Dissipating heat from an active region of an optical device
A device, such as an electroabsorption modulator, can modulate a light intensity by controllably absorbing a selectable fraction of the light. The device can include a substrate. A waveguide positioned on the substrate can guide light. An active region positioned on the waveguide can receive guided light from the waveguide, absorb a fraction of the received light, and return a complementary fraction of the received light to the waveguide. Such absorption produces heat, mostly at an input portion of the active region. The input portion of the active region can be thermally coupled to the substrate, which can dissipate heat from the input portion, and can help avoid thermal runaway of the device. The active region can be thermally isolated from the substrate away from the input portion, which can maintain a relatively low thermal mass for the active region, and can increase efficiency when heating the active region.
1. An optical device, comprising:
a substrate;
a waveguide positioned on the substrate and configured to guide light; and
an active region positioned on the waveguide and including an input portion that is configured to receive guided light from the waveguide, the active region configured to absorb a first fraction of the received light and return a second fraction of the received light to the waveguide, the active region being thermally coupled to the substrate at the input portion and thermally isolated from the substrate away from the input portion.
2. The optical device of claim 1 , wherein:
the substrate is etched to form an etched portion;
the etched portion extends under the waveguide from the input portion of the active region to an output end of the active region such that the etched portion thermally isolates the active region from the substrate; and
the etched portion of the substrate does not extend under the input portion of the active region such that the active region is thermally coupled to the substrate at the input portion.
3. The optical device of claim 2 , further comprising:
a heater configured to heat the active region;
a plurality of electrodes configured to produce an electric field in the active region; and
control circuitry configured to control the heater to selectively heat the active region, and supply a selectable voltage to the plurality of electrodes to control a strength of the electric field in the active region.
4. The optical device of claim 3 , wherein:
the active region includes a III-V semiconductor material having an absorption spectrum that is variable in response to the heat from the heater and to the electric field from the electrodes;
the III-V semiconductor material uses one of the quantum-confined Stark effect or the Franz-Keldysh effect; and
an absorption of the III-V semiconductor material increases with increasing temperature.
5. The optical device of claim 3 , wherein the control circuitry and electrodes are further configured to provide a first electric field in the input portion of the active region and provide a second electric field, greater than the first electric field, in the active region away from the input portion of the active region.
6. The optical device of claim 3 , wherein:
the waveguide tapers to a narrowed portion at a first taper and widens from the narrowed portion at a second taper;
the active region is parallel to and adjacent the narrowed portion; and
the active region extends beyond the narrowed portion to at least partially overlap with the first taper.
7. The optical device of claim 3 , wherein at least one of the plurality of electrodes is segmented proximate the input portion of the active region.
8. The optical device of claim 2 , further comprising at least one silicon rib oriented parallel to the active region and positioned between the active region and the etched portion of the substrate, the at least one silicon rib having a cross-sectional size that is smaller than a wavelength of the guided light.
9. The optical device of claim 1 , further comprising a metallic thermal shunt positioned at least partially on the input portion of the active region.
10. The optical device of claim 1 , wherein the active region has a cross-sectional size that is larger at the input portion of the active region than at an output end of the active region.
11. The optical device of claim 1 , further comprising:
a buried oxide layer positioned between the substrate and the waveguide; and
a cladding layer formed from a dielectric material, positioned directly adjacent the waveguide and the active region, and surrounding the heater.
12. A method, comprising:
guiding light from a waveguide into an input portion of an active region of an optical device, the waveguide and the active region positioned on a substrate, the active region being thermally coupled to the substrate at the input portion and thermally isolated from the substrate away from the input portion;
absorbing a fraction of the input light in the active region; and
returning a portion of the light to the waveguide.
13. The method of claim 12 , wherein:
the substrate is etched to form an etched portion;
the etched portion extends under the waveguide from the input portion of the active region to an output end of the active region, such that the etched portion thermally isolates the active region from the substrate; and
the etched portion of the substrate does not extend under the input portion of the active region, such that the waveguide is thermally coupled to the active region at the input portion.
14. The method of claim 12 , further comprising heating the active region with a heater controlled by control circuitry.
15. The method of claim 14 , further comprising producing an electric field in the active region with the control circuitry and a plurality of electrodes.
16. An optical device, comprising:
a substrate;
a waveguide positioned on the substrate and configured to guide light, the waveguide tapering to a narrowed portion at a first taper and widening from the narrowed portion at a second taper;
a buried oxide layer positioned between the substrate and the waveguide; and
an active region positioned on the waveguide and including an input portion that is configured to receive guided light from the waveguide at the first taper, the active region configured to absorb a first fraction of the received light and return a second fraction of the received light to the waveguide at the second taper,
the substrate being etched to form an etched portion,
the etched portion extending under the waveguide from the input portion of the active region to an output end of the active region such that the etched portion thermally isolates the active region from the substrate away from the input portion,
the etched portion of the substrate not extending under the input portion of the active region such that the active region is thermally coupled to the substrate at the input portion.
17. The optical device of claim 16 , further comprising:
a heater configured to heat the active region;
a plurality of electrodes configured to produce an electric field in the active region; and
control circuitry configured to control the heater to selectively heat the active region, and supply a selectable voltage to the plurality of electrodes to control a strength of the electric field in the active region.
18. The optical device of claim 17 , further comprising a cladding layer formed from a dielectric material, positioned directly adjacent the waveguide and the active region, and surrounding the heater.
19. The optical device of claim 17 , wherein:
the active region includes a III-V semiconductor material having an absorption spectrum that is variable in response to applied heat and to the electric field from the electrodes;
the III-V semiconductor material uses one of the quantum-confined Stark effect or the Franz-Keldysh effect; and
an absorption of the III-V semiconductor material increases with increasing temperature.
20. The optical device of claim 17 , wherein the control circuitry and electrodes are further configured to provide a first electric field in the input portion of the active region and provide a second electric field, greater than the first electric field, in the active region away from the input portion of the active region.