Effects of substrate birefringence and tilt on the irradiance and phase patterns of the return beam in magneto-optical disk data storage

Substrate birefringence in a magneto-optical disk system is shown to have a predictable effect on the return beam. The irradiance and phase patterns of the return beam at the exit pupil of the objective lens are calculated and experimentally verified for the cases of no substrate birefringence, birefringence aligned with the incident polarization, and birefringence aligned at 45° to the incident polarization. The irradiance at the exit pupil is also calculated (and experimentally verified) for a grooved substrate for various amounts of substrate tilt.     

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.     

Substrate noise in optical data-storage systems

Noise in optical disk readout has been examined for different polarizations of the incident beam. The disks studied are bare grooved glass substrates, having different groove shapes or differing jaggedness in the sidewalls. We perform measurements for the electric field of the incident laser beam parallel to the track and perpendicular to the track using both differential magneto-optical and conventional phase-change readout schemes. The incident beam of light is focused on the grooved surface of the (bare) substrate either through the substrate or directly from the air. Experiments reveal that the noise level is dependent on the state of polarization, the nature of the track (i.e., land or groove), and the medium of incidence. Surface roughness and sidewall jaggedness are two dominant contributors to the media noise in these substrates.     

Sources of noise in erasable optical disk data storage

Noise sources in the readback signal for phase-change and magneto-optical disks at red, green, and blue wavelengths are examined, and a simple model is presented to explain the observed noise spectra. For phase-change disks the media noise, which corresponds to ~0.4% fluctuation in the disk's amplitude reflection coefficient, is the limiting performance factor for the conventional detection scheme. In magneto-optical media the depolarization noise, whose fluctuations are ~0.05% of the disk's reflection coefficient, is the major contributor to the media noise in the differential detection scheme. In phase-change optical disks the main sources of noise are the roughness of the groove profiles and the graininess of the polycrystalline recording layer. In nongrooved regions of the disk the media noise measured with green light is found to be nearly the same as that obtained with the red light. In magneto-optical disks the scattering of light from the rough groove profiles, as well as media inhomogeneities, gives rise to depolarization. Measurements on nongrooved regions of a magneto-optical disk indicate that the media noise obtained with the green light is somewhat higher than that obtained with the red light.     

Analyzing combinations of circular birefringence,linear birefringence,and elliptical dichroism in magneto-optical rotators

We derive analytic expressions for the polarization characteristics of light emerging from a magneto-optical medium possessing arbitrary contributions from linear and circular birefringence as well as magnetic circular dichroism. The medium is placed inside a static magnetic field. The rotation of the plane of polarization and the ellipticity of the resultant light exhibit interesting characteristics that can be a useful guide in the design and analysis of new photonic devices. Furthermore, the Jones matrices are derived in all cases, including for elliptical dichroism, indicating the role of hyperbolic trigonometric functions in modeling the effects of dichroism. Finally, implications for experimental detection of the polarization state and the limits on the performance of optical isolators are discussed.     

Image contrast in polarization microscopy of magneto-optical disk data-storage media through birefringent plastic substrates

Using polarized-light microscopy, we have investigated the magnetic domains of perpendicularly magnetized media under several different conditions, including direct observation of the thin-film magnetic layer and observations through the glass or plastic substrates on which the magnetic film was deposited. The results show that the image contrast is reduced with an increasing numerical aperture of the objective lens. They also indicate that the polarization rotation caused by differences between the reflectivity-transmissivity of the p and s components of polarization deteriorate the magnetic image contrast. Furthermore, by comparing the image quality using the same objective lens on samples having different substrates, we found that the images obtained through plastic substrates are worse than those obtained through glass substrates. Birefringence of the plastic substrate is shown to be responsible for the additional degradation of the image contrast.     

Effects of substrate birefringence on focusing and tracking servo signals in magneto-optical disk data storage

Using computer simulations based on the diffraction theory of high-N.A. systems, we examine the effects of substrate birefringence on servo signals in magneto-optical disk drives. Our attention is confined to systems that use the methods of push-pull tracking and astigmatic focus-error detection. We show that the amounts of birefringence typically observed in polycarbonate substrates do not in themselves cause problems for the servo channels. However, the presence of residual aberrations in the optical path (astigmatism in particular) can have devastating effects on the magnitude of the track-error signal and the stability of the focusing servo. We show that the combination of substrate birefringence and residual beam astigmatism can either improve or deteriorate the performance of the focusing servo, depending on the orientation of the aberration relative to the principal axes of the substrate.     

Measurement of birefringence for optical recording of disk substrates

We experimentally investigate the birefringence of bare and coated substrates for magneto-optical recording, using ellipsometry at wavelengths of 632.8 and 780 nm. The polarization rotation and ellipticity of reflected or transmitted light are measured for different incident angles and for different orientations of the incident linear polarization. The measured data are then fitted by the MULTILAYER computer program, which solves the Maxwell equations for a plane wave propagating in a multilayer structure. This approach makes it possible to determine, with high accuracy, the orientations of the principal axes of the substrate and the corresponding refractive indices. The results show that one of the principal axes is always along the substrate's normal direction, but the orientations of the in-plane principal axes can be much different from the radial and track directions. A special feature of the ellipsometers that were used is that a glass hemisphere is placed in contact with the substrate to eliminate refraction of the incident beam. This enables a maximum propagation angle of 70° (with respect to the normal) in the substrate and hence increases the measurement sensitivity. Certain anomalies have been observed, which we believe are associated with the variation of birefringence properties along the thickness direction.     

Contrast mechanisms in polarization-sensitive Mueller-matrix optical coherence tomography and application in burn imaging

We investigate the various contrast mechanisms provided by polarization-sensitive (PS) Mueller-matrix optical coherence tomography (OCT). Our PS multichannel Mueller-matrix OCT is the first, to our knowledge, to offer simultaneously comprehensive polarization-contrast mechanisms, including the amplitude of birefringence, the orientation of birefringence, and the diattenuation in addition to the polarization-independent intensity contrast, all of which can be extracted from the measured Jones or the equivalent Mueller matrix. Theoretical analysis shows that when diattenuation is negligible, the round-trip Jones matrix represents a linear retarder, which is the foundation of conventional PS-OCT, and can be calculated with a single incident polarization state, although the one-way Jones matrix generally represents an elliptical retarder; otherwise, two incident polarization states are needed. The experimental results obtained from rat skin samples, which conform well with the histology, show that Mueller OCT provides complementary structural and functional information on biological samples and reveal that polarization contrast is more sensitive to thermal degeneration of biological tissue than amplitude-based contrast. Thus, Mueller OCT has significant potential for application in the noninvasive assessment of burn depth.     

Mirror magneto-optical trap using circularly polarized light-emitting optical fibers

A fiber-optic mirror magneto-optical trap (mirror-MOT) that uses a pair of circularly polarized light-emitting optical fibers as an optical access is demonstrated. The fiber is fabricated so that a length of birefringence fiber, designed to be a quarter wave retarder at both wavelengths of 780 and 852 nm, is attached directly onto a polarization-maintaining normal fiber. The polarization states of light emitted from the fibers are sufficiently circular for the operation of a mirror-MOT with 87Rb atoms. The mirror-MOT is able to capture approximately the same number of atoms obtainable with a conventional mirror-MOT. The technique makes it possible to fabricate a compact MOT apparatus by introducing the optical fibers directly into an ultrahigh-vacuum chamber.     

Retroreflecting ellipsometer for measuring the birefringence of optical disk substrates

A retroreflecting ellipsometer has been constructed for measuring the birefringence of optical disk substrates. In contrast to conventional ellipsometers with two mechanical arms, this system has only one arm along which both the incident and reflected beams travel. This construction eliminates the mechanical limitations of conventional ellipsometers, thereby permitting normal incidence on the sample. In addition, the single arm is adjustable in two dimensions, with the polar incident angle, θ(inc), varying from 0° to 70°, and the azimuthal incident angle, Φ(inc), varying from 0° to 360°. The condition of normal incidence permits accurate measurement of in-plane birefringence. The adjustability of both θ(inc) and Φ(inc) is necessary for the measurement of possible tilts of the index ellipsoid, and also for the variation of birefringence through the substrate thickness. Measurement results showing the useful features of the equipment are presented. The optics of the hemispherical assembly used for retroreflection as well as for the elimination of undesirable refractions are also studied by use of the ZEMAX lens design program.     

磁光盘记录光场畸变分析

运用光波衍射理论，针对磁光盘建立了记录光场的数学模型。该数学模型可全面描述基片杂质大小、位置、透射率对记录光场的影响。利用该数学模型可进一步研究基片杂质与磁畴特性，以及盘片误码的关系。     

Effect of tilted ellipsoid of birefringence on readout signal in magneto-optical disk data storage

In the 2× format for second-generation magneto-optical (MO) disks, there are recording tracks next to preformat pits. It has been observed that the MO signals from the tracks adjacent to the preformat pits can be affected. We present numerical modeling results on the effect of a tilted ellipsoid of substrate birefringence on the MO readout signal. Theoretical calculations show that the observed effects can be explained if a certain tilt of the ellipsoid of birefringence exists around the preformat pits.     

Simple birefringence measurement method for coated optical disks with a fixed incident angle ellipsometer

In a coated and laminated optical disk, only a reflectance method is available for oblique incidence retardation measurements to evaluate the birefringence in a polycarbonate substrate. I propose a simple measurement method that simultaneously measures both the lateral and vertical birefringence in a coated substrate. In this reflectance method, only two oblique incidence retardation values with a fixed incident angle are measured from the radial and the circumferential directions of the disk. Using the sum and the difference of these two retardation values, we can calculate both birefringences without any cumbersome curve-fitting procedure. This method can easily be introduced into the routine inspection for optical disk manufacturing.     

焊接微缺陷磁光成像检测有限元分析

目的 研究铁磁材料焊接微缺陷的磁光成像规律.方法 运用漏磁检测原理和法拉第磁致旋光效应,建立微缺陷三维有限元模型,分析微缺陷磁光成像过程与磁场之间的关联,研究不同提离值、励磁电流、缺陷宽度、缺陷深度下的磁光成像,以及探索这些因素对磁光图像特征的影响.在此基础上,对最小宽度为0.05 mm的微缺陷进行磁光成像检测实验,并...     

Theory of near-field magneto-optical imaging

Scanning near-field optical microscopy has been recently applied to the imaging of magnetic samples. It was shown experimentally that an apertureless microscope suffers a substantial loss of resolution when used for magneto-optical imaging compared with that for conventional imaging. No such change is observed for aperture microscopes. We explain this observation by developing a model for the imaging process that incorporates the response of the probe. We calculate real observable properties such as the rotation of polarization at the detector or the circular dichroism signal and thus simulate magneto-optical images of a domain structure in cobalt for both aperture and apertureless microscopes.     

Recording and readout using clock marks premastered by groovewobbling

A new recording and readout technique for land and groove recording on a magnetic super-resolution (MSR) disk is described. The technique uses specifically premastered clock marks. To generate a stable clock signal, the clock marks are fabricated with short bursts in grooves by wobbling at a different frequency from address information. The clock marks and the address information can be separated from a track error signal. When the extracted clock signal is applied to precise recording and readout of the magneto-optical data marks, the bit error rate becomes lower than that of the conventional clock recording/readout system. Additionally, the crosstalk of the wobbled address information to the magneto-optical signal can be canceled out electronically. This new format is not only suitable for high-density recording but also convenient in the disk manufacturing process     

Polarization microscopy by use of digital holography: application to optical-fiber birefringence measurements

We present a digital holographic microscope that permits one to image polarization state. This technique results from the coupling of digital holographic microscopy and polarization digital holography. The interference between two orthogonally polarized reference waves and the wave transmitted by a microscopic sample, magnified by a microscope objective, is recorded on a CCD camera. The off-axis geometry permits one to reconstruct separately from this single hologram two wavefronts that are used to image the object-wave Jones vector. We applied this technique to image the birefringence of a bent fiber. To evaluate the precision of the phase-difference measurement, the birefringence induced by internal stress in an optical fiber is measured and compared to the birefringence profile captured by a standard method, which had been developed to obtain high-resolution birefringence profiles of optical fibers.     

Subtle effects of the substrate in optical disk data storage systems

Much attention has been focused on the effects of the disk substrate in optical data storage. In particular, substrate birefringence has been studied extensively because it causes significant problems in magneto-optic systems. We investigate certain subtle effects of the substrate, such as feedback into the laser diode, in compact disk and phase-change systems. Our analysis of the compact disk system led us to discover a new technique for the rapid measurement of the substrate birefringence. We also address the question of how focusing the laser beam through the substrate will affect the depth of focus.     

IEEE Magnetics Society & The IEEE Press call for books

The following topic are dealt with: magnetic actuators; magnetic sensors; magneto-optical recording materials; ferromagnetism; magnetic disk drive technology; magnetic heads; magnetic media; magnetic interfaces; magnetic integration; and magnetic hysteresis.