Answer:
Please find attached an example project for a thin lens.
The Thin Lens is modeled by the thin lens approximation [1]. Thus, the input light is assumed to be directly incident on the lens and the output signal is given at a distance equal to the focal length.
To model the effect of free space propagation on a spatial profile please try the Spatial Connector. The Spatial Connector is used to translate in free space (z-axis) the transverse optical mode(s). This is done using the Distance parameter. For the example shown, the Spatial CW Laser is the source located 10 microns from the thin lens. The second Spatial Connector translates the optical wave another 65 microns. The model is thus showing a source wave-front (single or multiple transverse modes) propagating, using a diffraction integral model, a distance of 10 microns to the thin lens, undergoing an x-y spatial phase transformation, and then finally propagating another 65 microns to be viewed by a Spatial Visualizer. This visualizer provides an intensity distribution plot for the different propagating modes.
The Spatial Connectors can also be used to translate or rotate the x-y origin of the transverse optical modes, so that the x, y, and z offsets can be defined between, for example, a spatial laser source and a fiber or a waveguide.
To vary the lens diameter, check the Aperture effects box in the Main tab of the Thin Lens component. This will release the diameter and reflectance parameters.
[1] J. W. Goodman, “Introduction to Fourier Optics”, McGraw-Hill, New York, NY 1996.