Effects of EDFA Gain on RF Phase Noise in a WDM Fiber Optic Link

1
Effects of EDFA Gain on RF Phase Noise in a WDM
Fiber Optic Link

• John Summerfield , Mehdi Shadaram, and Jennifer
  Bratton
• Photonics Research Laboratory
• Department of Electrical and Computer Engineering
• University of Texas at San Antonio
• IEEE MILCOM 2006
• Washington, D.C.

2
Outline

• Transmission of Reference Signals via Optical
  Fibers
• Noise Sources
• Phase Noise
• Link Under Investigation
• Results

3
RF Transmission via Optical Fibers

• Phased Array Antenna
• Reference Signals for Timing and Synchronization
• Doppler Radar
• CATV
• Passband Signal Transmission

4
Noise Sources

• Optical Transmitter
• Fiber Cable
• Optical Amplifier
• Photo Receiver

5
Phase Noise
6
What Causes Phase Noise?

• Temperature fluctuation of the link
• Fluctuation of longitudinally applied stress
• Relative intensity noise of the laser
• Back reflections in the cable
• Bias fluctuations of the photodiode
• Bias fluctuations of either directly modulated
  laser or the external modulator
• Amplified spontaneous emission noise
• Etc.

7
Analysis(Additive phase noise by the optical
amplifier)
8
Analysis (Cont.)(Additive phase noise by the
optical amplifier)
9
Link Under Investigation

• 8.8 Km Fiber Link with an EDFA located at 4.4 Km
• Single Light source operating at 1560nm
• 100MHz RF reference signal

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Link Under Investigation (Cont.)
11
Link Under Investigation (Cont.)
12
EDFA Output Power vs. EDFA Gain
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Additive RF Phase noise vs. Log Offset Frequency
14
Gain Effects

• Starting at 44 dB EDFA gain, for every dB
  decrease in optical gain there is a 2 dB decrease
  in additive phase noise.
• Maximum additive phase noise occurs at the
  maximum gain with a value near -65 dBc/Hz.
• Decreasing EDFA gain below 33 dB will not
  necessarily result in further additive phase
  noise reduction, or will reduce at a different
  rate.

15
Additive Phase Noise vs. EDFA Gain
16
Conclusion

• In conclusion, the additive phase noise in a link
  with an EDFA is affected by the gain of the
  amplifier. In order to increase the phase
  stability of the link, the EDFA should be
  operated at higher optical input levels. These
  higher input power levels can be achieved by
  placing an EDFA as close as possible to the
  transmitter.

17
References

• M. Shadaram, C. Thomas, J. Summerfeld, P. Chennu,
  RF phase noise in WDM fiber optic links," 7th
  International Conference on Transparent Optical
  Networks, (Barcelona, Catalonia, Spain, 3-7 July.
  2005).
• M. Shadaram, V. Gonzalez, J. Ceniceros, N. Shah,
  J. Myres, S. A . Pappert, D. Law, "Phase
  stabilization of reference signal in analog
  fiberoptic links," Proceedings of International
  Topical Meeting on Microwave Photonics, (Kyoto,
  Japan, 1987).
• M. Shadaram, J. Medrano, S. A. Pappert, M. H.
  Berry and D. M. Gookin, "Technique for
  stabilizing the phase of the reference signals in
  analog fiber-optic links," Applied Optics, 34,
  (1987) 8283-8288.
• A. Hajimiri, T. H. Lee., A general theory of
  phase noise in electrical oscillators, IEEE
  Journal of Solid-State Circuits, 33
  (1998)179-194.