Spatial light modulator

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A spatial modulator for light (English: Spatial Light Modulator (SLM) ) is a device to impress a spatial modulation on light. A simple example is the slide of an overhead projector. In the 1980s, large SLMs were placed on overhead projectors in order to project the content of computer monitors onto a surface (wall or screen). There are now SLMs that are built into projectors . These are used for presentations, for example.

Usually, an SLM modulates the intensity of a light beam, but it is also possible to modulate the phase or the phase and the intensity at the same time.

Modulator types

Electronically controllable SLM (EASLM)

The pixels in an EASLM are addressed electronically and corresponding images are generated. Communication with one of the signal transmitters usually takes place via conventional interfaces (e.g. a VGA input). They are commonly available in resolutions up to SXGA (1280 * 1024). In contrast to conventional displays, they are much smaller (the size of the active area is 2 cm²). An example of an EASLM is the Digital Micromirror Device , which is located in a DLP display (better known as a projector).

Optically controllable SLM (OASLM)

In the case of an OASLM (also known as an optical valve ), a light beam with an embossed pattern is sent to the display, which is simulated by the modulator. The image information is usually retained on the display even without the presence of light, but it can be deleted optically or electronically.

OASLMs are often used as the second stage of a high resolution display, e.g. B. a computer generated holographic display . In a process called “active tiling”, images displayed on an EASLM are successively transferred to different parts of an OASLM. Then the image of the OASLM is shown to the viewer. Since EASLMs can generate up to 2500 frames / s, it is possible to copy approx. 100 copies of the EASLM's image to the OASLM, whereby a smoothly running video is still displayed on the OASLM. This enables resolutions of over 100 megapixels.

Applications

Pulse shaping

To short laser pulses to form in their temporal structure, you can use an SLM. Here, the pulse is first sent through a dispersive element (e.g. a diffraction grating or a prism ) in order to spatially separate the frequency components. With the aid of spatial phase modulation, the individual frequency components can now be delayed relative to one another. The division into the individual frequency components is reversed by sending the light to a dispersive element again. In principle, all possible pulse shapes can be generated in accordance with the phase modulation.

Holographic data storage

SLMs are used extensively in holographic data storage to encode information in a laser beam, similar to the way in which transparency is used in an overhead projector. They can also be used as part of a holographic display.

Optical grids

Likewise, modulators can be used to produce optical grids in which atoms can be trapped and localized with the help of the intensity pattern impressed on a laser beam .

literature

  • Digital Light Processing for High-Brightness, High-Resolution Applications , Larry J. Hornbeck ( TI ), 21st century Archives [1]
  • Optically addressed spatial light modulators for replaying computer-generated holograms , Coomber, Stuart D .; Cameron, Colin D .; Hughes, Jonathon R .; Sheerin, David T .; Slinger, Christopher W .; Smith, Mark A .; Stanley, Maurice ( QinetiQ ), Proc. SPIE Vol. 4457, p. 9-19 (2001)
  • Liquid Crystal Optically Addressed Spatial Light Modulator , [2]
  • Computer-Generated Holography as a Generic Display Technology , Slinger, C .; Cameron, C .; Stanley, M .; IEEE Computer, Volume 38, Issue 8, Aug. 2005, pp 46-53, PDF