Mode hybridization and conversion in silicon-on-insulator nanowires with angled sidewalls

Abstract

The mode property and light propagation in a tapered silicon-on-insulator (SOI) nanowire with angled sidewalls is analyzed. Mode hybridization is observed and mode conversion between the TM fundamental mode and higher-order TE modes happens when light propagates in a waveguide taper which is used very often in the design of photonic integrated devices. This mode conversion ratio is possible to be very high (even close to 100%) when the taper is long enough to be adiabatic, which might be useful for some applications of multimode photonics. When the mode conversion is undesired to avoid any excess loss as well as crosstalk for photonic integrated circuits, one can depress the mode conversion by compensating the vertical asymmetry in the way of reducing the sidewall angle or introducing an optimal refractive index for the upper-cladding. It is also possible to eliminate the undesired mode conversion almost and improve the desired mode conversion greatly by introducing an abrupt junction connecting two sections with different widths to jump over the mode hybridization region.

Mode hybridization analysis in thin film lithium niobate strip multimode waveguides

“…(1) for hybrids between TM 0 and TE 1 are 4.6843 × 10 4 /m and 4.7768 × 10 4 /m in Y-propagating and Z-propagating LN respectively. Modal hybridization becomes stronger with the introduction of angled sidewalls 6 , and the coupling factor increased to 6.7623 × 10 4 /m and 7.6152 × 10 4 /m respectively for Y-propagating and Z-propagating LN. Also, the number of hybrid points rose to 264 and 259 in Y-and Z-propagating LN respectively.…”

“…In this section, mode converters using tapers that utilize mode hybridization phenomena are designed on X-cut LNOI and analyzed numerically. The end widths of the taper are chosen such that w 1 < w H < w 2 , where w H is the waveguide width for a specific mode hybridization region, and w 1 , w 2 are taper end widths 6 .…”

“…In X-cut Z-propagating LN, with Z being the optic axis of the crystal, both qTE and qTM modes see an ordinary index (n o ), which is similar to isotropic material. Modal hybridness is quantized by a parameter called TE polarization fraction (γ TE ), which indicates the fraction of power in the horizontal component of a mode 6 , as in Eq. ( 2 ), when is close to 1, it represents a qTE mode, while a value close to 0 represents a qTM mode.…”

“…This phenomenon of mode hybridization happens when two orthogonally polarized modes with similar effective indices couple to each other 4 . Mode hybridization regions are used in designing tapers for TM-TE higher order mode conversions [5][6][7][8][9] in polarization splitter-rotators. These components are essential for the implementation of polarization diversity schemes, which circumvent the problem of polarization-dependent losses and dispersion in high-contrast waveguides 10 .…”

Design and performance analysis of integrated focusing grating couplers for the transverse-magnetic TM00 mode in a photonic BiCMOS technology

“…On the other hand, grating couplers for TM 00 are reported seldom. In linear GCs [12], the TM 00 mode passes through a long taper, where a mode hybridization takes place [13]. To avoid this effect, focusing GCs (FGCs) need to be designed instead, which also have the obvious advantage of having a reduced on-chip footprint.…”