THORLABS EXULUS-4K1 Instructions Manual page 11

Holographic Projection
We show here a holographic projection as one of the applications of the Exulus SLM.
Figure 2 shows a typical setup required to realize a 2D holographic projection using the
EXULUS-HD1. A collimated laser beam is incident on the SLM panel; best results for
holographic projection are obtained using a beam size just smaller than Ø7 mm. The
incident beam passes through a polarizer and a half-wave plate such that it is polarized
at 45°, the direction of the optical axis of the panel. The target projection image is first
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converted into a computer generated holography (CGH) pattern that is calculated by
Figure 2: Setup for CGH using the EXULUS-HD1 SLM. In this case, the
the bundled software (accessed through the CGH tab in the software). The output
focusing effect added to the CGH is set to a 100 000 mm focal length.
beam is separated from the incident beam with a beam splitter. Within the SLM
[APPLIST] [APPLIST]
software, a focusing effect is added to the CGH pattern; if you do not desire to have
These product lists do not include the optical breadboard, laptop, or
the SLM provide focusing, set the focal length to an extremely long value. In this screws used to mount the post holders to the breadboard.
example, we have set the focal length to 100 000 mm (at the laser wavelength of 635 nm), checked the "Invert Image" box to set the image outline to be bright in
the projected pattern, and set the position to "Fit" so that the entire image is visible on the preview. Since CGH relies on far-field diffraction, a set of imaging
lenses is required to produce a sharp holographic image on a screen (in this example, first lens: f = 50 mm, second lens: f = 75 mm). Figure 3 shows the
holographic image to be projected and the corresponding CGH pattern, which is calculated using the bundled software. The resulting holographic projection is
shown in Figure 4.
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Figure 4: CGH projection of an image using the setup
shown in Figure 2 and the image shown in Figure 3.
The central bright spot is a zero-order diffraction spot
due to the gaps between the SLM pixels.
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b.
Click for Full-Resolution Example Image
a.
c.
Figure 3: a. The image used to generate the holographic projection.
b. The corresponding CGH pattern generated by the Exulus software.
c. The CGH settings tab in the Exulus software.
Effects of Focusing and Tilt
Due to the fill factor of the SLM panel, there is a small gap between the pixels. This in turn causes higher diffraction orders and a high-energy zero-order spot
which is unaffected by the SLM, but inherently exists at the output. The center bright spot often overlaps with the holographic projection on the same image
plane, as is visible in Figure 4. It is highly preferable to remove this zero-order-spot in many applications.
In order to further enhance the holographic image, one can adjust the focusing effect added to the CGH pattern. This causes the CGH projection to focus itself
without the requirement of imaging lenses. Since the zero-order spot is not affected by the focusing parameter, the CGH projection will form an image while the