FRESNEL Quick Guide


Below Reader will find several examples of calculations that can be run and examined using FRESNEL.

Working with an existing Scheme

To open an existing scheme, do the following:

Choose File | Open Scheme. FRESNEL displays the Open File dialog box.
Specify the file containing the scheme you want to open, e.g., propagat.scm.
Press OK or Enter.

Example 1. Propagation

The Scheme Window appears. In the case of propagat.scm it contains Source, Target, and three View screens.

In propagat.scm each View screen has a label indicating the distance from the Source. All elements are sitting on a connecting line.
To open the element dialog box just double click its pictogram with your mouse. If you do this with View screens of our propagat.scm you will see real distances between previous and current elements (respectively 25, 25, and 50 cm). You will also notice that the function "pause" is on. It means that when the beam hits the View screen the propagation calculations are stopped till next Run command.
You can also double click the Target. You will see in this case the Target distance is 900 cm from the previous View screen, thus the total distance between Source and Target is 10 m. You will notice also that the Magnification is 1.
Now you can double click the Source. If you do this you will see the list of Input Beam Shapes.
You can choose any, but here you need file propagat.pls. So choose Load from file | propagat.pls. And in the "In" Beam Window you will get a beam of predefined type: flat-topped square beam of 0.44 by 0.66 cm given on a scale 1.28 cm (with pixels number 128x128), rotated by 30 deg (in respect to horizontal X-axis). The beam wavelength is 530 nm and the energy is 1 mJ. The softening scale (see Help for this term) is 0.015 cm. (As soon as propagat.pls is of the predefined type its parameters can be checked by double clicking Source again and choosing Flat-topped beam. Try this and return back pressing Cancel.).

Now you are ready to run the scheme. Choose Run or corresponding button on the Main FRESNEL Toolbar.
The calculations are started and in a second the Current Beam Window will appear. The picture will change after passing prescribed distance to the next element. When the calculation discretization is small and your computer is fast you will not be able to follow all the beam changes. Thus before or after an element you can arrange pauses or checkpoints (as we did in propagat.scm). Using buttons in the Beam Window you can examine beam distributions on a linear or logarithmic scale, take profiles along X and Y axes, observe temporal shape changes (if any), and so forth. Pressing Run again you will proceed to the next element. The color of the passed element turns green.
Note that FRESNEL uses "Automatic Scale Adjustment Procedure" to minimize the aliasing effect. So if you propagate the beam "too far" it can occupy only small portion of the Beam Window pane (as it is at the Target in propagat.scm).
You may apply Zoom = 8 and press button that will display zoom area, but this does not result in getting more information, just number of the pixels displayed will be 64 times less.
DO NOT WORRY! THE RESOLUTION CAN BE IMPROVED! But you will have to edit the Target parameters. Choose Magnification=8 and pause - "on". Restart calculations from the beginning. Now after finishing all calculation steps, in front of the Target you will see small size beam again, but pressing Run for the last time you trigger the Magnification 8X procedure. After finishing it (Target change color to green) you will have much more detailed picture than it was in the case when Zoom=8 was used. Now the number of the pixels in the image is 128x128 again. Just compare.

Zoom = 8 Magnification = 8

The pictures above exhibit pronounced diffraction of the beam.
More detailed study you can perform for the diffraction onto the sharp edge diaphragm. It is a "touchstone" for numerical calculations in optics.

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