Silicon Photonic Device, Optical Polarisation Beam Coupler and Optical Waveguide Coupler

Abstract

A silicon photonic device comprises a silicon core having a core refractive index and a structure formed in the silicon core. The structure comprises a first refractive index variation pattern across the core in a first direction (x) and having a first modulation depth (H 1 ), and a second refractive index variation pattern across the core in a second, orthogonal, direction (y) and having a second modulation depth (H 2 ), less than the first modulation depth. The first refractive index variation pattern overlays the second refractive index variation pattern, forming a three-dimensional structure. The first refractive index variation pattern only supports propagation of light having a TM mode between the first direction and a third direction (z) and the second refractive index variation pattern only supports propagation of light having a TE mode between the second direction and the third direction.

1 . A silicon photonic device comprising:
 a silicon core having a core refractive index; and   a structure formed in the silicon core, the structure comprising:
 a first refractive index variation pattern extending across the core in a first direction, the first refractive index variation pattern having a first modulation depth; and 
 a second refractive index variation pattern extending across the core in a second direction, substantially orthogonal to the first direction, the second refractive index variation pattern having a second modulation depth, less than the first modulation depth, 
 wherein the first refractive index variation pattern overlays the second refractive index variation pattern, thereby forming a three-dimensional grating structure, and 
 wherein the first refractive index variation pattern only supports propagation of light having a transverse magnetic mode between the first direction and a third direction, substantially orthogonal to the first direction and the second direction, and the second refractive index variation pattern only supports propagation of light having a transverse electric mode between the second direction and the third direction. 
   
     
     
         2 . The silicon photonic device of  claim 1 , wherein:
 the structure is a diffractive grating structure and the first refractive index variation pattern is a first periodic refractive index modulation extending across the core in the first direction, the first periodic refractive index modulation having a first grating period and the first modulation depth (H 1 );   the second refractive index variation pattern is a second periodic refractive index modulation extending across the core in the second direction (y), the second periodic refractive index modulation having a second grating period and the second modulation depth (H 2 );   the three-dimensional structure is a three-dimensional grating structure; and   the first grating period and the first modulation depth together cause the device only to support propagation of light having a transverse magnetic mode between the first direction and a third direction (z), substantially orthogonal to the first direction and the second direction, and the second grating period and the second modulation depth together cause the device only to support propagation of light having a transverse electric mode between the second direction and the third direction.   
     
     
         3 . The silicon photonic device of  claim 2 , wherein the silicon core has a length in the second direction, a width in the first direction and a height in the third direction and wherein the three-dimensional grating structure comprises:
 a plurality of first regions each having the length of the silicon core and a width substantially equal to one half of the first grating period, and each comprising a silicon core layer and a grating layer being of the first modulation depth, which is less than the height of the silicon core, and the grating layer comprising an optically transmissive medium having a lower refractive index than the core refractive index; and   a plurality of second regions each having the length of the silicon core and a width substantially equal to one half of the first grating period, and each comprising a silicon core layer and a grating layer comprising a plurality of high index grating elements and a plurality of low index grating elements arranged in an alternating series, each of the grating elements having a length substantially equal to one half of the second grating period, and the high index grating elements being formed of the silicon core and the low index grating elements comprising an optically transmissive medium having a lower refractive index than the core refractive index, and   wherein the first regions and the second regions are arranged in an alternating series across the width of the silicon core in the first direction.