Posted by Prateek Jain on May 27, 2020
CODE V’s Beam Synthesis Propagation (BSP) option simulates the general diffraction propagation of an optical field through an optical system. Its output can be as comprehensive as the complex 3D vector information of the optical field at any surface throughout the optical system.
BSP does this by first representing the input optical field as a coherent summation of a set of proprietary smaller beamlets. These individual beamlets are then propagated through the system. Where output is desired, the individual beamlets are coherently summed to obtain the optical field at the desired surfaces. Not only is this method very accurate, it is also extremely easy to setup. The ability to both export and import beamlet sets or a general complex optical field make this a very versatile analysis option. BSP has many built-in features for generating output, and similar to other options in CODE V, knowledge of Macro-PLUS can be very useful in maximizing the utility of the capability.
General beam propagation enables various analysis where the ray-based approaches (or ray-based analysis with diffraction at the system exit pupil) cannot achieve the required accuracy. An example is when we have a very slow beam as shown below.
Figure 1: Rays in the above plot obviously cannot represent the actual situation of the lower figure.
We need general beam propagation for these situations.
BSP can support several unique applications. PSF and MTF calculations can sometimes benefit from BSP. One method for computing the PSF in optical design software is to perform a Fourier Transform of the complex amplitude in the exit pupil. The amplitude and phase information in the exit pupil is determined by tracking the intensity and OPDs of ray grids traced through the system. However, sometimes this is not a sufficiently accurate, for example when there are multiple apertures separated by some distance. Physically, light is not simply blocked by apertures forward in the system, but these apertures act as sources of diffracted light, which can change the optical field at the next clipping aperture, and so on. A purely ray-based approach does not take such multiple-diffraction effects into account. BSP can be used to calculate the PSF for system including diffraction from all clipping apertures, and there is a supplied macro (CV_MACRO:MTF_FROM_BSP.SEQ) to calculate MTF from a PSF computed by BSP.
BSP can model Mid-Spatial-Frequency Surface Errors described by the Surface Power Spectral Density. These are random corelated surface errors with some Fourier signature as shown below.
Figure 2: PSD definition used in BSP.
BSP calculates the ensemble average effect of these surface errors on the optical field.
For some systems, such as interferometers or systems with birefringent materials, you may need to combine the results from several BSP runs. The versatility of BSP with the Macro-PLUS makes this easy. For example, you can model the results of metrology interferometer in CODE V by running BSP on the reference arm, and outputting the complex field at a reference surface; separately running BSP on the test path (with the surface to be measured) and outputting the complex field at the same reference surface on the same output grid; and then use Macro-PLUS to do a coherent summation of the two complex fields. This new complex field represents the interference fringes that the interferometer would produce.
Figure 3: Interferometer fringes modeled using BSP.
BSP is detailed in the CODE V Generalized Beam Propagation and Coupling Efficiency Reference Manual. For more information, check out the General Beam Propagation section from the CODE V Core Tools training material on the Customer Support Portal. Another place to look is the CODE V Feature Close-Up: Beam Synthesis Propagation. For any questions about BSP, contact us at firstname.lastname@example.org, and we’ll be happy to answer your questions about this CODE V feature.
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