1. 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

2. System overview & requirements;
Contents
1. Introduction;
System overview & requirements;
Optical beamformer - Ring resonator-based delays; - OBFN structure; - Chip fabrication; - OBFN control block; - E/O & O/E conversion; - System performance.
Conclusions
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3. 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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4. 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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5. 1. Beamforming: Uniform line array
s(t)
x0(t) d
x1(t) 
x2(t)
xM(t) 
(M-1)
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6. 1. Beamforminh: Delay-sum
When T=, the channels are all time aligned
wm are beamformer weights
Gain in direction  is wm. Less in other directions due to incoherent addition.
Timed array
(t-[M-1]T) x w0
(t-[M-2]T) x + w1
(t) x
wM-1
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7. 1. Beamforming: Similar to FIR filterbr
Let T=0 (broadside)
Represent signal delay across array as a delay line
Sample: x[n]=x0 (nT)
Looks like an FIR filter!
x[n]*w[n]
Design w with FIR methods x
(t-) w0 x +
y(t) w1
(t-) x
wM-1
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8. 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),
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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9. 1. Beamforming: Narrowband phased array
x0[n] x w0
x1[n] x + w1
xM-1[n] x
wM-1
m = amplitude weight for sensor m,
f0 = bandpass center frequency, Hz,
0 = direction of max response
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10. 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.
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11. 1. Beamforming: FFT implementation
many beams at once
x0[n]
Amplitude
Taper
(multiply by )
FFT
y0[n]
x1[n]
y1[n]
xM-1[n]
y(M-1)[n]
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12. 1. Beamforming: Examples
Electronic
Digital
Optical
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13. 1. Examples: Electronic beamforming
SiGe
H. Hashemi, IEEE Communications magazine, Sept 2008
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14. 1. Examples: Electronic beamforming
CMOS
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15. 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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16. 1. Examples: Digital beamforming
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17. 1. Examples: Digital beamforming
Frequency domain beamforming
Narrowband decomposition
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18. 1. Examples: 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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19. 1. Examples: Optical beamforming
+Easy
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20. 1. Int: aose
Smart
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21. 1. Introduction: aim and purpose
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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22. 1. Introduction: 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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23. 2. System overview & requirements
8x8
8x1
gain
40x40
optical
beam width
Antenna
array RF
front-end
beamformer
to receiver(s)
with amplitude tapering
scan angle
Frequency range: 10.7 – GHz (Ku band)
Polarization: 2 linear (H/V)
Scan angle: -60 to +60 degrees
Gain: > 32 dB
Selectivity: << 2 degrees
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 ! 1
 Continuous delay tuning required !
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24. 3. Optical beamformer: overview
plane position & angle
antenna viewing angle
feedback loop
control block
Antenna
elements
RF front-end
OBFN chip
E/O
O/E
to receiver(s) ? ? ?
tunable broadband delays & amplitude weights.
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25. 3. Optical beamformer: ORR-based delays
Single ring resonator:
 Phase
T : Round trip time;
 : Power coupling coefficient;
 : Additional phase.
 f
 Group delay
Periodic transfer function;
Flat magnitude response.
Phase transition around resonance frequency;
Bell-shaped group delay response;
Trade-off: delay vs. bandwidth
 f
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26. 3. Optical beamformer: ORR-based delays
Cascaded ring resonators:
ripple
bandwidth
 Group delay
Rippled group delay response;
Enhanced bandwidth;
Trade-off: delay vs. bandwidth vs. delay ripple vs. no. rings;
 f
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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27. 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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28. 3. Optical beamformer: chip fabrication
Fabrication process of TriPleX Technology
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29. 3. Optical beamformer: chip fabrication
TriPleXTM results
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
1: chip length 3 cm
2: here, small core fibers were used (MFD of 3.5 µm)
3: minimal bend radius ~400 µm
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30. 3. Optical beamformer: 8x1 OBFN chip
Single-chip 8x1 OBFN realized in CMOS-compatible optical waveguide technology
TriPleX Technology
1 cm
4.85 cm x 0.95 cm
Heater bondpad
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31. 3. Optical beamformer: OBFN control block
beam angle
delays + amplitudes
ARM7 microprocessor
chip parameters (is and is)
DA conversion & amplification
heater voltages
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32. 3. Optical beamformer: OBFN measurements
OBFN Measurement results
L. Zhuang et al., IEEE Photonics Technology Letters, vol. 19, no. 15, Aug. 2007
1 ns ~ 30 cm delay distance in vacuum
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33. 3. Optical beamformer: E/O and O/E conversion
E/O and O/E conversions?
low optical bandwidth;
high linearity;
low noise
RF front-end
LNA
photo-diode
DM laser
OBFN
chirp!
TIA
electrical » optical
optical » electrical
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34. 3. Optical beamformer: E/O and O/E conversion
E/O and O/E conversions?
low optical bandwidth;
high linearity;
low noise
RF front-end
LNA
photo-diode
CW laser
mod.
OBFN
beat noise!
TIA
electrical » optical
optical » electrical
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35. 3. Optical beamformer: E/O and O/E conversion
E/O and O/E conversions?
low optical bandwidth;
high linearity;
low noise
RF front-end
LNA
photo-diode
mod.
OBFN
CW laser
TIA
electrical » optical
optical » electrical
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36. 3. Optical beamformer: E/O and O/E conversion
spectrum
frequency
2 x = 25.5 GHz
12.75 GHz
RF front-end
10.7 GHz
LNA
photo-diode
mod.
OBFN
CW laser
TIA
electrical » optical
optical » electrical
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37. 3. Optical beamformer: E/O and O/E conversion
spectrum
frequency
2 x 2.15 = 4.3 GHz
2150 MHz
RF front-end
950 MHz
LNB
LNA
photo-diode
mod.
OBFN
CW laser
TIA
electrical » optical
optical » electrical
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38. 3. Optical beamformer: E/O and O/E conversion
spectrum
frequency
single-sideband modulation with suppressed carrier (SSB-SC)
Implementation of optical SSB modulation?
Optical heterodyning;
Phase shift method;
Filter-based method.
RF front-end
1.2 GHz
LNB
photo-diode
Balanced optical detection:
cancels individual intensity terms;
mixing term remains;
reduces laser intensity noise;
enhances dynamic range.
SSB-SC
mod.
filter
OBFN
CW laser
filter
TIA
electrical » optical
optical » electrical
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39. 3. Optical beamformer: E/O and O/E conversion
spectrum
frequency
single-sideband modulation with suppressed carrier (SSB-SC)
Filter requirements:
Broad pass band and stop band (1.2 GHz);
1.9 GHz guard band;
High stop band suppression;
Low pass band ripple and dispersion;
Low loss;
Compact;
Same technology as OBFN.
RF front-end
1.2 GHz
LNB
mod.
OBFN
CW laser
filter
TIA
electrical » optical
1 chip !
optical » electrical
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40. 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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41. 3. Optical beamformer: E/O and O/E conversion
Measured filter response
3 GHz
Bandwidth—Suppression Tradeoff
14 GHz
20 GHz
25 dB
FSR 40 GHz
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42. 3. Optical beamformer: E/O and O/E conversion
Spectrum measurement of modulated optical signal
Optical heterodyning technique:
 f f1
 f f1 f2
 f
 f
MZM
OSBF SA
fiber coupler RF
 f
f1 – f2
 f
 f f2
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43. 3. Optical beamformer: E/O and O/E conversion
Spectrum measurement of modulated optical signal
Measured filter response, and measured spectrum of modulated optical signal, with and without sideband-filtering
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44. 3. Optical beamformer: E/O and O/E conversion
RF phase response measurements
RF output
RF input 1-2 GHz
Measured RF phase response of one beamformer channel, for different delay values.
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45. 3. Optical beamformer: E/O and O/E conversion
Signal combination measurements
RF input 0-1 GHz
CH 1
CH 2
Output
CH 3
CH 4
Measured output RF power of beamformer with intensity modulation and direct detection, for - 1 channel, - 2 combined channels, - 4 combined channels
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46. 3. Optical beamformer: 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.
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