Integrated photonic beamformer employing
continuously tunable ring resonator-based
delays in CMOS-compatible LPCVD waveguide
technology
C. G. H. Roeloffzen*a, A. Meijerinka, L. Zhuanga, D. A. I. Marpaunga, R.
G. Heidemanb, A. Leinseb, M. Hoekmanb, W. van Ettena.
aUniversity
of Twente, Faculty of Electrical Engineering, Mathematics
and Computer Science, Telecommunication Engineering Group, P.O.
Box 217, 7500 AE Enschede, The Netherlands
bLioniX
B.V., P.O. Box 456, 7500 AH Enschede, The Netherlands
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Contents
1. Introduction;
2. System overview & requirements;
3. Optical beamformer
- Ring resonator-based delays;
- OBFN structure;
- Chip fabrication;
- OBFN control block;
- E/O & O/E conversion;
- System performance.
4. Conclusions
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1. Introduction: What is beamforming
Smart Antenna systems:
Switched beam: finite number of fixed predefined
patterns
Adaptive array: Infinite number of (real time)
adjustable patterns
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1. Introduction: Smart antenna
Smart Antenna systems:
Using a variety of new signal-processing algorithms,
the adaptive system takes advantage of its ability to
effectively locate and track various types of signals to
dynamically minimize interference and maximize
intended signal reception.
Beamforming: spatial filtering
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1. Beamforming: Uniform line array
s(t)
x0(t)
d
x1(t)
xm  t   s  t  m 


x2(t)
d
sin 
c
xM(t)

(M-1)
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1. Beamforminh: Delay-sum
• When T=, the
channels are all
time aligned
Timed array
(t-[M-1]T)
x
• wm are beamformer
weights
w0
(t-[M-2]T)
x
w1
+
M 1
y  t    wm xm  t   M  m  1T 
m0
(t)
x
wM-1
• Gain in direction  is
wm. Less in other
directions due to
incoherent addition.
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1. Beamforming: Similar to FIR filter
br
• Let T=0 (broadside)
• Represent signal delay
across array as a delay
line
x
(t-)
w0
x
w1
(t-)
+
y(t)
• Sample: x[n]=x0 (nT)
• Looks like an FIR filter!
x
wM-1
• x[n]*w[n]
• Design w with FIR
methods
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1. Beamforming: Narrowband
• Narrowband assumption: Let s(t) be bandpass with BW << c / (M-1)d Hz.
• This means the phase difference between upper and lower band edges for
propagation across the entire array is small, e.g. < /10 radian.
• Most communication signals fit this model.
• If signal is not narrowband, bandpass filter it and build a new beamformer
for each subband.
• Sample the array x[n]=x(nT),
T
2 f 0 d
 j  M 1
 j


x  n   1, e , , e
s
n
,


sin 
  
c
• We can now eliminate time delays and use complex weights, w= [w0, …,
wM-1] , to both steer (phase align) and weight (control beam shape)
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1. Beamforming: Narrowband phased array
x0[n]
x1[n]
x
w0
w1
xM-1[n]
y  n  w H x  n 
x
x
wM-1
+
w   0 , 1e
 
 j
, ,  M 1e
 j  M 1
T
 ,

2 f 0 d
sin  0
c
m = amplitude weight for sensor m,
f0 = bandpass center frequency, Hz,
0 = direction of max response
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1. Beamforming: FFT implementation
Suppose you want to for many beams at once, in
different directions.
If beam k steered to k , has strongest signal, we
assume sources is in that direction.
yk  n  w kH x  n
w k   0 , 1e , ,  M 1e
2 f 0 d
k 2
k 
sin  k 
c
M
 j k
 j  M 1 k
M 1
M 1
m0
m0
T
 ,

yk  n    m xm  n e jm k    m xm  n e
j
mk 2
M
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1. Beamforming: FFT implementation
many beams at once
x0[n]
x1[n]
y0[n]
Amplitude
Taper
FFT
y1[n]
(multiply by )
xM-1[n]
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y(M-1)[n]
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1. Beamforming: Examples
Beamforming:
Electronic
Digital
Optical
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1. Examples: Electronic beamforming
SiGe
H. Hashemi, IEEE Communications magazine, Sept 2008
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1. Examples: Electronic beamforming
CMOS
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1. Examples: Electronic beamforming
MMIC (CMOS & SiGe)
+ very small chip
+ TTD & phase shifters
- Switched delay
+ Large bandwidth
+ low power consumption
- Large coupling between channels
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1. Examples: Digital beamforming
Digital beamforming:
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1. Examples: Digital beamforming
Frequency domain beamforming
Narrowband decomposition
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1. Examples: Digital beamforming
Digital beamforming:
+Easy implementation of time delays
- Multiple data converters required
- Large sampling rate
- Large number of bits (large dynamic range)
- Large power consumption
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1. Examples: Optical beamforming
Optical beamforming:
+Easy
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1. Int: aose
Smart
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1. Introduction: aim and purpose
Aim:
Development of a novel Ku-band antenna for airborne
reception of satellite signals , using a broadband
conformal phased array
Purpose:
• Live weather reports;
• High-speed Internet access;
• Live television through Digital Video
Broadcasting via satellite (DVB-s)
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1. Introduction: specific targets
Specific targets:
• Conformal phased array structure definition;
• Broadband stacked patch antenna elements;
• Broadband integrated optical beamformer
based on optical ring resonators
in CMOS-compatible waveguide technology;
• Experimental demonstrator.
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2. System overview & requirements
8x8
40x40
Antenna
array
8x1
RF
front-end
optical
beamformer
with
amplitude
tapering
gain
to
receiver(s)
beam
width
scan
angle
Frequency range: 10.7 – 12.75 GHz (Ku band)
Polarization:
1 2 linear (H/V)
Scan angle:
-60 to +60 degrees
Gain:
> 32 dB
Selectivity:
<< 2 degrees  Continuous delay tuning required !
No. elements:
~1600
Element spacing: ~/2 (~1.5 cm, or ~50 ps)
Maximum delay: ~2 ns
Delay compensation by phase shifters?  beam squint at outer frequencies!
 (Broadband) time delay compensation required !
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3. Optical beamformer: overview
plane position &
angle
antenna
viewing angle
feedback
loop
control
block
Antenna
elements
RF
front-end
E/O
?
OBFN
chip
O/E
?
?
to
receiver(s)
tunable broadband delays
& amplitude weights.
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Single ring resonator:
T

T : Round trip time;
 : Power coupling
 0.4
 0.8
1.2
1.6
coefficient;
 : Additional phase.
0
 Group delay

 Phase
3. Optical beamformer: ORR-based delays
• Periodic transfer function;
• Flat magnitude response.
• Phase transition around
resonance frequency;
• Bell-shaped group delay response;
• Trade-off: delay vs. bandwidth
 2
1
10
T
2T
8T

1
4T
0
1
1
4T 1 2T
FSR 
 f
T
6T
4T
2T
0
1

2T

1
4T
MWP » MWP in PAAs » SMART » Conclusions » Questions »
0
1
4T
f
1
2T
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Cascaded ring resonators:


 Group delay
3. Optical beamformer: ORR-based delays
10T
8T
bandwidth
ripple
6T
4T
2T
0
1

2T

1
4T
0
1
4T
• Rippled group delay response;
f
• Enhanced bandwidth;
• Trade-off: delay vs. bandwidth vs. delay ripple vs. no. rings;
1
2T
Or in other words: for given delay ripple requirements:
Required no. rings is roughly proportional to product of
required optical bandwidth and maximum delay
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3. Optical beamformer: 8x1 OBFN
8x1 Optical beam forming network (OBFN)
combining network
with tunable combiners
(for amplitude tapering)
8 inputs
with
tunable
delays
1 output
12 rings
7 combiners
 31 tuning elements
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3. Optical beamformer: chip fabrication
Fabrication process of TriPleX Technology
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3. Optical beamformer: chip fabrication
TriPleXTM results
1-2 m
~1m
Group
birefringence
(Bg)
Channel
attenuation
(dB/cm)
Polarization
dependent
loss (PDL, in
dB)
Insertion loss
(IL) without
spot size
converter (dB)
≤ 1×10-4
≤ 0.10
0.12 1
1.4 2
1.1×10-1
0.12
0.20 1
8.0 2,3
2 m
~100 nm
1:
chip length 3 cm
2: here, small core fibers were used (MFD of 3.5 µm)
3: minimal bend radius ~400 µm
MWP » MWP in PAAs » SMART » Conclusions » Questions »
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3. Optical beamformer: 8x1 OBFN chip
Single-chip 8x1 OBFN realized in
CMOS-compatible optical waveguide technology
TriPleX Technology
1 cm
Heater bondpad
4.85 cm x 0.95 cm
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3. Optical beamformer: OBFN control block
1
1
beam angle
5
1
3
4
2
3
4
3
4
2
6
7
2
delays + amplitudes
chip parameters (is and is)
ARM7
microprocessor
DA conversion
& amplification
heater voltages
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3. Optical beamformer: OBFN measurements
OBFN Measurement results
stage 2
14
in
13
1
2
3
4
1
2
3
4
1
2
3
4
17
stage 3
7 out 1
5
6
5
6
5
6
9
10
9
10
9
10
15
16
18
7
out 2
7
8
out 3
8
out 4
1.4
8
11 out 5
11 out 6
11
12
19
1.2
out 7
12 out 8
1.0
12
Delay (ns)
stage 1
L. Zhuang et al., IEEE Photonics
Technology Letters, vol. 19,
no. 15, Aug. 2007
Out 2
Out 3
Out 4
Out 5
Out 6
Out 7
Out 8
2.5 GHz
Max. ripple 0.1 ns
0.8
0.6
0.4
0.2
0.0
1549.97
1 ns ~ 30 cm delay distance in vacuum
1549.98
1549.99
1550.00
1550.01
1550.02
1550.03
Wavelength (nm)
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3. Optical beamformer: E/O and O/E conversion
RF front-end
LNA
E/O and O/E conversions?
• low optical bandwidth;
• high linearity;
• low noise
photodiode
DM laser
chirp!
OBFN
TIA
electrical » optical
optical » electrical
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3. Optical beamformer: E/O and O/E conversion
RF front-end
LNA
CW laser
E/O and O/E conversions?
• low optical bandwidth;
• high linearity;
• low noise
photodiode
mod.
beat noise!
OBFN
TIA
electrical » optical
optical » electrical
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3. Optical beamformer: E/O and O/E conversion
RF front-end
LNA
E/O and O/E conversions?
• low optical bandwidth;
• high linearity;
• low noise
photodiode
mod.
CW laser
OBFN
TIA
electrical » optical
optical » electrical
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3. Optical beamformer: E/O and O/E conversion
RF front-end
spectrum
2 x 12.75 = 25.5
12.75GHz
GHz
10.7 GHz
LNA
frequency
photodiode
mod.
CW laser
OBFN
TIA
electrical » optical
optical » electrical
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3. Optical beamformer: E/O and O/E conversion
RF front-end
spectrum
2 x 2.15 = 4.3
2150
GHzMHz
950 MHz
LNB
LNA
frequency
photodiode
mod.
CW laser
OBFN
TIA
electrical » optical
optical » electrical
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3. Optical beamformer: E/O and O/E conversion
Implementation
of optical
RF front-end
1. Optical heterodyning;
LNB
single-sideband modulation with
SSB
modulation?
suppressed carrier (SSB-SC)
spectrum
1.2 GHz
2. Phase shift method;
frequency
3. Filter-based method.
SSB-SC
mod. filter
Balanced optical detection:
CW laser
 cancels individual intensity terms; filter
 mixing term remains;
photodiode
OBFN
TIA
 reduces laser intensity noise;
 enhances dynamic range.
electrical » optical
optical » electrical
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3. Optical beamformer: E/O and O/E conversion
RF front-end
Filter requirements:
spectrum
single-sideband modulation with
suppressed carrier (SSB-SC)
1.2 GHz
• Broad pass band andLNB
stop band (1.2 GHz);
• 1.9 GHz guard band;
frequency
• High stop band suppression;
• Low pass band ripple and dispersion;
mod.
• Low loss;
CW laser
• Compact;
• Same technology as OBFN.
electrical » optical
filter
OBFN
TIA
1 chip !
optical » electrical
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3. Optical beamformer: E/O and O/E conversion
Optical sideband filter chip
in the same technology as the OBFN
5 mm
MZI + Ring
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3. Optical beamformer: E/O and O/E conversion
Measured filter response
3 GHz
Bandwidth—Suppression
Tradeoff
14 GHz
20 GHz
0
0
Measurement
Simulation
Filter Power Response (dB)
Filter Power Response (dB)
-5
-10
-20
-30
-10
25 dB
-15
-20
-25
-30
-40
0
10
20
Frequency (GHz)
30
40
-35
1549.6
1549.7
1549.8
1549.9
1550.0
1550.1
1550.2
1550.3
1550.4
Wavelength (nm)
FSR 40 GHz
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3. Optical beamformer: E/O and O/E conversion
Spectrum measurement of modulated optical signal
Optical heterodyning technique:
f1
f1
f
f
MZM
RF
f
OSBF
f2
f
fiber
coupler
f2
f
SA
f1 – f2
f
f
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3. Optical beamformer: E/O and O/E conversion
Spectrum measurement of modulated optical signal
-30
RF signal Magnitude (dB)
0
DSB signal
SSB-SC signal
OSBF response
-5
-40
-10
-50
-15
-60
-20
-70
-25
-80
-30
4
5
6
7
8
9
OSBF Response (dB)
-20
10
Frequency (GHz)
Measured filter response, and measured spectrum of
modulated optical signal, with and without sideband-filtering
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3. Optical beamformer: E/O and O/E conversion
RF phase response measurements
s1(t)
MZM
s2(t)
OBFN
OSBF
MZM
laser
s3(t)
RF output
Iout(t)
MZM
s4(t)
MZM
0.0
Measured RF phase
response of one
beamformer channel,
for different delay
values.
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
Channel Group Delay Response (ns)
RF input 1-2 GHz
Signal Phase Shift ( x 360o)
for 0 ns
1.0
for 0.75 ns
2
Signal sideband
1.5 ns
for 1.5 ns
1
0.75 ns
Measured response
Desired response
0
0.5
0 ns
1.0
1.5
2.0
Relative Frequency ( GHz )
1.2
1.4
2.5
1.6
1.8
2.0
Signal Frequency (GHz)
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3. Optical beamformer: E/O and O/E conversion
Signal combination measurements
RF input 0-1 GHz
CH 1
CH 2
-20
ch 1
ch 2
ch 3
ch 4
ch 1&2
ch 3&4
ch All
Output
CH 4
Measured output RF power of
beamformer with intensity
modulation and direct
detection, for
- 1 channel,
- 2 combined channels,
- 4 combined channels
RF signal magtitude (dB)
-30
CH 3
-40
-50
-60
-70
0.0
0.2
0.4
0.6
0.8
Frequency (GHz)
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1.0
3. Optical beamformer: Conclusions
Conclusions
•
A novel squint-free, continuously tunable beamformer mechanism for a phased
array antenna system has been described and partly demonstrated. It is based
on filter-based optical SSB-SC modulation, and ORR-based OBFN, and
balanced coherent detection.
•
This scheme minimizes the bandwidth requirements on OBFN and enhances
the dynamic range.
•
Different measurements on optical beamformer chip successfully verify the
feasibility of the proposed system.
MWP » MWP in PAAs » SMART » Conclusions » Questions »
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