Beam Shaping with Extended Source

This example demonstrates how to use the ExtendedSourceOptic feature to model a collimated Gaussian beam (from a single-mode fiber) illuminating a beam shaping singlet. The singlet transforms the Gaussian input into a flat-top irradiance profile.

Key concepts: - Setting up an SMFSource (Single-Mode Fiber Source) - Wrapping an optical system with ExtendedSourceOptic - Visualizing the system with source rays - Computing the irradiance at the detector

import optiland.backend as be
from optiland.optic import Optic, ExtendedSourceOptic
from optiland.sources import SMFSource
from optiland.analysis import IncoherentIrradiance
from optiland.physical_apertures import RectangularAperture

Define the Optical System

We create a beam shaping singlet with a Forbes QBFS aspheric surface.

gaussian_beam_waist = 5.0  # in mm
wavelength_um = 0.55  # in µm

# Forbes QBFS surface parameters
forbes_terms = {
    0: 0.5414,
    1: 0.6689,
    2: 0.3409,
    3: -0.0537,
    4: -0.3960,
    5: -0.2991,
    6: 0.3921,
}
forbes_norm_radius = 30.3636
top_hat_radius = 25.0

lens = Optic()
lens.set_aperture(aperture_type='EPD', value=gaussian_beam_waist * 6)
lens.wavelengths.add(value=wavelength_um, is_primary=True)
lens.fields.set_type(field_type='angle')
lens.fields.add(y=0.0)

lens.surfaces.add(index=0, thickness=be.inf)
lens.surfaces.add(
    index=1, thickness=20.0, is_stop=True,
    aperture=RectangularAperture(x_min=-15, x_max=15, y_min=-15, y_max=15),
)
lens.surfaces.add(
    index=2, surface_type='forbes_qbfs', radius=5.7410, conic=-2.3165,
    thickness=15.0, material='N-BK7',
    radial_terms=forbes_terms, norm_radius=forbes_norm_radius, aperture=30.0,
)
lens.surfaces.add(index=3, surface_type='standard', radius=be.inf, thickness=70.0, material='air')
lens.surfaces.add(
    index=4,
    aperture=RectangularAperture(
        x_min=-top_hat_radius * 1.1, x_max=top_hat_radius * 1.1,
        y_min=-top_hat_radius * 1.1, y_max=top_hat_radius * 1.1,
    ),
)

Note on Normalization Radius (``norm_radius``):

In optiland, when you explicitly provide normalization parameters (like norm_radius for Forbes/Zernike surfaces, or norm_x/norm_y for Chebyshev surfaces) during geometry initialization, the surface’s normalization_mode is automatically set to 'manual'.

This means the radius is locked to your provided value and will not be automatically scaled during paraxial updates. If you omit the normalization parameters, the normalization_mode defaults to 'auto', and the normalization radius will dynamically scale to \(1.25 \times \text{semi-aperture}\).

Create the Extended Source

We use SMFSource with near-zero divergence to approximate a collimated Gaussian beam. Then we wrap the optic with ExtendedSourceOptic.

mfd_microns = gaussian_beam_waist * 2 * 1000  # MFD in µm

source = SMFSource(
    mfd_um=mfd_microns,
    wavelength_um=wavelength_um,
    divergence_deg_1e2=1e-10,  # Virtually zero divergence for collimation
    total_power=1.0,
    position=(0, 0, 0),
)

ext_optic = ExtendedSourceOptic(lens, source)
print(source)

SMFSource(mfd=10000.0µm, divergence=1e-10°, wavelength=0.55µm, power=1.0W, mode=extended, position=(0.0, 0.0, 0.0))

Draw the System

Visualize the optical system with the traced source rays.

fig, ax = ext_optic.draw(num_rays=1000, title='Beam Shaping Singlet')

Irradiance at the Detector

Compute and plot the irradiance distribution at the image plane.

analysis = IncoherentIrradiance(
    lens, source=source, num_rays=100_000,
    detector_surface=-1, res=(128, 128),
)
analysis.view(figsize=(7, 5), cmap='magma', normalize=False)

(<Figure size 700x500 with 2 Axes>,
 array([[<Axes: title={'center': '(User Rays: 0.0)'}, xlabel='X (mm)', ylabel='Y (mm)'>]],
       dtype=object))