Submillisecond Response Time Research Articles

Isothiocyanato-Tolanes Based High Birefringence and Fast Response Time Mixtures for Photonic Applications

We have developed some high birefringence (Δn ∼ 0.3 at λ = 633 nm) and relatively low viscosity nematic mixtures using isothiocyanato tolanes. These mixtures exhibit a high figure-of-merit (FoM) which is desirable for reducing the device response time. Using such a high FoM liquid crystal mixture in a 2-μm homogeneous cell, we demonstrate a 1π phase modulator at λ = 633 nm with submillisecond response time.

Polarization-independent and submillisecond response phase modulators using a 90° twisted dual-frequency liquid crystal

A polarization-independent phase modulator using a 90° twisted dual-frequency liquid crystal (DFLC) is demonstrated. In addition to being polarization independent, such a phase modulator exhibits many other advantages such as being scattering-free and having large phase change, low operating voltage, and submillisecond response time. Using a 15μm transmissive DFLC cell, the phase shift achieves 1π at λ=633nm and the applied voltage is lower than 25Vrms. Potential applications of such a phase modulator for laser beam steering, tunable-focus lenses, and switchable two-dimensional/three-dimensional liquid crystal displays are foreseeable.

Polarization-independent and fast-response phase modulators using double-layered liquid crystal gels

A polarization-independent phase modulator using two thin homogeneously stratified but orthogonally oriented liquid crystal (LC) gels is demonstrated. In addition to polarization independence and submillisecond response time, the LC gel possesses a much larger phase change and lower operating voltage than the nanosized polymer-dispersed liquid crystal. For a 16μm transmissive E48 LC gel, the phase shift exceeds 1π at λ=633nm with an electric field strength of ∼11Vrms∕μm. Potential applications of such a phase modulator for laser beam steering, microlens array, and switchable two- and three-dimensional LC displays are foreseeable.

P-131: Polarization Independent and Fast Response Phase Modulators Using Orthogonally Orientated Liquid Crystal Gels

A polarization independent phase modulator using two ultra-thin homogeneously stratified liquid crystal gels is demonstrated. In addition to the polarization independence and submillisecond response time, our liquid crystal gel possesses a much larger phase change and lower operating voltage than the nanosized polymer-dispersed liquid crystal.

Fast-response nematic liquid-crystal mixtures

— High birefringence and relatively low-viscosity isothiocyanate-based liquid-crystal compounds and mixtures were developed. A high figure-of-merit (FoM), which implies a fast response time of the described liquid crystals was observed. Using the new UCF mixture in a 2-μm cell, a submillisecond response time was obtained. The UV stability dilemma is discussed as a common concern for high-birefringence LC materials.

Polarization-independent phase modulation of a homeotropic liquid crystal gel

A homeotropic liquid crystal (LC) gel with submicron domain size is fabricated and its phase-only modulation property evaluated. The LC gel is highly transparent in the voltage-off state and exhibits pure phase modulation before light scattering occurs. Compared to a nanosized polymer-dispersed liquid crystal (nano-PDLC), our gel possesses a larger phase change but at a lower operating voltage because of a higher LC concentration. Similar to a nano-PDLC, our gel also exhibit submillisecond response time, hysteresis-free, and polarization-independent phase change.

Dual-Frequency Addressed Variable Optical Attenuator with Submillisecond Response Time

We demonstrated a submillisecond response time and low-voltage variable optical attenuator (VOA) using a dual-frequency liquid crystal. The dynamic range of the VOA reaches 43 dB at a 1.55 µm wavelength. A normally on VOA with a flat wavelength response is obtained using a phase compensation cell. Fast rise and decay times are achieved using low-frequency overdrive and high-frequency undershoot voltages. Other approaches to further improve the VOA's performances are discussed.

Dual-frequency liquid crystal gels with submillisecond response time

Two types of gels using dual-frequency liquid crystal are demonstrated. The one using a homogeneous cell shows anisotropic scattering behavior while the other prepared using a cell without polyimide alignment layers exhibits isotropic scattering properties. Both liquid crystal gels are highly transparent in the voltage-off state. Light scattering occurs when a high frequency voltage is applied. The isotropic gel exhibits a high contrast ratio and submillisecond response time. Potential applications of these gels for switchable polarizer, telecom optical switch, and reflective displays are emphasized.

P‐96: Electro‐optical Devices from Polymer‐stabilized Molecular Shape Polarity of a Cholesteric Liquid Crystal

AbstractWe present the electro‐optical devices prepared from polymer‐stabilized cholesteric liquid crystals. By using a small concentration of polymer we are able to stabilize two switching modes, phase‐only (in‐plane switching at low field) and birefringence (out‐of plane at high field), of the uniformly lying helix (ULH) texture of a cholesteric with a sub‐millisecond response time.

Total-internal-reflection mode in holographic polymer dispersed liquid crystals.

We have developed a holographic polymer dispersed liquid-crystal device based on total internal reflection (TIR). In the field-off state, the holographic planes are slanted such that the angle of the diffraction beam is beyond the TIR angle of the glass substrate. The beam is trapped in the sample and propagates to the edge. In the field-on state, the holographic grating is erased and the incident beam propagates directly through the sample. Contrast ratios of 10 dB, submillisecond response times, and an index modulation of 0.042 at 1550 nm are reported.

Active U-turn electrooptic switch formed in patterned holographic polymer-dispersed liquid crystals

We have developed an active U-turn device by spatially patterning two identical holographic polymer dispersed liquid-crystal gratings. In the field-OFF state, the holographic planes of both gratings are slanted such that the angle of the diffraction beam is beyond the total internal reflection angle of the air-glass interface. The diffraction beam is trapped in the device until diffracted by the second slanted grating. In the field-ON state, the holographic grating is erased and the incident beam propagates directly through the sample. The index modulation is determined to be 0.059 /spl plusmn/ 0.006 at 1.55 /spl mu/m. Low switching voltage and sub-millisecond response times are reported.

Adaptive optics based on analog parallel stochastic optimization: analysis and experimental demonstration

Wave-front distortion compensation using direct system performance metric optimization is studied both theoretically and experimentally. It is shown how different requirements for wave-front control can be incorporated, and how information from different wave-front sensor types can be fused, within a generalized gradient descent optimization paradigm. In our experiments a very-large-scale integration (VLSI) system implementing a simultaneous perturbation stochastic approximation optimization algorithm was applied for real-time adaptive control of multielement wave-front correctors. The custom-chip controller is used in two adaptive laser beam focusing systems, one with a 127-element liquid-crystal phase modulator and the other with beam steering and 37-control channel micromachined deformable mirrors. The submillisecond response time of the micromachined deformable mirror and the parallel nature of the analog VLSI control architecture provide for high-speed adaptive compensation of dynamical phase aberrations of a laser beam under conditions of strong intensity scintillations. Experimental results demonstrate improvement of laser beam quality at the receiver plane in the spectral band up to 60 Hz.

A rule-based machine vision system for fire detection in aircraft dry bays and engine compartments

In this paper, a rule-based machine vision approach is applied to detect and categorize hydrocarbon fires in aircraft dry bays and engine compartments. Images for computer analysis are provided by charge-coupled device imaging sensors placed inside dry bays and engine compartments. Using a set of heuristics based on statistical measures derived from the histogram and image subtraction analyses of successive image frames, we showed that it is possible to detect and categorize life-threatening fires from non-fire/non-lethal events accurately in sub-millisecond response time. Specifically, the median, standard deviation, and first-order moment statistical measures of the histogram data of each image frame are used to confirm the presence or absence of fire. Concurrently, another set of mean, median, and standard deviation statistical measures from the image subtraction of two successive frames are used to determine the growth and subsequently reaffirm the existence of a fire. This approach is also tested for false alarms such as those due to flashlights and high-power halogen lights.

Submillisecond response times of oxygen-quenched luminescent coatings

An apparatus and data analysis methodology is described which allows determination of response time to oxygen concentration changes of luminescent oxygen sensor coatings. Utilizing a solenoid valve, a sample chamber creates a pressure jump from 0.1 to 700 Torr in 600 μs that is followed by 15 ms of ringing. An optical detection system measures the response of porphyrin-based luminescent oxygen sensors to the pressure jump. The pressure in the chamber is measured simultaneously and independently with a piezoresistive pressure transducer. Data analysis techniques using nonlinear least-squares and numerical convolution of the luminescent response to the pressure rise allow determination of response times of the oxygen sensor. The response to pressure jumps of several luminescent oxygen sensitive coatings designed for video luminescent barometry are measured with this computer-controlled instrument. Several coatings were studied with response times of ∼2.5 s, ∼400 ms, 11 ms, 1.5 ms, and <25 μs. Studies of the system suggest that we can determine response times down to about 10 μs.