Waveguide type variable optical attenuator

1. A waveguide type variable optical attenuator, comprising:

a substrate for forming a waveguide for optical signal propagation;

a waveguide element comprising a first arm waveguide and a second arm waveguide arranged on a surface of the substrate for constituting a portion of the waveguide and cladding for covering the first and second arm waveguides and the surface of the substrate;

an output side waveguide connecting the first arm waveguide with the second arm waveguide and arranged to couple light from the first arm waveguide with light from the second arm waveguide; and

a heater arranged on a surface of the waveguide element for heating the first and second arm waveguides, wherein:

the first and second arm waveguides and the output side waveguide constitute one optical interference circuit, the first and second arm waveguides are connected thermally to each other by a high thermal-conductivity member that covers the heater, the high thermal-conductivity member being different from the heater.

2. The waveguide type variable optical attenuator according to claim 1 , wherein:

a length L of a spacing between the first and second arm waveguides at a center part is substantially equal to or more than that of a spacing in which optical characteristics of the first and second arm waveguides are affected by each other.

3. The waveguide type variable optical attenuator according to claim 1 , wherein:

when a temperature of the first arm waveguide adjacent to the heater during heater electrical conduction is T 1 , a temperature of the second arm waveguide during heater electrical conduction is T 2 , and

a temperature of the first and second arm waveguides during no heater electrical conduction is T 3 , T 1 >T 2 >T 3 .

4. The waveguide type variable optical attenuator according to claim 2 , wherein:

the length L is set to a range 30 μm≦L≦250 μm.

5. The waveguide type variable optical attenuator according to claim 1 , wherein:

the substrate comprises a quartz glass substrate.

6. The waveguide type variable optical attenuator according to claim 1 , wherein:

a thermal conductivity α of the high thermal-conductivity member is set to thermal expansivity ranging 100 W/mK≦α≦500 W/mK.

7. The waveguide type variable optical attenuator according to claim 1 , wherein:

the high thermal-conductivity member covers the first arm waveguide, and extends to a region adjacent to the second arm waveguide.

8. The waveguide type variable optical attenuator according to claim 1 , wherein:

the high thermal-conductivity member is arranged on the surface of the waveguide element via an electrical insulation layer, and the heater is covered with the electrical insulation layer, the electrical insulation layer arranged between the high thermal-conductivity member and the heater.

9. The waveguide type variable optical attenuator according to claim 1 , wherein:

the high thermal-conductivity member extends on or over a center line between the first and second arm waveguides.

10. The waveguide type variable optical attenuator according to claim 1 , wherein:

the heater is a single heater.

11. The waveguide type variable optical attenuator according to claim 1 , wherein:

the high thermal-conductivity member covers the first arm waveguide, but not the second arm wave guide.

12. The waveguide type variable optical attenuator according to claim 4 , wherein:

the high thermal-conductivity member extends over a half point of the spacing between the first and second arm waveguides.

13. A waveguide type variable optical attenuator, comprising:

a substrate for forming a waveguide for optical signal propagation;

a waveguide element comprising a first arm waveguide and a second arm waveguide arranged on a surface of the substrate for constituting a portion of the waveguide and cladding for covering the first and second arm waveguides and the surface of the substrate;

an output side waveguide connecting the first arm waveguide with the second arm waveguide and arranged to couple light from the first arm waveguide with light from the second arm waveguide; and

a single heater arranged on a surface of the waveguide element for heating the first and second arm waveguides, wherein:

the first and second arm waveguides and the output side waveguide constitute one optical interference circuit, the first and second arm waveguides are connected thermally to each other by the heater.

14. The waveguide type variable optical attenuator according to claim 13 , wherein:

the heater extends on or over a center line between the first and second arm waveguides.