Optical storage currently is the fastest growing segment of the data-storage hierarchy. In this presentation, we will review the technology improvements required to continue this growth.

Advances in hard disk magnetic recording technology are accelerating the increase in areal density. With the advent of MR heads and PRML channels, the drive industry is doubling the capacity of high performance drivers every twelve months. As a result, the cost of storage will reach $DLR .10/MB in 1997.

Extension of optical storage to the third dimension by reading and writing data at distinct levels throughout the volume of optical media will be discussed. An overview of the status, challenges, and limits of this approach will be presented.

PSR (premastered optical disk by super-resolution) is a super-resolution technique used to increase the recording density of ROM optical disks. Our PSR system uses a novel disk with a nonlinear optical layer, a second-harmonic-generation green laser ((lambda) equals 532 nm), 0.52NA objective lens and eight-to-fourteen modulation coding. By using PSR, a high C/N ration of 51dB was obtained for a pit length of 0.2 micrometers which is beyond the conventional optical limit. Furthermore, we obtained a bit error rate of less than 10^-5 for a linear density of 0.185 micrometers /bit and a track pitch of 0.8 micrometers , thus demonstrating the feasibility of our PSR system with six times the areal density compared to the current CD.

We developed the HDTV optical video disk recorder, which combines the HD-CODEC, compresses HD-TV signal to 94Mbps, and the VF-200 optical disk recorder, which can record noncompressed NTSC video data, 4-channel audio data, and user data to both sides of a 30cm(phi) magneto-optical disk in 32 minutes.

The system parameters and detection margins of the recently developed high-density multimedia compact disc system are discussed.

CD size video disc employing the MPEGG2 system for recording moving pictures, is highly desired. However, it has been difficult to record a movie of more than 2 hours without causing a picture quality degradation. This problem has been resolved by the authors by applying a variable bit rate coding scheme. The variable bit rate coding scheme allows to allocate less codes to simple pictures and more to the portions that are needed. Therefore, a total generated code amount can be reduced with the adoption of the variable scheme, this allows to realize more flexible formats in recorded program length, which have not been possible before. This paper covers the coding, recording, and reproduction technologies of the newly developed disc system employing the variable bit rate coding scheme realized this time.

CD-R media is increasingly being looked upon as an excellent archival media with an estimated storage life on the order of 100 years. Since the government will be making large investments in CD-R media, it is imperative that the characteristics of media, recorders, test devices, and compatibility with CD-ROM readers be understood. This article summarizes the results of a CD-R media evaluation conducted by SIGCAT (the Federal Special Interest Group for CD-ROM Applications and Technology) and follow-on evaluations performed by the Data Distribution Laboratory at JPL. For this evaluation, 250 media samples from various manufacturers were recorded on several recorders at different record rates. Every byte of every disc recorded in the study was retievable on some, but not all, CD-ROM readers. We are confident that any commercially available media recorded on 2X recorders will provide a reliable archival media. We have, however, identified incompatibilities between single-speed (1X) recorders and phthalocyanine-based media and between high speed recorders (i.e. 4X and 6X recorders) and cyanine-based media. We recommend that the industry address the recorder and media incompatibilities and that a specification be developed for robust CD-R reader for use in archival applications.

We have developed several HD-CD players, which can play 2X, 3.3X, and 4.2X higher density than normal disc respectively, to compare cost/performance characteristics.

To make higher density on optical disk, a new recording method- 2D recording-which can increase the track-density rather than the linear density was proposed. The performance of this method on increasing density is estimated by the simulation and confirmed by measuring the read-out signal. Consequently, it is clarified that this method can attain more than twice the areal density independently and realize the density of 2.2Gbit/in² which is 8 times as large as that of first generation using 680 nm laser diode.

Single carrier independent pit edge recording (SCIPER) stores data by modulating the position of the leading and trailing pit edges in very small (0.04 micrometers ) discrete steps. With this method we are able to achieve double the compact disc recording density while using a standard CD pick up. In this paper we review the principles of the SCIPER optical disc system and describe a noise reduction method based on the BCH error correction code. Incorporating of this noise reduction circuit improves the robustness of the SCIPER system against additive noise by 2.8 dB. The possibility of applying more advanced coding techniques such as Trellis coded modulation is also discussed.

We consider the problem of coding for a recording channel. Traditional magnetic or optical channels employ saturation recording, where the input to the channel is a two-level waveform. New optical recording channels have been developed that support unsaturated, M-ary (M >= 3) signal levels. In this paper we consider the analysis and design of partial response codes for these channels. Comparisons are made with M-ary runlength limited codes. The partial response codes presented here achieve the largest known storage density and achieve a coding gain of 3-6 dB over M-ary runlength limited codes.

We have developed a new MO recording method based on the single carrier independent pit edge recording (SCIPER) technique with either a 0.07 micrometers or a 0.1 micrometers modulation step and a conventional MO pickup ((lambda) equals 0.5). We have performed read/write experiments and measured both the write power and tilt margins for a 0.07 micrometers modualtion step.

Optical heads wherein a laser diode (LD) is optically coupled through a small aperture to a recording medium acting as an external mirror are described. A flying head and a fiber- coupled head are presented, and their high-sensitivity, wavelength-free detection characteristics are demonstrated. A micro-lens head is also investigated. Interference undulations of the coupled-cavity laser with various spatial frequencies due to variations in the external-cavity length are completely explained. Anti-reflection (AR) coatings on the LD facet with reflectivities of better than -40 dB greatly suppress these undulations. To achieve practical heads, fundamental fabrication techniques including AR coating and monolithic integration on laser wafers are briefly mentioned.

The system transfer function of magneto-optic data storage systems can be significantly improved by super-resolution. Super-resolution is achieved using optical and electronic filters. In our experiments the relative high frequency contrast improved from 10% to 75% after application of a combined optical and electronic filter. The carrier-to-noise ratio was slightly degraded at low frequencies, but was improved by 2dB to 3dB at high frequencies. Measurements indicate that the combined system improves jitter by a factor of three at high frequencies.

We have developed a brand-new method using a gyrator for magneto-optical pickup. This method provides a simple optical pickup with some unique characteristics. It achieves both higher optical coupling efficiency, higher readout signal level, and improved modulation transfer function called ultra-resolution. In this paper, we describe the principle of MO signal readout, theoretical analysis, some results of simulation and experiments.

A high reliability and low noise 680 nm optical head is described. This head enables a high data integrity, 14.8 GByte/disk product (KODAK optical disk system 2000). The laser intensity noise is measured to be -133 to -137 dB/Hz at the data frequencies. This allows phase margin for the system to exceed 50%. The laser with drive electronics is shown to have a pulse lifetime of greater than 40,000 hours at exaggerated drive conditions.

A new focus detection method based on filtering of the servo beam near the domain of frusterated total reflection is intoduced. The method naturally combines an unfocused, wide diameter servo beam with differential detection, making the focus servo of an optical drive extremely robust and dynamically stable.

A new tracking method for an optical tape recording system is described. Since it is difficult to form pits or grooves on tape media, we have employed crosstalk signals to detect tracking error. On the 2 tracks adjacent to the one that the focused spot must follow, pilot signals at 2 different frequencies are recorded respectively. When reading the track, the pilot signals from the 2 adjacent tracks are derived from the readout signal. The tracking error signal is generated by differential detection of their amplitudes. We carried out fundamental experiments using a rotating optical head and a magneto-optical recording medium sputtered on a flat glass substrate. A 5 micrometers mark length was available to detect the tracking error for a 2 micrometers track pitch. A total residual tracking error of less than 0.1 micrometers was obtained when the medium was displaced in a 2 micrometers peak to peak range.

A 2D finite difference time domain (FDTD) approach to modeling the push-pull tracking error signals in a rewritable optical drive is presented. The predictions of the model are experimentally confirmed.

This paper deals with the dynamic characteristics of a sliding, rotary type small lens actuator for high recording density disk system such as digital video disc. In particular the influence of the vibration modes of the lens holder on the frequency characteristics in the tracking direction is discussed. In order to understand the vibration mode phenomena, we performed numerical analysis by using the finite element method and observed actual vibration with a scanning type laser doppler velocimeter. As a result, we determined the rotational movement of the lens holder around the jitter axis at a few kHz, which turned out additional phase delay in the tracking servo loop and sometimes appears when the size of the actuator decreases. To suppress this movement, we considered the mechanism of this rotational movement and proposed a method of increasing the moment of inertia around the jitter axis. The effectiveness of this method was also confirmed experimentally.

Optical super-resolution is one of many techniques being explored to increase the storage density of optical disks. With optical super-resolution, the optical system is modified to reduce the diameter of the central lobe of the spot on the disk. This generally involves increasing the energy in the sidelobes of the spot. We will discuss the trade-offs of this approach.

High density magneto-optical recording was realized with the combination of a compact green laser and a very sensitive NdTbFeCo/TbFeCo MO disk for the wavelength of 532 nm. This media has a high Kerr rotation angle at 532 nm and maintains a stable recording domain. With an objective lens of 0.6 NA, CNR of 48 dB and jitter of 3.0 nsec by mark edge recording was obtained for a mark length of 0.45 micrometers (0.34 micrometers /bit) at a writing power of 4.5 mW.

A solid immersion lens (SIL) attached to a conventional objective could multiply the effective numerical aperture of an optical pick-up by a factor of refractive index n. A truncated hemispherical SIL (n equals 1.5) mounted on a slider, which was designed to follow the movement of the objective, kept good contact between the SIL and a disk substrate with a slight amount of air gap. Our simple configuration enabled the use of this method in an unsealed environment and extended the spatial cut-off frequency by 1.5 times. In addition, the combination with a second-harmonic generation green laser yielded high areal density optical data storage, six times that of current CD systems.

Current status of high density recording on phase change optical disks is summarized. More than 4.1 Mbits/mm² density of direct overwriting on phase change disks was achieved using red (680 nm) high power (35mW) LDs, large NA (0.6) optics, thin (t0.6 mm) substrates, mark-edge recording with write compensation, and land-groove recording technologies.

Phase-change optical disks where the cooling rate has been reduced by design show about double the sensitivity of conventional disks and are capable of more than 500,000 overwrites. The disk layer materials are the same as for conventional phase-change disks. The disk diameter is 120 mm and the capacity 9is 650 MB single sided, the same capacity as a conventional CD-ROM. The new 'PD' concept drive has a simple optical unit and supports both 4X data rate CD-ROMs and the new phase-change overwritable disks.

The attainable recording density of a thermally balanced phase-change optical disk with the mark position recording method have been studied, focusing on the viewpoint of designing an optical disk memory system. A new recording method with constant duration of the read power level after every write-pulse, called the off-pulse, was applied. A wide overwrite power margin from -20% to +10% was obtained for a bit length of 0.63 micrometers and a track pitch of 0.9 micrometers . These results mean that it is possible to attain 1.3 GByte capacity in a 90 mm double-sided disk with the zoned constant angular velocity format.

We have measured linear velocity dependency of overwriting erasability and jitter. It was found that overwriting jitter correlates with overwriting erasability which reflects the degree of mark distortion. A 5-layered disk was developed whose absorptivity of crystalline state was larger than that of amorphous state. This disk showed small jitter, which was less than 16% and 15m/s when the bit length was 0.45 micrometers /bit without applying a writing compensation. And we have studied land/groove recording at relativley low linear velocity of 4.5 m/s. Crosstalk both from land and groove were lower than -26db in the recording pitch of 0.7 micrometers , which is sufficiently small for practical use. The bit length of 0.45 micrometers /bit and the recording pitch of 0.7 micrometers realize 2.6 Gbytes in 120 mm, supposing that user data capacity is three quarters of the total capacity.

For high-density recording at a wavelength of 690 nm, we developed a phase-change optical disk with a Si reflective layer. We estimated the effect of interference layer by calculating optical properties. The absorption control required for mark edge recording and a 2 dB C/N improvement was obtained by forming a ZnS-SiO₂ interference layer on the Si layer. Under recording conditions with a minimum bit length of 0.335 micrometers and a track pitch of 1.2 micrometers , a sufficient C/N and a BER less than 10^-4 were confirmed. This result indicates that the recording capacity of the new disk is more than 4 GB.

Tamarack's progress in integrating key holographic storage technologies will be discussed. Specifically, development of the holographic optical head, the 3D storage media and the image-wise data recovery techniques toward a 5 1/4 inch drive with 20 gigabytes user capacity will be reported.

Holographic techniques and materials have matured sufficiently to allow high capacity in practical systems. We demonstrate a holographic memory with storage density of 10 bits/micrometers ². Novel techniques, such as shift multiplexing, can be used to attain even higher capacity with simpler implementation.

We describe a page-formatted random-access holographic memory capable of storing up to 160,000 holograms. A segmented mirror array allows a 2D angle scanner to provide access to any of the stored holograms. High-speed random access can be achieved with a nonmechanical angle scanner. We demonstrate holographic storage and high-speed retrieval using an acousto- optic deflector.

Land/groove recording in an MO system was demonstrated using MFM recording technology and a 690 nm wavelength diode laser. The CNR of 55 dB and cross-talk of about -35 dB were obtained at a 1.88 micrometers recorded mark length and a 0.7 micrometers yielded a 0.35 micrometers bit length and a 0.7 micrometers track pitch were 8.1% on land and 8.7% on groove. The disk tilt margins were radially +/- 0.42 deg and tangentially +/- 0.45 deg when reading out on land, and +/- 0.40 deg and +/- 0.44 deg on the groove. The recording power margins were estimated to be +/- 8.0% on land and +/- 9.0% on groove considering over-writability and cross-write characteristics.

As one of the most promising schemes for high density MO recording, a new media for magnetic multivalued recording has been proposed. By switching the external field on four levels, the quadri-valued signal was obtained. On the MMV media, field sensitivity is improved by controlling composition of PtCo capping layer coupled to TbFeCo. The external filed for recording four levels is reduced down to 350 Oe and direct overwriting capability is confirmed on 0.8micrometers mark length recording.

Direct overwrite (DOW) at 2X operating densities has been demonstrated utilizing an IBM 2X drive and exchange coupled multilayered media. To generate the writing waveforms required for DOW, modifications of the laser driver electronics were implemented. The drive was integrated into a test bed to study the light intensity modulation direct overwrite media writing and overwriting characteristics. In this paper, we show that utilizing simple write waveforms consisting of P_Hi superimposed on a pedestal power, does not permit the attainment of satisfactory figure of merits (FOMs). We show that by modifying the write waveform we increase the temperature gradients that precisely define the written marks. Consequently, it is possible to achieve good FOMs even at OD conditions, and this is done by implementing straightforward modifications to the laser drive electronics of an IBM 2X drive.

A magneto-optical disk drive was developed to be capable of reading/writing 230 MB and 128 MB disks at a disk rotation of 3600 rpm. In order to improve the data transfer speed, a new disk cache algorithm was applied to the drive using its internal buffer. This cache algorithm achieved speed-up by 10-50% even under 'smartdrv'.

An optical disk array using four magneto-optical disk drives has been developed. This system uses ISO standard format 130 mm disks and has a standard SCSI-2 interface to the host computer. The system functions as a fast, large-capacity single-disk subsystem for host computers.

We have modified drives of 2Gbyte/130 mm non-DOW MO disk to be drives of 2GByte/130 mm DOW-MO disk. We have demonstrated that we have already reached the practical stage showing that it is possible to produce 2 GByte/130 mm DOW-MO disk and drive.

We consider the design and analysis of read-back symbol discriminator for multilevel codes on an electron trapping optical memory recording channel. The linear amplitude response of this new media provides a method of increasing the density of stored data by using multilevel or M-ary runlength limited (RLL) channel codes. In this paper, we consider the problem of symbol detection for a high density M-ary channel that encodes 3 user data bits per minimum sized feature to achieve a maximum channel capacity of 3.4 bits/sec/Hz. A dynamic model of the optical/electronic read channel is constructed to generate the expected amplitude waveform. These signals are discriminated by a maximum likelihood detector that operates on a small number of quantized amplitude values. Another model is constructed to study the effect of signal amplitude quantization on the error performance for M-ary RLL channel codes. Included in the analysis are the effects due to variations in the offset of both sampling phase and amplitude levels. Comparisons of the proposed symbol detection method are presented for two different M-ary RLL channels operating at user data rates from 25 Mbits/sec to 68 Mbits/sec.

This paper gives optical and magneto-optical (MO) characterization results (index of refraction, absorption coefficient, thickness, Faraday rotation, and coercivity) obtained from bismuth-doped dysprosium iron garnet samples. These samples were prepared under differing sol-gel processing conditions and were characterized using the Ultimate Ellipsometer. After first describing the design and measurement capabilities of the Ultimate Ellipsometer (Simpson, 1995), we then describe how various processing parameters, such as spin speed and annealing temperatures, affect the bismuth iron garnet material's optical and MO properties.

Ultrafast photon-mode recording based on photoinduced electron transfer reaction was proposed using ion-pair charge-transfer (IPCT) complexes of 4,4-bipyridinium salts. Results with two kinds of counter anions, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and iodide, were reported. These anions made electronic interactions with 4,4-bipyridinium ions in solutions and in solid films to give characteristic absorption in the visible region. The dynamics of color changes from pale yellow or orange to blue upon excitation of IPCT bands of these complexes in solutions were studied by femtosecond (fs) pulsed laser. Transient absorption at about 600 nm appeared in about 0.3 ps in both samples, which was controlled by the time-resolution of our fs laser system. Such extremely fast color changes were due to the fact that the IPCT absorption band is associated with the electronic transition from a partially charge-transferred ground state to an alomost completely charge-separated excited state. The decay behavior was totally different between two salts. Tetrakis[3,5- bis(trifluoromethyl)phenyl]borate salts showed a decay curve composed of a fast component with 80 ps lifetime and of an extremely slow one corresponding to steady and reversible color changes.

We have developed CD-R media using phthalocyanine dye for the recording layer. This phthalocyanine CD-R has good recording performance in high speed and normal speed recording. We analyzed this behavior compared to cyanine CD-R, and found that even the shortest pits (3T) in phthalocyanine CD-R were sufficiently formed in high speed recording. On the other hand, it is difficult for cyanine CD-R to form the short pit sufficiently by irradiation of a short laser pulse corresponding to the high speed recording. We conclude that the difference in the shortest pit (3T) formation between phthalocyanine CD-R and cyanine CD-R for high speed recording is due to the difference in the dye decomposition process. That is, the phthalocyanine dye is decomposed more rapidly than the cyanine dye for less than 0.5 microsecond(s) of the laser pulse irradiation, resulting the effective formation of the shortest pit (3T) in high speed recording.

The noise power spectrum was calculated for groove wall undulation based on the Fraunhofer diffraction theory. Groove profile fluctuations were modeled as fluctuations in the groove profile parameters. Changes in the diffracted light field were assumed to vary linearly with the changes in the groove profile. The expression for the noise power spectrum was obtained by statistical averaging, and the contribution of the undulation to the noise power spectrum was found to depend on the depth of the undulating area.

In an optical pick-up for high areal density recording using Nd:YAG SHG (second harmonic generation) green laser, the stability of a servo mechanism may be deteriorated easily due to many optical elements and long optical path. We propose a new stable servo method employing a differential critical-angle prism. This method showed one order of higher stability than conventional astigmatic servo method to the chief ray tilt. We obtained more than a CNR of 48dB even in case of the large chief ray tilt with 0.5 micrometers mark length on MO recording layer, which is close to the case of no chief ray tilt.

The process of optical recording on ablation mechanism carriers shows a peculiar microrelief. The peculiarity of the latter is in the presence of an annular rim around the pit, of the carrier material arising by action of surface tension. This force on the liquid phase of microregion was subjected to the thermal action of a laser beam. During reading for increasing the signal- to-noise ratio, it is necessary to achieve optimal relations for parameters of reading laser beam, and geometrical pit dimensions surrounded by the annular rim. The method of scalar theory of diffraction did not allow to investigate the influence of geometrical sizes rims of pits on scattering characteristics of the laser beam from pits. In this connection, the mathematical simulation problem of the informaiton reading process from the surface with such form of units is of current concern.

Wideband semiconductor based on the alkali-earth metals and doped with the rare-earth ions are sensitive to the laser irradiation in the blue spectral range. An alkali-earth sulphide doped with the rare-earth ions could be used as recording reversible media for data storage.

We propose a system that combines both the multilayer storage and the wavelength multiplexing afforded by using volume holograms to enhance by several fold, the optical data storage capacity of compact discs. The storage capacity and interchannel crosstalk are analyzed as a function of data-channel wavelength separation and reading beam spot size. A storage capacity of 9.7 Gbytes per disc is provided when 6 layers and 6 wavelengths are used.

This paper discussed the further development of magnetic force microscopy to observe successive changes in recorded marks resulting from heating cycles in the presence of an external field. Our results on a conventional TbFeCo-based medium indicate very stable domains up to relatively high temperatures, which then rapidly collapse once wall movement starts. The images show that due to variations of the media on the local scale, the marks diminish in a highly nonuniformity manner. Inhomogeneities of the magnetic and mechanical properties of the medium play a crucial role in retarding wall motion and lead to formation of complex domain shapes prior to collapse. Pinning at the grooves has been similarly observed. Quantitative analysis of the results provide additonal insights concerning mark size reduction and edge sharpening as a function of temperature. This analysis also leads to the derivation of a critical radius for this media and biasing field combination.

CD-ROM has established itself as the preferred medium for the cost-effective distribution of software and various types of multimedia information. The ongoing price reductions of write- once CD-R media and drives are broadening the application range of CD-R from CD- prototyping and small-scale data distribution towards data archiving and bck-up at the desktop. The addition of erasability would increase the attractiveness of recordable CD technology and enable its wide-spread use for personal recording of all types of data files with convenient data interchange. A CD-erasable system (CD-E) with the following characteristics would be very desirable: 1) compatible with existing computer systems and software, 2) erasable with direct overwrite (DOW), 3) recordable using CD-R recorders with minimum modification, 4) readable on CD-ROM players with minimum modification, and 5) reliable. The potential of Ag-In-Sb-Te phase-change materials for CD-E discs and some basic studies have been previously reported by Ricoh. In this paper, we present detailed results of DOW recording on Ricoh's CD-E discs at 2.4-2.8 m/s (CD double speed). We conclude that reliable writing and reading as outlined in the concept above can be achieved with this CD-E system.