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USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Photonic time-stretching of 102 GHz millimeter waves
using 1.55 µm nonlinear optic polymer EO modulators
H. Erlig
Pacific Wave Industries
H. R. Fetterman and D. Chang
University of California Los Angeles
M. Oh, C. Zhang, W. H. Steier, and L. R. Dalton
University of Southern California
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Outline
• Time-stretch concept
• Experimental description
• Polymer modulator
• Data and data analysis
• Dispersion penalty & SSB modulation
• Conclusions
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Time-Stretching Concept
BroadbandFemtosecond
SourceL1, Dλ
Time
Optical PulseTime Domain
Mach-ZehnderModulator
Applied RFSignal, 50 GHz
Time
Optical PulseAfter Modulator
L2, Dλ
Time
StretchedModulated
Optical Pulse
OpticalAmplifier
Photodetector
Detected RFSignal, 10 GHz
BroadbandFemtosecond
SourceL1, Dλ
Time
Optical PulseTime Domain
Mach-ZehnderModulator
Applied RFSignal, 50 GHz
TimeTime
Optical PulseAfter Modulator
L2, Dλ
TimeTime
StretchedModulated
Optical Pulse
OpticalAmplifier
Photodetector
Detected RFSignal, 10 GHz
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Time-Stretching - Intensity
( ) ∝tI ( )tf
π
tMf2
cos m
βπ 2m
2''
2 fML
2cos
PulseEnvelope Time-Stretched
WaveformDispersion
Penalty,Leads to amplitude
attenuation
1
2
LL
1M += Stretch Ratio
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Experimental Arrangement
optic
FiberSpool L2
optic
FiberSpool L1
MM-WaveSource
8-12GHz32dB Amp
Photo-detector
ND-Filter +Adj. Attenuator
Modelocked1.55um Laser
optic
EDFA
SamplingScope
SpectrumAnalyzer
PolymerModulator
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Polymer Modulator
• PC/CLD polymer traveling wave modulators
• Optical network analyzer: 1 dB down at 20 GHz compared to 2 GHz
• Modeled effects: velocity mismatch and electrical loss
• Vπ ~ 7 V, 1.3 cm interaction length
• W-band response relatively flat
Measured and Calculated Optical Response
-12
-9
-6
-3
0
3
2 4 6 8 10 12 14 16 18 20
Frequency (GHz)
Op
tica
l Re
spo
nse
(d
B)
Measured Optical Response
Calculated Optical Response 0
5
10
15
20
25
30
70 80 90 100 110 120
Frequency (GHz)
Rel
ativ
e R
esp
on
se (
dB
)
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Transmission Through Modulator
Influence Of Modulator 5 On Optical Spectrum
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600
Wavelength (nm)
Norm
aliz
ed In
tensi
ty
After Modulator
Before Modulator
Influence Of Modulator 3 On Optical Spectrum
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600
Wavelength (nm)
Norm
aliz
ed In
tensi
ty
After Modulator
Before Modulator
• Mach-Zehnders used with broadband femtosecond source
• Modulators reshape 50 nm spectrum• Different modulators on chip introduce
different amount of spectral reshaping• Slight optical path mismatch• Highly chirp pulses key• Effect also observed in LiNbO3
modulators
Light
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Pulse Shape After System
• Two interfering highly chirped optical pulses
( )( )
δ
⋅
β
−∝
tcz
b21
cos
TL
texptI
2
20
''
2
For system parameters and period of 1.5 ns
• Calculated effective optical path mismatch of 100 µm
• Calculated effective index mismatch 0.005
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
33 To 50 GHz Time-Stretched Signals
• Sweep oscillator
• L1=1.5 Km
• L2=4.5 Km
• Measured Meff 3.86
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
61.8 GHz Time-Stretched Signal
• Source GUNN diode at 61.8 GHz
• L1=1.5 Km
• L2=6.5 Km
• Measured Meff 5.13
• PSD shape not significantly changed
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
101.7 GHz Time-Stretched Signal
• Source Klystron at 101.7 GHz
• L1=0.5 Km
• L2=5.0 Km
• Measured Meff 9.8
• Change in input pulse chirp
• Drop in signal level
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Stretch Ratios
• Discrepancies between calculated M and measured Meff
• Need to account for dispersive elements ahead of first fiber spool such as 50m fiber patch cord
• Then:
• where δ has length units and represents dispersion equivalent to that length of fiber
• Solutions for δ are correct in magnitude and self-consistent:
f (GHz) L1 (Km) L2 (Km) M Meff δ δ (m)33-50 1.5 4.5 4.00 3.86 7361.8 1.5 6.5 5.33 5.1 74
101.7 0.5 5 11.00 9.8 68
δ++=
1
2
LL
1Meff
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Dispersion Penalty
• Sidebands slip out of phase in L2due to group velocity dispersion
• Result: dispersion penalty (Coppinger et. al.)
• Tradeoff: M vs. aperture time vs. bandwidth.
• Practical limit for A/D preprocessing: “must stay in 1st lobe”
• Modulator operation exceeds this limit in our experiment
βπ 2m
2''
2 fML
2cos
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
SSB Modulation
Optical Field,Frequency Ω
tsinmV)t(V ω=
Γ+ω= VtcosmV)t(V
Mach-ZehnderDriven To ProduceSingle Side Band
Frequency
Amplitude
Ω Ω+ω
Optical SpectrumAt Output Of Modulator
VΓ chosen to produce additional π/2 opticalphase shift
( )( )
( )( )
π+ω
β+
ω∆∆
⋅
τ−⋅∝
4M2L
tM
cosJJ
M
t2exp
M1
tI 2m
2''
m
0
12
2
• Amplitude limitation imposed by dispersion penalty removed
USCUNIVERSITY
OF SOUTHERNCALIFORNIA
Conclusions
• Demonstrated time-stretching of 102 GHz signal
• Enabled by broadband 1.55 µm polymer modulator
• Modulator performance spans useful bandwidth range determined by dispersion penalty
• Importance of optical path length mismatch observed
• Single-Sideband Modulator removes high-frequency attenuation