Laser Beam “Clean-Up” with Spatial Filter Abstract

To obtain good beam quality is important for many applications, and a typical experimental method to obtain good beam quality is the spatial filtering. Within a spatial filtering system, a pinhole is placed on the intermediate focal plane (i.e. the Fourier plane) to get rid of the unwanted spatial frequency components. To model such systems, one must consider the from the pinhole and the diffraction property of the laser beams, and we demonstrate the spatial filtering effect in this example.

2 Modeling Task

How does the size of the pinhole, located at the intermediate focal plane, influence the output beam? ? input field - #1 lens #2 632.8 nm feff = 25 mm feff = 25 mm - multimode laser D = 12.5 mm D = 12.5 mm with “noise” pinhole circular opening

Fourier plane

3 Spatial Filter with 7.5µm-Diameter Opening

A relatively large pinhole does not filter out much noise on the Fourier plane.

7.5 µm

behind pinhole

in front of pinhole

input field

pinhole 7.5 µm diameter lens #1 lens #2

4 Spatial Filter with 7.5µm-Diameter Opening

7.5 µm

behind pinhole

in front of pinhole

input field

output field pinhole 7.5 µm diameter lens #1 lens #2

5 Spatial Filter with 5.0µm-Diameter Opening

5.0 µm

behind pinhole

in front of pinhole

input field

output field pinhole 5.0 µm diameter lens #1 lens #2

6 Spatial Filter with 2.5µm-Diameter Opening

2.5 µm

behind pinhole

in front of pinhole

input field

output field pinhole 2.5 µm diameter lens #1 lens #2

7 Output Beam Profile and Power Comparison

lens #1 lens #2 pinhole varying diameter Spatial filter in the Fourier plane helps reduce the higher-frequency noise, therefore leads to better output beam profiles.

pinhole diameter 7.5µm pinhole diameter 5.0µm pinhole diameter 2.5µm

8 Output Beam Profile and Power Comparison

lens #1 lens #2 pinhole varying diameter Relatively small pinhole limits the transmitted and, as a consequence, leads to relatively large loss of power.

pinhole diameter 7.5µm pinhole diameter 5.0µm pinhole diameter 2.5µm

9 Peek into VirtualLab Fusion

flexible consideration of diffraction via Fourier transform settings

real lens modeling for laser beams

10 Workflow in VirtualLab Fusion

• Set up input Gaussian field − Basic Source Models [Tutorial Video] • Import lens systems from Zemax OpticStudio® − Import Optical Systems from Zemax [Use Case] • Set the position and orientation of components − LPD II: Position and Orientation [Tutorial Video] • Set the Fourier transforms properly

11 VirtualLab Fusion Technologies

nonlinear free crystals & components space prisms, anisotropic plates, components cubes, ... 1 waveguides & & fibers freeforms 1 1 2 1 1 & 3 scatterer Field 3 boundaries Solver

diffusers gratings 2 2 diffractive diffractive, beam Fresnel, meta splitters lenses SLM & micro lens & HOE, CGH, adaptive freeform DOE components arrays

# idealized component

12 Document Information title Laser Beam “Clean-Up” with Spatial Filter document code MISC.0082 version 1.0 edition VirtualLab Fusion Basic software version 2020.1 (Build 1.202) category Application Use Case - Pinhole Modeling in a Low-Fresnel-Number System further reading - Automatic Selection of Fourier Transform Techniques in Free-Space Propagation Operator

13 www.LightTrans.com