RF Phase Noise in WDM Fiber Optic Links

Report
RF Phase Noise in WDM Fiber Optic Links
Mehdi Shadaram, Cecil Thomas*, John Summerfield, and Pushkar Chennu
Department 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
Power Spectral Density
PC (W)
PSSB (W/Hz)
fc
fc+fm
Frequency
 PSSB 

S ( f m )  10 log
 PC  dBc/Hz
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)
E t ( t )  2 GP in cos(  o t )  E ASE ( t )
E ASE ( t ) 
M

i M
2 S ASE  f cos  o  2 i  f t   i 
S ASE  ( G  1 ) n SP  hf
Analysis (continued)
n SP 
G
N2
N 2  N1
Go
1( Pin / Psat )
Fn = F0 +

Fn
2 
 1( G0Pin )
Go
Pm
dB
dB
K1()exp[K2()(G0 –G )]
Analysis (continued)
M
I t   Et (t )  GPin  2  GPin S ASEf cos2if t  i 
2
iM
Pin  Po 1   cos  c t 
M
I t  P0G  GP0 cos(ct )  2  GP0 S ASEf cos(2ift  i )
i  M
{ cos ct  cosct 

2
2
2
8
3 3
48
cos3 ct  ............}
Analysis (continued)
Assume <<1
it  GPo cos( ct )   GPo S ASE f
 S ASE  
S ( f )  10 log 

 GPo 
dB c / Hz
M
 cos
iM
c
 2if t  i
dB
Saturated gain and corresponding noise figure with
respect to input signal power at wavelength of 1550 nm.
40
35
30
25
20
15
10
5
0
Gain
Noise Figure
-35 -30 -25 -20 -15 -10 -5 0
Input Power (dBm)
5
10 15
Small signal gain (GodB)=40 dB
Phase Noise (dBc/Hz)
-120
-125
-130
-135
-140
-145
-150
-155
-160
-35
-30
-25
-20
-15
-10
-5
Input Power (dBm)
0
5
10
15
Experimental Setup
WDM
Laser
Source
Modulator
RF Signal
Generator
Fiber
EDFA
PhotoDetector
RF
Amplifier
PhaseDetector
O-Scope
RF phase noise for different output powers of
source
-70
1530nm
1535nm
1540nm
1545nm
1550nm
1555nm
1
2
1560nm
1565nm
Phase Noise (dBc/Hz)
-72
-74
-76
-78
-80
-82
-84
-86
-88
-4
-3
-2
-1
0
Output Power of Laser (dBm)
3
4
5
RF Phase noise against EDFA’s input
power
-70
Phase Noise (dBc/Hz)
1530 nm
-71
1535 nm
1540 nm
-72
1545 nm
1550 nm
-73
1555 nm
1560nm
-74
1565 nm
-75
-18
-15
-12
-9
EDFA Input Power (dBm)
-6
-3
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 MQAM 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.

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