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. = w + E ( t ) 2 GP cos( t ) E ( t ) t in o ASE M [ ] ( ) å = d w + p d + f E ( t ) 2 S f cos 2 i f t ASE ASE o i = - i M = - c S ( G 1 ) n hf ASE SP Analysis(Additive phase noise by the optical amplifier)

8. a Assume << 1 M [ ] ( ) å = Â a w + Â a d w + p d + f i GP cos( t ) GP S f cos 2 i f t t o c o ASE c i = - i M ( ) é ù S = ASE S ( f ) 10 log dB / Hz ê ú f c GP ë û o 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

10. Link Under Investigation (Cont.)

11. Link Under Investigation (Cont.)

12. 16 15 14 EDFA output Power dBm 13 12 11 27 29 31 33 35 37 39 41 43 EDFA Gain dB EDFA Output Power vs. EDFA Gain

13. -60 -65 -70 -75 -80 dBc/Hz -85 -90 -95 -100 -105 1 10 100 1000 Log Offset Freq 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. -60 -65 -70 -75 -80 dBc/Hz -85 -90 -95 -100 -105 27 29 31 33 35 37 39 41 43 45 EDFA Gain dB 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.