Libraries and projects · 3 of 3

Project: splitter tree

The last project uses nearly everything in the tour: a splitter tree that divides one input into 2^level outputs, built from 1×2 MMI splitters joined by S-bends.

MMI1x2 is given; its two outputs sit out_offset above and below its axis. The tree grows one level at a time. At each level it places a splitter on every port that still needs feeding and fans the two outputs of that splitter apart with S-bends:

the first level has one splitter, at the input, and spreads its outputs to spread / 2 above and below its axis;
each following level has twice as many splitters and half the spread;
after level levels, neighbouring outputs are pitch apart, so the first spread is pitch × 2^(level − 1).

The key is a list of ports. It starts as [self.origin]; at every level, a loop over it places the splitters and collects the outputs of their S-bends in a new list, which replaces the old one for the next level.

The S-bend that goes up shifts by spread / 2 − mmi.out_offset, since the output it starts from is already out_offset above the axis; the one that goes down shifts by the negative of that.

Exercise

Write the body of MMITree so that it builds the tree for any level and pitch, starting with a splitter at the origin.

That is the end of the tour. The language reference and the playground are good places to go on from here.

loading compiler…

import "components.lyl"

# A 1x2 MMI splitter; its outputs are `out_offset` above and below its axis.
cell MMI1x2 {
    param {
        wg_len     size = 10um
        wg_width   size = 500nm
        mmi_len    size = 30um
        mmi_width  size = 6um
        out_offset size = 1.5um
    }

    marker {
        input {
            at inp_wg.left_center
            with type = "waveguide", direction = "input", width = wg_width
        }
        output {
            at {
                out_wg1.right_center
                out_wg2.right_center
            }
            with type = "waveguide", direction = "output", width = wg_width
        }
    }

    layer si {
        shape rect with wg_len, wg_width as inp_wg
        shape rect with mmi_len, mmi_width as mmi_area
        clone inp_wg as out_wg1
        clone inp_wg as out_wg2

        place {
            inp_wg.left_center   at self.origin
            mmi_area.left_center at inp_wg.right_center
            out_wg1.left_center  at mmi_area.right_center offset (0, out_offset)
            out_wg2.left_center  at mmi_area.right_center offset (0, -out_offset)
        }
    }
}

# A binary tree of splitters: 2^level outputs, `pitch` apart.
cell MMITree {
    param {
        level integer = 3
        pitch size    = 10um
    }

    marker {
        input { at self.origin with type = "waveguide", direction = "input" }
    }

    place {
        MMI1x2 at self.origin as mmi
    }
}

import "components.lyl"

# A 1x2 MMI splitter; its outputs are `out_offset` above and below its axis.
cell MMI1x2 {
    param {
        wg_len     size = 10um
        wg_width   size = 500nm
        mmi_len    size = 30um
        mmi_width  size = 6um
        out_offset size = 1.5um
    }

marker {
        input {
            at inp_wg.left_center
            with type = "waveguide", direction = "input", width = wg_width
        }
        output {
            at {
                out_wg1.right_center
                out_wg2.right_center
            }
            with type = "waveguide", direction = "output", width = wg_width
        }
    }

layer si {
        shape rect with wg_len, wg_width as inp_wg
        shape rect with mmi_len, mmi_width as mmi_area
        clone inp_wg as out_wg1
        clone inp_wg as out_wg2

place {
            inp_wg.left_center   at self.origin
            mmi_area.left_center at inp_wg.right_center
            out_wg1.left_center  at mmi_area.right_center offset (0, out_offset)
            out_wg2.left_center  at mmi_area.right_center offset (0, -out_offset)
        }
    }
}

# A binary tree of splitters: 2^level outputs, `pitch` apart.
cell MMITree {
    param {
        level integer = 3
        pitch size    = 10um
    }

marker {
        input { at self.origin with type = "waveguide", direction = "input" }
    }

place {
        MMI1x2 at self.origin as mmi
    }
}

Edit the code, watch the layout, then press Submit to check your solution against the exercise.

Compiling…