Transition device for optical waveguides

1. A transition part ( 1 , 101 , 201 , 301 , 401 ) between two optical waveguides ( 2 , 3 ) with different index contrast, the transition part ( 1 , 101 , 201 , 301 , 401 ) comprising:

a non-adiabatically and continuously up-tapered longitudinal section ( 8 );

an adiabatically down-tapered longitudinal section ( 10 ), arranged after the non-adiabatically up-tapered longitudinal section ( 8 ) as seen in a main propagation direction (L) of light, between said longitudinal sections a transition ( 7 ) between the two waveguides ( 2 , 3 ) is arranged along the main propagation direction (L) of the light; and

a non-adiabatically down-tapered longitudinal section ( 9 ), arranged immediately after the non-adiabatically up-tapered longitudinal section ( 8 ) as seen in the main propagation direction (L) of the light, and the transition ( 7 ) between the two waveguides ( 2 , 3 ) is arranged between the non-adiabatically up-tapered longitudinal section ( 8 ) and the non-adiabatically down-tapered longitudinal section ( 9 ).

2. The transition part ( 1 , 201 , 301 , 401 ) according to claim 1 , wherein the transition ( 7 ) between the two waveguides ( 2 , 3 ) is arranged on a border between the non-adiabatically up-tapered longitudinal section ( 8 ) and the non-adiabatically down-tapered longitudinal section ( 9 ).

3. The transition part ( 201 ) according to claim 1 , wherein a longitudinal section ( 11 ) with essentially constant width is arranged immediately after the non-adiabatically up-tapered longitudinal section ( 8 ) as seen in the main propagation direction (L) of the light, and the transition ( 7 ) between the two waveguides ( 2 , 3 ) is arranged along the longitudinal section ( 11 ) with essentially constant width.

4. A transition part ( 1 , 101 , 201 , 301 , 401 ) between two optical waveguides ( 2 , 3 ) with different index contrast, the transition part ( 1 , 101 , 201 , 301 , 401 ) comprising:

a non-adiabatically and continuously up-tapered longitudinal section ( 8 );

an adiabatically down-tapered longitudinal section ( 10 ), arranged after the non-adiabatically up-tapered longitudinal section ( 8 ) as seen in a main propagation direction (L) of light, between said longitudinal sections a transition ( 7 ) between the two waveguides ( 2 , 3 ) is arranged along the main propagation direction (L) of the light,

wherein both the waveguide ( 2 ) with lower index contrast and the waveguide ( 3 ) with higher index contrast comprise a ridge structure, in both cases comprising a lower ridge part ( 5 ) with relatively high refractive index and an upper ridge part ( 4 ) with relatively low refractive index, and the lower ridge part ( 5 ) is comparatively wider in the waveguide ( 2 ) with lower index contrast.

5. The transition part ( 1 , 101 , 201 , 301 , 401 ) according to claim 4 , wherein the refractive index of the lower ridge part ( 5 ) is between 3.4 and 3.6.

6. The transition part ( 1 , 101 , 201 , 301 , 401 ) according to claim 4 , wherein the refractive index for the upper ridge part ( 4 ) is between 3.0 and 3.3.

7. A method for manufacturing a transition part ( 501 ) between two optical waveguides ( 2 , 3 ) with different index contrasts, the transition part comprising:

a transition ( 7 ) between the waveguides ( 2 , 3 ), where both the waveguide ( 2 ) with lower index contrast and the waveguide ( 3 ) with higher index contrast comprise a ridge structure, in both cases comprising a lower ( 5 ) ridge part having relatively high refractory index and an upper ridge part ( 4 ) having relatively low refractory index, where both the upper ( 4 ) and lower ( 5 ) ridge parts, respectively, are common for both the waveguides ( 2 , 3 ), the method comprising:

manufacturing the transition part ( 501 ) from a wafer ( 502 ) with at least one upper layer ( 503 ) having comparatively lower refractory index and one lower layer ( 504 ) having comparatively higher refractory index, where the lower ridge part ( 5 ) is made wider than the upper ridge part ( 4 ) in the waveguide ( 2 ) with lower index contrast, but as wide as the upper ridge part ( 4 ) in the waveguide ( 3 ) with higher index contrast, where the transition ( 7 ) between the waveguides ( 2 , 3 ) comprises a step in the width of the lower ridge part ( 5 ), where the width of the upper ridge part ( 4 ) is varied along a main propagation direction (L) of the light in the transition part ( 501 ), so that at least one non-adiabatically up-tapered longitudinal section ( 8 ) is formed, which longitudinal section ( 8 ) is followed by the transition ( 7 ) between the waveguides ( 2 , 3 );

etching parts of the upper layer ( 503 ) initially using a first etching mask ( 506 ), which corresponds to a geometrical form of the upper ridge part ( 4 ), so that the upper ridge part ( 4 ) of both waveguides ( 2 , 3 ) is formed; after which

etching parts of the lower layer ( 504 ), using a second etching mask ( 507 ) in combination with the first etching mask ( 506 ), where the second etching mask ( 507 ) corresponds to the geometrical form of the transition ( 7 ), so that the lower ridge part ( 5 ) is formed, where an edge ( 507 a ) of the second etching mask ( 507 ) corresponds to the transition ( 7 ) between the waveguides ( 2 , 3 ) is placed upon the wafer ( 502 ) at an angle in relation to the main propagation direction (L) of the light which is caused to be different from 90 degrees,

wherein the edge ( 507 a ) comprises a zigzag pattern, essentially extending in a direction perpendicular to the main propagation direction (L) of the light.

8. The transition part ( 1 , 101 , 201 , 301 , 401 ) according to claim 4 , wherein the ridge parts ( 4 , 5 ) each comprise a plurality of different layers with different optical properties.

9. The transition part ( 1 , 101 , 201 , 301 , 401 ) according to claim 4 , wherein the upper ridge part ( 5 ) is a quantum well stack having between 10 and 20 periods of wells and barriers having different band gaps.

10. The transition part ( 1 , 101 , 201 , 301 , 401 ) according to claim 4 , wherein the ridge parts ( 4 , 5 ) are manufactured from InP.