Mode conversion in tapered submicron silicon ridge ... - OSA Publishing

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mode and higher-order TE modes is observed in tapered submicron silicon- on-insulator ridge optical waveguides due to the mode hybridization resulting from ...
Mode conversion in tapered submicron silicon ridge optical waveguides Daoxin Dai,1,2,* Yongbo Tang,2 and John E Bowers2 1

Centre for Optical and Electromagnetic Research, State Key Laboratory for Modern Optical Instrumentation, Zhejiang Provincial Key Laboratory for Sensing Technologies, Zhejiang University, Zijingang Campus, Hangzhou 310058, China 2 Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, USA * [email protected]

Abstract: The mode conversion in tapered submicron silicon ridge optical waveguides is investigated theoretically and experimentally. Two types of optical waveguide tapers are considered in this paper. One is a regular lateral taper for which the waveguide width varies while the etching depth is kept the same. The other is a so-called “bi-level” taper, which includes two layers of lateral tapers. Mode conversion between the TM fundamental mode and higher-order TE modes is observed in tapered submicron siliconon-insulator ridge optical waveguides due to the mode hybridization resulting from the asymmetry of the cross section. Such a mode conversion could have a very high efficiency (close to 100%) when the taper is designed appropriately. This enables some applications e.g. polarizer, polarization splitting/rotation, etc. It is also shown that this kind of mode conversion could be depressed by carefully choosing the taper parameters (like the taper width, the etching depth, etc), which is important for the applications when low-loss propagation for the TM fundamental mode is needed. ©2012 Optical Society of America OCIS codes: (130.0130) Integrated optics; (230.5440) Polarization-selective devices.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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1. Introduction Optical waveguide taper is a fundamental element for photonic integrated circuits (PIC’s). It is often used to change the light spot size in order to have better coupling efficiency between two sections with different cross sections (e.g., a planar optical waveguide and a singlemode or lens fiber) [1–8]. In order to achieve a low-loss taper, one usually makes the taper long enough to be adiabatic so that higher-order modes are not excited [9–12]. This design rule works well usually especially for low index-contrast (∆) optical waveguides (e.g., SiO2-on-Si buried waveguides). However, the situation becomes complicated for small-sized high-∆ optical waveguides, e.g., submicron silicon-on-insulator (SOI) waveguides, which have been used widely for ultra-compact CMOS-compatible PIC’s in the recent years [13–24]. In a high-∆ optical waveguide, the mode hybridization is significant at some special waveguide widths [25–30] and consequently mode conversion may happen in a tapered structure [29,30]. In Ref [29,30], the authors give a discussion on the mode conversion in a tapered SOI strip

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nanowire. When a SOI nanowire has a SiO2 under-cladding and an air upper-cladding, which makes the SOI nanowire asymmetrical in the vertical direction, the mode conversion between the TM fundamental mode and the first-order TE mode is observed when light propagates along a taper structure. Such a mode conversion is not desired usually because it introduces some serious excess loss as well as crosstalk due to the excited higher order modes, e.g., in AWG (arrayed-waveguide grating) demultiplexer [31]. Such undesired mode-conversion could be minimized by using several kinds of modified tapered structures suggested in Ref [29]. A simple and easy way to depress such a mode conversion in a SOI-nanowire taper is to introduce a SiO2 upper-cladding (instead of air) to make the SOI nanowire symmetrical in the vertical direction [30]. On the other hand, such a kind of mode conversion could be very useful. For example, in our previous paper a SOI-nanowire taper was designed to have an almost 100% mode conversion efficiency from the TM fundamental (TM0) mode to the first higher-order TE (TE1) mode so that polarization splitter-rotators could be realized with a very simple design and easy fabrication process [30]. In this paper, we focus on the mode conversion in submicron SOI rib waveguides (other than SOI nanowires), which is also very popular for silicon-based integrated optoelectronics [19–24]. One should note that there is a significant difference between an SOI rib waveguide and a SOI strip nanowire. A SOI strip nanowire could be symmetrical or asymmetrical in the vertical direction by simply choosing an appropriate material for the upper-cladding so that the mode conversion could be eliminated or enhanced accordingly [30]. In contrast, for a SOI rib waveguide, it is still asymmetrical in the vertical direction even when having the same material for the upper-cladding and the under-cladding. Therefore, when it is desired to have a mode conversion between the TM0 mode and the higher-order TE mode for the case of using a SOI rib waveguide, it is not necessary to choosing different materials for the uppercladding and the under-cladding. On the other hand, such an asymmetry also makes that one cannot yet avoid the mode conversion in a taper section due to the mode hybridization by simply choosing the same material for the upper-cladding and the under-cladding. Such a mode conversion will introduce a significant excess insertion loss as well as some channel crosstalk due to the excited higher-order modes (e.g., in AWG demultiplexers [31]). In the following section, we give a detailed analysis for light propagation in SOI rib waveguide tapers and present the mode conversion numerically. The experimental observation for the mode conversion has also been presented. Two types of taper structures are considered here. One is a regular lateral taper for which the waveguide width varies while the etching depth is kept the same. Since the structure and the fabrication are very simple, a regular lateral taper is very popular for modifying the waveguide mode size in the lateral direction. The other is a so-called “bi-level” taper, which includes two layers of lateral tapers and consequently a double-etching process is needed for the fabrication. A bi-level taper is often used to connect two sections with different etching depths, e.g., from a shallowly-etched rib waveguide to a deeply-etched rib waveguide [7–8, 32–33]. For example, bi-level taper are very useful for the case when a singlemode rib waveguide is needed at the input/output ends of a chip while a strong confinement is desired for e.g., sharp bending. Our experimental and theoretical results show that one should be very careful when designing an adiabatic lateral taper or bi-level taper with a small-sized high-∆ optical waveguide, e.g., submicron SOI ridge waveguides considered in this paper. 2. Structure and analysis In this paper, we consider tapered submicron SOI rib waveguides, which has been used very widely for silicon optoelectronics [19–24]. Two types of taper structures are analyzed here. The first one is a regular lateral taper, and the other is the so-called bi-level taper [7–8, 32– 33]. In the present example, the SOI wafer has a 400nm-thick top Si layer and the refractive indices of Si and SiO2 are nSi = 3.455, and nSiO2 = 1.445, respectively. A finite-difference method (FDM) mode-solver (from Fimmwave) is used to calculate the mode field profiles and the effective indices for all eigenmodes.

#166272 - $15.00 USD Received 9 Apr 2012; revised 19 May 2012; accepted 22 May 2012; published 31 May 2012

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A. Regular lateral taper Figure 1(a) and 1(b) show the 3D-view for the regular lateral taper and the cross section for the SOI rib waveguide. In this taper section, the waveguide width varies while the etching depth is kept the same. Such a taper is often used when it is needed to modify the mode size, e.g., at the input/output ends of a silicon photonic integrated chip in order to enhance the coupling efficiency between fibers and the chip. Regarding that the spot size of a commercialized lens fiber is usually around 3µm, in our calculation we give a modal analysis for a SOI rib waveguide whose core width varies from 3µm to 0.5µm in order to characterize the mode conversion in a waveguide taper to match the lens fiber [6].

Fig. 1. (a) The schematic configuration of a regular lateral taper; (b) the cross section for a SOI rib waveguide.

Fig. 2. The calculated effective indices for the eigen modes of SOI rib waveguide with different etching depths. (a) het = 0.4H; (b) het = 0.5H; (c) het = 0.6H. Here the total height of the Si layer is H = 400nm.

Figure 2(a)-2(c) show the effective indices for SOI rib waveguides with different etching depths het as the core width wco increases from 0.5µm to 3µm. Here the etching depth is chosen as het = 0.4H, 0.5H, and 0.6H, respectively. Particularly, for the case of het = 0.4H, one should note that the TM0 mode becomes leaky and is to be cutoff in the range of wco