SOA
want fixed gain to eliminate XGM
Note: a laser has fixed gain above threshold (gain clamping)

39. Gain Clamped SOA
gain medium is shared between SOA and a laser. lasing at a different wavelength.

40. In-Line Optical Amplifier
Distance
Noise Figure can limit performance of links
* B. Verbeek, JDSU