Analysis of Phase Noise in a fiber-optic link

Cecil D. Thomas Aug 20 ’04

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

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.

Major problems in a fiber-optic link

 Attenuation = deterioration in signal strength  Dispersion = pulse broadening (causes ISI)  Phase Noise  Etc.

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.

Definition of Phase Noise

Power Spectral Density P C (W) P SSB (W/Hz) f

c

f

c

+f

m

S

 (

f m

)  10 log

P SSB P C

Frequency dBc/Hz

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.

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

Block diagram of experimental setup

Laser Modulator EDFA Photo detector Phase detector Filter RF Amp Phase shift = 90 degrees

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.

Average noise floor = -85.66dBm/Hz

-60 -65 -70 -75 -80 -85 -90 -95 -100 Noise floor and Electrical Phase noise Blue:Noise floor; Avg= - 85.6627dBm/Hz Green: Electric phase noise; Avg= - 85.6092

10 1 10 2 frequency (Hz) 10 3

Effect of wavelength selection (no optical amplification)

-71 Laser Output = 5dBm -71.5

-72 -72.5

-73 -73.5

-74 1525 1530 1535 1540 1545 1550

Wavelength(nm)

1555 Series1 1560 1565 1570

Effect of varying the output power of laser source

-60 -65 -70 -75 -80 -85 -90 -6 1530nm -4 1535nm 1540nm -2 0 2

Output Power of Laser (dBm)

1545nm 1550nm 1555nm 4 1560nm 1565nm 6

Gain vs. Input Power of EDFA

30 27 24 21 18 -16 -14 1530nm 1535nm -12 1540nm -10

Input Power(dBm)

1545nm 1550nm -8 1555nm -6 1560nm 1565nm -4

27 25 23 21 35 33 31 29 19 17 15 0

Output power vs. Input Power of EDFA

0.3

0.35

0.4

0.05

1530 0.1

1535 0.15

1540 0.2

0.25

Pin EDFA (mW)

1545 1550 1555 1560 1565 0.45

Effect of EDFA

Phase noise for different input powers

-66 -68 -70 -72 -74 -76 -78 -80 -16 -14 1550nm 1530nm -12 1535nm -10 -8

EDFA Input Power (dBm)

1540nm 1545nm 1555nm -6 1560nm -4 1565nm -2

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).