Multiplexing Methods for Surface-Stabilized Ferroelectric Liquid Crystal Devices

Doktorsavhandling, 1995

The present thesis deals with the electronic addressing of Surface-Stabilized Ferroelectric Liquid Crystal (SSFLC) displays. Basically, this means to investigate how a certain information can be produced serially in the shape of long square pulse waveforms and transferred to a high resolution SSFLC matrix, via the specific linear electro-optic response exhibited by this material, to form a desired image on the screen at any time. To accomplish this, the thesis also includes the actual construction of the necessary hardware and software. The work combines various aspects and approaches. Since the SSFLC does not react on the rms values of the pulse, the starting point was to correctly analyze how to use the polar ferroelectric interaction, with its unconventional threshold characteristics and memorized states, in order to write a dynamic picture in black and white, corresponding to the digital nature of the optical states. After demonstrating a virtually unlimited multiplexability due to the bistability of the SSFLC, the conflicting aspect of producing grey shades was also considered. In the early addressing schemes consideration was mainly given to the ferroelectric interaction, which is permissible as long as the applied voltages are low. The dielectric interaction, which is quadratic in the field, was mainly seen as a disturbance. Gradually, however, with the general recognition of the importance of the chevron local layer structure as well as the high values of the dielectric biaxiality, the dielectric interaction has been included in the analysis, resulting in a variety of possible new addressing schemes characterized by a much higher writing speed. In order to test these unorthodox or speculative schemes it was necessary to design and build a new type of electronic waveform generator. This work has been a major part of the thesis and resulted in an advanced electronic instrument distinctly different from the commercially available equipment and ideally suited for research on the addressing of all kinds of liquid crystal displays. The dedicated waveform generator has the special ability to instantaneously vary different parameters in an applied waveform, without disconnecting the display, allowing immediate observation of the change in optical response. It has been used for comparing practically all known drive schemes with each other and with some very new ones, on three reference cells fabricated with different well-characterized FLC materials. The evaluation of their performance gives guidelines for future materials in combination with future drive schemes.