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Fast, large and controllable phase modulation using dual frequency liquid crystals

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Abstract

We report on a method for high speed, large stroke phase modulation using dual frequency control of liquid crystals. Our system uses an all-electronic feedback system in order to simplify the control. We show half wave phase modulations of ~120Hz with the operating point varying over nearly the full dynamic range of the device, and demonstrate larger phase shifts (2.5 waves) at a frequency of 37Hz. For large phase shifts, the speeds are an order of magnitude faster than existing techniques.

©2004 Optical Society of America

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Figures (5)

Fig. 1.
Fig. 1. (a) A simple LC cell structure schematic, and (b) the approximate equivalent circuit, where C is the equivalent parallel capacitance and G is the equivalent conductance
Fig 2.
Fig 2. Block diagram showing the simplified electronic closed loop system. The cell capacitance is measured and is compared to the control input to generate an error signal. A comparator, operating on inputs from the error signal and a saw-tooth generator, provides the appropriate mark-space ratio between the high and low frequency voltages to be applied to the LC cell.
Fig. 3.
Fig. 3. Oscillogram illustrating high-speed large phase modulation with closed-loop control. Upper trace shows input (control) signal. Lower trace shows measured optical signal. The time-base is 5ms/division.
Fig. 4.
Fig. 4. A comparison of maximum achievable phase shift, with respect to frequency, for LC1001 in closed loop dual frequency control (triangles) and LC-E49 using the transient nematic effect (diamonds).
Fig 5.
Fig 5. A comparison of maximum frequency of operation for a half wave phase shift versus the center operating point (bias) on the phase-voltage range, for LC1001 operated with closed-loop dual frequency control (diamonds) and LC-E49 operating in transient nematic mode (triangles). A nominal operating point of zero is defined when the cell is fully on. Note different frequency axes for dual frequency and transient nematic results.

Tables (1)

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Table 1. Niopik LC1001 parameters.

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