Planar optical waveguide element, chromatic dispersion compensator, optical filter, optical resonator and methods for designing the element, chromatic dispersion compensator, optical filter and optical resonator

Abstract

There is provided a planar optical waveguide element in which an optical waveguide comprises a core, and a gap portion that is positioned in a center of a width direction of the core so as to extend in a propagation direction of guided light, and that has a lower refractive index than that of the core; and wherein the core comprises two areas that are separated by the gap portion, and a single mode optical waveguide, in which a single mode is propagated span crossing these two areas, is formed.

1 . A planar optical waveguide element wherein an optical waveguide comprises:
 a core, and a gap portion that is positioned in a center of a width direction of the core so as to extend in a propagation direction of guided light, and that has a lower refractive index than that of the core; and   wherein the core comprises two areas that are separated by the gap portion, and a single mode optical waveguide, in which a single mode is propagated so as to span across these two areas, is formed.   
     
     
         2 . The planar optical waveguide element according to  claim 1 , wherein
 a first Bragg grating pattern and a second Bragg grating pattern are respectively formed in areas which are mutually parallel in the propagation direction of guided light when viewed in a cross-section which is perpendicular to the propagation direction of guided light;   the first Bragg grating pattern comprises recessed and protruding portions that are formed on both outer side walls of the core of the optical waveguide along the propagation direction of guided light;   the second Bragg grating pattern comprises recessed and protruding portions that are formed, along the propagation direction of guided light, on both inner side walls of a groove that is formed on a top portion of the core at the center of the width direction of the core; and   when viewed in the propagation direction of guided light, portions of the first Bragg grating pattern where a core width is wide correspond with portions of the second Bragg grating pattern where a groove width is narrow, and portions of the first Bragg grating pattern where the core width is narrow correspond with portions of the second Bragg grating pattern where the groove width is wide.   
     
     
         3 . The planar optical waveguide element according to  claim 1  or  2 , wherein
 the first Bragg grating pattern and the second Bragg grating pattern comprise a plurality of isolated single coordinate points where a sign of a gradient of an envelope curve of an amplitude of a Bragg grating is inverted. 
 
     
     
         4 . The planar optical waveguide element according to any one of  claims 1  through  3 , wherein
 the optical waveguide comprises a Bragg grating pattern; 
 the Bragg grating pattern only uses discrete values of three or more different pitches; 
 these discrete values are each present in a plurality of locations over an entire length of the optical waveguide; and 
 if a value which has the highest distribution frequency among all of these discrete values is taken as M, and if a closest value to the M which is larger than the M is taken as A, and if a closest value to the M which is smaller than the M is taken as B, then a difference expressed as A−M is equal to a difference expressed as M−B. 
 
     
     
         5 . The planar optical waveguide element according to any one of  claims 1  trough  4 , wherein
 the core of the optical waveguide comprises an inner side core having a projection which forms a rib structure, and an outer side core which is provided on top of the inner side core and covers a circumferential surface of the projection; and 
 a refractive index of the outer side core is lower than an average refractive index of the inner side core. 
 
     
     
         6 . A chromatic dispersion compensator which comprises a Bragg grating pattern in an optical waveguide; and a chromatic dispersion and a dispersion slope in the optical waveguide are compensated by differing a distance over which signal light is propagated in the optical waveguide between entering the optical waveguide and being reflected in accordance with a wavelength, in a plurality of wavelength channels,
 wherein the chromatic dispersion compensator comprises the planar optical waveguide element according to any one of  claims 1  through  5 .   
     
     
         7 . A design method of the chromatic dispersion compensator according to  claim 6 , wherein
 the chromatic dispersion compensator comprises an optical waveguide in which a first Bragg grating pattern and a second Bragg grating pattern are each formed in areas which are mutually parallel in a propagation direction of guided light when viewed in a cross-section which is perpendicular to the propagation direction of guided light,   wherein the design method comprising:   an optical waveguide cross-sectional structure design process in which, by changing dimensions, in a cross-section which is perpendicular to the propagation direction of guided light, of the two areas which form the first Bragg grating pattern and the second Bragg grating pattern, and thus equalizing effective refractive indices of the optical waveguide for two mutually independent polarizations that are guided on the optical waveguide, and by then determining a common effective refractive index for the two polarizations, a relationship between the dimensions of the two areas and the common effective refractive index is obtained;   a Bragg grating pattern design process in which, after a predetermined complex reflectance spectrum is calculated by specifying three of a chromatic dispersion, a dispersion slope, and a reflectance as parameters, a profile of an effective refractive index along the propagation direction of guided light is obtained for the optical waveguide from the complex reflectance spectrum and a desired optical waveguide length; and   a chromatic dispersion compensator design process in which, by converting the profile of the effective refractive index obtained in the Bragg grating pattern design process into a profile of the dimensions of the two areas based on the relationship between the dimensions of the two areas and the common effective refractive index obtained in the optical waveguide cross-sectional structure design process, the first Bragg grating pattern and the second Bragg grating pattern which are formed by the changes in the dimensions of the two areas are obtained.   
     
     
         8 . The design method of the chromatic dispersion compensator according to  claim 7 , wherein
 the Bragg grating pattern design process further comprises a coarse graining process in which a resolution of discretization of a coordinate axis is taken as more than an amount of change in a pitch which corresponds to a half value of a width of a reflection band; and   an optical waveguide is created by the coarse graining process, which includes a plurality of isolated single coordinate points where a sign of a gradient of an envelope curve of an amplitude of a Bragg grating is inverted.   
     
     
         9 . An optical filter that comprises the planar optical waveguide element according to any one of  claims 1  through  5 . 
     
     
         10 . A design method of the optical filter according to  claim 9 , wherein
 the optical filter comprises an optical waveguide in which a first Bragg grating pattern and a second Bragg grating pattern are mutually parallel in a propagation direction of guided light when viewed in a cross-section which is perpendicular to the propagation direction of guided light;   wherein the design method comprising:   an optical waveguide cross-sectional structure design process in which, by changing dimensions, in a cross-section which is perpendicular to the propagation direction of guided light, of two areas which form the first Bragg grating pattern and the second Bragg grating pattern, and thus equalizing effective refractive indices of the optical waveguide for two mutually independent polarizations that are guided on the optical waveguide, and by then determining a common effective refractive index for the two polarizations, a relationship between the dimensions of the two areas and the common effective refractive index is obtained;   a Bragg grating pattern design process in which, after a predetermined complex reflectance spectrum is calculated by specifying both a reflectance and a phase as parameters, a profile of an effective refractive along the waveguide direction of the optical waveguide is obtained from the complex reflectance spectrum and a desired optical waveguide length; and   an optical filter design process in which, by converting the profile of the effective refractive index obtained in the Bragg grating pattern design process into a profile of the dimensions of the two areas based on the relationship between the dimensions of the two areas and the common effective refractive index obtained in the optical waveguide cross-sectional structure design process, the two Bragg grating patterns which are formed by the changes in the dimensions of the two areas are obtained.   
     
     
         11 . The design method of the optical filter according to  claim 10 , wherein
 the Bragg grating pattern design process further comprises a coarse graining process in which a resolution of discretization of a coordinate axis is taken as more than an amount of change in a pitch which corresponds to a half value of a width of a reflection band; and   an optical waveguide is created by the coarse graining process, which includes a plurality of isolated single coordinate points where a sign of a gradient of an envelope curve of an amplitude of a Bragg grating is inverted.   
     
     
         12 . An optical resonator comprising:
 a first optical waveguide which forms a first reflection mirror;   a second optical waveguide which forms a second reflection mirror; and   a third optical waveguide which is sandwiched between the first optical waveguide and the second optical waveguide,   wherein the first optical waveguide, the third optical waveguide, and the second optical waveguide are connected in series, so that a single planar optical waveguide is formed; and   wherein the first optical waveguide and the second optical waveguide comprise the planar optical waveguide element according to any one of  claims 1  through  5 .   
     
     
         13 . A design method of the optical resonator according to  claim 12 , wherein
 the reflection mirrors comprise an optical waveguide in which a first Bragg grating pattern and a second Bragg grating pattern are mutually parallel in a propagation direction of guided light when viewed in a cross-section which is perpendicular to the propagation direction of guided light, and   wherein the design method comprising:   an optical waveguide cross-sectional structure design process in which, by changing dimensions, in a cross-section which is perpendicular to the propagation direction of guided light, of two areas which form the first Bragg grating pattern and the second Bragg grating pattern, and thus equalizing effective refractive indices of the optical waveguide for two mutually independent polarizations that are guided on the optical waveguide, and by then determining a common effective refractive index for the two polarizations, a relationship between the dimensions of the two areas and the common effective refractive index is obtained;   a Bragg grating pattern design process in which, after a predetermined complex reflectance spectrum is calculated by specifying both a reflectance and a phase as parameters, a profile of an effective refractive index along the waveguide direction of the optical waveguide is obtained from the complex reflectance spectrum and a desired optical waveguide length; and   a reflection mirror design process in which, by converting the profile of the effective refractive index obtained in the Bragg grating pattern design process into a profile of the dimensions of the two areas based on the relationship between the dimensions of the two areas and the common effective refractive index obtained in the optical waveguide cross-sectional structure design process, the two Bragg grating patterns which are formed by the changes in the dimensions of the two areas are obtained.   
     
     
         14 . The design method of the optical resonator according to  claim 13 , wherein
 the Bragg grating pattern design process further comprises a coarse graining process in which a resolution of discretization of a coordinate axis is taken as more than an amount of change in a pitch which corresponds to a half value of a width of a reflection band; and   an optical waveguide is created by the coarse graining process, which comprises a plurality of isolated single coordinate points where a sign of a gradient of an envelope curve of an amplitude of a Bragg grating is inverted.