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Ultracompact polarization converter with a dual subwavelength trench
built in a silicon-on-insulator waveguide
Velasco, Aitor, V.; Calvo, Maria L.; Cheben, Pavel; Ortega-Monux, Alejandro;
Schmid, Jens H.; Ramos, Carlos Alonso; Fernandez, Inigo Molina; Lapointe,
Jean; Vachon, Martin; Janz, Siegfried; Xu, Dan-Xia
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Ultracompact polarization converter with a dual
subwavelength trench built
in a silicon-on-insulator waveguide
Aitor V. Velasco
2,4
, María L. Calvo
2
, Pavel Cheben
1,5
, Alejandro Ortega-Moñux
3
, Jens H. Schmid
1
,
Carlos Alonso Ramos
3
, Íñigo Molina Fernandez
3
, Jean Lapointe
1
, Martin Vachon
1
,
Siegfried Janz
1
, and Dan-Xia Xu
1
1
Institute for Microstructural Sciences, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada
2
Departamento de Optica, Facultad de Ciencias Fisicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
3
ETSI Telecomunicación, Universidad de Málaga, 29071 Málaga, Spain
4
e-mail: avillafr@pdi.ucm.es
5
e-mail: pavel.cheben@nrc.ca
Received October 24, 2011; revised December 1, 2011; accepted December 5, 2011;
posted December 7, 2011 (Doc. ID 156835); published January 20, 2012
The design and fabrication of an ultracompact silicon-on-insulator polarization converter is reported. The polar-
ization conversion with an extinction ratio of 16 dB is achieved for a conversion length of only 10 μm. Polarization
rotation is achieved by inducing a vertical asymmetry by forming in the waveguide core two subwavelength
trenches of different depths. By taking advantage of the calibrated reactive ion etch lag, the two depths are imple-
mented using a single mask and etching process. The measured converter loss is −0.7 dB and the 3 dB bandwidth is
26 nm. © 2012 Optical Society of America
OCIS codes: 130.3120, 130.5440, 230.7380.
Recent developments in integrated silicon photonic
devices have demonstrated the remarkable capabi-
lities of the silicon-on-insulator (SOI) platform to imple-
ment a wide range of app lications, such as optical
interconnects [
1], nonlinear photonics [2], biological sen-
sors [
3], and microspectrometers [4]. However, while the
high refractive index contrast of the SOI platform is ad-
vantageous for high-density photonics integration, it
comes with a drawback of largely disparate propagation
characteristics for the TE- and TM-like modes. As a
result, most of the photonic devices in SOI have been
designed for a single mode and a single polarization,
typically TE. To mimic polarization independent opera-
tion, polarization diversity schemes are typically used
with the polarization splitters and rotators as the key
components.
Recently, several SOI polarization rotators (also
known as mode converters) have been proposed, includ-
ing desig ns based on triangular waveguides [
5], asym-
metric waveguides with two etch depths [
6], coupling
via an intermediate multimode waveguide [
7], wave-
guides with vertical and sloped sidewalls [
8], and a com-
bination of horizontal and vertical waveguides [
9].
However, fabrication of these devices is often complex,
with multiple patterning and etching steps involved.
Some single-etch devices have also been reported, such
as a polarization rotator based on an adiabatic taper and
an asymmetrical directional coupler [
10] (conversion
length 100 μm), and a mode converter based on the
cross-polarization coupling effect [
11] (conversion length
44 μm). Additionally, mode converters with single [
12]or
multiple [
13] trenches etched to create an asymmetric
waveguide have been implemented in gallium arsenide
(GaAs) waveguides, reaching a 96% mode conversion
within a device of 150 μm. Some numerical simulations
have also been performed in an attempt to extend the
single-trench design to silicon waveguides [14].
In this Letter, we report the design and fabrication of
an ultracompact dual-trench polarization rotator in SOI,
with a conversion length as short as 10 μm, for a wave-
length of 1.5 μm. The converter schematic is shown in
Fig.
1. The converter exploits the asymmetry induced
by two adjacent subwavelength trenches, resulting in
two orthogonal hyb rid modes (Fig.
1, inset) with optical
axes rotated 45º with respect to the x and y axes, that is,
hybrid modes consisting of 50% TE and 50% TM polariza-
tion. This geometry allows both hybrid modes to be ex-
cited with equal efficiency by a TE (or TM) polarize d
input. Th e two hybrid modes propagate with different
propagation constants along the device, resulting in a
90° polarization rotation at each half-beat length L
1∕2
:
L
1∕2
π
Δβ
; (1)
SiO
Si
Cladding
2
(b)
(a)
Fig. 1. (Color online) Schematic structure of the proposed
polarization converter. (Inset, color online) Simulated contour
plot of the (a) X and (b) Y components of the field of the
first hybrid mode for a dual-trench structure with trench
widths 60 nm and 85 nm and trench depths 210 nm and
235 nm, respectively.
February 1, 2012 / Vol. 37, No. 3 / OPTICS LETTERS 365
0146-9592/12/030365-03$15.00/0 © 2012 Optical Society of America
where Δβ β
1
− β
2
is the difference between the propa-
gation constants of the two hybrid modes, β
1
and β
2
, re-
spectively. Additionally, in orde r to reach conversion
efficiency values close to 100%, the power distributions
of the two hybrid modes should efficiently overlap, and
the higher order mode conversion needs to be minimized.
Indeed, the operation mechanism of the converter en-
sures reciprocity between TE-TM and TM-TE mode con-
version. Th e dual-trench structure is chosen, as opposed
to a single-trench design, in order to optimize mode con-
version and dramatically reduce device length.
To design the polarization rotator, we define a dual-
trench subwavelength structure in the waveguide
with variable depths and widths, and we perform two-
dimensional (2D) mode computation with a commercial
software (FIMMWAVE, Photon Design, UK). The full
width of the Si-wire waveguide was chosen as 450 nm,
for single mode operation near 1.5 μm wavelength. The
Figure
1 inset shows the calculated hybrid mode profiles
for this geometry. 2 μm-long linear inverse taper mode
adaptation sections were included at both ends of the po-
larization rotator to ensure a smooth transition between
the wire waveguide and the dual-trench waveguide, and
to generate equal excitation of the two hybrid modes.
Insofar as the width and depth of the two trenches are
considered independent design parameters, modes with
a 45° offset can be obtained for many combinations of
these parameters (Fig.
2(a)). For a given wavelength,
greater trench widths require shallower trench depths
and result in shorter conversion lengths (Fig.
2(b)). Also,
a greater disparity in the depths of the two trenches re-
sults in greater conversion lengths (Fig.
2(c)). Simula-
tions predict that if the width and depth of one of the
trenches are increased, the size of the other trench
has to be reduced to achieve maximum conversion effi-
ciency, and conversion length is altered. The problem to
minimize the device size and/or to maximize the conver-
sion efficiency can be solved numerically. However, only
a restricted set of solutions is suitable for device fabrica-
tion. In order to fabricate two narrow trenches of differ-
ent depths using a single-etch step, we take advantage of
the reactive ion etch (RIE) lag effect, i.e., the etch depth
dependence on the trench width for small feature sizes.
For our fabrication process, a reduced etched depth is
observed for a feature size smaller than ∼140 nm. We
calibrated the RIE lag effect using scanning electron
microscopy (SEM) measurements on a set of reference
trenches of varying width, obtaining the calibration
curves shown in inset of Fig. 3. By applying the calibra-
tion curve to the set of theoretical solutions for trench
widths and depths yielding hybrid modes with 45° offset,
we found the optimized values for the two trench widths
of 60 nm and 85 nm and trench depths of 210 nm and
230 nm, respectively. This structure yields a final device
length of 10.3 μm. Parameter combinations that result in
shorter conversion lengths cannot be fabricated, as they
do not match the RIE lag calibration curve, and resulting
trench depths would be too shallow.
The operation wavelength can be increased by increas-
ing the trench width and depth and/or the overall device
length. According to our dimension tolerance study, a
13 nm shift of the design central wavelength is predicted
for fabrication errors of 9 nm in the depths of the
trenches or 5 nm in the widths of the trenches. Our
calculations show that the design can be optimized for
1550 nm wavelength by enlarging the outer trench to a
width of 90 nm and a depth of 240 nm. For that wave-
length and topology, conversion length is increased to
12.74 μm.
Samples were fabricated using SOI substrates with a
0.26 μm thick silicon and a 2 μm thick buried oxide layer.
Waveguides with the polarization rotator structure were
defined in a single patterning step by electron beam litho-
graphy with high contrast hydrogen silsesquioxane
(HSQ) resist. Inductively coupled plasma reactive ion
185 190 195 200 205 210 215
9
11
13
Trench 1 depth (nm)
Half-beat length (
µ
m)
185 190 195 200 205 210 215
220
240
260
Trench 1 depth (nm)
Trench 2 depth (nm)
(a)
(b)
40 50 60 70
10
12
14
Trench 1 width (nm)
Half-beat length (
µ
m)
(c)
W = 65 nm, W = 90 nm
W = 60 nm, W = 85 nm
W = 55 nm, W = 80 nm
1
1
1
2
2
2
W = 65 nm, W = 90 nm
W = 60 nm, W = 85 nm
W = 55 nm, W = 80 nm
1
1
1
2
2
2
D = 210 nm, D = 230 nm
D = 200 nm, D = 240 nm
D = 190 nm, D = 260 nm
1
1
1
2
2
2
Fig. 2. (a) Combinations of trench depths (D
1
, D
2
) that yield
hybrid modes with 45° offset, for different combinations of
trench widths (W
1
, W
2
). (b) L
1∕2
versus depth of the first trench
and (c) L
1∕2
versus width of the first trench, for parameter com-
binations yielding hybrid modes with 45° offset.
1 µm
0 100 200 300
0
100
200
300
Ga
p
width (nm)
Etch depth (nm)
trench 1
trench 2
Fig. 3. SEM of the fabricated dual-trench polarization conver-
ter. (Inset) Etch depth versus gap width due to RIE lag effect.
366 OPTICS LETTERS / Vol. 37, No. 3 / February 1, 2012
etching (ICP-RIE) was used to transfer the resist pattern
into the silicon layer. Figure
3 shows a SE M image of the
fabricated device.
The polarization rotator was characterized using a tun-
able laser over a wavelength range of 1460– 1580 nm, with
polarization control optics. The light was coupled both at
the input and the output of the chip using lens fibers fa-
cing the chip facets terminated at both sides with subwa-
velength grating mode converters [
15]. The output signal
was separated into the TE and TM components with a
polarizing beam splitter, and both polarization compo-
nents were measured independently.
The measured extinction ratio (Eq. (
2)) is shown in
Fig. 4.
ER
TE-TM
10 · log
P
TM
P
TE
; (2)
where P
TM
and P
TE
are the output powers of the TM and
TE polarized fundamental modes. Measured extinction
ratio was corrected by calibrating polarization dependent
loss in the 60 μm-long strip waveguide between the de-
vice and the chip facet (0.4 dB∕cm excess loss for TE
polarization). Furthermore, the polarization dependence
of the output coupling efficiency between the subwave-
length grating mode converter and the lensed fiber of the
light collecting system was compensated (0.3 dB excess
loss for TM polarization). Polarization conversion effects
in the subwavelength grating mode converter were
calibrated by performing polarization-resolved measure-
ments of a bare waveguide with the mode transformers,
showing that polarization was maintained with an extinc-
tion ratio exceeding 35 dB. Polarization conversion ef-
fects in the adaptation stages were characterized by
measuring the polarization rotation angle for a set of test
devices with different lengths, showing a rotation in the
adaptation stage of 2º, which was corrected by a 0.22 μm
reduction in the length of the rotator. Trench dep ths mea-
surements of the adaptation section, determined by RIE
lag effect, confirmed shallow trenches during most of the
adaptation stage.
The peak extinction ratio for TE-TM conversion is
16 dB (97.5% conversion efficiency, defined as η
TE-TM
P
TM
∕P
TM
P
TE
× 100%) at the nominal wavelength of
1500 nm. A peak extinction ratio of 14 dB is observed
for TM-TE conversion, showing a slight drop due to po-
larization dependent losses in the device and to fabrica-
tion imperfections. The measured extinction ratio 3 dB
bandwidth is 26 nm, while a conversion efficiency over
90% is reached within a bandwidth of 47 nm. Polarization
converter loss was estimated by subtracting the insertion
loss of a reference waveguide without a polarization con-
verter, to compensate for the fiber-chip coupling losses
and the loss of the Si-wire interconnecting waveguide.
The meas ured polarization converter excess loss is
−0.7 dB , with an adaptation stage excess loss of −0.4 dB.
In conclusion, we have demonstrated the design and
fabrication of an ultracompact and efficient polarization
rotator using a dual subwavelength trench in an Si-wire
waveguide. The device is implemented in a single-etch
step standard fabrication process. Extinction ratios up
to 16 dB were reached for an ultrashort conversion
length of 10 μm, opening promising prospects for device
applications for efficient polarization management in
integrated optoelectronic circuits.
Financial support from the Spanish Ministry of Science
and Innovation (MICINN) is acknowle dged under grants
TEC2008-04105 and TEC2009-10152.
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6
10
14
18
Extinction ratio (dB)
1460 1480 1500 1520 1540 1560 1580
Wavelength (nm)
Fig. 4. Measured ER versus wavelength of the fabricated
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February 1, 2012 / Vol. 37, No. 3 / OPTICS LETTERS 367
