Polarization splitter using horizontal slot
waveguide
Huijuan Zhang,1,* Ying Huang,1 Suchandrima Das,2,3 Chao Li,1 Mingbin Yu,1 Patrick
Guo-Qiang Lo,1 Minghui Hong,2 and John Thong2
1
2
Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 11 Science Park Road,
Singapore Science Park II, 117685, Singapore
Department of Electrical and Computing Engineering, National University of Singapore, 21 Lower Kent Ridge
Road, 117583, Singapore
3
Advanced Micro Devices Singapore, 469032, Singapore
*
zhangh@ime.a-star.edu.sg
Abstract: In this paper a compact and efficient polarization splitter using
horizontal slotted waveguides is presented. The splitter is designed by
finite-difference-time-domain simulation and realized experimentally. The
splitter is built by using a direction coupler consisting of two horizontal
slotted waveguides and achieves a high extinction ratio of 14.1 and 16.8 dB
for cross and through ports. The optimal coupling length is found to be 15
µm. The device exhibits a good response of extinction ratio across C + L
broadband. The splitter obtained is readily used for a polarization diversity
circuit, particularly for platforms with horizontal slot waveguides.
©2012 Optical Society of America
OCIS codes: (230.3120) Integrated optics devices; (230.5440) Polarization-selective devices;
(230.7370) Waveguides.
References and links
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
B. Jalali, V. Raghunathan, D. Dimitropoulos, and O. Boyraz, “Raman-based silicon photonics,” IEEE J. Sel.
Top. Quantum Electron. 12(3), 412–421 (2006).
S. Nakamura, C. Tao, M. Ishizaka, M. Tokushima, Y. Urino, M. Sakauchi, I. Nishioka, and K. Fukuchi, “Ultrasmall one-chip color-less multiplexer/ demultiplexer using silicon photonic circuit,” Proceedings of ECOC, 175–
176(2008).
H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S. Itabashi, “Silicon photonic circuit
with polarization diversity,” Opt. Express 16(7), 4872–4880 (2008).
T. Barwicz, M. R. Watts, M. A. Popovic, P. T. Rakich, L. Socci, F. X. Kartner, E. P. Ippen, and H. I. Smith,
“Polarization-transparent microphotonic devices in the strong confinement limit,” Nat. Photonics 1(1), 57–60
(2007).
M. R. Watts and H. A. Haus, “Integrated mode-evolution-based polarization rotators,” Opt. Lett. 30(2), 138–140
(2005).
J. Zhang, H. Zhang, S. Chen, M. Yu, G. Q. Lo, and D. L. Kwong, “A tunable polarization diversity silicon
photonics filter,” Opt. Express 19(14), 13063–13072 (2011).
C. Li, J. H. Song, J. Zhang, H. Zhang, S. Chen, M. Yu, and G. Q. Lo, “Silicon polarization independent
microring resonator-based optical tunable filter circuit with fiber assembly,” Opt. Express 19(16), 15429–15437
(2011).
J. Zhang, H. Zhang, S. Chen, M. Yu, G. Q. Lo, and D. L. Kwong, “A polarization diversity circuit for silicon
photonics,” Optical Fiber Communication Conference, JThA019 (2011).
J. Zhang, M. Yu, G.-Q. Lo, and D.-L. Kwong, “Silicon-waveguide-based mode evolution polarization rotator,”
IEEE J. Sel. Top. Quantum Electron. 16(1), 53–60 (2010).
J. Feng and Z. Zhou, “Polarization beam splitter using a binary blazed grating coupler,” Opt. Lett. 32(12), 1662–
1664 (2007).
Y. Yue, L. Zhang, J.-Y. Yang, R. G. Beausoleil, and A. E. Willner, “Silicon-on-insulator polarization splitter
using two horizontally slotted waveguides,” Opt. Lett. 35(9), 1364–1366 (2010).
I. Kiyat, A. Aydinli, and N. Dagli, “A compact silicon-on-insulator polarization splitter,” IEEE Photon. Technol.
Lett. 17(1), 100–102 (2005).
H. Fukuda, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Shinojima, and S. Itabashi, “Ultrasmall polarization
splitter based on silicon wire waveguides,” Opt. Express 14(25), 12401–12408 (2006).
W. N. Ye, D.-X. Xu, S. Janz, P. Waldron, P. Cheben, and N. G. Tarr, “Passive broadband silicon-on-insulator
polarization splitter,” Opt. Lett. 32(11), 1492–1494 (2007).
M. R. Watts, H. A. Haus, and E. P. Ippen, “Integrated mode-evolution-based polarization splitter,” Opt. Lett.
30(9), 967–969 (2005).
#178136 - $15.00 USD
(C) 2013 OSA
Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013
11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3363
16. L. Liu, Y. Ding, K. Yvind, and J. M. Hvam, “Silicon-on-insulator polarization splitting and rotating device for
polarization diversity circuits,” Opt. Express 19(13), 12646–12651 (2011).
17. V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding and confining light in void nanostructure,” Opt.
Lett. 29(11), 1209–1211 (2004).
18. Q. Xu, V. R. Almeida, R. R. Panepucci, and M. Lipson, “Experimental demonstration of guiding and confining
light in nanometer-size low-refractive-index material,” Opt. Lett. 29(14), 1626–1628 (2004).
19. H. Zhang, J. Zhang, S. Chen, J. Song, J. S. Kee, M. Yu, and G.-Q. Lo, “CMOS-compatible fabrication of silicon
based sub-100nm slot waveguide with efficient channel-slot coupler,” IEEE Photon. Technol. Lett. 24(1), 10–12
(2012).
20. A. Spott, T. Baehr-Jones, R. Ding, Y. Liu, R. Bojko, T. O’Malley, A. Pomerene, C. Hill, W. Reinhardt, and M.
Hochberg, “Photolithographically fabricated low-loss asymmetric silicon slot waveguides,” Opt. Express 19(11),
10950–10958 (2011).
21. R. Ding, T. Baehr-Jones, W.-J. Kim, B. Boyko, R. Bojko, A. Spott, A. Pomerene, C. Hill, W. Reinhardt, and M.
Hochberg, “Low-loss asymmetric strip-loaded slot waveguides in silicon-on-insulator,” Appl. Phys. Lett. 98(23),
233303 (2011).
22. X. Tu, X. Xu, S. Chen, J. Yu, and Q. Wang, “Simulation demonstration and experimental fabrication of a
multiple-slot waveguide,” IEEE Photon. Technol. Lett. 20(5), 333–335 (2008).
23. S.-H. Yang, M. L. Cooper, P. R. Bandaru, and S. Mookherjea, “Giant birefringence in multi-slotted silicon
nanophotonic waveguides,” Opt. Express 16(11), 8306–8316 (2008).
24. C. Koos, P. Vorreau, T. Vallaitis, P. Dumon, W. Bogaerts, R. Baets, B. Esembeson, I. Biaggio, T. Michinobu, F.
Diederich, W. Freude, and J. Leuthold, “All-optical high-speed signal processing with silicon–organic hybrid slot
waveguides,” Nat. Photonics 3(4), 216–219 (2009).
25. C. A. Barrios and M. Lipson, “Electrically driven silicon resonant light emitting device based on slotwaveguide,” Opt. Express 13(25), 10092–10101 (2005).
26. A. Martínez, J. Blasco, P. Sanchis, J. V. Galán, J. García-Rupérez, E. Jordana, P. Gautier, Y. Lebour, S.
Hernández, R. Spano, R. Guider, N. Daldosso, B. Garrido, J. M. Fedeli, L. Pavesi, and J. Martí, “Ultrafast alloptical switching in a silicon-nanocrystal-based silicon slot waveguide at Telecom Wavelengths,” Nano Lett.
10(4), 1506–1511 (2010).
27. S. Lee, S. C. Eom, J. S. Chang, C. Huh, G. Y. Sung, and J. H. Shin, “Label-free optical biosensing using a
horizontal air-slot SiNx microdisk resonator,” Opt. Express 18(20), 20638–20644 (2010).
28. S. Lin, J. Hu, and K. B. Crozier, “Ultracompact, broadband slot waveguide polarization splitter,” Appl. Phys.
Lett. 98(15), 151101 (2011).
29. RSoft, http://www.rsoftdesign.com/
30. V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28(15),
1302–1304 (2003).
31. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Low-loss amorphous silicon wire waveguide for integrated photonics:
effect of fabrication process and the thermal stability,” Opt. Express 18(24), 25283–25291 (2010).
32. H. Zhang, S. Das, J. Zhang, Y. Huang, C. Li, S. Chen, H. Zhou, M. Yu, P. G.-Q. Lo, and J. T. L. Thong,
“Efficient and broadband polarization rotator using horizontal slot waveguide for silicon photonics,” Appl. Phys.
Lett. 101(2), 021105 (2012).
1. Introduction
Silicon-on-insulator (SOI) has attracted tremendous research efforts recently as a promising
platform for photonic circuits, mainly because of the high refractive-index contrast of the
waveguide structure enabling compact foot-prints [1,2]. Its compatibility with CMOS
fabrication technology makes the mass production of devices cost–effective. However, the
high-index contrast in SOI waveguides also causes polarization-mode dispersion,
polarization-dependent loss, and polarization-dependent wavelength characteristics. This
renders SOI an inconvenient platform for integrated photonic circuits, such as optical-fiber
networks, where polarization independence is required. Polarization control and manipulation
are crucial in the design and the operation of optical devices. In order to circumvent this
problem, a polarization diversity scheme needs to be implemented [3–8]. A polarization
splitter is a key element in the polarization diversity circuit, which aims to separate the two
orthogonally polarized transverse-electric (TE) and transverse-magnetic (TM) components for
them to be processed individually in the following circuit. Different configurations of
polarization splitters such as the grating coupler [9], directional coupler [10–13], and arrayed
waveguide grating (AWG) [14], have been reported. A MIT group has demonstrated an
integrated polarization splitter and rotator with an asymmetric core cross section for silicon
nitride waveguides [4,15]. However, its double core process faces fabrication challenges due
to the three-dimensional structure. L. Liu et al. have also reported an integrated splitter and
rotator using directional coupler consisting of channel waveguides, but it has stringent
requirements on waveguide dimensions due to its underlying phase-matching principle [16].
#178136 - $15.00 USD
(C) 2013 OSA
Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013
11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3364
H. Fukuda et al. [13] have demonstrated an ultra-small polarization splitter for SOI platform.
The device has a size of 7 × 16 μm2 and can achieve 15 dB extinction ratio.
In a slotted waveguide, a layer of low refractive index material is introduced between two
layers of high refractive index materials so that light is tightly confined in the slot region [17].
The slotted waveguides can be divided into two categories: vertically-oriented slotted
waveguides for TE mode strong confinement, and horizontally-oriented ones for the TM case
[18–23]. The strong confinement of light in the slot region thereby potentially provides
enhanced nonlinearities, modulation, light emission, switching or possible sensor functions
[24–27]. It is then necessary and desirable to have polarization functional devices including
polarization splitter and rotator for platforms using slot waveguides due to their strong
polarization sensitivity. In addition, the introduction of the silicon oxide (SiO2) slot enhances
the polarization sensitivity of the waveguide. S. Lin et al. have demonstrated a polarization
splitter by using a directional coupler consisting of a vertical slot waveguide and a channel
waveguide [28]. A high extinction ratio of 20 dB for through port has been achieved.
However, an additional sharp bend is required to improve the extinction ratio at cross port.
Simulation work has been reported to realize high extinction ratio by the horizontal slotted
waveguide [11].
Here we present an efficient and broadband polarization splitter based on a directional
coupler consisting of horizontally slotted waveguides. Based on mode coupling theory,
polarization splitters are designed and demonstrated experimentally. Simulation results and
experimental evaluation of the designed components are presented.
2. Principle and simulation
The splitter is designed based on the mode-coupling theory. TE and TM modes experience
different coupling when propagating along a waveguide. By employing proper coupling
length, splitting of polarization is achieved based on this intrinsic polarization dependence.
Introducing a horizontal slot further enhances the polarization dependence of the coupling
[11]. TM mode, which is confined mostly within the slot, experiences a low contrast of
refractive index between the waveguide and the cladding (SiO2). The coupling of this mode to
the adjacent waveguide is thus enhanced compared to that with normal silicon (Si)
waveguides.
The design of the splitter, as shown in Fig. 1(a), involves two identical slotted waveguides
with slot thickness (tslot), Si thickness (tSi) and width (w) that are placed parallel to one another
with a coupling gap (g). The coupling length of the splitter is denoted as L. In an ideal case,
when the power of the TM mode is completely transferred from the input waveguide to the
adjacent waveguide, almost all of the power of the TE mode remains in the input waveguides.
Now keeping Si thickness at 250 nm, a common thickness for an SOI platform, both slot
thickness t and waveguide width w can be varied, which affect the refractive index difference
(∆n = Neff(TE) – Neff(TM)) between TE and TM modes and the power confinement in the slot.
A high ∆n is preferred since it allows the TM mode to couple over to the adjacent waveguide
without affecting the TE mode. Figure 1(b) shows the variation of ∆n with respect to the
width at a slot thickness of 60 nm. It is observed that ∆n increases with waveguide width.
However, increasing the width of the waveguide beyond 500 nm leads to possible nonfundamental modes propagating along the waveguide. Hence, a width of 500 nm is chosen for
our splitter. The choice of slot thickness as 60 nm will be explained below. Figures 1(c) and
1(d) are the mode distribution of TE and TM, respectively.
#178136 - $15.00 USD
(C) 2013 OSA
Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013
11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3365
Fig. 1. (a) Schematic of a polarization splitter using horizontal slot waveguides. (b) Plot of
effective index difference between TE and TM as a function of waveguide width. (c) Mode
profile of TE. (d) Mode profile of TM showing confinement in slot region.
The performance of the polarization splitter was studied using three-dimensional finitedifference-time-domain (FDTD) simulation by RSoft [29]. The structural dimensions were
chosen as follows: tSi = 250 nm, tslot = 60 nm, w = 500 nm, g = 200 nm, and L = 20 μm. Both
bottom and top Si layers are amorphous Si to match the experimental condition. The cladding
material and slot is SiO2. To simulate the polarization effect, both TE and TM modes were
launched at the input plane z = 0 μm and outputs at the ends of the two waveguides were
monitored. Figure 2(a) and 2(b) show the TE and TM waves along the 20 μm polarization
splitter. It is observed that TM quickly couples into the adjacent waveguide in less than 15 μm
transition region. In contrast, when TE mode was launched at the input, the wave mainly
propagates along the input waveguides. In this way, the polarization at the input is separated
into two waveguides. The extinction ratio obtained from simulation is 9.2 dB and 17.2 dB at
through port and cross port, respectively, for a coupling length of 13 μm. The extinction ratio
decreases at through port and increases at cross port as the coupling length increases beyond
13 μm. The better performance at through port compared with that at cross port is attributed to
the efficient coupling of TM to cross port and higher insertion loss of TE when propagating,
as observed in the simulations.
The effect of slot thickness on the splitter performance is also analyzed. The plot of ∆n
with varying slot thickness at 500 nm waveguide width is shown in Fig. 2(c). It is noticed that
∆n increases when the slot thickness increases. The coupling length of the TM mode is
expected to decrease as the slot thickness increases. For a small slot thickness, owing to
tighter confinement of the TM mode, the light wave has to propagate for a longer length
before almost all the TM mode energy is coupled to the adjacent waveguide. Simulation for
different slot thickness also confirms the expected result (Fig. 2(d)). However, the thicker the
slot is, the higher propagation loss. Hence a preferred thickness of 60 nm is chosen for the
splitter to take into account of mode confinement, optimal splitting length as well as ease of
fabrication.
#178136 - $15.00 USD
(C) 2013 OSA
Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013
11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3366
Fig. 2. Simulation results of (a) TE propagation along the splitter and (b) TM propagation
along the splitter; (c) Plot of effective index difference between TE and TM with slot
thickness; (d) Plot of varying coupling lengths at different slot thickness
3. Fabrication
Devices were fabricated on an 8-inch Si wafer with a 2 μm SiO2 layer. The process started
with a multilayer deposition using plasma enhanced chemical vapor deposition (PECVD): 250
nm amorphous Si as the bottom Si layer followed by 60 nm SiO2 as the slot, and finally 250
nm amorphous Si as the top Si layer. The Si layer thickness could be chosen and varied
depending the device application and photonic circuit design. Then waveguide structures were
patterned by optical lithography using a 248 nm Nikon Scanner. The wafer was etched using
reactive ion etching (RIE) down to the buried oxide surface. Figure 3 shows the polarization
splitter fabricated. The inset at the top left corner shows a cross section view of the horizontal
slot waveguide. Finally, 2 μm SiO2 was deposited on the wafer as the top cladding.
Fig. 3. SEM image of the polarization splitter obtained. Inset shows a side view of horizontal
slot waveguide.
4. Experiment and discussion
The device performance was firstly characterized using a tunable laser source at 1550 nm.
Polarization of the light was controlled by a polarization controller and polarization
maintaining (PM) fibers were used throughout the tests. Light was coupled into the
waveguide through an inverted taper structure at a tip size of 180 nm [30]. The light was then
guided into the horizontal slot waveguides. The output light was coupled back into a PM
fiber. At the output fiber end, a second polarizer was connected to measure the power of TE
and TM components separately to examine the polarization splitting performance of the
#178136 - $15.00 USD
(C) 2013 OSA
Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013
11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3367
device (Fig. 4). Input light was first adjusted to TE mode and the output polarizer was also set
to TE-pass condition, and TE power at through port and cross port was recorded. Then light
input for TM condition was measured in a similar manner. The polarization extinction ratio is
defined as:
 TEThrough
ERThroughPort = 10 log 
 TM
Through




 TM Cross 
ERCrossPort = 10 log 

 TECross 
(1)
(2)
Fig. 4. Schematic of experimental setup. Measurements were done for TE and TM input
separately.
The coupling length of the splitters varied from 10 to 20 μm. Figure 5(a) shows the
polarization extinction ratios of the splitters at the cross port and through port, respectively.
We observed polarization splitting effect in all the measurements using the fabricated device
for different coupling lengths. In general, the performance progressively improves as coupling
length increases from 10 μm. The extinction ratio balances and achieves an optimal value
when the coupling length is 15 μm. Optimal splitting of 14.1 and 16.8 dB extinction ratio at
cross port and through port, respectively, is obtained at 15 μm coupling length. This length is
very close to the simulation value of optimal coupling length 13 μm (Fig. 5(b)).
To characterize the broadband response, the polarization splitter at the optimal coupling
length was also tested from 1530 to 1600 nm using an Amplified Spontaneous Emission
(ASE) source and an optical spectrum analyzer (OSA). Figures 5(b) and 5(c) show the
measured polarization components from a polarization splitter with a 15 μm transition length
when launching TM and TE, respectively. Our polarization splitter has an almost flat response
from 1530 to 1600 nm with a small fluctuation. The splitting performance at the through port
is relatively better than that at the cross port, which is attributed to the efficient coupling of
TM mode to the cross port. Test waveguides were included in the device chip to investigate
individual loss contribution. Coupling loss from fiber to waveguide having a nanotaper size of
180 nm is about 2~2.5 dB per facet. The propagation loss of the horizontal slot waveguide
fabricated was experimentally characterized to be 11.3 dB/cm for TE and 4.2 dB/cm for TM.
The relatively larger propagation loss for TE mode of horizontal slot waveguide is attributed
the modes splitting in the top and bottom waveguides. The propagation loss of the channel
waveguide (bottom amorphous Si layer) is obtained as 5.2 dB/cm for TE and 2.4 dB/cm for
TM. This propagation can be further reduced by process control and thermal treatment as
reported by S. Y. Zhu et al. [31]. The horizontal slot part is bridged to the normal channel
waveguide using a taper structure for mode conversion with an insertion loss of ~0.5 dB [32].
In this way, the relatively high propagation loss of the horizontal slot waveguide is minimized
in the circuit.
#178136 - $15.00 USD
(C) 2013 OSA
Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013
11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3368
Fig. 5. (a) Extinction ratio at various coupling lengths when input wavelength is 1550 nm.
ERThroughPort = 10log(TEThrough/TMThrough) and ERCrossPort = 10log(TMCross/TECross) (b) Broadband
response at the through port for a coupling length of 15 μm from 1530 nm to 1600 nm. (c)
Broadband response at the cross port of the same device in (b).
5. Conclusions
A compact polarization splitter with a coupling length of 15 μm is demonstrated in this paper.
The polarization extinction ratio reaches > 16 dB for a single-stage coupler configuration at
1550 nm. The performance is expected to be further improved using a double-stage
configuration. The efficient wavelength range covers the C + L band. The presented
polarization splitter exhibits competitive performance with reported values and is particularly
useful for a horizontal slot waveguide platform. Together with the polarization rotator we
have reported recently [32], a polarization diversity scheme for Si photonics is ready for use.
Acknowledgment
This work was supported by the Science and Engineering Research Council of A*STAR
(Agency for Science, Technology and Research), Singapore. The SERC grant number is
1122804038.
#178136 - $15.00 USD
(C) 2013 OSA
Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013
11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3369