Multi-mode interference coupler

1. A multimode interference coupler comprising:

a multimode interference (MMI) region including:

first and second region ends separated by a region length along a longitudinal axis, and

first and second edges between the first and second region ends defining the MMI region, which varies in width along the region length;

wherein each of the first and second edges includes at least three critical points in slope relative to the longitudinal axis defining at least three local minima or maxima;

at least one port optically coupled to the first region end; and

at least one port optically coupled to the second region end;

wherein the at least three local minima or maxima comprises at least three maxima and at least two minima in each of the first and second edges; wherein a first derivative of a slope of the first and second edges at each local minima or maxima relative to the longitudinal axis is substantially 0.

2. The coupler according to claim 1 , wherein the MMI region includes a plurality of different segment widths, perpendicular to the longitudinal axis, each segment width corresponding to one of the critical points, spaced apart along the region length, defining adjacent segments; and wherein the first and second edges provide gradual transitions between the different widths.

3. The coupler according to claim 2 , wherein the plurality of different widths corresponds to 5 to 20 local maxima and minima spaced along each of the first and second edges.

4. The coupler according to claim 2 , wherein the plurality of different widths are equally spaced along the longitudinal axis.

5. The coupler according to claim 4 , wherein the plurality of different widths are symmetrical about a midpoint of the longitudinal axis.

6. A multimode interference coupler comprising:

a multimode interference (MMI) region including:

first and second region ends separated by a region length along a longitudinal axis, and

first and second edges between the first and second region ends defining the MMI region, which varies in width along the region length;

wherein each of the first and second edges includes at least three critical points in slope relative to the longitudinal axis defining at least three local minima or maxima;

at least one port optically coupled to the first region end; and

at least one port optically coupled to the second region end;

wherein the at least three local minima or maxima comprises at least two maxima and at least three minima in each of the first and second edges; wherein a first derivative of a slope of the first and second edges at each local minima or maxima relative to the longitudinal axis is substantially 0.

7. A multimode interference coupler comprising:

a multimode interference (MMI) region including:

first and second region ends separated by a region length along a longitudinal axis, and

first and second edges between the first and second region ends defining the MMI region, which varies in width along the region length;

wherein each of the first and second edges includes at least three critical points in slope relative to the longitudinal axis defining at least three local minima or maxima;

at least one port optically coupled to the first region end; and

at least one port optically coupled to the second region end;

wherein each of the first and second edges also includes at least one transition point with a change in curvature as the width transitions from a wider width to a narrower width, wherein a second derivative of a slope of the first and second edges at the transition point equals relative to the longitudinal axis is substantially 0.

8. The coupler according to claim 7 , wherein the at least three local minima or maxima comprises at least three maxima and at least two minima in each of the first and second edges; wherein a first derivative of a slope of the first and second edges at each local minima or maxima relative to the longitudinal axis is substantially 0.

9. The coupler according to claim 7 , wherein each of the at least one port optically coupled to the first and second region ends comprises two ports, wherein the first and second edges are symmetrical to each other about the longitudinal axis; and

wherein the plurality of different widths corresponding to the 5 to 20 local maxima and minima or other transition points spaced along each of the first and second edges;

whereby light entering one of the two ports optically coupled to the first region end is substantially equally split into the two ports optically coupled to the second region end.

10. The coupler according to claim 9 , wherein each of the plurality of different widths is substantially equal to 1.5 μm+/−10%.

11. The coupler according to claim 10 , wherein the two ports coupled to the first region end are spaced apart by a first port gap; wherein the two ports coupled to the second region end are spaced apart by a second port gap; and wherein the first port gap is substantially equal to the second port gap, which is substantially equal to 0.2 pm+/−50%.

12. The coupler according to claim 11 , wherein each of the ports coupled to the first and second region ends comprises:

a respective outer port side for optical coupling to an optical waveguide; and

a tapered section with a taper length equal to 1.0 μm+/−50%, a first taper width bordering the multimode interference region equal to 0.7 μm+/−20%, and a second taper width at the outer port side equal to 0.5 μm+/−20%.

13. A method of manufacturing a multimode interference (MMI) coupler comprising:

a multimode interference (MMI) region including:

first and second region ends separated by a region length along a longitudinal axis, and

first and second edges between the first and second region ends, defining the MMI region, which varies in width along the region length, each of the first and second edges includes at least three critical points defining at least three local minima or maxima;

at least one port optically coupled to the first region end; and

at least one port optically coupled to the second region end;

said method comprising:

determining the first and second edges for a predefined set of design parameters, using a computerized optimization algorithm; and

fabricating the MMI coupler with the first and second edges;

wherein each of the first and second edges also includes at least one transition point with a change in curvature as the width transitions from a wider width to a narrower width, wherein a second derivative of a slope of the first and second edges at the transition point equals relative to the longitudinal axis is substantially 0.

14. A method of manufacturing a multimode interference (MMI) coupler comprising:

a multimode interference (MMI) region including:

first and second region ends separated by a region length along a longitudinal axis, and

first and second edges between the first and second region ends, defining the MMI region, which varies in width along the region length, each of the first and second edges includes at least three critical points defining at least three local minima or maxima;

at least one port optically coupled to the first region end; and

at least one port optically coupled to the second region end;

said method comprising:

determining the first and second edges for a predefined set of design parameters, using a computerized optimization algorithm; and

fabricating the MMI coupler with the first and second edges;

wherein determining the first and second edges comprises:

a) generating a set of region widths, perpendicular to and spaced apart along the longitudinal axis, corresponding to the plurality of local minima and maxima, using the computerized optimization algorithm, thereby forming first and second interim edges; and

b) smoothing the first and second interim edges to generate the first and second edges including at least two local minima and at least three local maxima in each of the first and second edges.

15. The method according to claim 14 , wherein each of the first and second edges also includes at least one transition point with a change in curvature as the width transitions from a wider width to a narrower width, wherein a second derivative of a slope of the first and second edges at the transition point equals relative to the longitudinal axis is substantially 0.

16. The method according to claim 14 , further comprising setting the set of region widths to between 8 to 20 region widths corresponding to between 5 and 20 local maxima and minima.

17. A method of manufacturing a multimode interference (MMI) coupler comprising:

a multimode interference (MMI) region including:

first and second region ends separated by a region length along a longitudinal axis, and

first and second edges between the first and second region ends, defining the MMI region, which varies in width along the region length, each of the first and second edges includes at least three critical points defining at least three local minima or maxima;

at least one port optically coupled to the first region end; and

at least one port optically coupled to the second region end;

said method comprising:

determining the first and second edges for a predefined set of design parameters, using a computerized optimization algorithm; and

fabricating the MMI coupler with the first and second edges;

wherein determining the first and second edges comprises:

i) randomly generating a set of interim region widths, perpendicular to and spaced apart along the longitudinal axis, corresponding to the plurality of local minima and maxima, within a predefined widths range approximate to a predefined initial region width, using the computerized optimization algorithm, thereby forming first and second interim edges;

ii) smoothing the first and second interim edges;

iii) determining output optical power at each of the ports optically coupled to the second region end relative to a predefined input optical power at one of the ports optically coupled to the first region end, by simulating the MMI coupler for the set of interim widths and the predefined set of design parameters;

iv) calculating a figure of merit from the output optical powers and the predefined input optical power;

v) repeating steps i) to iv) until the figure of merit exceeds a predefined performance limit for a working set of interim widths; and

vi) setting the working set of interim widths as the set of region widths.

18. The method according to claim 17 , wherein the step of smoothing the first and second interim edges comprises interpolating the working set of interim widths, using a computerized interpolation algorithm.

19. The method according to claim 17 , wherein the computerized optimization algorithm comprises one of a particle swarm optimization algorithm and a genetic optimization algorithm.

20. The method according to claim 17 , further comprising setting the predefined widths range to between 80% and 120% of the predefined initial region width.