RF Phase Noise in WDM Fiber Optic Links

Mehdi Shadaram, Cecil Thomas*, John Summerfield, and Pushkar ChennuDepartment of Electrical and Computer Engineering, University of Texas at San Antonio, San Antonio, TX 78249* Department of Electrical and Computer Engineering, University of Texas at El Paso, El Paso, TX 79968

Abstract

A WDM fiber optic link is considered for transmitting the reference signals for distances of less than 10 km. The quantification and analysis of phase noise caused by the link is the main objective of this work. The RF phase noise is investigated under different experimental setups so that the additive effect of different components in the link can be examined. Moreover, the influence of wavelength selection and power on the noise is also studied.

RF Transmission via Optical Fibers

Phased Array Antenna Reference Signals for Timing and

Synchronization Doppler Radar CATV Passband Signal Transmission

RF Phase Noise

P o w e r S p e c t r a l D e n s i t y

f c f c + f m

F r e q u e n c y

C

SSBm P

PfS log10)(

d B c / H z

P C ( W )

P S S B ( W / H z )

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.

Analysis(additive phase noise by the optical amplifier)

)()cos(2)( tEtGPtE ASEoint io

M

MiASEASE tfifStE

2cos2)(

hfnGS SPASE )1(

Analysis (continued)

F n = F 0 + K 1 ( ) e x p [ K 2 ( ) ( G 0d B – G d B ) ]

212

2

Fn

NN

NSPn

)(1)/(1 0

PmPinGoo G

PsatPin

GG

Analysis (continued)

iASE

M

Miinintt tfifSGPGPtEI

2cos2)(2

tPP coin cos1

..}..........coscoscos{

)2cos(2)cos(

348

38

22

000

32

ttt

ftifSGPtGPGPI

ccc

i

M

MiASEct

Analysis (continued)

A s s u m e < < 1

M

MiicASEocot tfifSGPtGPi 2cos)cos(

Hz

GP

SfS

o

ASE /dB log10)( c

Saturated gain and corresponding noise figure with respect to input signal power at wavelength of 1550 nm.

05

10152025303540

-35 -30 -25 -20 -15 -10 -5 0 5 10 15Input Power (dBm)

dB

Gain

Noise Figure

Small signal gain (GodB)=40

dB

-160

-155

-150

-145

-140

-135

-130

-125

-120

-35 -30 -25 -20 -15 -10 -5 0 5 10 15

Input Power (dBm)

Pha

se N

oise

(dB

c/H

z)

Experimental Setup

Modulator

O-Scope

RF Amplifier

WDM Laser Source

Phase-Detector

RF SignalGenerator

Fiber EDFA Photo-Detector

RF phase noise for different output

powers of source

-88

-86

-84

-82

-80

-78

-76

-74

-72

-70

-4 -3 -2 -1 0 1 2 3 4 5

Output Power of Laser (dBm)

Ph

as

e N

ois

e (

dB

c/H

z)

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

RF Phase noise against EDFA’s input

power

-75

-74

-73

-72

-71

-70

-18 -15 -12 -9 -6 -3

EDFA Input Power (dBm)

Phas

e N

oise

(dB

c/H

z)

1530 nm

1535 nm

1540 nm

1545 nm

1550 nm

1555 nm

1560nm

1565 nm

Conclusion

In conclusion, the 1550 - 1555 nm wavelength range is the optimum range for analog signal transmission through fiber, since it offers lowest values for phase noise. It is a matter of fact that the light signal also has the lowest attenuation in this range. The RF phase noise exhibits significant variations (>10dB) as the output power of the laser source is varied. The reason for this trend is that these variations in power increase thermal and shot noise in the photo-receiver which in turn causes an increase in the average RF phase noise of the system. In addition, including the EDFA in the link increases the average phase noise in the system by 2 to 3 dB.

References Shah, N, Shadaram, Phase stabilization of reference signals in analogue fiber

optic links, Electronics Letters, Volume: 33, Issue: 13, 1164 – 1165, June 1997. Shadaram M., Mody, A.N.; Usevitch, B.; Lafaw, D Performance Evaluation Of M-

QAM Fiber-Optic Links in the Presence of the External Modulator Bias Fluctuations, Microwave Photonics, 1997. MWP '97, International Topical Meeting, September 3-5, 1997.

Shadaram, M.; Gonzalez, V.; Ceniceros, J.; Shah, N.; Mayers, J.; Pappert, S.; Lafaw, D., Phase stabilization of reference signals in analog fiber-optic links, Microwave Photonics, 229 – 232,1996. MWP '96. Technical Digest. 1996 International Topical Meeting on , 3-5 Dec. 1996.

R. K. Khan, H. Kabir, S. M. Shamim Hasan, T. X. Wu, and M. Shadaram, “Performance evaluation of multi-wavelength M-QAM signal transmitted through fiber optic link with EDFA,” Optical Fiber Technology Journal, Vol. 10, Issue 3, July 2004, pp. 266-274.

L. Angrisani, A. Baccigalupi, M. D’Arco, A Digital Signal Processing Approach for Phase Noise measurement, IEEE Transactions on Instrumentation and Measurement Vol. 50, No. 4, 930 – 935, August 2001.