PhotoCAD Introduction
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**PhotoCAD** is a Python-based computer-aided design platform for photonics chip layout design. **fnpcell** and **link** are the two main tools of **PhotoCAD**, which correspond to the parametric cell layout and circuit-level layout in photonics chip layout design.


fnpcell
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**fnpcell** is a tool for parametric component layout design in **PhotoCAD** platform, covering active and passive components commonly used in current photonics chips, and the waveguides used in the devices cover ``FWG`` (Full etch waveguide), ``MWG`` (Medium etch waveguide) and ``SWG`` (Shallow etch waveguide). The parameterizable devices (i.e. Parameterized Cell) in **fnpcell** are generated by Python scripts, and the user has the flexibility to adjust the variable parameters in the python scripts to achieve custom single components.

link
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**link** is a design tool for automatic routing of devices in the **PhotoCAD** platform. The user can not only call the device and set the device position in the script, but also finish the automatic routing of the device through a simple port connection function **link**. 

**link** will first call the corresponding waveguide transition unit automatically according to the waveguide type of the device port to achieve the transition of the waveguide type at the port, and then call the corresponding straight, bend waveguide according to the location of the device port to complete the connection between the device ports. 

In addition, **link** also supports the automatic routing of the metal pins of the device and the metal layer transition in the process of implementation.