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

1Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 11 Science Park Road, Singapore Science Park II, 117685, Singapore

2Department of Electrical and Computing Engineering, National University of Singapore, 21 Lower Kent Ridge Road, 117583, Singapore

3Advanced 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

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#178136 - $15.00 USD Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013(C) 2013 OSA 11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3363

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“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 Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013(C) 2013 OSA 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 non-fundamental 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 Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013(C) 2013 OSA 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 finite-difference-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 Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013(C) 2013 OSA 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 Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013(C) 2013 OSA 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:

10log ThroughThroughPort

Through

TEER

TM

=

(1)

10log CrossCrossPort

Cross

TMER

TE

=

(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 Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013(C) 2013 OSA 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 Received 16 Oct 2012; revised 18 Nov 2012; accepted 24 Nov 2012; published 4 Feb 2013(C) 2013 OSA 11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3369