Mode converter and method of fabricating thereof

Abstract

An optical mode converter and method of fabricating the same from wafer including a double silicon-on-insulator layer structure. The method comprising: providing a first mask over a portion of a device layer of the DSOI layer structure; etching an unmasked portion of the device layer down to at least an upper buried oxide layer, to provide a cavity; etching a first isolation trench and a second isolation trench into a mode converter layer, the mode converter layer being: on an opposite side of the upper buried oxide layer to the device layer and between the upper buried oxide layer and a lower buried oxide layer, the lower buried oxide layer being above a substrate; wherein the first isolation trench and the second isolation trench define a tapered waveguide; filling the first isolation trench and the second isolation trench with an insulating material, so as to optically isolate the tapered waveguide from the remaining mode converter layer; and regrowing the etched region of the device layer.

1 . A method of fabricating an optical mode converter from a wafer including a double silicon-on-insulator (DSOI) layer structure, comprising the steps of:
 providing a first mask over a portion of a device layer of the DSOI layer structure;   etching an unmasked portion of the device layer down to at least an upper buried oxide layer, to provide a cavity;   etching a first isolation trench and a second isolation trench into a mode converter layer, the mode converter layer being:
 on an opposite side of the upper buried oxide layer from the device layer and between the upper buried oxide layer and a lower buried oxide layer, the lower buried oxide layer being above a substrate; 
   wherein the first isolation trench and the second isolation trench define a tapered waveguide;   filling the first isolation trench and the second isolation trench with an insulating material, so as to optically isolate the tapered waveguide from the remaining mode converter layer; and   regrowing the etched region of the device layer.   
     
     
         2 . The method of  claim 1 , further comprising a step of:
 etching a rib waveguide from the regrown region of the device layer.   
     
     
         3 . The method of  claim 1 , wherein the step of etching the unmasked portion of the device layer down to at least the upper buried oxide layer comprises:
 a first etching step, etching from an upper surface of the device layer to an upper surface of the upper buried oxide layer; and   a second etching step, etching from an upper surface of the upper buried oxide layer to an upper surface of the mode converter layer.   
     
     
         4 . The method of  claim 3 , wherein the second etching step does not remove all of the buried oxide layer in the cavity. 
     
     
         5 . The method of  claim 1 , further comprising a step, between the steps of etching the unmasked portion and etching the first and second isolation trenches, of:
 depositing an oxidation barrier over: (i) the first mask and (ii) the cavity, wherein the cavity is defined by sidewalls and a bed.   
     
     
         6 . The method of  claim 5 , wherein the step of filling the first isolation trench and the second isolation trench comprises:
 thermally oxidizing the mode converter layer, so as to fill the first isolation trench and the second isolation trench with an oxide.   
     
     
         7 . The method of  claim 1 , further comprising a step, after regrowing the etched region of the device layer, of:
 planarizing the regrown region of the device layer such that it is coplanar with an uppermost surface of the unetched region of the device layer.   
     
     
         8 . The method of  claim 1 , wherein the tapered waveguide is provided with a first width of between 9 μm and 15 μm and a second width of less than 1 μm. 
     
     
         9 . The method of  claim 1 , wherein a width of the cavity etched is substantially wider than a widest width of the tapered waveguide. 
     
     
         10 . The method of  claim 1 , further comprising a step of:
 etching a v-groove interface at a first end of the mode converter, such that an input facet of the tapered waveguide overhangs the v-groove interface, so as to allow passive alignment of a fiber optical cable to the tapered waveguide.   
     
     
         11 . The method of  claim 1 , further comprising a step of:
 polishing a first end of the tapered waveguide, so as to provide a planar input facet for active alignment to a fiber optic cable.   
     
     
         12 . An optical mode converter, formed on a wafer including a double silicon-on-insulator (DSOI) layer structure, comprising:
 a substrate, above which is a lower buried oxide layer;   a mode converter layer, which is above the lower buried oxide layer, and includes:
 a tapered waveguide, cladded by an insulator disposed in a first isolation trench and a second isolation trench; and 
 a bulk region, adjacent to the insulator and on an opposing side thereof to the tapered waveguide, formed of a same material as the tapered waveguide; 
   an upper buried oxide layer, which is above the mode converter layer and has a gap therein above the tapered waveguide; and   a device layer, which is above the upper buried oxide layer;   wherein the device layer includes two etched portions which define a rib waveguide, and an uppermost surface of the rib waveguide is co-planar with an uppermost surface of the device layer.   
     
     
         13 . The optical mode converter of  claim 12 , wherein the tapered waveguide has a first width of between 9 μm and 15 μm and a second width of less than 1 μm. 
     
     
         14 . The optical mode converter of  claim 12 , further comprising a v-groove interface at a first end of the mode converter, wherein an input facet of the tapered waveguide overhangs the v-groove interface so as to allow passive alignment of a fiber optical cable to the tapered waveguide. 
     
     
         15 . The optical mode converter of  claim 12 , further comprising a polished first end of the tapered waveguide, providing a planar input facet for active alignment to a fiber optic cable. 
     
     
         16 . The optical mode converter of  claim 12 , wherein the insulator is silicon dioxide. 
     
     
         17 . The optical mode converter of  claim 12 , wherein the first isolation trench and the second isolation trench respectively have a width of between 0.4 μm and 1.0 μm.