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.

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

RF Transmission via Optical Fibers Phased Array Antenna Phased Array Antenna Reference Signals for Timing and Synchronization Reference Signals for Timing and Synchronization Doppler Radar Doppler Radar CATV CATV Passband Signal Transmission Passband Signal Transmission

Noise Sources Optical Transmitter Fiber Cable Optical Amplifier Photo Receiver

Phase Noise

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

Analysis (Additive phase noise by the optical amplifier) )()cos(2)(tEtGPtE ASEoint    io M Mi ASE tfifStE    2cos2)( hfnGS SPASE  )1( 

Analysis (Cont.) (Additive phase noise by the optical amplifier) Assume  << 1      M Mi icASEocot tfifSGPt i  2cos)cos(  Hz GP S fS o ASE /dB log10)( c        

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

Link Under Investigation (Cont.)

EDFA Output Power vs. EDFA Gain EDFA Gain dB EDFA output Power dBm

Additive RF Phase noise vs. Log Offset Frequency dBc/Hz Log Offset Freq

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. 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. 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. Decreasing EDFA gain below 33 dB will not necessarily result in further additive phase noise reduction, or will reduce at a different rate.

Additive Phase Noise vs. EDFA Gain dBc/Hz EDFA Gain dB

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. 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.

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