Waveguide mode converter

Abstract

A SOI bent taper structure is used as a mode convertor. By tuning the widths of the bent taper and the bend angles, almost lossless mode conversion is realized between TE0 and TE1 in a silicon waveguide. The simulated loss is <0.05 dB across C-band. This bent taper can be combined with bi-layer TM0-TE1 rotator to reach very high efficient TM0-TE0 polarization rotator. An ultra-compact (9 μm) bi-layer TM0-TE1 taper based on particle swarm optimization is demonstrated. The entire TM0-TE0 rotator has a loss <0.25 dB and polarization extinction ratio >25 dB, worst-case across the C-band.

What is claimed is: 
     
         1 . A waveguide mode converter comprising:
 a first port;   a second port; and   a bilayer taper optically connecting the first port and the second port, the bilayer taper comprising a first layer and a second layer vertically coupled to the first layer;   wherein the first layer increases in width in a first direction from the first port to the second port; and   wherein the second layer comprises a first section that decreases in width in the first direction, and a second section that increases in width in the first direction.   
     
     
         2 . The waveguide mode converter of  claim 1  wherein the first section is disposed between the first port and the second section. 
     
     
         3 . The waveguide mode converter of  claim 1  wherein the bilayer taper comprises a length wherein the first layer increases in width in the first direction and the second layer decreases in width in the first direction. 
     
     
         4 . The waveguide mode converter of  claim 1  wherein the bilayer taper is configured to couple a TM mode at the first port to a TE mode at the second port. 
     
     
         5 . The waveguide mode converter of  claim 1  wherein the bilayer taper is configured to convert light propagating in a TM0 mode at the first port into a higher-order TE mode at the second port. 
     
     
         6 . The waveguide mode converter of  claim 5  wherein the higher-order TE mode comprises a TE1 mode. 
     
     
         7 . The waveguide mode converter of  claim 1  wherein the second layer is disposed over the first layer. 
     
     
         8 . The waveguide mode converter of  claim 1  wherein the first layer exceeds the second layer in width. 
     
     
         9 . The waveguide mode converter of  claim 7  wherein the first layer is disposed over a planar substrate. 
     
     
         10 . The waveguide mode converter of  claim 9  wherein the planar substrate comprises a semiconductor material. 
     
     
         11 . The waveguide mode converter of  claim 9  wherein the planar substrate comprises silicon. 
     
     
         12 . A waveguide polarization rotator comprising:
 the waveguide mode converter of  claim 5 ; and   a TE mode converter optically coupled to the second port of the waveguide mode converter and configured to convert the higher-order TE mode into a TE 0 mode.   
     
     
         13 . The waveguide polarization rotator of  claim 12  wherein the TE mode converter comprises a bent waveguide taper. 
     
     
         14 . The waveguide polarization rotator of  claim 13  wherein the bent waveguide taper comprises a first end, a second end, and two or more bent waveguide segments optically connected in series between the first end and the second end. 
     
     
         15 . The waveguide polarization rotator of  claim 14  wherein the bent waveguide taper has a width that gradually increases from the first end toward the second end along at least a portion of a length of the bent waveguide taper. 
     
     
         16 . The waveguide polarization rotator of  claim 14  wherein the two or more bent waveguide segments are configured so that light that enters the bent waveguide taper from the second end in the higher order TE mode exits the second end in the TE0 mode. 
     
     
         17 . The waveguide polarization rotator of  claim 14  wherein the two or more bent waveguide segments comprise two oppositely-curved waveguide bends. 
     
     
         18 . The waveguide polarization rotator of  claim 12  wherein the waveguide mode converter and the TE mode converter are disposed on a same substrate.