1. No category

Reconfigurable thermo-optic polymer switch based true-time-delay
network utilizing imprinting and inkjet printing
Zeyu Pana, Harish Subbaramanb,*, Cheng Zhangc, Ashwin Pandayc, Qiaochu Lic, Xingyu Zhanga, Yi
Zoua, Xiaochuan Xub, L. Jay Guoc, Ray T. Chena,b,*
a
Department of Electrical and Computer Engineering, The University of Texas at Austin, 10100
Burnet Rd, PRC/MER 160, Austin, TX 78758, USA
b
Omega Optics, Inc., 8500 Shoal Creek Blvd, Building 4, Suite 200, Austin, TX 78757, USA
c
Department of Electrical Engineering and Computer Science, University of Michigan, 1301 Beal
Ave, Ann Arbor, MI 48109, USA
ABSTRACT
Previously, we introduced a novel and an etch-free solution based procedure utilizing a combination of imprinting and
inkjet printing for developing polymer photonic devices to overcome the limitations of conventional polymer photonic
device fabrication techniques, such as RIE or direct pattern writing. In this work, we demonstrate the feasibility of
developing very large-area photonic systems on both rigid and flexible substrates. Specifically, a complete
reconfigurable 4-bit true-time-delay module, comprising of an array of five interconnected TO switches and polymer
delay lines, with a dimension of 25 mm × 18 mm is developed. Because of the roll-to-roll (R2R) compatibility of the
employed solution processing techniques, photonic system development over large areas at high-throughput on rigid or
flexible substrates is possible, which will lead to tremendous cost savings. Moreover, these devices can be integrated
with other printed photonic and electronic components, such as light sources, modulators, antennas, etc., on the same
substrate, thus enabling integrated systems that can be conformably integrated on any platform.
Keywords: Polymer, waveguide, thermo-optic, switch, imprinting, inkjet printing, true time delay, phased array antenna
1. INTRODUCTION
Integrated optical switches are important building blocks in optical links and systems [1-5]. Among various
optical switches, polymer-based thermo-optic (TO) switches have been found very attractive, owing to the advantages of
1) high thermo-optic coefficient (-1~3 x10-4 K-1) [6-8], 2) high transparency in the telecommunication wavelength
windows, and 3) fabrication feasibility over large areas on PCBs and other kinds of substrates. With these special
features, TO polymer switches have enabled widespread applications in several areas, such as communication and radar,
add/drop multiplexing, bypass switching in the event of a network failure or network jam, packet switching, etc. [6-20].
However, until now, the most common methods for polymer optical device fabrication includes either using Reactiveion Etching (RIE) to define the pattern into a resist, and transferring the pattern to the optical polymer via plasma etching
[21, 22], or directly writing the pattern in a low-loss UV/Ebeam curable polymer using lithography [18, 20]. Although
these methods are straightforward, they are not a cost-effective way due to complicated fabrication process and low
throughput. Previously, we introduced a novel and an etch-less solution processing technique utilizing a combination of
imprinting and ink-jet printing for developing photonic devices [16, 19, 23]. In this work, we demonstrate the feasibility
of developing very large-area photonic systems. Specifically, a complete 4-bit true-time-delay reconfigurable module
with a dimension of 25 mm × 18 mm, comprising of an array of five interconnected TO switches and polymer delay
lines [9-11, 24-27], is developed. Thanks to the roll-to-roll (R2R) compatibility of the employed solution processing
techniques, photonic system development over large areas on either rigid or flexible substrates, and at high-throughput,
is possible which will lead to tremendous cost savings. Moreover, these devices can be integrated with other printed
photonic and electronic components, such as light sources, modulators, antennas on the same substrate, thus achieving
an integrated system that can be conformably integrated on any platform.
Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications VIII,
edited by Laurence P. Sadwick, Tianxin Yang, Proc. of SPIE Vol. 9362, 936214
© 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2080115
Proc. of SPIE Vol. 9362 936214-1
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2. POLYM
MER BASED
D THERMO
O-OPTIC SW
WITCH
A siingle TO polym
mer switch is first
f
introducedd in this sectioon, followed byy the developm
ment of the enttire TTD
module in section 3. A scchematic of a single 2x2 TO
T switch is shown
s
in Figuure 1. In this design,
d
a singgle mode
waveguide comprising of 3.5
3 µm thick UV15DC80LV
U
V (n=1.501 @ 1.55 µm) bottoom cladding; 3 µm thick UF
FC-170A
(n=1.496 @ 1.55 µm) top cladding;
c
and 2.3
2 µm thick (00.5 µm rib heigght, 1.8 µm slabb) and 8.5 µm wide SU8 (n=1.575 @
1.55 µm) corre layer, is con
nsidered. An 8 µm
µ wide and 500
5 µm long goold heating eleectrode is used to heat the polymer in
the center off the junction.
Ch. A
25
t[bar
port)
sp
4° half;
branch ani
T
horn\
lµm wide
heating
electrode
ontact
pads
-
f
i wide ;;(
Gold
40µm
center
width
;Junction
iength:
4110 µm
?guides
I
Iriput pori
Figure 1. Schematic of a single 2×2 TO polymer
p
switch [19]. Dependingg on whether a voltage
v
is applieed across the heaating
electrode to heat the junction region, lighht exits from the bar port (no appplied voltage) annd cross port (witth applied voltagge).
Norrmally, when th
here is no heatt applied at thee junction regiion, light from the input portt will exit from
m the bar
port. By heaating the junctiion region in the switch, thee refractive inndex of the polymer underneeath is reducedd, which
creates a totaal-internal-refleection (TIR) coondition, thus directing the liight to output from the crosss port. Figures 2(a) and
2(b) show thhe simulation reesults performeed by the beam
m propagating method
m
(Beam
mPROP from RSoft
R
Suite) in the OFF
state (no volttage across the electrode) andd the ON state (voltage acrosss the electrode) of the TO sw
witch, respectively.
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150
(a)
-150
1
1
2000
2500
3500
4000
3500
4000
x (un)
(b)
Figure 2.. Light propagattion through thee switch in the (a) OFF and (b) ON states of the switch. Heaating in the juncction
causes a local decrease in the refractive index, thus leadding to total-inteernal-reflection (TIR) condition in the center off the
horn struccture.
We fabricated a siingle switch deevice in order to
t characterize its performancce. First, a flexxible mold conttaining a
single TO sw
witch core pattern is replicateed from a silicon hard mold. The core wavveguide patternn is then defineed in the
UV15DC80L
LV bottom claadding layer, using UV impriinting techniquue. The SEM cross-section
c
o printed layers in the
of
polymer wavveguide [19] is
i shown in Fiigure 3(a). Ann atomic forcee microscope (AFM)
(
image of the imprinnted core
pattern of thee waveguide in
n UV15DC80L
LV bottom claadding layer is shown in Figuure 3(b). The measured
m
rougghness is
1.45 nm, which is comparaable to our preevious results [16]. The corre layer trench is filled by innkjet-printing the SU8
material. Inkkjet printing off SU8 automatiically produces a flat surfacee profile on topp, which can be
b used for subbsequent
material prinnting. The top UFC-170A
U
claadding layer is then coated onn top of the corre layer. Finallly, gold metal heater is
deposited onn the top cladding layer usingg photolithograaphy, ebeam evaporation,
e
annd lift-off [21, 22]. Alignmennt marks
are utilized too aid in the heaater placement. A microscopee image of a fuully fabricated TO switch is shown
s
in Figure 4.
(b)
x (µm)
Figure 3.. (a) SEM crosss-section view of
o printed layerss in the polymeer waveguide [119]. (b) An AFM
M measured botttom
cladding of the polymer waveguide.
w
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Figure 4. A microscope im
mage of a fully fabricated
f
TO Sw
witch.
Nexxt, the static an
nd dynamic chharacteristics of
o the fabricateed TO polymerr switch are measured.
m
Lighht from a
1550 nm tunnable laser sou
urce (Santec ECL-200)
E
is cooupled into andd out of the device
d
using leensed fibers (O
OzOptics
TSMJ-3U-15550-9/125-0.25
5-7-2.5-14-2). The normalizeed output optiical power from the bar porrt versus the electrical
e
power consuumed by the heeater is plottedd in Figure 5(aa). The switch consumes aboout 160 mW of
o the electricaal power.
Next, the dyynamic characteristics are tessted. A 100 Hzz square wave signal generaated by a functtion generator (Agilent
33120A) is applied
a
across the heating ellectrode, and the
t output optiical response from
f
the bar port
p is obtainedd from a
digital oscilloscope (HP 16
660ES), as shoown in Figure 5(b). The risee and fall timees for the switcch are measurred to be
0.49 ms and 0.35 ms, respeectively.
(a)
(b)
Figure 5. (a) The normalized output optical power of barr port shows thee TO switch withh a power consuumption of 160 mW.
m
(b) Opticcal response with
h square wave function
f
applied across the heatiing electrode at 100 Hz frequenncy (CH1 repressents
the applieed voltage and CH2
C represents bar
b port).
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3. LARGE AR
REA FABRIC
CATION FO
OR THE REC
CONFIGURA
ABLE TTD NETWORK
N
K
Upoon characterizin
ng the perform
mance of a singgle switch, we developed
d
a laarge-area 4-bit TO-switch bassed TTD
module. A scchematic of succh module is shown in Figurre 6. It consistss of five 2×2 TO
T polymer sw
witches, intercoonnected
via judicioussly chosen leng
gths of polymeer waveguide delay
d
lines. Thhe minimum leength delay steep (ΔL) determ
mines the
minimum achhievable time delay step (Δττ) according to Δτ = neff· (ΔL//c), where neff is the effectivee index of the mode in
the waveguidde and c is the speed of light in vacuum. Att the first switcch (n=1), the opptical signal iss delivered to either
e
the
reference waaveguide (leng
gth L) or the delay
d
line (lenngth L+ΔL), depending on the
t chosen ON
N or OFF statee of TO
polymer swittch. Then, the second switchh (n=2) coupless the optical signal into two more
m
waveguiddes with lengthhs L and
L+2·ΔL. This sequence is continued untiil the last switcch delivers thee optical signaal to two wavegguides with leengths L,
or a 4-bit delayy network). Thhe last switch (n=5)
(
of the 4-bit delay TTD
D line is used too control
and delay linne, L+23 ΔL (fo
the optical signal to couplle into the outpput waveguidee. Table 1 listss the delay coonfiguration for a 4-bit TTD module
capable of prroviding up to ±60° steeringg angle for an X-band
X
Phasedd Array Antennna (PAA). It shhould be noted that this
upper limit in
i the steering
g angle is not limited by ourr technology, but by the characterization setup availablle in our
laboratory foor conducting antenna
a
patternn measurementss.
n=1
L+0 L
/wr
+eguide
ay Lines
N
*.'
1
&L
t Waveguide
I
n=5
Figure 6. Schematic of a reconfigurable 4-bit TTD unit comprising of 2×2
2
TO polymeer switches and polymer wavegguide
delay linees.
Table 1. Delay
D
configurattion for each eleement in a 4-bit X-Band
X
PAA.
Steering Angle
A
(deg)
Modu
ule #1
Module #2
#
M
Module
#3
Modulee #4
6 °
60
0
5⋅ΔL
10⋅ΔL
15⋅ΔL
L
433.85 °
0
4⋅ΔL
8⋅ΔL
12⋅ΔL
L
31.31 °
0
3⋅ΔL
6⋅ΔL
9⋅ΔL
L
200.27 °
0
2⋅ΔL
4⋅ΔL
6⋅ΔL
L
9..97 °
0
1⋅ΔL
2⋅ΔL
3⋅ΔL
L
0°
0
0
0
0
-9.97 °
3⋅Δ
ΔL
2⋅ΔL
1⋅ΔL
0
-200.27 °
6⋅Δ
ΔL
4⋅ΔL
2⋅ΔL
0
-311.31 °
9⋅Δ
ΔL
6⋅ΔL
3⋅ΔL
0
-43.85 °
12 ⋅Δ L
8 ⋅Δ L
4 ⋅Δ L
0
- 660 °
15 ⋅Δ L
10 ⋅Δ L
5 ⋅Δ L
0
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An essential
e
consideeration in fabricaating such a 4-biit TTD device iss to ensure the waveguides
w
are defect-free
d
over the entire
device area, which
w
is a non-ttrivial challengee. Hard mold im
mprinting gives satisfactory
s
resuults for small-areea devices, but lacks the
capability to provide
p
uniform imprinting over a large area, witth de-molding coonstituting the greatest
g
challenge [28, 29]. In coomparison,
soft mold givees better uniform
mity over a large area and an eassier de-molding process.
p
Howeveer, a suitable maaterial system neeeds to be
selected in ordder to provide a reliable mold that
t
can repeateddly be utilized. In this work, PD
DMS (mixed att a 10:1 ratio wiith curing
agent) was choosen as the imprrinting soft moldd material. First,, the 4-inch Si mold
m
is fabricated using conventtional RIE methood, and is
then thoroughly cleaned and coated
c
with a suurfactant to loweer its surface eneergy. Then, PDM
MS is used to duuplicate the TTD
D patterns
from a 4-inchh Si mold. A piccture of a succeessfully developeed large-area PD
DMS flexible mold
m
is shown inn Figure 7(a). The
T mold
showed no deffects when inspeected under a miicroscope over thhe entire area. Next,
N
the PDMS mold is used to imprint the pattern in the
UV15DC80LV
V bottom cladd
ding layer. Figuure 7(b-d) show
ws the microscope image of a TO polymer switch, curved polymer
waveguide, annd alignment marks
m
on the UV
U imprinted UV15DC80LV
U
r
respectively,
whhich demonstrattes a defect-freee surface
achieved from
m such a large-a
area imprintingg process. The core layer, top cladding, and electrodes
e
are fabricated
fa
using the same
process as desscribed in sectio
on 2. A picture of the fully fabrricated 4-bit TT
TD module is shhown in Figure 7(e). We are currently
c
characterizinng the performaance of the TTD
D module, whiich will be sharred in a future publication.
Figure 7. (a) The picture of a large-area PDMS
P
flexible mold
m
fabricated from
f
a 4-inch sillicon wafer. Thee microscope im
mages
mer switch, (c) cuurved polymer waveguides,
w
andd (d) the alignment marks provee the
of (b) thee horn structure of a TO polym
defect-freee imprinting pro
ocess over a largge area (4-inch sample). (e) Pictuure of the fully fabricated
f
full 4--bit TTD lines.
4. CONCLUSIO
ON
A sccheme to utilizze an etch-freee and roll-to-rooll compatible fabrication prrocess for fabriicating very laarge-area
polymer photonic systems is
i demonstrateed in this work.. Utilizing a coombination of PDMS
P
imprintiing and inkjet printing,
p
mer switch is fiirst developed and characterrized. The TO switch providdes over 35 dB
B extinction raatio, and
a TO polym
consumes 1660 mW of elecctrical power. A defect-free large area PDM
MS mold contaaining a 4-bit TTD
T
module pattern
p
is
successfully replicated from
m a silicon maaster mold. Utiilizing large-arrea imprinting (using the larrge-area flexiblle mold)
T
baseed TTD moduule capable of providing suffficient time delay
d
for
and inkjet printing processes, a 4-bit TO-switch
achieving ±60° steering an
ngle in an X-band PAA systeem is successfuully fabricatedd. This R2R com
mpatible proceess holds
great promise for scalable and low-cost manufacturingg of true-time-delay feed nettworks for phaased array anteennas on
rigid as well as on flexible substrates.
Proc. of SPIE Vol. 9362 936214-6
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ACKNOWLEDGEMENTS
The authors would like to acknowledge the Air Force Office of Scientific Research (AFOSR) for supporting this work
under the Small Business Technology Transfer Research (STTR) program (Grant No. FA9550-14-C-0001), monitored
by Dr. Gernot Pomrenke.
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