ELECTROCLINIC LIQUID CRYSTAL VARIABLE ATTENUATOR FOR LASER ROS
Original Publication Date: 1992-Dec-31
Included in the Prior Art Database: 2004-Apr-06
Xerox Disclosure Journal
AbstractLiquid crystal devices are employed as variable attenuators to control the intensity of a laser used in a Raster Output Scanner. Fast response times are not required, but the device must be controllable in a linear fashion and possess a high transmittance to contrast ratio. The twisted nematic type of liquid crystal device is commonly used as an attenuator wherein its typical response time falls within the range of 10 to 100 milliseconds.
XEROX DISCLOSURE JOURNAL
ELECTROCLINIC LIQUID Proposed Classification CRYSTAL VARIABLE U.S. C1.355/233 ATTENUATOR FOR LASER ROS
Virgil J. Hull
Joseph J. Wysocki
Int. C1. G03g 15/28
Attenuation versus Voltage of an Electroclinic Device
0 10 20 30 40 Applied Voltage (V)
XEROX DISCLOSURE JOURNAL Vol. 17 No 6 Sovember/December 1992 451
ELECTROCLINIC LIQUID CRYSTAL VARIABLE ATTENUATOR FOR LASER ROS(Cont'd)
Liquid crystal devices are employed as variable attenuators to control the intensity of a laser used in a Raster Output Scanner. Fast response times are not required, but the device must be controllable in a linear fashion and possess a high transmittance to contrast ratio. The twisted nematic type of liquid crystal device is commonly used as an attenuator wherein its typical response time falls within the range of 10 to 100 milliseconds.
Alternatively, the Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) effect of Smectic C liquid crystals is used where higher speed is required. These devices have reported times of less than 1 microsecond, although typical room temperature devices are currently within the 100 to 500 microsecond range. The SSFLC effect, however, is not suitable for use as an attenuator since it is inherently bistable and does not have linearly controllable switching characteristics.
A new liquid crystal effect has been discovered which has high speed and a linear attentuation. The electroclinic effect, which occurs in Smectic A crystals with optically active molecules, was originally reported to be in an arrangement such that the Smectic layers are parallel to a substrate. The same effect has been adapted to the device configuration used in SSFLC. In this arrangement, the effect displayed an attenuation which is linear with respect to an applied voltage. A graph for Attenuation versus Voltage in the SA phase of a ferroelectric liquid crystal mixture is shown in the illustration.
Further development in materials are required to reduce the temperature and voltage requirements of the electroclinic effect to values suitable for room temperature devices driven by conventional integrated circuits. The major limitation of the electroclinic effect is the small modulation depth obtainable. The largest molecular tilt attainable is in the 5" range. Since the transmittance of the device varies as a function of the sin 24, where 4 is the deviation of the optic axis from one of the polarizer directions, the maximum available attentuation from a device configured for maximum transmittance is about 5 percent. By operating the device in a minimum transmittance configuration, higher contrast is obtainable, but the throughput then becomes too low for the present application.
The present invention...