Joris J. Vrehen

The dependence of the electro‐optic effect on electric field distribution and light beam propagation direction

A calculation of the electro‐optic effect in cubic materials is presented on the basis of Nelson’s perturbation approach and the calculated field distribution in an electro‐optic tip used for measurements in integrated circuits. In a strict sense, longitudinal electro‐optic sampling is not possible for this application, and it is found that the magnitude of the electro‐optic effect depends on both the field distribution and propagation direction of the light beam. This has important consequences for the interpretation of the signal amplitude obtained with longitudinal electro‐optic sampling. We show that the adverse effects can be circumvented in a configuration suitable for a null measurement.

Liquid crystal aberration compensation devices

The application of aberration compensation devices can support the trend to higher information densities in optical data storage. The information content of a disc may be increased while maintaining acceptable tolerance levels. For example, disc tilt dependence may require coma compensation, whereas double layer formats require spherical aberration compensation. Liquid crystals (LCs) can be successfully applied in such devices. The design of two practical implementations is discussed; a coma compensating LC-cell with special electrode-design for optimized tolerances w.r.t. decentering, and a lens made of photopolymerizable LC, which can be used to compensate for spherical aberration.

Optical tape system: evaluation of recorder and media

A linear optical tape recorder has been developed and its performance is evaluated. Write-once and rewritable tape samples are tested and on the basis of these results the expected performance of the recorder is concluded. In the recorder data are written and read back with the spot from a single laser that is scanned over the surface with a 10- faced polygon, rotating at up to 3500 Hz. The data density is 1.6 micrometers 2/user-bit, offering 70 Gbyte storage capacity in a VHS-C like cassette. Read and write data rates up to 4.4 and 3.2 Mbytes respectively can be achieved. A novel tape guiding systems winds the tape at a velocity exceeding 20 m/s, offering an average access-time of 1.5 s.

Advances in liquid crystal tilt compensation

Coma generating Liquid Crystal cells can compensate for the coma arising form disc tilt. The segmentation of the ITO- layer determines the effectiveness of this compensation. Different segmentations based on a minimum overall RMS wavefront aberration are described and tested. The tilt window for rewriteable DVD conditions is increased from +/- 0.6 deg to +/- 1.6 deg, and the decrease in the modulation width of the high-frequency eye pattern with tilt is reduced.

The transient behaviour of an input protection

The high frequency behaviour of input protections has been measured with electro-optic sampling. These measurements allow the determination of the time dependence of the voltages at internal nodes as well as the time dependence of the current through the input protection. Simulations are performed using a detailed model of the input protection and a simplified model of the integrated circuit, including connections. The measurements are vital to the development of the model, which ultimately aims at explaining the differences found in Human Body Model testing and Charged Device Model testing.

Evaluation of the prototype optical tape recorder as a bit-machine

In 1996 an improved design for a prototype Optical Tape Recorder was presented. The recorder is now operational and is currently being evaluated. We report on the results of tests of the various servo-systems and of the recording and play-back functions. The tracking and focus servo both have 1 kHz bandwidth which proves to be more than sufficient in our configuration. Recording and play-back experiments are currently run on write-once tape. The data density is currently still half CD-density, to allow for sufficient margins during testing of the equipment, but we have shown in off-line experiments that full CD-density recording on tape is possible.