Phase-only Light Modulator Research Articles

Fast and light-efficient wavefront shaping with a MEMS phase-only light modulator

Over the last two decades, spatial light modulators (SLMs) have revolutionized our ability to shape optical fields. They grant independent dynamic control over thousands of degrees-of-freedom within a single light beam. In this work we test a new type of SLM, known as a phase-only light modulator (PLM), that blends the high efficiency of liquid crystal SLMs with the fast switching rates of binary digital micro-mirror devices (DMDs). A PLM has a 2D mega-pixel array of micro-mirrors. The vertical height of each micro-mirror can be independently adjusted with 4-bit precision. Here we provide a concise tutorial on the operation and calibration of a PLM. We demonstrate arbitrary pattern projection, aberration correction, and control of light transport through complex media. We show high-speed wavefront shaping through a multimode optical fiber – scanning over 2000 points at 1.44 kHz. We make available our custom high-speed PLM control software library developed in C++. As PLMs are based upon micro-electromechanical system (MEMS) technology, they are polarization agnostic, and possess fundamental switching rate limitations equivalent to that of DMDs – with operation at up to 10 kHz anticipated in the near future. We expect PLMs will find high-speed light shaping applications across a range of fields including adaptive optics, microscopy, optogenetics and quantum optics.

Efficient freeform lens optimization for computational caustic displays.

Phase-only light modulation shows great promise for many imaging applications, including future projection displays. While images can be formed efficiently by avoiding per-pixel attenuation of light most projection efforts utilizing phase-only modulators are based on holographic principles which rely on interference of coherent laser light and a Fourier lens. Limitations of this type of an approach include scaling to higher power as well as visible artifacts such as speckle and image noise. We propose an alternative approach: operating the spatial phase modulator with broadband illumination by treating it as a programmable freeform lens. We describe a simple optimization approach for generating phase modulation patterns or freeform lenses that, when illuminated by a collimated, broadband light source, will project a pre-defined caustic image on a designated image plane. The optimization procedure is based on a simple geometric optics image formation model and can be implemented computationally efficient. We perform simulations and show early experimental results that suggest that the implementation on a phase-only modulator can create structured light fields suitable, for example, for efficient illumination of a spatial light modulator (SLM) within a traditional projector. In an alternative application, the algorithm provides a fast way to compute geometries for static, freeform lens manufacturing.

Holographic projection of images with step-less zoom and noise suppression by pixel separation

A new method of projection of color images without color-sequential technique is proposed. It is a combination of the spatial division of the phase-only light modulator with a pixel separation noise suppression technique and an efficient propagation method called a scaled Fresnel diffraction. The unique property of a step-less and lossless zooming of the projected image is shown. The experimental demonstration of the method is presented, showing high quality, low noise images of a variable size at reasonable frame rates.

Minimized speckle noise in lens-less holographic projection by pixel separation

Images displayed by holographic methods on phase-only light modulators inevitably suffer from speckle noise. It is partly caused by multiple uncontrolled interferences between laser light rays forming adjacent pixels of the image while having a random phase state. In this work the experimental proof of concept of an almost speckle-less projection method is presented, which assumes introducing a spatial separation of the image pixels, thus eliminating the spurious interferences. A single displayed sub-frame consists of separated light spots of very low intensity error. The sub-frames with different sampling offsets are then displayed sequentially to produce a non-fragmented color final image.

A liquid crystal spatial phase-only light modulator and its application to optical wavefront control

A liquid crystal spatial phase-only light modulator and its application to optical wavefront control

Oblique-Incidence Characteristics of a Parallel-Aligned Nematic-Liquid-Crystal Spatial Light Modulator

We have developed optically-addressed and electrically-addressed liquid crystal spatial phase-only light modulators having no pixelized structures. We obtained a large depth of phase-only modulation and high diffraction efficiency based on the electro-optical characteristics of a parallel-aligned nematic liquid crystal. These spatial light modulators (SLM) are of the reflection type, so there would be a loss of power in the readout light from the half mirror, which was set up so as to separate the incident and reflected lights. To optimize the characteristics of a reflection type spatial phase-only light modulator, we have proposed an oblique incident optical readout setup. We have examined the effect of conditions such as the polarization direction and the incidence angle of the readout light, and the orientation of liquid crystal molecules in the SLM. High diffraction efficiency close to the theoretical maximum value was obtained by adjusting the above conditions. The simulation analysis can well explain the experimental results of phase modulation.

Phase Modulation Characteristics Analysis of Optically-Addressed Parallel-Aligned Nematic Liquid Crystal Phase-Only Spatial Light Modulator Combined with a Liquid Crystal Display

An optically addressed parallel-aligned nematic-liquid-crystal spatial light modulator (PAL-SLM) has been studied as a dynamic phase-only light modulation device. The phase modulation characteristics of the PAL-SLM using a liquid crystal display (LCD) as an addressing mask were investigated by analyzing diffraction efficiencies resulting from binary gratings projected from the LCD. A more than 2π phase-only modulation depth was achieved. The highest first-order diffraction efficiency of approximately 38% was also obtained; this is close to the theoretical limit. The experimental results of diffraction efficiencies depending on the phase modulation depth are in good agreement with the simulation for the system operation.