Spatial LIGHT Modulators BUYER ' S GUIDE beam, enabling continuous, pixelfree modulation, but at the cost of a more complex optical architecture.
SLM MAJOR FAMILIES( COMMERCIALLY-AVAILABLE, ZOOM ON LCOS) Most commercial SLMs are electrically addressed, driven through a standard digital video interface in which each grey level corresponds to a given voltage applied to the LC layer. They fall into two broad geometries, transmissive and reflective, with the reflective LCoS architecture dominating the scientific and industrial market. Transmissive SLMs. Transmissive devices confine a nematic LC layer between two transparent conductive windows, at least one of which carries a segmented electrode array providing individual pixel control. The most widespread example is the liquid crystal display( LCD), based on a twisted-nematic( TN) LC configuration placed between parallel or crossed polarizers, primarily used for amplitude modulation. Transmissive SLMs offer the advantage of a straightforward in-line optical geometry, but are limited to larger pixel pitches( typically a few tens of ยต m) and lower fill factors than their reflective counterparts. Reflective SLMs: LCoS. The dominant technology for scientific and industrial applications is Liquid Crystal on Silicon( LCoS), in which a nematic LC layer is included between a transparent electrode and a CMOS silicon backplane carrying the pixel addressing circuit( Figure 3). Light passes through the LC layer twice upon reflection from the mirror integrated into the CMOS matrix, increasing the achievable phase modulation range for a given LC thickness. This geometry also enables very high pixel densities. Within the LCoS family, it is worth distinguishing two addressing schemes. Most devices use a digital drive, which can introduce phase fluctuations at the frame rate, the so-called flicker. An alternative
Figure 2. Electro-optical properties of nematic liquid crystal layers enable to locally change the phase of the propagating polarized light.( a) Nematic mesophase.( b) Typical LC cell. Planar anchoring conditions can be vertical( VAN) or horizontal( PAN) or both, e. g. twisted( TN).( c) The application of the electric field induces an average molecular rotation, which in return changes the refractive index.( d) Such a simple device allows for the modulation of the phase, amplitude or polarization of light according to the design details and the presence or absence of additional polarizing elements.
analogue drive, recently brought to market by Holoeye, applies a continuous voltage to each pixel, delivering superior phase stability and reduced flicker at the cost of a more complex driver design. Optically addressed SLMs( OASLMs). A distinct and more specialized family is that of optically addressed light valves, in which a photo-conducting layer replaces the pixelated electrode. The local voltage across the LC layer is controlled by an auxiliary writing beam, and the spatial modulation pattern is imprinted optically rather than electrically. This eliminates pixelization, yielding a 100 % fill factor and continuous phase modulation, properties particularly attractive for applications where interpixel gaps and diffraction from the pixel structure are problematic.
Figure 3. Cross-section of a LCoS SLM and driving waveforms for analogue( top) and digital( bottom) addressing.
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