Characterization of a bit-wise volumetric storage medium for a space environment.

We report playback performance results of volumetric optical data storage disks that are made from a class of light-absorbing (photo-chromic) compounds. The disks are exposed to a simulated space environment with respect to temperature and radiation. To test for temperature sensitivity, a vacuum oven bakes the disks for certain amount of time at a designated temperature. Radiation exposure includes heavy ions and high energy protons. Disks fail in high temperature and large proton-dose conditions. Heavy ions do not cause significant disk failure. The prevention of disk failure due to harsh space environments is also discussed.

Determination of optical constants of thin films and multilayer stacks by use of concurrent reflectance, transmittance, and ellipsometric measurements.

Using measurements of reflectance, transmittance, and the ellipsometric parameter D, we have determined the thickness, refractive index, and the absorption coefficient of various thin films and thin-film stacks. (D, the relative phase between the p- and s-polarized components, is measured for both reflected and transmitted light.) These optical measurements are performed with a specially designed system at the fixed wavelength of lambda = 633 nm over the 10 degrees -75 degrees range of angles of incidence. The examined samples, prepared by means of sputtering on fused-silica substrates, consist of monolayers and trilayers of various materials of differing thickness and optical constants. These samples, which are representative of the media of rewritable phase-change optical disks, include a dielectric mixture of ZnS and SiO(2), an amorphous film of the Ge(2)Sb(2.3)Te(5) alloy, and an aluminum chromium alloy film. To avoid complications arising from reflection and transmission losses at the air-substrate interface, the samples are immersed in an index-matching fluid that eliminates the contributions of the substrate to reflected and transmitted light. A computer program estimates the unknown parameters of the film(s) by matching the experimental data to theoretically calculated values. Although our system can be used for measurements over a broad range of wavelengths, we describe only the results obtained at lambda = 633 nm.

Crystallization studies on phase-change optical recording media by use of a two-dimensional periodic mark array.

We present the results of crystallization studies in thin-film samples of amorphous and crystalline Ge(x)Sb(y)Te(z). The experiments, conducted at moderately elevated temperatures, are based on measurements of the first-order diffraction efficiency from a two-dimensional periodic array of recorded marks. When the samples are slowly heated above room temperature, changes in the efficiencies of various diffracted orders give information about the on-going crystallization process within the sample. Two different compositions of the GeSbTe alloy are used in these experiments. Measurements on Ge(2)Sb(2.3)Te(5) films show crystallization dominated by nucleation. For the Sb-rich eutectic composition Ge-(SbTe), crystallization is found to be dominated by growth from crystalline boundaries. We also show that crystalline marks written by relatively high-power laser pulses are different in their optical properties from the regions crystallized by slow heating of the sample to moderate temperatures.

High-performance readout and recording by a combination aperture.

A solid immersion lens combined with a conical dielectric tip exhibits good resolution and efficiency in reading and recording data marks on optical storage media. We demonstrate a combination aperture that produces ~200-nm full-width 1/e(2) spot size and achieves 50% optical efficiency in an edge-scan experiment. A comparison of recording with the combination aperture, with an unmodified solid immersion lens, and with a far-field system is made.

Static tester for characterization of phase-change, dye-polymer, and magneto-optical media for optical data storage.

We have designed and built a static tester around a commercially available polarized light microscope. This device employs two semiconductor laser diodes (at 643- and 680-nm wavelengths) for the purpose of recording small marks on various media for optical data storage and for the simultaneous monitoring of the recording process. We use one of the lasers in the single-pulse mode to write a mark on the sample and operate the other laser in the cw mode to monitor the recording process. The two laser beams are brought to coincident focus on the sample through the objective lens of the microscope. The reflected beams are sent through a polarizing beam splitter and thus divided into two branches, depending on whether they are p or s polarized. In each branch the beam is further divided into two according to the wavelength. The four beams thus produced are sent to four high-speed photodetectors, and the resulting signals are used to monitor the reflectance as well as the polarization state of the beam on reflection from the sample. We provide a comprehensive description of the tester's design and operating principles. We also report preliminary results of measurements of phase-change, dye-polymer, and magneto-optical samples, which are currently of interest in the areas of writable and rewritable optical data storage.

Versatile polychromatic dynamic testbed for optical disks.

A dynamic testbed for the evaluation of optical disks has been designed and constructed. The system is achromatic within the wavelength range 440-690 nm, allowing any light source in this range to be utilized for read-write-erase experiments. In addition, the system accepts disks with substrate thicknesses ranging from 0 to 1.7 mm. The polarization handling capabilities of the testbed are such that, with the turn of a knob, one can generate either linearly polarized or circularly polarized light at the disk surface. This feature permits the testing of both magneto-optical and phase-change disks, in addition to compact disks and digital versatile disks, without any modifications to the system. A leaky polarizing beam splitter (LPBS) has been specially designed and built for this tester. The LPBS allows continuous adjustment of the ratio between p- and s-polarized components of the reflected beam that reach the detectors. This feature is especially useful for magneto-optical disks, where one can achieve an optimum signal-to-noise ratio by adjusting the relative amounts of the two components of polarization at the detection module. Focus-error detection is based on the astigmatic method, and the primary track-error detection scheme is the push-pull method, although other focusing and tracking schemes may also be implemented. The rf data signal and the focusing and tracking servo signals are all derived from the same detectors, thus allowing the optical power returning from the disk to be used in its entirety for these multiple purposes. The detection channel consists of two high-speed quad detectors mounted on the two arms of a differential detection module. By combining the various outputs of these detectors it is possible to generate the astigmatic focus-error signal, the push-pull track-error signal, the differential magneto-optical readout signal, the conventional sum signal for phase-change disk readout, and the differential edge-signal for mark-edge detection on various types of optical media.