Design and analysis of polarization independent MMI based power splitter for PICs

Abstract

An ultracompact and low loss polarization-independent 1 ​× ​2 power splitter based on multimode interference (MMI) is designed and analyzed using Eigenmode expansion (EME) and variational FDTD method on silicon on insulator. The numerical tool shows that the excess loss of the power splitter is 0.04 ​dB for TE and 0.06 ​dB for TM polarization at wavelength 1550 ​nm while the polarization differential loss (PDL) is ​< ​0.07 ​dB over wavelength 1500 ​nm–1600 ​nm. Moreover, the optimized MMI structure has a fabrication tolerance of width ±150 ​nm and a length of ±800 ​nm. In addition, polarization-independent arbitrary power splitter (APS) is designed using cascaded MMI and tapered waveguide as a phase shifter having excess loss ​< ​0.16 ​dB for TE and <0.21 ​dB for TM mode at wavelength 1550 ​nm. The proposed devices have broad applications in optical interconnect and switches for PICs.

Introduction

In recent years, silicon on insulator (SOI) technology is a promising platform for the development of ultra-compact and high-performance optical components for photonic integrated circuits (PICs). It has drawn an impressive property of high-density compatibility and integrability with complementary metal-oxide-semiconductor (CMOS) [1]. In PICs, silicon-based optical splitters are fundamental components for the optical splitting and routing application, which are widely used in optical switches [2,3], Mach–Zehnder Interferometer (MZI) modulators [4] and optical sensors [5]. However, silicon photonics devices have strong polarization dependency due to the high refractive index contrast of the SOI platform. Thus, it is highly desirable to design optical devices with ultracompact size, low loss, and polarization-insensitive properties simultaneously. Previously, various approaches have been reported for the design of polarization independent optical splitter using Y-branches [6], directional couplers [7], adiabatic coupler [8], MMI couplers [9,10]. In these structures, there have been different techniques used to minimize polarization dependency, such as silicon-based strip to slot power splitter [11], double strip silicon nitride [12], slot waveguide [13], grating structures [14], Subwavelength gratings [15]. However, we have minimized the polarization sensitivity of the device by increasing the thickness of the silicon layer to support higher order TE and TM modes and subsequently optimizing the width and length of MMI. The proposed MMI based device has a compact size, low polarization differential loss, and low excess loss than Y-branch structure, adiabatic coupler, and MMI based sandwiched slot waveguide. Moreover, it can be easily fabricated with various CMOS fabrication foundries, and also it has higher fabrication tolerances than directional coupler and grating structures.

In this paper, a polarization independent power splitter has been designed and analyzed using a full-vectorial 3D Eigenmode expansion (EME) and variational Finite Difference Time Domain (varFDTD) [16]. The strong polarization sensitivity in the device is compensated by optimizing the thickness of the silicon core layer. The silicon thickness plays a crucial role in designing a photonic device, and it is chosen according to the application. The silicon core thickness of 340 ​nm has been used to realize the polarization beam splitter for compact photonic integrated circuits [17]. The silicon thickness of 220 ​nm is not chosen since it does not support higher order TE and TM modes. Now, various silicon fabrication foundries are fabricating the device of different silicon thickness [18]. We have chosen an optimal core thickness of 340 ​nm for the specific width of the MMI region to reduce the polarization dependency. The EME method is used since it considered the vectorial properties of light for the high-index contrast (SOI) system. The Variational FDTD is used to achieve accuracy comparable to 3D-FDTD with computational times nearly equal to 2D-FDTD. The simulations are performed using EME numerical tools and also verified by the varFDTD method to ensure accuracy. Firstly, we have designed a polarization-insensitive 1 ​× ​2 power splitter having an ultracompact multimode region (2.6 ​× ​6.6 ​μm²) and low polarization differential loss and low excess loss for TE and TM polarization. For arbitrary power splitter (APS), various structures have been reported using an asymmetrical Y-branch [19], asymmetrical MZI [20], and MMI [21]. These structures are suffered from polarization and wavelength sensitivity, which causes polarization control and narrow bandwidth. For polarization-insensitive arbitrary power splitter, we have used 1 ​× ​2 symmetric interference and 2 ​× ​2 general interference principle of MMI coupler and tapered waveguide as a phase shifter. Our proposed MMI based polarization independent optical splitter has a compact footprint, low excess loss and broad bandwidth.