Athermalized multi-path interference filter

Abstract

A multi-path interference filter. The multi-path interference filter includes a first port waveguide, a second port waveguide, and an optical structure connecting the first port waveguide and the second port waveguide. The optical structure has a first optical path from the first port waveguide to the second port waveguide, and a second optical path, different from the first optical path, from the first port waveguide to the second port waveguide. The first optical path has a portion, having a first length, within hydrogenated amorphous silicon. The second optical path has a portion, having a second length, within crystalline silicon, and the second optical path has either no portion within hydrogenated amorphous silicon, or a portion, having a third length, within hydrogenated amorphous silicon, the third length being less than the first length.

What is claimed is: 
     
         1 . An arrayed waveguide grating, comprising:
 a first star coupler;   a second star coupler;   an array of waveguides connecting the first star coupler and the second star coupler;   one or more first port waveguides connected to the first star coupler; and   one or more second port waveguides connected to the second star coupler,   wherein:
 a first optical path, from a first waveguide of the first port waveguides, through a first waveguide of the array of waveguides, to a first waveguide of the second port waveguides, includes a portion, having a first length, within hydrogenated amorphous silicon, 
 the remainder of the first optical path is within crystalline silicon, 
 a second optical path, from the first waveguide of the first port waveguides, through a second waveguide of the array of waveguides, to the first waveguide of the second port waveguides, includes a portion, having a second length, within hydrogenated amorphous silicon, 
 the remainder of the second optical path is within crystalline silicon, and 
 the second length is different from the first length. 
   
     
     
         2 . The arrayed waveguide grating of  claim 1 , wherein a rate of change, with temperature, of a center wavelength of a channel of the arrayed waveguide grating is less than 70 pm/° C. 
     
     
         3 . The arrayed waveguide grating of  claim 1 , wherein:
 the first waveguide of the array of waveguides includes a first portion, having a length equal to the first length, composed of hydrogenated amorphous silicon,   the remainder of the first waveguide of the array of waveguides is composed of crystalline silicon,   the second waveguide of the array of waveguides includes a portion, having a length equal to the second length, composed of hydrogenated amorphous silicon, and   the remainder of the second waveguide of the array of waveguides is composed of crystalline silicon.   
     
     
         4 . The arrayed waveguide grating of  claim 3 , wherein an interface between the first portion of the first waveguide and a portion of the remainder of the first waveguide is a substantially planar surface having a surface normal, an angle between the surface normal and a longitudinal direction of the first portion being greater than 0.1 degrees. 
     
     
         5 . The arrayed waveguide grating of  claim 4 , wherein the angle between the surface normal and the longitudinal direction of the first portion is less than 30 degrees. 
     
     
         6 . The arrayed waveguide grating of  claim 1 , wherein the first star coupler includes a free propagation region including an area composed of hydrogenated amorphous silicon, the remainder of the free propagation region of the first star coupler being composed of crystalline silicon, the area including a wedge-shaped portion. 
     
     
         7 . A multi-path interference filter, comprising:
 a first port waveguide;   a second port waveguide; and   an optical structure connecting the first port waveguide and the second port waveguide,   the optical structure having:
 a first optical path from the first port waveguide to the second port waveguide, and 
 a second optical path, different from the first optical path, from the first port waveguide to the second port waveguide, 
   the first optical path having a portion, having a first length, within hydrogenated amorphous silicon,   the second optical path having a portion, having a second length, within crystalline silicon, and   the second optical path having either
 no portion within hydrogenated amorphous silicon, or 
 a portion, having a third length, within hydrogenated amorphous silicon, the third length being less than the first length. 
   
     
     
         8 . The multi-path interference filter of  claim 7 , wherein an optical path delay difference between the first optical path and the second optical path has a rate of change, with temperature, of less than 2e-5 radians/° C. 
     
     
         9 . The multi-path interference filter of  claim 7 , wherein the optical structure comprises a Mach-Zehnder interferometer having:
 a first coupler,   a second coupler,   a first waveguide connecting the first coupler and the second coupler, and   a second waveguide connecting the first coupler and the second coupler,   wherein:
 a portion of the first optical path is within the first waveguide, and 
 a portion of the second optical path is within the second waveguide. 
   
     
     
         10 . The multi-path interference filter of  claim 7 , wherein the optical structure comprises a generalized Mach-Zehnder interferometer having:
 a first coupler,   a second coupler,   a first waveguide connecting the first coupler and the second coupler,   a second waveguide connecting the first coupler and the second coupler,   a third waveguide connecting the first coupler and the second coupler, and   wherein:
 a portion of the first optical path is within the first waveguide, and 
 a portion of the second optical path is within the second waveguide. 
   
     
     
         11 . The multi-path interference filter of  claim 7 , wherein the optical structure comprises two concatenated Mach-Zehnder interferometers. 
     
     
         12 . The multi-path interference filter of  claim 7 , wherein the optical structure comprises an echelle grating. 
     
     
         13 . The multi-path interference filter of  claim 7 , wherein an optical path delay difference between the first optical path and the second optical path exhibits a maximum change, over a temperature range extending from 20° C. to 70° C., of less than 2e-3 radians.