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.

• Problems in a fiber link•> Attenuation•> Dispersion•> Phase noise ( How to quantify this problem in different applications?)•> etc.

* Conclusions

* Where do we use optical fibers?

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

fc fc+fm

Frequency

C

SSBm P

PfS log10)(

dBc/Hz

PC (W)

PSSB (W/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

ModulatorLaser EDFA

Phase shift = 90 degrees

Photo detector

RF Amp

Phase detector Filter

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

101

102

103

-100

-95

-90

-85

-80

-75

-70

-65

-60Noise floor and Electrical Phase noise

frequency (Hz)

PS

D(d

Bm

/Hz)

Blue:Noise floor; Avg= - 85.6627dBm/HzGreen: Electric phase noise; Avg= - 85.6092

Effect of wavelength selection (no optical amplification)

-74

-73.5

-73

-72.5

-72

-71.5

-71

1525 1530 1535 1540 1545 1550 1555 1560 1565 1570

Wavelength(nm)

Ph

ase n

ois

e (

dB

c/H

z)

Series1

Laser Output = 5dBm

Effect of varying the output power of laser source

-90

-85

-80

-75

-70

-65

-60

-6 -4 -2 0 2 4 6

Output Power of Laser (dBm)

Ph

ase n

ois

e (

dB

c/H

z)

1530nm 1535nm 1540nm 1545nm 1550nm 1555nm 1560nm 1565nm

Gain vs. Input Power of EDFA

18

21

24

27

30

-16 -14 -12 -10 -8 -6 -4

Input Power(dBm)

Gai

n(d

B)

1530nm 1535nm 1540nm 1545nm 1550nm 1555nm 1560nm 1565nm

Output power vs. Input Power of EDFA

15

17

19

21

23

25

27

29

31

33

35

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Pin EDFA (mW)

Po

ut

ED

FA

(m

W)

1530 1535 1540 1545 1550 1555 1560 1565

Effect of EDFA

Phase noise for different input powers

-80

-78

-76

-74

-72

-70

-68

-66

-16 -14 -12 -10 -8 -6 -4 -2

EDFA Input Power (dBm)

Ph

as

e n

ois

e (

dB

c/H

z)

1550nm 1530nm 1535nm 1540nm 1545nm 1555nm 1560nm 1565nm

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).