1. Analysis of Phase Noise in a fiber-optic link
Cecil D. Thomas
Aug 20 ’04

2. Outline
Introduction Why optical fibers? > Huge bandwidth > Immunity to interference > Low attenuation > Etc.
* Where do we use optical fibers?
Problems in a fiber link
> Attenuation
> Dispersion
> Phase noise ( How to quantify this problem in
different applications?)
> etc.
* Conclusions

3. Analog fiber-optic links are used in
Distribution of reference signals like local oscillators.
Video transmission as in Cable TV.
Antenna remoting for radar systems.
Etc.

4. Major problems in a fiber-optic link
Attenuation = deterioration in signal strength
Dispersion = pulse broadening (causes ISI)
Phase Noise
Etc.

5. Significance of Phase Noise
A high merit frequency distribution system should perform with a phase fluctuation of less than 1 degree over several days of operation.
Detection range, dynamic range, range resolution etc. are some of the radar parameters affected by phase instabilities.
Poor phase noise degrades the quality of television pictures and data transmission.

6. Definition of Phase Noise

7. What causes phase noise in a fiber-optic link?
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.

8. Our Tasks Quantify Phase Noise in the fiber-optic link
Study the effect of Wavelength selection
Study the effect of optical amplification
Assumptions
External modulation and direct detection
Optical amplification

9. Block diagram of experimental setup
Laser
Modulator
EDFA
Photo detector
Phase detector
Filter
RF Amp
Phase shift = 90 degrees

10. Methodology
Signal from the RF source traverses two separate paths before reaching the phase-detector
> 8.8 Km of fiber after modulating the laser output
> One meter of electric cable
The length of the electric cable is adjusted so that the phase difference between the two paths is 90 degrees.
Time samples from the digital oscilloscope
are downloaded to a PC.
Matlab is then used to calculate Power Spectral
Density from the time-voltage samples.

11. Average noise floor = -85.66dBm/Hz

13. Effect of wavelength selection (no optical amplification)

14. Effect of varying the output power of laser source

16. Output power vs. Input Power of EDFA

17. Effect of EDFA

18. Phase noise for different input powers

19. Conclusions Phase noise in an optical fiber-link was quantified
Wavelength selection does not have much effect on phase noise (< 2dB).
As laser power output increases, phase noise increases almost linearly (effect of shot noise, thermal noise).
Average phase noise increases by about 2.7dB with the addition of the EDFA.
For EDFA, phase noise decreases as input power increases (matches with theory).