add models for lattice

[joamatab]

Summary by Sourcery

Adds a new mzi_lattice_sc cell and a corresponding example script for circuit simulation. Updates coupler models.

New Features:

• Introduces a mzi_lattice_sc cell, a Mach-Zehnder interferometer lattice structure, to the cspdk.si220.cells.primitives module.
• Adds a new example script, circuit_lattice.py, to demonstrate circuit simulation of the mzi_lattice_sc cell.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request introduces a new mzi_lattice_sc cell for creating MZI lattices, updates the coupler models to use sm.coupler, and provides a sample script for simulating the MZI lattice. The mzi_lattice_sc cell simplifies the creation of MZI lattices by pre-configuring an MZI with lattice-specific settings. The update to coupler models ensures the use of the correct simulation model. The circuit_lattice.py script demonstrates how to simulate the new cell.

Sequence diagram for simulating the MZI lattice

sequenceDiagram
    participant circuit_lattice.py
    participant cells.mzi_lattice
    participant PDK.models
    participant sax.circuit
    participant circuit

    circuit_lattice.py->>cells.mzi_lattice: cells.mzi_lattice(delta_length=10)
    circuit_lattice.py->>PDK.models: models = PDK.models
    circuit_lattice.py->>sax.circuit: circuit, _ = sax.circuit(netlist, models=models)
    circuit_lattice.py->>circuit: S = circuit(wl=wl)

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Class diagram for the new mzi_lattice_sc cell

classDiagram
    class mzi_lattice_sc{
        +delta_length: float
        +bend: str
        +straight: str
        +splitter: str
        +combiner: str
        +cross_section: CrossSectionSpec
        +gf.Component mzi_lattice_sc()
    }
    note for mzi_lattice_sc "New cell for creating MZI lattices"

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File-Level Changes

Change	Details	Files
Added a new cell mzi_lattice_sc for creating a Mach-Zehnder interferometer (MZI) lattice.	Defined a new mzi_lattice_sc cell that instantiates an MZI with specific parameters. The cell takes parameters such as delta_length, bend, straight, splitter, combiner, and cross_section to customize the MZI. It uses the existing gf.c.mzi component as a building block, pre-configuring it with specific port names and other settings suitable for lattice applications.	cspdk/si220/cells/primitives.py
Updated the coupler_sc, coupler_rc, coupler_so, and coupler_ro models to use sm.coupler instead of sm.mmi2x2.	Replaced sm.mmi2x2 with sm.coupler in the definitions of coupler_sc, coupler_rc, coupler_so, and coupler_ro models. This change likely reflects a correction or update in the preferred model for couplers within the PDK.	cspdk/si220/models.py
Added a new sample script circuit_lattice.py to demonstrate circuit simulation of the MZI lattice.	Created a new file circuit_lattice.py. The script instantiates the mzi_lattice cell. It performs a circuit simulation using sax and plots the transmission spectrum. The script showcases how to use the new mzi_lattice cell in a circuit simulation workflow.	cspdk/si220/samples/circuit_lattice.py

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