Spatial light modulators (SLM)

Recently, I have got interested in spatial light modulators (SLMs). SLMs are to optics what programmable logic is to computing. They allow modulation of certain properties of light (e.g., amplitude, phase, polarization).  I thought of noting down my finds about available technologies and update them as I stumble upon new information. A good question to ask is – ‘what do we precisely mean  by modulation’? Modulation here means changing two dimensional distribution of amplitude, phase or polarization of light at given plane. I am particularly interested in amplitude modulators for automating our new quantitative imaging method based on oblique-illuminaiton.  Here are links to our conference abstracts and a journal paper:

• “Quantitative phase-gradient imaging at high resolution with asymmetric illumination-based differential phase contrast” in Optics Letters 2009
• “Linear Phase-Gradient Imaging with Asymmetric Illumination Based Differential Phase Contrast (AIDPC)” @ Novel Techniques in Microscopy 2009
• “Full-field phase-gradient contrast methods for label-free quantitative imaging of cellular morphology: AIDPC and DIC” @ Focus on Microscopy 2009
• “Asymmetric illumination based differential phase contrast (AI-DPC) for full-field transmission imaging of phase information in biological specimens” @ Focus on Microscopy 2008

I classify the spatial light modulators depending on the type of modulation they are designed to provide. Most of the time, one would like to modulate either amplitude, phase, or polarization without affecting the other properties. In basic optics research, SLMs are widely used for phase modulation in areas of holography, optical communication, optical trapping, beam shaping, adaptive optics etc. In commercial applications, amplitude modulation dominates where SLMs are used for writing dynamic patterns that are projected. Polarization modulation has been used in design of programmable spectral filters called Lyot filter and quantitative birefringence imaging devices called Polscope. As noted next,  all SLM technologies except for digital micro-mirror devices (DMD) and deformable mirrors employ birefringence modulation under electronic control for achieving amplitude, phase or polarization modulation. Most of the birefringent reflective devices seem to use LCoS (liquid crystal on silicon) technology. Reflection based geometry allows putting control logic behind the liquid crystals leading to high density and possibility of calibration. In transmission SLMs, the logic is implemented around the pixels which limits amount of intelligence (e.g. calibration circuit) that can be built in and causes spurious diffraction.

Amplitude modulators:

If you want to perform binary modulation (black and white), digital micro-mirror devices (DMD) and ferroelectric liquid crystals (FLC) are the technologies of the day. For grayscale amplitude modulation, twisted nematic liquid crystal (TNLC) devices seem to be the most suitable.

• DMDs are micro-mechanical devices in which a tiny mirror is mounted on a semiconductor chip whose orientation is controlled by currents produced on the chip under programmable control. Texas Instruments invented DMDs for projection applications with goal of substituting the roll of film by this single device. DMDs are fast and one can update the patterns at the rate of MHz. Advantages of DMDs include nearly-polarization-insensitive modulation, fast switching time, high damage threshold, and low cost. But, they are bit awkward to use because of their reflection based geometry which requires oblique illumination to separate unmodulated and modulated light. There are some prism-based optical modules noted on TI website which provide easy integration of DMD in light path.
• FLC devices use bistable liquid crystal whose birefringence can be switched at MHz rate. By placing FLC between two crossed or parallel polarizers, one can perform binary  modulation. They are little pricier than DMDs and available from Displaytech. Depending on your application, FLCs polarization sensitive operation may be an advantage or disadvantage. They operate at lower power than DMDs and allow smaller pixel sizes. They can be used in reflection (which is frequent) or transmission.
• Both types of black and white technologies can mimic grayscale modulation due to their fast switching speed. Grayscale modulation can be achieved by temporal modulation  at rates much faster than integration time of the detector. This works fairly well with commecial projection applications as visual integration time is around 33ms (1/30 s).
• True grayscale modulation can be obtained with twisted-nematic liquid crystal (TNLC) devices, which again can be used in transmission or in reflection between polarizers.  Holoeye and Meadowlark both supply twisted-nematic LCDs. Holoeye is very active in design and support of TNLC SLMs. Their reflection-type device LC-R 1080 is based on liquid crystal on silicon (LCoS) technology and designed for amplitude modulation. Most of the commercial displays are based on TNLCs, so you might source one from an unused display projector. Holoeye  provides an OEM kit (HEO 0017) based on a commercial display from Sony and costs nearly half in comparison to its reflection devices.

Phase modulators

• (Thanks to Laura, friend at MIT, for reminding me about deformable mirrors) The most extensively used devices for phase-modulation are deformable mirros (DM).  There are several technologies for realizing DMs.  Conceptually, DM can be thought of as a flat stretched reflective membrane mounted on actuators. The actuators deform the membrane as per required phase modulation. They are widely used in astronomy, retinal imaging, microscopy research, and pulse shaping. Several companies provide DM and phase-modulation kits suitable for above application, a noteworthy among them being Boston Micromachines.
• With TNLCs, one can control the phase distribution of light but perhaps accompanied by spurious amplitude modulation. However, Holoeye combines twisted nematic LCs with intelligent voltage control to negate spurious amplitude modulation with its pluto and HEO 1080P devices.
• A competing technology is  Parallel aligned liquid crystal (PALC). PALCs twist along the propagation axis rather than around it. They do not rotate polarization of input light but merely change refractive index seen by it. Hence, they allow pure phase modulation. Such devices are available from Hamamatsu.

Polarization modulators

• These modulators are usually used for variable retardation, i.e. to control ellipticity of light. TNLCs are again widely used for this purpose. In fact, the basic mechanism of TNLC modulation is to alter the 2D distribution of polarization of light, which is then converted to amplitude or phase modulation with help of polarizer on the output side. Arcoptix and Meadowlark supply devices meant for programmable retardation, generation of radially or azimuthally polarized light, etc.