HIGH-EFFICIENCY OPTICAL WAVEGUIDE TRANSITIONS
Embodiments describe high-efficiency optical waveguide transitions—i.e., creating heterogeneous transitions between Si and III-V semiconductor regions or devices with minimal reflections. This is advantageous for III-V device performance, e.g. for an on-chip lasers achieving lower relative intensity noise (RIN) and lower phase noise by avoiding reflections, higher gain and reduced gain-ripple from an semiconductor optical amplifier (SOA) by avoiding internal reflections in the SOA. Furthermore, in some embodiments, generated photocurrent can be used as a monitor signal for control purposes, thereby avoiding the use of separate tap-monitor photodetectors, which provide additional link loss.
1 . A waveguide coupling apparatus, comprising:
a first waveguide including a tapered coupling region that extends to a termination region;
a second waveguide positioned parallel to and directly adjacent to the first waveguide in the tapered coupling region of the first waveguide, the first and the second waveguides being heterogeneous semiconductor materials and the tapered coupling region of the first waveguide configured to couple light from the first waveguide to the second waveguide, the termination region of the first waveguide positioned away from the second waveguide and operatively minimizing reflections at transitions between the heterogeneous materials of the first and second waveguides; and
a structure coupled to the termination region of the first waveguide and configured to reduce back-reflections of light from the termination region toward the tapered coupling region.
2 . The waveguide coupling apparatus of claim 1 , wherein the structure includes a passive structure configured to dissipate light received at the termination region.
3 . The waveguide coupling apparatus of claim 2 , wherein the passive structure includes an angled edge positioned to terminate the first waveguide and couple the received light to a radiation mode of a surrounding cladding.
4 . The waveguide coupling apparatus of claim 2 , wherein the passive structure includes a polygonal structure having a plurality of vertices configured to couple the received light to one or more radiation modes of a surrounding cladding.
5 . The waveguide coupling apparatus of claim 2 , wherein the passive structure includes a free propagation region.
6 . The waveguide coupling apparatus of claim 1 , wherein the structure includes an active structure configured to absorb light received at the termination region.
7 . The waveguide coupling apparatus of claim 6 , wherein the active structure includes electrical contacts configured to receive a reverse bias to enhance an absorption of the received light.
8 . The waveguide coupling apparatus of claim 6 , wherein the active structure includes a photodetection layer configured to absorb the received light.
9 . The waveguide coupling apparatus of claim 8 , wherein the photodetection layer is further configured to produce a photocurrent in response to the absorbed light, the photocurrent being suitable for monitoring an optical device that couples light into the first waveguide.
10 . The waveguide coupling apparatus of claim 1 , wherein:
the first waveguide includes a first material, the first material being a semiconductor or a dielectric; and
the second waveguide includes a second material different from the first material, the second material being a semiconductor or a dielectric.
11 . The waveguide coupling apparatus of claim 10 , wherein the first material and the second material each include at least one of III-V semiconductor material, silicon semiconductor material, or silicon nitride dielectric material.
12 . The waveguide coupling apparatus of claim 10 , wherein the first material has a higher refractive index than the second material.
13 . The waveguide coupling apparatus of claim 1 , wherein the first waveguide includes a taper proximate the coupling region, the taper shaped to match a mode size in the first waveguide to a mode size in the second waveguide.
14 . The waveguide coupling apparatus of claim 1 , wherein the second waveguide includes a taper proximate the coupling region, the taper shaped to match a mode size in the second waveguide to a mode size in the first waveguide.
15 . A method for coupling light between waveguides, the method comprising:
propagating light in a first waveguide to a coupling region of the first waveguide, the coupling region of the first waveguide being parallel to and directly adjacent to a second waveguide;
coupling a first fraction of the propagating light from the first waveguide into the second waveguide at the coupling region of the first waveguide;
retaining a second fraction of the propagating light in the first waveguide beyond the coupling region of the first waveguide, the first fraction and the second fraction summing to a value less than or equal to 100%;
directing the second fraction of the light onto a structure coupled to first waveguide and configured to reduce back-reflections of light toward the coupling region.
16 . The method of claim 15 , wherein the structure is a passive structure configured to dissipate light received at the passive structure, the passive structure including at least one of:
an angled edge positioned to terminate the first waveguide and couple the received light to a radiation mode of a surrounding cladding,
a polygonal structure having a plurality of vertices configured to couple the received light to one or more radiation modes of a surrounding cladding, or
a free propagation region.
17 . The method of claim 15 , wherein the structure is an active structure configured to absorb light received at the active structure, the active structure including at least one of:
electrical contacts configured to receive a reverse bias to enhance an absorption of the received light, or
a photodetection layer configured to absorb the received light.
18 . A waveguide coupling apparatus, comprising:
a first waveguide including silicon semiconductor material, the first waveguide including a tapered coupling region that extends to a termination region;
a second waveguide including silicon nitride dielectric material, the second waveguide positioned parallel to and directly adjacent to the first waveguide in the coupling region of the first waveguide, the tapered coupling region of the first waveguide being configured to couple light from the first waveguide to the second waveguide, the termination region of the first waveguide positioned away from the second waveguide and operatively minimizing reflections at transitions between the silicon semiconductor material and the silicon nitride dielectric material of the first and second waveguides; and
a structure coupled to the termination region of the first waveguide and configured to reduce back-reflections of light from the termination region toward the tapered coupling region.
19 . The waveguide coupling apparatus of claim 18 , wherein the structure is a passive structure configured to dissipate light received at the termination region, the passive structure including at least one of:
an angled edge positioned to terminate the first waveguide and couple the received light to a radiation mode of a surrounding cladding,
a polygonal structure having a plurality of vertices configured to couple the received light to one or more radiation modes of a surrounding cladding, or
a free propagation region.
20 . The waveguide coupling apparatus of claim 18 , wherein the structure is an active structure configured to absorb light received at the termination region, the active structure including at least one of:
electrical contacts configured to receive a reverse bias to enhance an absorption of the received light, or
a photodetection layer configured to absorb the received light.