Steven A. Serati

Advanced liquid crystal on silicon optical phased arrays

Non-mechanical beamsteering eliminates the need for massive optomechanical components to steer the field of view of optical systems. This benefit is to come in the form of compact, low-power, light-weight optical phased arrays (OPAs) that provide better control with greater flexibility in their steering capability than their mechanical counterparts. Due to such benefits, there is a need to develop technologies that provide this capability without greatly sacrificing other parameters such as aperture size, efficiency, and scanning range. One technology being explored for OPA implementation is liquid crystal on silicon (LCoS). The LCoS technology provides a means for manufacturing high-resolution backplanes using high-volume semiconductor processes commonly used for very large scale integrated (VLSI) circuits. VLSI production minimizes the cost of backplane fabrication and allows integration of electronic circuits into the backplane structure to provide individual addressing of each pixel while minimizing interconnects to the OPA. Since each pixel is individually addressed, the phase modulation is not restricted to phase ramps but provides any type of phase profile. This capability is useful for dynamic correction of phase distortions across the aperture due to heating effects, for example. Also, it allows the reset period to be randomized to minimize sidelobe amplitudes. However, VLSI technology has its own set of limitations that have slowed the development of highspeed, high-resolution, non-mechanical beamsteerers having good optical efficiency and a large scanning range. This paper discusses the benefits and limitations of the LCoS approach and methods for improving the state-of-the-art.

Advances in liquid crystal based devices for wavefront control and beamsteering

New devices and approaches are being developed for controlling beam direction and shape using liquid crystal based assemblies. This paper discusses recent advancements in these areas including improvements in zero-order diffraction efficiency, broadband wide field-of-regard steering, wavefront correction using in-line configurations and high average power handling.

Analog optical phase modulator based on chiral smectic and polymer cholesteric liquid crystals.

A high-speed analog optical phase modulator based on chiral smectic and cholesteric liquid crystals is discussed. The chiral smectic liquid-crystal device functions as a variable-orientation half-wave retarder, whereas the polymer cholesteric liquid-crystal film acts as a polarization-preserving mirror. We use circular Jones calculus to describe optical phase modulation, using a half-wave retarder of variable orientation acting on circularly polarized light. The phase induced by this modulator is achromatic. Analog phase modulation of nearly 360° is demonstrated with a device switching time of 200 μs at 25 °C.

Beam combining using a phased array of phased arrays (PAPA)

Mobile free space optical (MFSO) networks introduce new challenges for laser communication terminals. Network capabilities, such as packet routing where different information packets are sent to different locations, require switchable links. Also, switchable links are needed to route an information packet through different channels to the same destination. For MFSO networks, the ability to route information through different channels greatly aids reliability because any one channel can be disrupted by weather conditions. To have the ability to route around obstructions, the mobile platforms have to act as hubs, communicating with a number of remote terminals. Since these terminals are all in motion, the challenge is to acquire and track multiple terminals from a single platform without requiring a full duplication of hardware for each link. To reduce hardware duplication, a significant improvement in scanning flexibility is needed. Improvement is being pursued through the development of nonmechanical optical phased arrays (OPAs). This work discusses the ability to combine several OPAs to generate and independently control one or more beams from a common aperture.

Influence of the interpixel region in liquid crystal diffraction gratings

An important problem associated with multiple pixel liquid crystal (LC) modulators is the incidental diffraction due to amplitude and phase gratings formed by the improperly modulated regions between electrodes. This problem becomes more of an issue as the resolution of the SLM increases and the size of the pixels begins to approach the size of the inter-pixel spacing. We perform 2D director profile modeling in LC diffraction gratings to take into account the electrode structure and fringing electrostatic fields. The electrooptical properties of the grating are simulated and compared with experimental data. This information can be used to design addressing structures with enhanced fill factor. The results obtained could be beneficial for applications in image processing, laser beam control, steering and adaptive optics.

Transmissive spatial light modulators with high figure-of- merit liquid crystals for foveated imaging applications

Unique liquid crystal (LC) spatial light modulators (SLM) are being developed for foveated imaging systems that provide wide field-of-view (FOV) coverage (±60° in azimuth and elevation) without requiring gimbals or other mechanical scanners. Recently, a transmissive-SLM- based system operating in the visible (532 nm) has been demonstrated. The LC SLM development is addressing implementation issues through the development of high figure-of-merit (FoM) LC materials and transmissive high-resolution SLMs. Transmissive SLM operation allows the foveated imaging configuration to be very compact using a very simple lens system. The reduction in the size, weight and cost of the imaging optics and in data acquisition/processing hardware makes the foveated approach attractive for small platforms such as unmanned airborne vehicles (UAVs) or missile seekers.

Spatial light modulator considerations for beam control in optical manipulation applications

Holographic beam forming to generate and control multiple optical traps has proved successful using high-resolution spatial light modulators (SLMs). This type of beam control allows a multitude of traps to be independently controlled in three dimensions. Also, exotic beam shapes and profiles can be generated, which gives the optical trapping system even greater flexibility. Until recently, the generation of high resolution phase patterns has limited the speed of dynamic holographic optical trapping (HOT) systems. Today, video rate operation controlling hundreds of traps using 512x512 phase masks is possible and significantly faster operation is possible with fewer traps using less phase resolution. Therefore, phase-only liquid crystal modulator response is becoming the bottleneck. This paper discusses recent advances in SLM developments which address this issue.

Polarization gratings for non-mechanical beam steering applications

Over the last few years, Boulder Nonlinear Systems (BNS) and North Carolina State University (NCSU) have developed a new beam steering technique that uses a stack of thin liquid crystal polarization gratings (LCPGs) to efficiently and non-mechanically steer a beam over a large field-of-regard (FOR) in discrete steps. This technology has been successfully transferred to BNS through an exclusive license agreement, and a facility has been completed to enable commercial production of these devices. This paper describes the capabilities enabled by both the LCPGs and the successful transfer of this technology.

Advances in polarization-based liquid crystal optical filters

Liquid crystal tunable filters are gaining wide acceptance in such diverse areas as optical fiber communications, astronomy, remote sensing, pollution monitoring, color generation for display and medical diagnostics. The large aperture and imaging capability of liquid crystal tunable filters represent a distinct advantage over conventional dispersive spectral analysis techniques. Furthermore, benefits of liquid crystal tunable filters over acousto-optic tunable filters include low power consumption, low addressing voltage, excellent image quality and large clear aperture. We discuss polarization interference filters based on liquid crystal tuning elements. While liquid crystal tunable filters based nematic liquid crystal, using Fabry-Perot and polarization interference effects are commercially developed, only recently has the emphasis been on liquid crystal tunable filters to include current novel developments in high-speed, analog ferroelectric-liquid crystals (FLCs). Compared to nematic liquid crystal, FLC-based tunable optical filters offer fast response time and increased field-of-view.

Programmable 128 x 128 ferroelectric liquid crystal spatial light modulator compact correlator

This paper details a compact two-dimensional optical correlator based on 128 X 128 ferroelectric liquid crystal spatial light modulators in both the image and filter planes. A complete description of this compact correlator is given. The correlator is fully programmable and performs automatic pattern recognition functions at 500 frames per second. Key design parameters and results of performance analysis are presented.