Spatial light modulator (SLM) is a general term describing devices that are used to modulate amplitude, phase, or polarization of light waves in space and time. HOLOEYE´s Spatial Light Modulator systems are based on translucent (LCD) or reflective (LCOS) liquid crystal microdisplays.
The use of LC materials in SLMs is based on their optical and electrical anisotropy. A certain gray level represents a defined average voltage across the LC cell. This voltage leads to a variable tilt of the LC molecules due to their electrical anisotropy. As LC molecules also show optical anisotropy this tilt changes the refractive index of the LC molecules (for suitable incident polarization, dependent on device version) which causes a modified optical path length within the LC cell. The addressed gray level is now converted into a phase level.
HOLOEYEs SLMs are based on vertical aligned nematic (VAN),parallel aligned nematic (PAN) or twisted nematic (TN) microdisplay cells. In a twisted cell, the orientation of the molecules differs by typically 45°/90° between the top and the bottom of the LC cell and is arranged in a helix-like structure in between. In VAN / PAN cells the alignment layers are parallel to each other, so the LC molecules have the same orientation.
To use an SLM in amplitude modulation mode you need linear incident polarisation. The transmitted or reflected light has to be guided through a 2nd polariser (analyser) that is crossed to the incident polarisation. For phase modulation a setup without an analyser is used. With devices based on twisted nematic LC or LCOS displays the twist always causes a polarisation effect (amplitude modulation) and no phase only modulation is possible (phase mostly modulation).
With VAN / PAN displays it is possible to modulate the phase without influence on the polarisation / amplitude (phase only modulation) by using incident polarization alang the LC director axis.
HOLOEYE Spatial Light Modulators:
|Device||SLM Type||Resolution||Pixel Pitch||Fill Factor||Wavelengths Range|
|GAEA-2||Reflective LCOS SLM||max. 4160×2464 (4000×2160)||3.74 µm||90%||420 – 1100 nm (Different Versions),
|PLUTO-2||Reflective LCOS SLM||1920 x 1080 Pixel||8 µm||93%||350 – 1700 nm (Different Versions)|
|LETO||Reflective LCOS SLM||1920 x 1080||6.4 µm||93%||420 – 1100 nm (Different Versions)|
|LC 2012||Transmissive LC SLM||1024 x 768 Pixel||36 µm||58%||420 – 800 nm|
Definitions of Specs
- Resolution: Number of pixels (width x height)
- Pixel Pitch: Size of a pixel including the interpixel gap
- Fill Factor: Surface area of the display which can actively used. There are gaps between the pixels at which the incident light is scattered.
- Active Area: Size of the actual adressable/usable display area.
- Addressing: Number of gray levels / phase levels that can be addressed. This can vary with addressing sequences.
- Signal Formats: Input signal format. Typically HDMI or DVI.
- Input Frame Rate: Addressing speed of the input signal (typically DVI / HDMI video frame rates of 60 Hz for monochrome applications).
- Response Time: The response time is defined as the switching time from 10% to 90% and from 90% to 10 % (rise and fall time). The actual response time of the liquid crystal is determined by the properties of the used liquid crystal material, the thickness of the LC layer, the used drive sequence / calibration (the actual voltages applied to a pixel) and temperature.
For phase SLMs the response time typically is below the Input frame rate.
- Reflectivity: Amount of light which is directly reflected (0-order of a non-addressed display). The reflectivity is not 100% as some of the light is diffracted into higher orders due to the grating like structure of the pixel matrix. Some part of the light is also scattered and absorbed at the interpixel gaps. In addition the reflectivity of the aluminum mirror is limited (dependent on wavelength).
Addressing the SLM:
The optical function or information to be displayed on HOLOEYE Spatial Light Modulators can be taken directly from an optic design software or an image source and can be transferred by a computer interface. Using DVI/HDMI ports of standard PC graphics cards, the Spatial Light Modulator can be used just like an external plug & play monitor. No additional software or dedicated hardware is needed to operate the SLM. In many cases even no additional optics are necessary.
Possible Spatial Light Modulator Applications:
- Imaging & Projection
- Display Applications
- Holography (Display holography, holographic memory, holographic recording and security systems, including digital holography)
- Holographic Projection
- Beam Splitting
- Laser Beam Shaping
- Coherent Wavefront Modulation
- Phase Shifting
- Optical Tweezers
- Laser Pulse Modulation