1 / 32 Integrated photonic beamformer employing continuously tunable ring resonator-based delays in...

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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 / 30MWP » MWP in PAAs » SMART » Conclusions » Questions

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

3 / 46MWP » MWP in PAAs » SMART » Conclusions » Questions »

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


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


1. Beamforming: Uniform line array

x0(t)d

x1(t)

x2(t)

xM(t)

s(t)

(M-1)

sin

mx t s t m

d

c


1. Beamforminh: Delay-sum

Timed array

+

(t-[M-1]T)

(t-[M-2]T)

(t)

w0

x

w1

x

wM-1

x

• 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.

1

0

1M

m mm

y t w x t M m T


1. Beamforming: Similar to FIR filter

br

+

w0

x

w1

x

wM-1

x

• 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

(t-)

(t-)

y(t)


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)

1 021, , , , sin

Tj Mj f d

n e e nc

x s


1. Beamforming: Narrowband phased array

+

w0

x

w1

x

wM-1

x

m = amplitude weight for sensor m,

f0 = bandpass center frequency, Hz,

0 = direction of max response

x0[n]

x1[n]

xM-1[n]

Hy n nw x

10 1 1

00

, , , ,

2sin

Tj MjMe e

f d

c

w


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.

Hk ky n nw x

10 1 1

0

, , , ,

2 2sin

kkTj Mj

k M

k k

e e

f d k

c M

w

21 1

0 0

k

mkM M jjm Mk m m m m

m m

y n n e n e

x x


1. Beamforming: FFT implementation

many beams at oncex0[n]

x1[n]

xM-1[n]

Amplitude

Taper

(multiply by )

FFT

y0[n]

y1[n]

y(M-1)[n]


1. Beamforming: Examples

Beamforming:

Electronic

Digital

Optical


1. Examples: Electronic beamforming

SiGe

H. Hashemi, IEEE Communications magazine, Sept 2008



CMOS



MMIC (CMOS & SiGe)

+ very small chip+ TTD & phase shifters- Switched delay+ Large bandwidth+ low power consumption- Large coupling between channels


1. Examples: Digital beamforming

Digital beamforming:



Frequency domain beamforming

Narrowband decomposition



Digital beamforming:

+Easy implementation of time delays- Multiple data converters required- Large sampling rate- Large number of bits (large dynamic range)- Large power consumption


1. Examples: Optical beamforming

Optical beamforming:

+Easy


1. Int: aose

Smart


1. Introduction: aim and purpose

Aim:

Development of a novel Ku-band antenna for airborne reception of satellite signals

Purpose:• Live weather reports;• High-speed Internet access;• Live television through Digital Video

Broadcasting via satellite (DVB-s)

, using a broadband conformal phased array


1. Introduction: specific targets

Specific targets:

• Conformal phased array structure definition; • Broadband stacked patch antenna elements;• Broadband integrated optical beamformer

based on optical ring resonatorsin CMOS-compatible waveguide technology;

• Experimental demonstrator.


2. System overview & requirements

beamformerAntennaarray

RFfront-end to

receiver(s)

Frequency range: 10.7 – 12.75 GHz (Ku band)Polarization: 2 linear (H/V)Scan angle: -60 to +60 degreesGain: > 32 dBSelectivity: << 2 degreesNo. elements: ~1600Element spacing: ~/2 (~1.5 cm, or ~50 ps)Maximum delay: ~2 nsDelay compensation by phase shifters? beam squint at outer frequencies! (Broadband) time delay compensation required !

40x40

Continuous delay tuning required !

gain

beam width

scan angle

optical

with amplitude tapering

8x8

1

8x1


3. Optical beamformer: overview

OBFN chip

Antennaelements

RFfront-end to

receiver(s)

E/O O/E

? ?

tunable broadband delays& amplitude weights.

?

controlblock

antenna viewing angle

plane position & angle

feedbackloop


3. Optical beamformer: ORR-based delays

T

T4

1 0

T4

1

T2

1

2

T8

T4

T2

T6

0

2.1

T10

6.1

0

T2

1

8.0

4.0

f

Gro

up

d

ela

y

P

has

e

TFSR

1

Single ring resonator:

T : Round trip time;

: Power coupling coefficient;

: Additional phase.

• Periodic transfer function;• Flat magnitude response.• Phase transition around

resonance frequency;• Bell-shaped group delay response;• Trade-off: delay vs. bandwidth

T4

1 0

T4

1

T2

1

T2

1

f


3. Optical beamformer: ORR-based delays

Cascaded ring resonators:

• Rippled group delay response;• Enhanced bandwidth;• Trade-off: delay vs. bandwidth vs. delay ripple vs. no. rings;

ripple

bandwidth

T4

1 0

T4

1

T2

1

T2

1

f

T8

T4

T2

T6

0 G

rou

p

del

ay

T10

Or in other words: for given delay ripple requirements:

Required no. rings is roughly proportional to product of required optical bandwidth and maximum delay


3. Optical beamformer: 8x1 OBFN

8x1 Optical beam forming network (OBFN)

8 inputswith

tunabledelays 1 output

combining networkwith tunable combiners(for amplitude tapering)

12 rings7 combiners 31 tuning elements


3. Optical beamformer: chip fabrication

Fabrication process of TriPleX Technology


3. Optical beamformer: chip fabrication

TriPleXTM resultsGroup

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 cm2: here, small core fibers were used (MFD of 3.5 µm)3: minimal bend radius ~400 µm

1-2 m

~1m

2 m

~100 nm


3. Optical beamformer: 8x1 OBFN chip

4.85 cm x 0.95 cm

1 cm

Heater bondpad

TriPleX Technology

Single-chip 8x1 OBFN realized inCMOS-compatible optical waveguide technology


3. Optical beamformer: OBFN control block

beam angle

delays + amplitudes

chip parameters (is and is)

heater voltages

DA conversion & amplification

ARM7microprocessor

436

43

43

1

1

1

2

2

2

5

7


3. Optical beamformer: OBFN measurements

OBFN Measurement results

1 ns ~ 30 cm delay distance in vacuum

5 6 16

5 6

5 6

7

7

out 27

8

8

8

15

out 1

out 4

out 314

9 10 19

9 10

9 10

11

11

out 611

12

12

12

18

out 5

out 8

out 7173 4

3 4

3 4

1 2

1 2

1 2

in

13

stage 1 stage 2 stage 3

L. Zhuang et al., IEEE Photonics Technology Letters, vol. 19,no. 15, Aug. 2007


3. Optical beamformer: E/O and O/E conversion

OBFN

electrical » optical optical » electrical

RF front-end

LNA

TIA

DM laser

chirp!

E/O and O/E conversions?• low optical bandwidth;• high linearity;• low noise

photo-diode



OBFN


RF front-end

LNA

TIA

CW laser

beat noise!

mod.


photo-diode



OBFN


RF front-end

LNA

TIA

mod.

CW laser


photo-diode



mod.

OBFN

RF front-end

LNA

spectrum

frequency

TIA


CW laser

10.7 GHz

12.75 GHz2 x 12.75 = 25.5 GHz

photo-diode



mod.

OBFN

RF front-end

LNALNB

spectrum

frequency

TIA


CW laser

2 x 2.15 = 4.3 GHz

950 MHz

2150 MHz

photo-diode



mod.

OBFN

RF front-end

filter

filter

LNB

spectrum

frequency

TIA

single-sideband modulation withsuppressed carrier (SSB-SC)


CW laser

1.2 GHz

SSB-SCBalanced optical detection:

cancels individual intensity terms;

mixing term remains;

reduces laser intensity noise;

enhances dynamic range.

Implementation of optical SSB modulation?

1. Optical heterodyning;

2. Phase shift method;

3. Filter-based method.

photo-diode



mod.

OBFN

RF front-end

filter

LNB

spectrum

frequency

TIA

single-sideband modulation withsuppressed carrier (SSB-SC)


CW laser

1.2 GHzFilter 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.

1 chip !



MZI + Ring

5 mm

Optical sideband filter chip in the same technology as the OBFN



Measured filter response

Wavelength (nm)

1549.6 1549.7 1549.8 1549.9 1550.0 1550.1 1550.2 1550.3 1550.4

Filt

er P

ower

Res

pons

e (d

B)

-35

-30

-25

-20

-15

-10

-5

0

MeasurementSimulation

20 GHz14 GHz

3 GHz

Frequency (GHz)

0 10 20 30 40

Filt

er

Po

we

r R

esp

on

se (

dB

)

-40

-30

-20

-10

0

Bandwidth—Suppression Tradeoff

25 dB

FSR 40 GHz



Spectrum measurement of modulated optical signal

Optical heterodyning technique:

f

f f f

f1

f

f1 f2

MZM OSBF SAfiber coupler

RF

f

f1 – f2

f

f2



Spectrum measurement of modulated optical signal

Measured filter response, and measured spectrum of modulated optical signal, with and without sideband-filtering



RF phase response measurementsMZM

MZM

I tout( )MZM

MZM

OBFNOSBF

s1( )t

s3( )t

s2( )t

s4( )t

laser

RF input 1-2 GHz

RF output

Measured RF phase response of one beamformer channel, for different delay values.



Signal combination measurements

Frequency (GHz)

0.0 0.2 0.4 0.6 0.8 1.0

RF

sig

nal m

agtit

ude

(dB

)

-70

-60

-50

-40

-30

-20

ch 1ch 2ch 3ch 4ch 1&2ch 3&4ch All

CH 1

CH 2

CH 3

CH 4

Output

RF input 0-1 GHz

Measured output RF power of beamformer with intensity modulation and direct detection, for - 1 channel, - 2 combined channels, - 4 combined channels


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.

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