Laser radar for entry, descent, and landing (EDL) applications as well as the space docking problem could benefit from a low size, weight, and power (SWaP) beam control system. Moreover, an inertia free approach employing non-mechanical beam control is also attractive for laser radar that is intended to be employed aboard space platforms. We are investigating a non-mechanical beam steering (NMBS) sub-system based on liquid crystal polarization grating (LCPG) technology with emphasis placed on improved throughput and significant weight reduction by combining components and drastically reducing substrate thicknesses. In addition to the advantages of non-mechanical, gimbal free beam control, and greatly improved SWaP, our approach also enables wide area scanning using a scalable architecture. An extraterrestrial application entails additional environmental constraints, consequently an environmental test plan tailored to an EDL mission will also be discussed. In addition, we will present advances in continuous fine steering technology which would complement the coarse steering LCPG technology. A low-SWaP, non-mechanical beam control system could be used in many laser radar remote sensing applications including meteorological studies and agricultural or environmental surveys in addition to the entry, descent, and landing application.
Liquid crystal polarization gratings (LCPGs) represent a relatively new technology capable of nonmechanically and efficiently steering light over a large field-of-regard in discrete steps. Due to their reliance on thin liquid crystal cells instead of mechanical moving parts, LCPG beam steering systems are attractive options for steering both active and passive optical sensors, especially in size, weight, and power (SWaP)-constrained platforms. This paper describes recent developments in large-aperture LCPG steering systems and summarizes the performance being achieved.
Liquid Crystal on Silicon micro-displays are the enabling components on a variety of commercial consumer products
including high-definition projection televisions, office projectors, camera view-finders, head-mounted displays and picoprojectors.
The use and potential application of LCOS technology in calibrated scene projectors is just beginning to be
explored. Calibrated LCOS displays and projectors have been built and demonstrated not only in the visible regime, but
also in the SWIR, MWIR and LWIR. However, LCOS devices are not only capable of modulating the intensity of a
broadband illumination source, but can also manipulate the polarization and/or phase of a laser source. This opens the
possibility of both calibrated polarization displays and holographic projection displays.
A hyperspectral image projector (HIP) is introduced that is built with liquid crystal based spatial light modulators (SLM)
as opposed to micromirror arrays. The use of an SLM as a broadband intensity modulator presents several benefits to this
application. With slight modifications to the SLM design, SLMs can be built for a wide range of spectral regimes,
ranging from the ultraviolet (UV) to the long-wavelength infrared (LWIR). SLMs can have a large pixel pitch,
significantly reducing diffraction in the mid-wavelength infrared (MWIR) and LWIR. Liquid crystal based devices offer
direct analog intensity modulation, thus eliminating flicker from time sequential drive schemes. SLMs allow for an on-axis
configuration, enabling a simple and compact optical layout. The design of the HIP system is broken into two parts
consisting of a spectral and spatial engine. In the spectral engine a diffraction grating is used to disperse a broadband
source into spectral components, where an SLM modulates the relative intensity of the components to dynamically
generate complex spectra. The recombined output is fed to the spatial engine which is used to construct two-dimensional
scenes. The system is used to simulate a broad range of real world environments, and will be delivered to the National
Institute of Standards and Technology as an enabling tool for the development of calibration standards and performance
testing techniques for multispectral and hyperspectral imagers. The focus of this paper is on a visible-band HIP system;
however, related work is presented with regard to SLM use in the MWIR and LWIR.
Liquid crystal spatial light modulators are emerging as a viable alternative to emitter arrays as the display engine for
infrared scene projection. Some benefits of liquid crystal spatial light modulators include low cost, light weight (to
enable portable test engines) flickerless scene generation with no dead pixels. Other possible advantages include high
efficiency operation, scalable architecture and potential for high apparent temperature simulation. We discuss a recently
developed high voltage 512x512 liquid crystal on silicon spatial light modulator. Design considerations and
experimental data on device performance are presented.
Commercially available Liquid Crystal on Silicon (LCoS) Optical Phase Arrays (OPA) are
capable of non-mechanically beamsteering up to ±3 degrees at 1550 nm. While the existing
technology is useful for many applications such as laser communications and pulse-shaping, it is
desirable to increase the steer angle and decrease the response time of the OPA. This was
accomplished through a research effort funded by Langley Research Center at NASA. Under this
research effort Boulder Nonlinear Systems (BNS) designed a new 1x12288 pixel OPA. In the
new backplane design the pixel pitch was decreased from 1.8 um to 1.6 um, the backplane voltage
was increased from 5 volts to 13 volts, and the aperture was increased from 7.4 x 6.0 mm to 19.66
x 19.66 mm. The OPA, when built with new liquid crystals and calibrated with new automated
calibration procedures demonstrated a greater than 2x improvement in steer angle. The OPA that
was tested, which was built for operation at 1550 nm, demonstrated the ability to steer to ±6.95
degrees. Additionally the relaxation time of the OPA was improved to 24.8 ms. This paper
discusses the benefits of the new backplane design, the liquid crystal (LC) properties that are most
desirable for beamsteering, the implementation of the automated calibration procedures, and the
Recent interest in liquid crystal spatial light modulators as a potential replacement to traditional optical beam steering methods have engendered experiments to determine the technology's resistant to gamma radiation such as may be encountered in a space environment. We previously investigated the effects of exposure of liquid crystal devices to ionizing radiation to total dose levels consistent with a 14-year mission at geostationary orbits (GEO). We reported on the parameters of retardation, contrast ratio and primary power current, which were monitored at various dosing intervals for liquid crystal cells and a spatial light modulator. Here we present for the first time measurements of spatial light modulators' beam steering characteristics taken while they are undergoing gamma irradiation. We examine data on angular deflection, intensity, and beam spread for the liquid crystal spatial light modulators obtained during irradiation. The modulators were in continuous operation during irradiation at approximately 23 Rad (Si)/s, and, again the total ionizing dose reached levels consistent with 14 years at GEO. We observed minimal to no degradation in performance, either from dose rate effects or from total ionizing dose, in these environments.
There are basically two types of high-resolution spatial light modulators (SLMs): reflective and transmissive. Obviously, the two types lend themselves to different optical configurations, where one might have advantages over the other for generating the dynamic multi-spot patterns needed by the micromanipulator. In addition, there are inherent performance and operational differences between the two, which has several implications. This paper compares reflective and transmissive high resolution SLMs as well as optical and electrical addressing schemes for operation in optical manipulation applications.
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.
A space platform for optical communications could benefit from nonmechanical beam steering in which no inertia is used to redirect the laser communications link. This benefit is to come in the form of compact, low-power, light-weight optical phased arrays that provide greater flexibility in their steering capability. Non-mechanical beam steering eliminates the need for massive optomechanical components to steer the field of view of optical systems. A phased array approach also allows for random access beam steering. This paper discusses nonmechanical beam steering based on liquid crystal on silicon optical phased array technology. Limitations of the current technology and improvements are presented.
Liquid crystal spatial light modulators are emerging as a potential replacement to traditional optical beam steering methods. The performance of these devices for optical communication systems in the radiation environment for geostationary orbits (GEO) are of interest for applications in the next generation of satellites. As an initial investigation to the study presented, several liquid crystals were irradiated to total dose levels consistent with expected GEO environments. While prior irradiation work has been done on spatial light modulators none is known to include a first look at a liquid crystal and CMOS backplane. Parameters of retardation, contrast ratio and primary power current were monitored at incremental stages during the test and are presented.
Novel tunable polarization interference filters (PIF) employing active liquid crystal devices are presented, and the principles of operation are described. Filter designs are presented based on a requirement for tunable nulls in the visible and near infrared spectral regions, of high optical density, for protection from intense electromagnetic radiation outside of the spectral range of interest which can saturate an imaging or sensor system. Two types of PIFs are presented with their modeled results and device performances. Analog filters in a generalized Lyot-Ohmann geometry are presented which are capable of tuning an optical null through 260 nm, by employing a single active device per filter stage. Binary filters are also presented which can switch between two complimentary and non-overlapping spectral states. Both types of filter can operate in a “normally on” state with a broadband “white light” throughput.
The advantages of laser communications including high bandwidth, resistance to jamming and secure links have made it a key technology for current and future C4ISR capabilities. Laser Communications between space/air/ground/sea-based assets may require multiple links. One advantage of this redundancy is that the signal is more likely to reach the intended receiver even if the environmental conditions are poor for laser transmission. In addition, multiple links provide simultaneous receipt of information to various assets engaged in activities that may need to be coordinated. That is, multibeam laser communication mimics the "broadcast" advantage of RF communications but with less likelihood of jamming or intercept. Liquid Crystal spatial light modulators are a versatile optical head that can be used for multispot beam steering applications. One advantage of the liquid crystal approach to multibeam laser communication is that the device is a modulator in addition to a mirror, so that one could conceivably send different signal amplitudes to different locations simultaneously. This paper discusses recent improvements to a 5 12x5 1 2 spatial light modulator that is specifically implemented as a multispot beamsteerer. This will include characterization of the device, analysis of its performance, and what improvements should be incorporated into the next generation device.
High resolution devices, using liquid crystal phase modulators, have the ability to correct large phase variations across the aperture. These phase variations are corrected using discrete (pixelated) modulo-2π phase shifts, which simplifies the control scheme by eliminating inter-actuator influence and decreases the response time of the phase modulator by reducing the required stroke. Even with these advantages, modulo-2π phase correction is generally not used if the source is broadband due to degradation from chromatic dispersion. This paper discusses techniques for reducing the angular dispersion using new modulators which are being developed.
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.
Polymer dispersed liquid crystals are generally described as a system with an isotropic liquid crystal (LC) droplet distribution in a polymer matrix. Using masked ultraviolet light and/or applied electric field a structured polymer/LC phase separation can be achieved. One technological advantage is the potential for integrated polymer/LC devices. This approach can be used to manufacture miniature switchable optical components such as diffractive gratings and switchable microlenses. We investigate switchable diffractive gratings based on a structured polymer/LC system. LC director modeling is used to take into account the polymer regions and electric field orientation when the device thickness is comparable with the electrode period. Optical diffraction properties are compared with results of the theoretical modeling.
Currently VLSI foundries are pushing for small feature size and higher operating voltages. These ongoing developments in integrated circuit fabrication processes result in devices exhibiting excellent electrical performance, but poor optical quality. Increased reflectivity and planarization are necessary to produce high efficiency beam steering devices. Steps taken to improve optical performance of silicon backplanes will also be discussed.
We discuss a white-light processing system that produces a dynamic, achromatic Fourier transformation over the visible spectrum. The system includes an achromatic Fourier transform lens system and a low-dispersion spatial light modulator. A programmable phase mask can only write patterns with a spatial frequency appropriate for one wavelength. However, this problem is resolved by scaling broadband light from a point source to a common spatial frequency using an achromatic Fourier transformer. Then, the programmable phase mask must produce the same phase profile for all wavelengths. Using a chiral smectic liquid crystal (CSLC) spatial light modulator can minimize the wavelength dependence of the phase shifting elements. Phase modulation is accomplished by re-orientation of the optic axis in a plane transverse to the direction of propagation in a manner similar to mechanical rotation of a waveplate. The position of the optic axis is the same for all wavelengths and ideally so is the induced phase shift. We present experimental far field diffraction patterns due to a CSLC spatial light modulator that produces a binary broadband phase mask and an achromatic Fourier transform lens system. An analog modulator is also introduced. Applications for this technology include optical process, beam steering and adaptive optics.
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.
A solid-state broad band beam deflector is described. This non-mechanical system steers spatially coherent broad band light to a common location in the far field. The components include a liquid crystal grating and achromatic Fourier transformer. The liquid crystal grating employs a polarization modulation scheme which produces a wavelength independent phase shift. The achromatic Fourier transformer eliminates grating dispersion. The modulation theory for the liquid crystal grating is introduced. Observations of the far field patterns for white light illumination of a binary liquid crystal grating and the design for the achromatic Fourier transformer are presented. Future research, including mid- infrared implementation is discussed.
An analog 128 X 128 spatial light modulator (SLM) has been designed and constructed using liquid crystal on silicon technology. This device is loaded with eight-bit grey-level data in 100 microsecond(s) . Its pixel pitch is 40 micrometers giving an array size of 5.12 X 5.12 mm. Low-voltage ferroelectric liquid crystals are used for the electro-optic modulator. These analog materials have 50 microsecond(s) to 100 microsecond(s) switching times, implying a frame rate of approximately 5 kHz. This paper presents results of the analog SLM and discusses modulation enhancements for improving correlator performance.