Universal Detector: Gridless Data
In VirtualLab Fusion, the propagation of a field through space or another homogeneous medium is evaluated in the k-domain. When the field arrives at a detector and diffraction effects are negligible, the software automatically selects a pointwise inverse Fourier transform to calculate the field in the x-domain. This transformation yields gridless data at discrete points on the detector.
By default, gridless data is interpolated onto a regular grid and presented as a colormap in the general profile. To plot the gridless data directly, double-click on the Universal Detector in your optical setup and select the Gridless Data tab. Then, click the button labeled “Yes,” as illustrated below.
In the following example, we have set up a Gaussian Wave source with a wavelength of 532 nm and a waist radius of 5 µm. The farther the Universal Detector is positioned from the beam waist, the less significant diffraction becomes. Beyond a certain distance, the software automatically selects the pointwise inverse Fourier transform.
We have positioned a Universal Detector 30 mm away from the input plane and ensured that the propagation to the detectors is set to Automatic.
Running the General Profile gives the following result for the gridless data:
When the Universal Detector is placed closer to the source (e.g., at 10 mm), the software selects a more accurate inverse Fourier transform that operates on a regular grid. Consequently, the data is no longer available as gridless data, and the Universal Detector will display the result as a colormap.
Tips & Tricks
It is also possible to enforce pointwise propagation to the detector.
However, this will also enforce a pointwise Fourier transform at the previous component, in this case, the Gaussian source. In the present example, this would result in a large error, as the small waist of the source leads to strong diffraction and subsequent expansion of the Gaussian beam. Ignoring diffraction would produce a collimated beam with a 5 µm radius, which is entirely incorrect.
Nevertheless, enforcing pointwise propagation can be beneficial, for instance, when assessing the importance of diffraction effects or during the design phase, where a quick test run is required.
Selected Use Cases
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