GPU-Accelerated Optical Propagation Demo (LG Beam, ASM, 2-Step Fresnel)

This repository contains a compact MATLAB demonstration of
GPU-accelerated scalar optical field propagation using two independent methods:

• Angular Spectrum Method (ASM)
• Two-Step Fresnel Propagation with Scaling (Voelz formulation)

The workflow generates a Laguerre–Gaussian beam, propagates it through free space or a thin lens, and produces quantitative intensity and phase comparisons between the two propagation techniques.

(The images above were generated directly from the provided MATLAB scripts.)

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✦ Features

• Laguerre–Gaussian (LG) mode generator (lg_mode_2d.m)
• GPU-accelerated FFT propagation using gpuArray
• Angular Spectrum Method (ASM)
• Two-Step Fresnel Scaling Propagator (Voelz, Appendix B)
• Thin-lens quadratic phase operator (Goodman)
• Sampling & Fresnel number diagnostics
• Optional ring aperture mask
• Visualization of intensity & phase differences

All propagation is performed on the GPU, and results are gathered only for visualization, minimizing unnecessary data transfers.

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✦ File Overview

main_propagation_demo.m     → Main script (LG beam, propagation, plots)
lg_mode_2d.m                → LG(p,l) mode generator
two_step_fresnel_gpu.m      → Fresnel scaling GPU propagator (Voelz)
angular_spectrum_gpu.m      → Angular Spectrum Method (ASM)
thin_lens_phase_gpu.m       → Thin-lens phase operator
sampling_criterion.m        → Sampling check (Voelz)
fresnel_number_criterion.m  → Fresnel number diagnostic
ring_aperture_mask_gpu.m    → Optional ring aperture (annulus)
draw_circle.m               → Helper for mask generation
center_overlay.m            → Helper for mask placement

/assets
    intensity_ASM_vs_2step.png
    phase_ASM_vs_2step.png

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✦ Requirements

• MATLAB R2020b or later
• Parallel Computing Toolbox
• NVIDIA GPU with CUDA support
• (Optional) Image Processing Toolbox for enhanced visualization

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✦ Usage

Run the main script:

main_propagation_demo

Toggle main options:

use_lens     = 1;     % include thin lens operator
use_pup      = 0;     % enable ring aperture mask
save_figures = 0;     % save PNG figures

Modify LG beam:

p = 0;    % radial index
l = 1;    % azimuthal index

Adjust simulation window and sampling:

L_input = 1.28e-2;   % meters
M = 2048;            % samples per dimension
z = 0.30;            % propagation distance

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✦ References

• D. G. Voelz, Computational Fourier Optics, SPIE Press

  • Two-step Fresnel scaling (Appendix B)
  • Thin-lens transform (Eq. 6.12)

• J. W. Goodman, Introduction to Fourier Optics

  • Thin-lens phase transmittance
    • 2nd Ed., Sec. 5-2
    • 3rd Ed., Sec. 4.2.2

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✦ License

MIT License