Tunable, Fourth-Order Silicon Microring-Resonator Add-Drop Filters

Abstract

We demonstrate the first tunable, high-order channel add-drop filters based on silicon microring resonators. They meet rigorous, telecom-grade spectral requirements for microphotonic R-OADMs (reconfigurable optical add-drop multiplexers). The design addresses 100GHz-spaced, 40GHz-wide channels over 16-32nm.

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Tunable, Fourth-Order Silicon Microring-Resonator Add-Drop Filters
Miloš A. Popović, Tymon Barwicz, Marcus S. Dahlem, Fuwan Gan, Charles W. Holzwarth, Peter T. Rakich,
Henry I. Smith, Erich P. Ippen and Franz X. Kärtner
Research Laboratory of Electronics, Massachusetts Institute of Technology,
77 Massachusetts Avenue, Cambridge, MA 02139 USA, email: mpopovic@alum.mit.edu

Abstract We demonstrate the first tunable, high-order channel add-drop filters based on silicon microring
resonators. They meet rigorous, telecom-grade spectral requirements for microphotonic R-OADMs (reconfigura-
ble optical add-drop multiplexers). The design addresses 100GHz-spaced, 40GHz-wide channels over 16-32nm.


1. Introduction
We demonstrate the first tunable high-order channel
add-drop filters based on silicon microring resonators
for chip-scale microphotonic reconfigurable optical
add-drop multiplexers (R-OADMs). Microphotonic R-
OADMs for dynamic transparent optical networks
promise dense integration of add-drop filters, low-
power tuning and switching, minimal filter cascading
losses and the benefit of complex and phase-
coherent device architectures not realizable in bulk
optics. Important strides have been made through
the demonstration of a high-order C-band-tunable
(drop-only) demultiplexer [1], a 4th-order tunable
(drop-only) filter in Si (for microwave photonics, with
15GHz FSR) [2] and a single-ring-filter tunable R-
OADM [3]. However, telecom applications require
add-drop filters with a large through-port extinction,
large FSR (THz), and wide tunability.
In this paper, we report the first Si tunable high-
order add-drop filter with suitable drop- and through-
port characteristics for telecom applications. The filter
is compatible with a dense wavelength-division
multiplexed (DWDM) optical network with 40 GHz-
wide channels and 100 GHz channel spacing. Based
on strong-confinement silicon-wire waveguides and
designed for full tunability of each of two adjacent
resonances over its 16nm free spectral range (FSR),
the filters can route channels on a 0.8/1.6 Tbps
(20/40 channel x 40Gbps) aggregate R-OADM.
2. Optical and Thermal Device Design
The add-drop filter was designed for a 40GHz clear
channel with a flat drop-port, >24dB through-port
extinction and <30ps/nm in-band dispersion (leading
to a ~70 GHz-wide passband), and >30dB extinction
at 100GHz spaced adjacent channel edges. The filter
employs a series-coupled design [4] (Fig. 1(a)). Fig.
1(b) is an optical micrograph of the fabricated filter in
a folded, compact geometry, showing four Ti heaters
(one per ring), and an outline of the waveguides.
A novel silicon waveguide design with 600x100nm
cross-section (Fig. 1c), optimized for thermal tuning,
was chosen [5]. It has reduced sensitivity to
dimensional variation and sidewall roughness by a
factor of ~3 in comparison to conventional cross-
sections, and supports high microring Q’s with
proximate heaters. Cross-sections of 500x100nm are
used for bus waveguides, and the microrings have a
7μm radius. An efficient electromagnetic filter design
was produced by designing
asymmetric couplers to suppress spurious mode
coupling losses [6], arriving at device dimensions by
3D finite-difference modesolver and time-domain
(FDTD) simulations. In spite of the strong
confinement that leads to small critical dimensions, all
coupling gaps are wider than 175 nm making these
designs compatible with deep-UV lithography.
The filters are tuned thermally. Titanium heaters
(Fig. 1(b)) were designed to raise the ring core
temperature to about 250°C, permitting tuning over a
full FSR with about 40 mW of power per ring.
resonators and




Fig. 1. (a) Fourth-order filter design configuration, having
coupling coefficients {κ1
0.36, 0.65, 18.6}% at the center wavelength; (b) optical
micrograph of fabricated tunable 4th-order microring-
resonator filter based on Si waveguides, showing Ti heaters;
(c) material layer stackup, showing Si guide cross-sections.
2, κ2
2, κ3
2, κ4
2, κ5
2} = {18.6, 0.65,

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3. Fabrication of Optical Devices and Heaters
The devices were fabricated on a Unibond silicon-on-
insulator (SOI) wafer with 3μm buried-oxide under-
cladding and a 220 nm silicon layer, thinned to 106
nm by calibrated steam oxidation and HF stripping.
The waveguides were defined by e-beam lithography
using 60-nm-thick hydrogen silsesquioxane (HSQ) as
e-beam resist and mask for reactive-ion etching in
pure HBr. The e-beam exposed HSQ was removed
and the structure was spin-coated with a 1μm layer of
HSQ used as overcladding [7]. Next, 100-nm-thick Ti
heaters were formed on top of the HSQ by aligned
contact photolithography, e-beam evaporation and
liftoff. A second photolithography and liftoff step
defined 100-nm-thick gold contact pads. The wave-
guide material stackup is shown in Fig. 1(c). A 100
nm SiO2 layer was sputtered as crude passivation to
slow the oxidation of the Ti heaters when in operation.
4. Experimental Results
Characterization of fabricated weakly-coupled (loss-
dominated) Si microrings showed loss Q’s of ~250k
without and ~130k with an overhead Ti heater present
(as designed), translating to about 2-2.5 dB/cm and
4.5 dB/cm propagation loss in the rings, respectively.
In the drop- and through-port responses in Fig.
2(a), this is consistent with the ~1dB drop loss seen in
the 4th-order add-drop filter. There is general agree-
ment with the design response. Each ring requires
<50mW to tune the full 16nm FSR due to the high Si
thermooptic coefficient and a thermooptically efficient
heater-waveguide system design. Detailed inspection
of the Si filter passbands (Fig. 2(a)) shows that the
filter meets all spectral criteria required for telecom
applications, as summarized in Table 1. The through-
port extinction reaches ~20dB. The commonly used
two-stage, add-after-drop configuration then provides
~40dB as needed for telecom applications [6]. In-
band dispersion was estimated by taking the Hilbert
transform of the measured amplitude response and
verified to be near the expected design value. In
these filters, about 4nm tuning bias needed to be
applied to the middle two rings to align the
resonances and form the passband, correspondingly
reducing the tuning range. This frequency mismatch
can be corrected in future fabrication as previously
demonstrated [8].
5. Conclusions
The first high-order silicon-microring-resonator add-
drop filters meeting telecom specifications for DWDM
applications and supporting full-FSR tunability were
demonstrated. Since these filters were designed to
meet the requirements in Table 1 over two FSRs
(starting at 1534 and at 1550nm) these filter may be
combined with an FSR doubling scheme [9] to enable
C-band operation over 32nm.
References
1. S.T. Chu et al., in Opt. Fiber Commun. Conference
2004 Technical Digest, postdeadline paper PDP9.
2. M.S. Rasras et al., in Opt. Fiber Commun. Conf.
2006 Technical Digest, postdeadline paper PDP13.
3. E.J. Klein et al., IEEE Photon. Technol. Lett. 17,
2358 (2005).
4. B.E. Little et al., IEEE Photon. Technol. Lett. 16,
2263 (2004).
5. M.A. Popović et al., in Conf. Lasers and Electr-Opt.
(CLEO), Long Beach, USA, paper CTuCC1 (2006).
6. M.A. Popović et al., Opt. Lett. 31, 2571 (2006).
7. C.W. Holzwarth et al., submitted to EIPBN 2007,
Denver, CO.
8. T. Barwicz et al., J. Lightwave Technol. 24, 2207
(2006).
9. M.A. Popović et al., Opt. Lett. 31, 2713 (2006);
M.R. Watts et al., in Proc. Conf. on Lasers and
Electro-Optics (CLEO), vol. 1, pp. 273 (2005).


Fig. 2. (a) Tuning spectra of the first fabricated higher-order, tunable Si microring-resonator add-drop filter show: high-quality
responses (see Table), matching design and experiment, efficient thermal tuning, and (b) two 16nm-FSR spectral bands.
Property
Drop loss
Drop-port out-of-band rejection
Single-stage through-port
extinction center (edge)
FSR
3dB bandwidth
Dispersion (drop, inband)
Wavelength tuning range
Tuning: power per ring
Tuning: ring-core temp.

Table 1. Summary of filter characteristics in Fig. 2.
Design
n/a
> 30 dB
Experiment
1 - 1.5 dB
> 32 dB
29 (24) dB 20 dB
2125 GHz
74 GHz
< 30ps/nm <45ps/nm(KK)
2125 GHz
35 mW/THz
105 K/THz
2050 GHz
66 GHz
TBD
28 mW/THz
TBD