Experimental demonstration of a reconfigurable
silicon thermo-optical device based on spectral
tuning of ring resonators for optical signal
processing
William S. Fegadolli,
1,2,3,*
Liang Feng,
1,4
Muhammad Mujeeb-U-Rahman,
1
José E. B.
Oliveira,
2
Vilson R. Almeida,
2,3
and Axel Scherer
1
1
Department of Physics and Electrical Engineering, California Institute of Technol
ogy - Caltech, Pasadena,
California, USA
2
Department of Electronics Engineering, Instituto Tecnológico de Aeronáutica – ITA, Brazil
3
Division of Photonics, Instituto de Estudos Avançados – IEAv, Brazil
4
Department of Electrical Engineering, The State University of New York at Buffalo, Buffalo, New York, USA
*fegadoli@caltech.edu
Abstract:
We have experimentally demonstrated a reconfigurable silicon
thermo-optical device able to tailor its intrinsic spectral optical response by
means of the thermo-optical control of individual and uncoupled resonant
modes of micro-ring resonators. Preliminarily results show that the device’s
optical response can be tailored to build
up distinct and reconfigurable logic
levels for optical signal processing, as well as control of overall figures of
merit, such as free-spectral-range, extinction ratio and 3dB bandwidth. In
addition, the micro-heaters on top of the ring resonators are able to tune the
resonant wavelength with efficiency of 0.25 nm/mW within a range of up to
10 nm, as well as able to switch the resonant wavelength within fall and rise
time of 15
μ
s.
©2014 Optical Societ
y of America.
OCIS codes:
(130.3120)
Integrated optics devi
ces; (130.7408) Wavelengt
h filtering devices;
(160.6840) Thermo-optical materials.
References and links
1. L. Pavesi and G. Guillot,
Optical Interconnects - The Silicon Approach
(Springer-Verlag, Heidelberg, 2006).
2. M. Lipson, “Guiding, modulating
and emitting light on silicon - Challenge
s and opportunities,” J. Lightwave
Technol.
23
(12), 4222–4238 (2005).
3. V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett.
28
(15),
1302–1304 (2003).
4. Q. Xu, B. Schmidt, S. Pradhan,
and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature
435
(7040), 325–327 (2005).
5. M. Liu, X. Yin, E. Ulin-Avila, B. Geng, T. Zentgr
af, L. Ju, F. Wang, and X. Zhang, “A graphene-based
broadband optical modulator,” Nature
474
(7349), 64–67 (2011).
6. D. J. Thomson, F. Y. Gardes, Y. Hu, G. Mashanovich
, M. Fournier, P. Grosse, J.-M. Fedeli, and G. T. Reed,
“High contrast 40Gbit/s optical m
odulation in silicon,” Opt. Express
19
(12), 11507–11516 (2011).
7. D. T. H. Tan, P. C. Sun, a
nd Y. Fainman, “Monolithic nonlinear pul
se compressor on a silicon chip,” Nat
Commun
1
(8), 116 (2010).
8. T. Barwicz, M. A. Popovi
ć
, M. R. Watts, P. T. Rakich, E. P. Ippen,
and H. I. Smith, “Fabrication of add-drop
filters based on frequency-ma
tched microring resonators
,” J. Lightwave Technol.
24
(5), 2207–2218 (2006).
9. W. S. Fegadolli, J. E. B. Oliveira, V. R. Almeid
a, and A. Scherer, “Compact and low power consumption
tunable photonic crystal nanobeam
cavity,” Opt. Express
21
(3), 3861–3871 (2013).
10. M. Erdmanis, L. Karvonen, A. Säynätjoki, X. Tu, T.
Y. Liow, Q. G. Lo, O. Vä
nskä, S. Honkanen, and I.
Tittonen, “Towards broad-bandwidth
polarization-independent nanostrip waveguide ring resonators,” Opt.
Express
21
(8), 9974–9981 (2013).
11. A. W. Fang, H. Park, O. Cohen, R. Jones, M. J. Pa
niccia, and J. E. Bowers, “Electrically pumped hybrid
AlGaInAs-silicon evanescent laser,” Opt. Express
14
(20), 9203–9210 (2006).
12. X. Sun, A. Zadok, M. J. Shearn, K. A. Diest, A. Ghaffari
, H. A. Atwater, A. Scherer,
and A. Yariv, “Electrically
pumped hybrid evanescent Si/I
nGaAsP lasers,” Opt. Lett.
34
(9), 1345–1347 (2009).
#204880 - $15.00 USD
Received 16 Jan 2014; accepted 24 Jan 2014; published 5 Feb 2014
(C)
2014
OSA
10
February
2014
| Vol.
22,
No.
3 | DOI:10.1364/OE.22.003425
| OPTICS
EXPRESS
3425
13. W. S. Fegadolli, S. H. Kim, P. A.
Postigo, and A. Scherer, “Hybrid singl
e quantum well InP/Si nanobeam lasers
for Silicon Photonics,” Opt. Lett.
38
(22), 4656–4658 (2013).
14. T. Creazzo, E. Marchena, S. B. Krasulick, P. K. L. Yu, D.
V. Orden, J. Y. Spann, C. C. Blivin, L. He, H. Cai, J.
M. Dallesasse, R. J. Stone, and A. Mizrahi, “I
ntegrated tunable CMOS laser,” Opt. Express
21
(23), 28048–
28053 (2013).
15. T. Yin, R. Cohen, M. M. Morse, G. Sarid, Y. Chetrit, D. Rubin, and M. J. Paniccia,
“31 GHz Ge n-i-p waveguide
photodetectors on Silicon-on-Insulato
r substrate,” Opt. Express
15
(21), 13965–13971 (2007).
16. S. Sahni, X. Luo, J. Liu, Y. H. Xie, and E. Ya
blonovitch, “Junction field-effect-transistor-based germanium
photodetector on silicon-on-in
sulator,” Opt. Lett.
33
(10), 1138–1140 (2008).
17. S. Assefa, F. Xia, and Y. A. Vlasov, “Reinventing
germanium avalanche photodetector for nanophotonic on-chip
optical interconnects,” Nature
464
(7285), 80–84 (2010).
18. F. Xia, T. Mueller, Y. M. Lin, A. Valdes-Garcia,
and P. Avouris, “Ultrafast gr
aphene photodetector,” Nat.
Nanotechnol.
4
(12), 839–843 (2009).
19. I. Goykhman, B. Desiatov, J. Khurgin, J. Shappir,
and U. Levy, “Locally oxidized silicon surface-plasmon
Schottky detector for telecom regime,” Nano Lett.
11
(6), 2219–2224 (2011).
20. L. Feng, Y. L. Xu, W. S. Fegadolli, M. H. Lu, J. E.
B. Oliveira, V. R. Almeida, Y. F. Chen, and A. Scherer,
“Experimental demonstration of a unidi
rectional reflectionless parity-time metamaterial at optical frequencies,”
Nat. Mater.
12
(2), 108–113 (2012).
21. L. Bi, J. Hu, P. Jiang, D. H. Kim, G. F. Dionne, L. C. Kimerling, and C. A. Ross,
“On-chip optical isolation in
monolithically integrated non-reciprocal
optical resonators,” Nat. Photonics
5
(12), 758–762 (2011).
22. H. Lira, Z. Yu, S. Fan, and M. Lipson, “Electri
cally driven nonreciprocity induced by interband photonic
transition on a silicon chip,” Phys. Rev. Lett.
109
(3), 033901 (2012).
23. W. S. Fegadolli, V. R. Almeida, and J. E. Olivei
ra, “Reconfigurable silicon th
ermo-optical device based on
spectral tuning of ring resonators,” Opt. Express
19
(13), 12727–12739 (2011).
24. C. K. Madsen and G. Lenz, “Optical All-Pass F
ilters for Phase Response Design with Applications for
Dispersion Compensation,” IEEE Photon. Technol. Lett.
10
(7), 994–996 (1998).
25. W. S. Fegadolli, G. Vargas, X. Wang, F. Valini, L. A. M. Ba
rea, J. E. B. Oliveira, N. Frateschi, A. Scherer, V. R.
Almeida, and R. R. Panepucci, “Reconfigurable silicon
thermo-optical ring resonator switch based on Vernier
effect control,” Opt. Express
20
(13), 14722–14733 (2012).
26. M. R. Watts, J. Sun, C. DeRose, D. C. Trotter, R. W. Young, and G. N. Nielson, “Adiabatic thermo-optic Mach-
Zehnder switch,” Opt. Lett.
38
(5), 733–735 (2013).
27. A. H. Atabaki, A. A. Eftekhar, S. Yegnanarayanan,
and A. Adibi, “Sub-100-nanosecond thermal reconfiguration
of silicon photonic devices,” Opt. Express
21
(13), 15706–15718 (2013).
28. P. Dong, W. Qian, H. Liang, R. Shafiiha, D. Feng,
G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M.
Asghari, “Thermally tunable silicon racetrack resonators with ultralow tuning power,” Opt. Express
18
(19),
20298–20304 (2010).
29. A. H. Atabaki, E. Shah Hosseini,
A. A. Eftekhar, S. Yegnanarayanan, a
nd A. Adibi, “Optimization of metallic
microheaters for high-speed reconfigurable silicon photonics,” Opt. Express
18
(
17), 18312–18323 (2010).
1. Introduction
Silicon photonics has been considered a promising technology owing to its intrinsic
characteristic of allowing high integration of optical devices in small footprints, as well as to
its synergy with existing CMOS (Complementary Metal-Oxide-Semiconductor) processes.
The applications have promised to cover a wide spectrum, comprising conventional long-
distance down to intra-chip communications [1, 2].
For the past decade, several research groups have demonstrated essential building blocks
to process optical signals, for example: efficient and broadband input/output coupling systems
from optical fibers to optical waveguides [3], high-speed electro-optic modulators [4–6],
chip-scale ultrafast pulse compressor [7], tuna
ble filters [8,9], polarization-independent
devices [10], heterogeneous integration on silicon-on-insulator (SOI) to produce light sources
and photodetectors [11–19], as well as unidirectional [20] and nonreciprocal devices [21, 22],
amongst others.
Nowadays, one particular challenge of highly importance in telecommunications has been
highly discussed by the technical community; it consists on the development of
reconfigurable devices to allow a degree of freedom on optical signal processing and then
allow the development of intelligent networks with higher performance.
In this letter, we experimentally demonstrat
e a reconfigurable silic
on thermo-optical filter
based on spectral tuning of separate ring resonators; this approach was previously reported on
#204880 - $15.00 USD
Received 16 Jan 2014; accepted 24 Jan 2014; published 5 Feb 2014
(C)
2014
OSA
10
February
2014
| Vol.
22,
No.
3 | DOI:10.1364/OE.22.003425
| OPTICS
EXPRESS
3426
a theoretical ground in our previous work [23], where its principle of operation was named as
the
Persiana
effect [23].
The proposed device consists of a series of uncoupled ring resonators, which in turn are
coupled to the same bus waveguide [24] and integrated with micro-heaters atop [23], as
shown in Fig. 1. The principle of operation of the device is based on the tuning of the
resonance of each ring resonator to tailor the
device’s optical response, as desired, by means
of thermo-optical effect induced by the indi
vidual control of the micro-heaters atop of each
ring resonator [23]. It is worth pointing out that a detailed analysis of the device was
presented in our previous work [23], where we identified optimal theoretical parameters for
particular applications.
2. Fabrication
Initially, we optimistically tried to fabricate the device on the grounds of the theoretical
design, which was validated by FDTD in our
previous work [23]. However we faced two
experimental limitations: i) imprecision on the fabrication process, mostly owing to
imperfections of E-Beam lithography, side wall roughness caused by Etch and imperfections
caused by PECVD deposition, which generated overall random deviation of the optical length
of the ring resonator within a range from 4nm to 38nm instead of the required precision
(2nm); and ii) we experimentally observed that the micro-heater with radii of approximately
5um melted down before providing enough required shift of the resonant mode to
demonstrate the “Persiana effect” [23].
The imprecision of the fabrication process in our ring resonator is the limited by the
capabilities of our facilities but we experimentally adjusted the fabrication parameters of
micro-heaters to support a higher shift before melting down and allow us to compensate the
intrinsic fabrication deviation by applying a bias current and tune the resonant wavelength on
the desired wavelength.
Therefore, the present device structure wa
s slightly redesigned to overcome these
experimental limitations not predicted in the previous report [23]. We modified the design
parameters as follows: 10 uncoupled identical Si micro-ring resonators are spatially separated
and connected to the same Si bus waveguide (Fig. 1). The gap distance between the micro-
ring resonators and the bus waveguide is 250 nm; the radius of the ring resonators is 10 μm;
the width and the height of the bus waveguide and ring resonators are 400 nm and 220 nm,
respectively; and the widths of the heaters atop of the ring resonators are all set to be 2.5 μm,
similar to the parameter used in our previous works [9, 25].
The device was fabricated by means of two
distinct layers: the optical layer and the
thermal layer. The optical layer was fabricat
ed using direct E-beam lithography over a SOI
substrate with negative-tone E-beam resist (HSQ), followed by dry etch using a mixture of
C
4
F
8
and SF
6
to form the ten ring resonators and the bus waveguide. Next, a thick layer of 1.5
μ
m of silicon dioxide was deposited on the sample using plasma-enhanced chemical vapor
deposition (PECVD). The purpose of this oxide layer is not only to make the optical mode
symmetric, but also to optically isolate the optical layer from the metal layer, required by the
micro-heaters. This isolation helps to avoid ab
sorption loss from metal layer, for both light
propagating inside ring resonators and the waveguide; however, the oxide layer still allows
enough heat transfer from the thermal layer to the optical layer as previously demonstrated
[9].
The thermal layer was then built in two steps using aligned photolithography and positive
photoresist with an inversion process: the firs
t step consists of photolithography of the micro-
heaters on top of the fabricated and passivated ring resonators, followed by 200-nm thick
Nichrome deposition and then lift-off; the second step consists of the photolithography for
both contact pads and power feed
-lines, followed by (5 nm / 270 nm) Ti/Au deposition and,
finally, lift-off. Various stages of the fabricated device can be seen in Fig. 1.
#204880 - $15.00 USD
Received 16 Jan 2014; accepted 24 Jan 2014; published 5 Feb 2014
(C)
2014
OSA
10
February
2014
| Vol.
22,
No.
3 | DOI:10.1364/OE.22.003425
| OPTICS
EXPRESS
3427
Fig. 1. Scanning electron microscope microgr
aph of (a) single ring resonator and (b) bus
waveguide after exposed and etched; (c) final de
vice passivated with a thermal oxide layer and
integrated with micro-heaters
and pad contacts atop; and (d) the scaled region of a ring
resonator seen in (c).
3. Measurements and results
We began our measurements characterizing the
electrical properties of our heaters using a
semiconductor analyzer and scanned the electric
current versus voltage in order to measure
the resistance of our heaters, which was found to be around 700
Ω
. In optical measurements,
we used nano-positioners to align and efficiently couple light from lensed optical fibers into
the silicon waveguides with inverted nanotapers [3]. An Agilent tunable laser model 81980A
was used as light source, and an Agilent fiber-coupled power meter model 81636B was used
to measure transmitted signals. A Keithley precision current source model 2400 was used to
control the electric current on the micr
o-heaters for thermo-optical control.
Moreover, we characterized one of the ring resonators in order to obtain the power
consumption efficiency and time dependency.
The results for the single ring resonator are
shown in Fig. 2. Figure 2(a) shows the optical response for different bias current values, Fig.
2(b) shows the resonant wavelength as a fu
nction of electric current and electric power,
indicating that the ratio of resonant wavelength per electrical power is around 0.25 nm/mW.
In addition to the electrical characterization of the thermo-optical properties of the device, we
also investigated the required fall and rise time to switch “on” and “off” the resonance
condition for the resonant wavelength; this resu
lt is shown in Fig. 2(
c) where we observe 15
μ
s for the fall and rise times. The electrical power was set to 300
μ
W.
#204880 - $15.00 USD
Received 16 Jan 2014; accepted 24 Jan 2014; published 5 Feb 2014
(C)
2014
OSA
10
February
2014
| Vol.
22,
No.
3 | DOI:10.1364/OE.22.003425
| OPTICS
EXPRESS
3428
1533.44
1533.482
1533.524
1533.566
1533.608
1533.65
Optical Power (dBm)
Resonant wavelength (nm)
Normalized Power
Fig. 2. (a) Optical response of a single ring
resonator as a function of the electric current
applied to the micro-heaters; (b) resonant wave
length as a function of the electrical power and
electric current applied to the micro-heaters.
(c) Temporal behavior of the modulation and
detected signals
Our micro-heaters were experimentally optimized to provide high resonant shift; however,
it is worthy pointing out that there is enough room for further optimization regarding power
efficiency and switching speed, as previously demonstrated by other authors [26–29].
Finally, we measured the optical response of the device shown in Fig. 1(c) with and
without appropriate bias currents applied to the
micro-heaters, as depicted in Fig. 3. Without
bias currents, it can be seen that, although
all the 10 ring resonators were designed and
fabricated to be identical, the randomness in fabrication made all the ring resonators slightly
off-resonance among each other and
with different extinction ratio, thus resulting in a series
of different resonant dips in the transmission spectrum from 1545 nm to 1560 nm.
To compensate the adverse effect from such
fabrication randomness, we applied distinct
and appropriate bias currents to each one of
the micro-heater; the overall dissipated power in
the micro-heater to form this optical sintonized state was 132 mW, this condition allows us to
flexibly control the thermo-optical blue and red-shifts of each resonant wavelength, and thus
reconfigure the entire device as desire
d to tailor the Free-Spectral-Range (
FSR
), extinction
ration, and 3dB bandwidth.
Consequently, we successfully realized all the re
sonances from different ring resonators to
coincide at the same wavelength, behaving like 10 identical micro-ring resonators in terms of
optical length (not with same figure of merit)
. This condition is referred as “Level 0” [23].
Figure 3 (b) shows a comparison between the optical response vs. normalized wavelength
of a single resonator and all the ten ring resonators tuned at the same wavelength in order
show the increasing of bandwidth, we compared
when all resonant modes are tuned at the
same wavelength with a single resonant mode
from our best ring resonator. Because of the
cascaded effect, the extinction ratio of the re
sonance becomes larger compared to those
without bias currents (increasing of 10 dB) and the 3dB bandwidth is increased by
approximately 10 times the value of a single ring resonators.
#204880 - $15.00 USD
Received 16 Jan 2014; accepted 24 Jan 2014; published 5 Feb 2014
(C)
2014
OSA
10
February
2014
| Vol.
22,
No.
3 | DOI:10.1364/OE.22.003425
| OPTICS
EXPRESS
3429
Optical Power (dBm)
Optical Power (dBm)
Fig. 3. (a) Optical response of the device in transmission under two conditions: no bias current
applied, and appropriate bias cu
rrents applied such that the “Level 0” was established. (b)
Extinction ratio and bandwidth comparis
on between a sing ring resonator and
Persiana
structrure properly biased.
To realize the
Persiana
effect based on Level 0, we focused on the resonance dip located
around the wavelength of 1558 nm in Fig. 3. Based on the bias current condition on Level 0,
we applied additional sets of el
ectric current values to the heat
ers atop the ring resonators,
which were classified into two separate gr
oups: additional + 4 mW (overall power) are
applied on heaters for the first five ring resonato
rs, creating a red shift of the resonance dip,
while
−
4 mW (overall power) are applied on the last five ring resonators, causing a blue shift
of the resonance dip. This condition is called Level 1 [23]. Therefore, instead of one single
resonance observed on Level 0 (Fig. 4), the overall resonance spectrum splits into two,
demonstrating the expected
Persiana
effect, as shown in Fig. 4(
a), in which the transmission
spectra for the quasi-TE
00
polarization state is plotted. In addition, Fig. 4(b) shows the
extinction ration between both levels as a function of the wavelength.
Optical Power (dBm)
Extinction ratio(dB)
Fig. 4. (a) Optical response of the device operati
ng on Levels 0 and 1, (b) extinction ration as a
function of wavelength.
It is worth pointing out that no thermal crosstalk was observed in the measurements and it
is evident that the transmission spectrum of the device can be reconfigurable and desirably
tailored, as well as the corresponding optical signal can be efficiently filtered or slowly
modulated thermo-optically between Level 0 and Level 1. The extinction ratio modulation
can be as high as 30 dB, as shown in Fig. 4 and the overall Free-Spectral-Range (FSR) can be
tailored accordingly.
#204880 - $15.00 USD
Received 16 Jan 2014; accepted 24 Jan 2014; published 5 Feb 2014
(C)
2014
OSA
10
February
2014
| Vol.
22,
No.
3 | DOI:10.1364/OE.22.003425
| OPTICS
EXPRESS
3430
Conclusion
In summary, we have experimentally demonstrated a reconfigurable Si thermo-optical device
able to tailor its spectral optical response, allowing several degrees of reconfigurable control,
such as
FSR
, bandwidth, extinction ratio, and spectral shape. The device brings unique
functionalities on optical signal processing that may open the doors for fundamental
applications on the next generation of intelligent and reconfigurable networks.
Acknowledgment
Authors thank the NSF CIAN ERC (Grant
EEC-0812072), CAPES and CNPQ (Brazilian
Foundations) for the financial support and Kavli Nanoscience Institute at Caltech for
technical support.
#204880 - $15.00 USD
Received 16 Jan 2014; accepted 24 Jan 2014; published 5 Feb 2014
(C)
2014
OSA
10
February
2014
| Vol.
22,
No.
3 | DOI:10.1364/OE.22.003425
| OPTICS
EXPRESS
3431