Photonic chip
A photonic chip including an optical coupler capable of transferring an optical signal between a first waveguide made of III-V material and a second waveguide made of silicon, this optical coupler including a first extension made of III-V material which extends the core of the first waveguide, a second extension made of silicon which extends the core of the second waveguide, and a SiGe inclusion buried inside of the second extension, this inclusion being made of SiGe whose chemical formula is Si 1−x Ge x , where x is in the range between 0.2 and 0.5, and being optically coupled, on a first side, to the first waveguide and, on a second opposite side, to the second waveguide.
1. A photonic chip comprising:
a substrate which mainly extends in a plane called “plane of the substrate”,
a first waveguide which mainly extends parallel to the plane of the substrate and whose core is made of a III-V material,
a second waveguide which mainly extends parallel to the plane of the substrate and whose core is made of silicon,
an optical coupler capable of transferring, between a start and an end of a coupling region, at least a part of an optical signal which is propagating, at the start of the coupling region, in one of the first and second waveguides towards the other of the first and second waveguides, said optical coupler comprising for said purpose:
a first extension made of III-V material which extends the core of the first waveguide into the coupling region,
a second extension made of silicon which extends the core of the second waveguide into the coupling region, said second extension runs facing the first extension,
wherein the optical coupler comprises a SiGe inclusion buried inside of the second extension, said inclusion:
being made of SiGe whose chemical formula is Si 1−x Ge x , where x is in the range between 0.2 and 0.5, and
being optically coupled, on a first side, to the first waveguide and, on a second opposing side, to the second waveguide.
2. The chip according to claim 1 , wherein the inclusion is accommodated within a recess formed in the second extension, a bottom of said recess being formed by a silicon base layer whose thickness is less than a thickness of the second waveguide and on which the SiGe inclusion is formed.
3. The chip according to claim 1 , wherein:
the first waveguide and the first extension are entirely situated between first lower and upper planes parallel to the plane of the substrate, the first waveguide having lower and upper faces contained, respectively, in the first lower and upper planes,
the second waveguide and the second extension are entirely situated between second lower and upper planes parallel to the plane of the substrate, the second waveguide having lower and upper faces contained, respectively, in the second lower and upper planes,
the first and second lower and upper planes are stacked one on top of the other in the following order: the second lower plane, the second upper plane, the first lower plane then the first upper plane, and
the inclusion is entirely situated between the second lower and upper planes.
4. The chip according to claim 3 , wherein:
on the first side, the inclusion comprises a first pointed termination which optically connects the inclusion to the first waveguide, a width of said first termination decreasing progressively and continuously from a maximum width down to a minimum width going in a direction of the first waveguide, the width of the first termination being measured in a direction parallel to the plane of the substrate and perpendicular to a direction of propagation of the optical signal in said first termination,
an effective propagation index of said first termination at a location where its width is maximum is greater than an effective propagation index of a portion of the first extension situated facing said location of the first termination where its width is maximum, and
an effective propagation index of said first termination at a location where its width is minimum is less than an effective propagation index of a portion of the first extension situated facing said location of the first termination where its width is minimum.
5. The chip according to claim 1 , wherein:
on the second side, the inclusion comprises a second pointed termination which optically connects the inclusion to the second waveguide, a width of said second termination decreasing progressively and continuously from a maximum width down to a minimum width going in a direction of the second waveguide, vertical sides of said second termination being in direct contact with vertical walls of a recess formed in the second extension, these vertical sides and these vertical walls each extending mainly in a plane perpendicular to the plane of the substrate, the width of the second termination being measured in a direction parallel to the plane of the substrate and perpendicular to a direction of propagation of the optical signal in said second termination,
an effective propagation index of said second termination at a location where its width is maximum is greater than an effective propagation index of the second waveguide, and
an effective propagation index of said second termination at a location where its width is minimum is less than the effective propagation index of the second waveguide.
6. The chip according to claim 3 , wherein a distance which separates the second lower plane from the second upper plane is less than or equal to 310 nm.
7. The chip according to claim 1 , wherein the inclusion is made of SiGe whose chemical formula is Si 1−x Ge x , where x is in the range between 0.4 and 0.5.
8. The chip according to claim 1 , wherein:
the first waveguide is made of III-V gain material, and
the chip comprises:
a semiconductor laser source capable of generating the optical signal, said laser source comprising the first waveguide, the second waveguide and the optical coupler which optically connects the first and second waveguides to each other through a layer of dielectric material, and
a phase and/or amplitude modulator formed on the same substrate and configured to modulate the optical signal generated by the semiconductor laser source, said modulator comprising:
a first electrode made of P- or N-doped silicon entirely situated between the second lower and upper planes, and
a second electrode having a doping of sign opposite to that of the first electrode.
9. The chip according to claim 1 , wherein the second waveguide has an effective propagation index less than an effective propagation index of the first waveguide.
10. A method of fabrication of a photonic chip according to claim 1 , wherein the method comprises:
providing the substrate which extends mainly in the plane of the substrate,
forming the first waveguide which extends mainly parallel to the plane of the substrate
forming the second waveguide which extends mainly parallel to the plane of the substrate and whose core is made of silicon, and
forming the optical coupler capable of transferring, between the start and the end of the coupling region, at least the part of the optical signal which is propagating, at the start of the coupling region, in one of the first and second waveguides towards the other of the first and second waveguides,
wherein forming the optical coupler comprises forming of the SiGe inclusion buried inside the second extension.
11. A method according to claim 10 , wherein forming the SiGe inclusion comprises an operation for epitaxial growth of the SiGe inclusion on a silicon base layer.