Polarization Characteristics of a silicon-on-insulator rib waveguide Directional Coupler

The polarization characteristics of a silicon-on-insulator (SOI) rib waveguide directional coupler are studied. The fundamental TE and TM-polarized modes are calculated by a semivectorial beam propagation method (BPM). The phase difference (Φ) between the coupling coefficients of TE and TM modes depends on the rib parameters of the SOI waveguide and also the gap (G) between the waveguides. The variation of the coupling lengths Ls (splitting lengths) with the reduction of rib width W, slab height H, the normalized slab height r, and G is shown. The simulated result shows the remarkable decrease of the value of Ls when H is decreased and also with the lower values of r and G. The negative and positive values of the phase difference Φ between the coupling coefficients of TE and TM modes depend appreciably on G with the fixed values of R = W/H. It shows that under certain condition of rib parameters, Φ becomes zero or very small to obtain a nonbirefringent SOI rib waveguide coupler. The slope of the curves decrease when G increases up to a certain critical value of Go. The results also show that for each value of R there is a value of G = Go, when we get the nonbirefringent SOI rib waveguide coupler with larger fabrication tolerance. The effect of extinction ratios with the variation of these parameters, and their improvements with the decrease of the normalized slab height are discussed. The fabrication tolerance of the splitters is also discussed.

Introduction

A polarization-independent performance is often desirable for waveguide type devices used in photonic integrated circuits (PIC), especially for applications in optical-fiber system or a network, as polarization state of the propagating light in a standard single-mode fiber is random. Again, TE/TM mode splitters are also most important devices in optical fiber communications, data storage, imaging, signal processing as well as in fiber sensors, where polarization plays an important role. To reduce the polarization dependence in PIC device, the methods, such as using polarization splitters [1]–[3] where separation of orthogonal polarization states is a straightforward solution or with a half-wave plate [4] or a undercladding ridge waveguide [5]. These methods are costly as well as complex, but a simple and practical method is to use planar waveguide structure. However, such splitters are required to separate TE and TM polarizations at the output with higher cross-talk. Interference-based directional couplers, asymmetrical Y-branch structures with mode sorting effect, and multimode interference couplers are used for these devices [6], [7]. Many kinds of structure such as buried waveguides, rib waveguides, and strip-loaded waveguides have been developed, but the rib structure has many advantages over the choice of output device characteristics. Highly biréfringent materials such as LiNb03, or III-V compound semiconductors such as GaAs-GaAlAs, InGaAsP, polymers, and silica materials are commonly used for these devices. Using a low priced glass waveguide in ion exchanged , exploiting the stress-induced birefringence, polarization splitting is realized with a symmetric directional coupler where an extinction ratio of 18.2 dB was obtained [1]. However, a silicon-on-insulator (SOI) waveguides having high- with Si slab ) as a core and SiO₂ () as a cladding layer is used in the transparent wavelength range of . Of course, this wafer is used for electronic circuits and thus the same SOI substrate can be a platform to realize compact opto-electronic circuits of mm in total area. The rib-type waveguides in using SOI have been demonstrated with a low propagation loss less than 0.5 dB/cm. Recently, a compact rib structure directional coupler-based polarization splitter is realized by using in SOI waveguides where the reduced waveguide dimension producing high birefringence property is utilized. The extinction ratios of 20.8 dB for TE and 9.7 dB for TM polarizations are obtained with the coupling length of 2450 for 1.4 gaps when the normalized slab height r = 0.6. For shorter size of the device with lower gaps of 0.7 , the extinction ratios were ~ 18 dB for TE and 8.0 dB for TM polarizations [8]. However, the variation of the waveguide birefringence of a single nonbirefringent SOI rib structure with the change of r and its reduction with the normalized rib width are analyzed by using the semivectorial finite difference method [9]. A general relation between the two normalized parameters is shown.