April 10, 2009 / Vol. 7, No. 4 / CHINESE OPTICS LETTERS 291
Tailoring the spectral response of add/drop single and
multiple resonators in silicon-on-insulator
Invited Paper
B. Timotijevic
1
, G. Mashanovich
1
, A. Michaeli
2
, O. Cohen
2
, V. M. N. Passaro
3
,
J. Crnjanski
4
, and G. T. Reed
1∗
1
Advanced Technology Institute, University of Surrey, Guildford, GU2 7XH, UK
(B. Timotijevic is now with CSEM SA, Rue Jaquet-Droz 1, CH-2002 Neuchatel, Switzerland)
2
Intel Corporation, Kyriat Gat 82109, Jerusalem, Israel
3
Politecnico di Bari, via E. Orabona 4, 70125 Bari, Italy
4
Faculty of Electrical Engineering, University of Belgrade, Bulevar kralja
Aleksandra 73b, 11120 Belgrade, Serbia
∗
E-mail: g.reed@surrey.ac.uk
Received February 13, 2009
Channel dropping waveguide filters based on single and multiple resonators in silicon-on-insulator (SOI)
technology are of great interest due to their compactness and high wavelength selectivity, which is a desir-
able feature for photonic modulators, detectors, and other optically integrated components in telecommu-
nication systems, in particular for wavelength division multiplexing (WDM) systems. Particular advantage
of these filters is that they are capable of producing relatively large free spectral range (FSR) as well as
narrow 3-dB bandwidth of the filter resonances. Herein we report experimental results and discuss the pos-
sibility of designing mono-mode and (nearly) polarization independent SOI ring and racetrack resonators
with the FSR in excess of 30 nm.
OCIS codes: 130.0130, 250.0250, 230.7370, 260.3060, 260.5430.
doi: 10.3788/COL20090704.0291.
Miniaturization of photonic devices has been signif-
icantly intensified in the last decade in an attempt
to improve a footprint and performance of integrated
optical components. Silicon-on-insulator (SOI) has
emerged as a promising material choice for various
integrated optoelectronic devices
[1]
. It is attractive for
complex optical systems as the cost can be significantly
reduced due to the compatibility with complementary
metal oxide semiconductor (CMOS) technology
[2]
. It
also has a high refractive index contrast between the
core and the cladding, which is an important property
for good confinement of light and efficient guiding and
coupling in sub-micron waveguides. However, for those
devices that are intended to be part of broadband optical
networks, for example multiplexers and de-multiplexers,
it is desirable to demonstrate a high selectivity and
a tunable response. Thus, it is necessary to provide
wavelength selective elements with the ability to filter
input data streams producing a large free spectral range
(FSR), a small full-width at half-maximum (FWHM),
and a high quality factor (Q), and all conditions set by
communication standards. Owing to the generic and
adaptable operation, ring-resonator-types of filters in
SOI are often considered as candidates to meet these
demands.
In silicon photonics, there is a particular focus on
two waveguide architectures upon which the devices
are typically built. Rib waveguides have proven to be
particularly useful regarding polarization properties
[3]
.
Single-mode and polarization independent (PI) res-
onators on rib waveguides have already been experi-
mentally demonstrated
[4]
, but their FSR is usually small
for applications in optical networks. On the other hand,
strip waveguides, or photonic wires, allow small bend
radii
[5,6]
, which in turn results in the improved FSR.
They have to be rather small in cross-section to prevent
higher-order modes (HOMs) from propagation
[7]
, and
they exhibit polarization dependence and loss issues.
In majority of applications, single-mode devices are re-
quired in order to minimize dispersion caused by multiple
spatial modes which, in turn, allows for more information
to be transmitted per unit time giving a higher band-
width. It is also desirable that TE and TM modes result
in similar transfer functions, i.e., a filter should perform
as a PI device as, otherwise, two separate devices, each
optimized for operation for one particular polarization,
need to be provided
[8]
.
In this letter, the structures are composed of Si-
waveguide and SiO
2
top and b ottom claddings. Further-
more, the waveguide height (WGH) is a fixed parame-
ter, dictated in advance by established wafer processing
procedures. The rib WGH is set to 1.35 µm and the
strip waveguides WGH = 0.29 or 0.34 µm, while the
waveguide width (WGW) and the etch depth (ED) are
variables used to target the best performance.
Single-mode rib waveguides with relatively large cross
sections have been studied extensively by a number of
researchers to find single-mode behavior at the same
time as low propagation loss
[9,10]
. They were recog-
nized as promising candidates due to the compatibility
with single-mode fiber dimensions and the possibility to
control the polarization. The single-mode behavior and
PI propagation depend both on geometrical parameters
of the waveguide (WGH, WGW, ED, sidewall angle)
1671-7694/2009/040291-05
c
° 2009 Chinese Optics Letters
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