Advanced examples

The options for placement and routing are more extensive than what are covered in the tutorials. In these examples we will see several powerful methods that provide additional tools for circuit layout design.

Generalized splitter tree

../../../_images/splitter_tree_parametric.png — Three-level splitter tree made in the tutorial

In the getting started tutorials we designed a splitter tree circuit. That design is already parametric, with the spacing between various components and the type of splitter used all being parameters. We can extend this approach even further, creating a circuit that is parametric to the number of levels in the splitter tree. To do this, all the instance creation, placement and routing must be parametric so that we simply pass in the size of splitter tree we want and the circuit is built for us. Whilst this may sound complicated, there are actually very few methods needed for this that we have not already seen. We will use a combination of f-strings and for loops with some basic logic to achieve the desired circuit.

By nesting a for loop within another for loop, we can iterate though each splitter in each level and add them to our instances dictionary.

luceda-academy/training/getting_started/2_circuit_layout/3_advanced_examples/explore_generalized_splitter_tree.py

        for level in range(self.n_levels):
            for splitter_no in range(2**level):
                insts[f"sp_{level}_{splitter_no}"] = self.splitter
        return insts

The placement is done in a similar way, where using nested for loops we can place all the MMIs at once.

luceda-academy/training/getting_started/2_circuit_layout/3_advanced_examples/explore_generalized_splitter_tree.py

        for level in range(self.n_levels):
            for splitter in range(2**level):
                x_coord = level * self.spacing_x
                y_coord = self.spacing_y * (
                    -0.5 * 2 ** (self.n_levels - 1) + ((splitter + 0.5) * 2 ** (self.n_levels - level - 1))
                )
                specs.append(i3.Place(f"sp_{level}_{splitter}", (x_coord, y_coord)))

We can also make use of Python logic to add another layer of control to our design. Since each splitter has two outputs, we can iterate through the splitters at each level and determine their connections based on whether the splitter number is even or odd. This allows us to connect all the splitters in each level with the splitters in the following level (except the last one) in a single for loop.

luceda-academy/training/getting_started/2_circuit_layout/3_advanced_examples/explore_generalized_splitter_tree.py

        for level in range(1, self.n_levels):
            for splitter in range(2**level):
                if splitter % 2 == 0:
                    in_port = f"sp_{level-1}_{int(splitter/2)}:out1"
                else:
                    in_port = f"sp_{level-1}_{int(splitter/2)}:out2"
                out_port = f"sp_{level}_{splitter}:in1"
                specs.append(i3.ConnectBend(in_port, out_port))

In this way, we can build a very large circuit with hundreds of outputs from the same code just by changing the number of splitter levels. You could extend this example to take the number of outputs as a parameter instead of the number of levels as an additional exercise.

../../../_images/four_level_splitter_tree.png — Four-level splitter tree made using the GeneralizedSplitterTree class

Advanced layout options

Using the parametric splitter tree, we can now create a more complicated circuit, which connects the splitter tree outputs to grating couplers. Waveguide connections from the splitter tree to the gratings is achieved using bundle routing and control points. To reuse the splitter tree circuit, we first import it from the other script and then rename it to Tree.