Erasable Optical Liquid Crystal Disc Memory

Summary and conclusionsIn the experiments described above we have shown that, in spite of their liquid natureand long relaxation times, liquid crystalline media can be used for storing data at densi-ties and data rates compatible with computer mass storage systems. The 13 MHz data rateand 1.25 micron bit spacing achieved in these experiments, are limited by the system charac-teristics rather than by the media. The statistical formation of the scattering centerscan be controlled, so that raw bit error rates of less than 10 -5 are obtained. Anchoringof these centers on the disc substrate prevents their motion and renders the informationstable. No sign of fatigue has been detected under repeated write -erase cycling.LC media coupled with a simple dark field optical system offer high contrast readout.High sensitivity detectors, rather than high resolution ones, are needed and the shot noise 196 the domains grow slightly as evidenced by a slight increase in the width of the scanned scattering profiles. However, their diameters remain less than that of the beam.In order to preclude electrochemical degradation of the LC media, an alternating field is used for erasure. Since the local erase process is fast (a few microseconds), a square wave voltage was utilized in all experiments. No difference in erase characteristics was ob­ served for erase frequencies between 0.5 and 10 kHz. Typical erase characteristics are shown in Figure 8. The erase window, i.e., the voltage range for which local erasure can be performed without affecting other domains, is relatively small for the LC materials that we investigated. The narrow erase window imposes limits on the LC layer thickness tolerance of the device. This window decreases as the temperature increases.Figure 8 also shows the results of repeated write-erase cycling of the LC sample. Since the medium is liquid, it does not exhibit the fatigue problems usually associated with struc­ tural transitions in solids.5. Results of experiments on a rotating discFor these experiments a 3" diameter LC disc was spun at up to 6000 RPM by a small DC motor. The speed of the motor was controlled by a signal derived from an optical encoder patterned at the periphery of the disc. This encoder was read with an auxiliary He-Ne laser beam and provided the on-disc positional information used by the writing electronics. Data were coded in ASCII format with 1 corresponding to the presence of the domain and 0 to its absence. The laser power required to write saturated spots was about twice that required in the static experiments.Figure 9 shows the results of writing on a rotating disc. It also demonstrates the bit by bit local erasure capability of the LC media. Spots were written at 13 MHz on 1.25 micron centers. Unfortunately electronic and mechanical system -jitter did not allow measure­ ment of bit error rates at this data rate. However, measurements of the bit error rate at 6.5 MHz with 2.5 micron bit spacing showed a raw bit error rate of less than 10~5 .6. Focusing and track followingA dynamic focus servo was implemented for the rotating disc experiments. In addition to the usual focusing problems associated with laser power variations encountered in other recording systems, the LC media pose unique considerations. The written domains and the isotropic bubbles appearing during writing act as lenses which modify the focus error signal. To circumvent this, an auxiliary beam falling in the intertrack region was used to derive the focus correction signal. This was achieved by using a He-Ne laser beam coupled into the optical system with a dichroic mirror. The error signal is derived from the far field size of the reflected beam. The electronic signal was derived from a split detector offset from the beam center so that at focus 1/3 of the beam fell on one-half of the detector and 2/3 on the other. The lens was maintained in focus by a piezoelectric bimorph bender mem­ brane with total travel of 10 microns. Long term focus drifts were compensated by a step­ ping motor driven translation stage.Though no attempt was made in these experiments to follow the track on which the informa­ tion was written, the feasibility of following the written data was investigated. Figure 10 shows the track error signal derived from the scattered radiation detected by a differential, split photodetector positioned so that the gap is parallel to the track. If the bright field detection system is used, the error signal can be again derived from a differential, split detection scheme5 . No attempt has yet been made to evaluate whether it is possible to groove the substrate in order to follow the clear tracks. The groove structure, particu­ larly their edges, may interfere with the layer structure of the smectic liquid crystal. This interaction may result in an increased number of defect domains and thus adversely affect the signal to noise. It is clear that the shape of the groove is going to be impor­ tant and that more research is needed in this area.7. Summary and conclusionsIn the experiments described above we have shown that, in spite of their liquid nature and long relaxation times, liquid crystalline media can be used for storing data at densi­ ties and data rates compatible with, computer mass storage systems. The 13 MHz data rate and 1.25 micron bit spacing achieved in these experiments, are limited by the system charac­ teristics rather than by the media. The statistical formation of the scattering centers can be controlled, so that raw bit error rates of less than 10"5 are obtained. Anchoring of these centers on the disc substrate prevents their motion and renders the information stable. No sign of fatigue has been detected under repeated write-erase cycling.LC media coupled with a simple dark field optical system offer high contrast readout. High sensitivity detectors, rather than high resolution ones, are needed and the shot noise