Jay E. Stockley

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.

Advances in optical phased array technology

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 results.

Advances in liquid crystal beam steering

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.

Broadband beam steering

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.

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.

Fabry–Perot etalon with polymer cholesteric liquid-crystal mirrors

We present what we believe to be the first implementation of a Fabry-Perot (FP) etalon using polymer cholesteric liquid-crystal mirrors. These polymer mirrors have each been fabricated onto a single substrate, which allows the FP cavity spacing to be only a few micrometers wide. For the experimental results presented, cavity lengths of 13.8 and 7.6 microm yield near-infrared free spectral ranges of 24.8 and 45.6 nm, respectively. The measured finesse of 14.31 is approaching the limitation imposed by the reflectivity of the mirrors.

128 x 128 analog liquid crystal spatial light modulator

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.

High-speed analog achromatic intensity modulator.

We report what is to our knowledge the first implementation of a broadband analog intensity modulator composed of two chiral smectic liquid-crystal half-wave retarders. A reflection-mode intensity modulator employing a single active device has also demonstrated achromatic transmission. A quantitative theory for chromatic compensation is presented. By optimum selection of liquid-crystal retardance and orientation, intensity transmission is uniform throughout the visible. The chiral smectic liquid-crystal devices used in the implementation are capable of switching in less than 20 micros.