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.     

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.     

Measurements and simulations of differential phase-tracking signals in optical disk data storage

The tracking-error signal generated in differential phase detection (DPD) is theoretically analyzed and numerically simulated. Experimental measurements of the DPD signal versus the tracking offset obtained on compact read-only and phase-change disks are also reported. The signal is sensitive to the geometry of the marks, intersymbol interference along the track, and cross-track cross talk. A characteristic parameter is introduced to relate the DPD signal to the reflectivities of the mark and the spacer. For read-only disks such as CD-ROM and DVD-ROM, the magnitude of the DPD signal does not seem to depend on the reflectivity of the disks, nor does it depend on the pit depth. As for the influence of the various aberrations on the DPD signal, coma in the cross-track direction is shown to give rise to significant tracking offset, whereas defocus and spherical aberrations reduce the magnitude of the DPD signal appreciably.     

Optimization of groove depth for cross-talk cancellation in the scheme of land-groove recording in magneto-optic disk systems

In the scheme of land-groove recording in magneto-optic disk data-storage systems, it has been shown that an optimum groove depth exists at which the cross talk from adjacent tracks diminishes. Crosstalk cancellation, however, is very sensitive to various parameters of the system, and, in particular, the presence of substrate birefringence can have devastating effects on system performance. We analyze the origin of the observed effects by using scalar diffraction theory, and we show the reasons behind cross-talk cancellation. We also explain the relation between substrate birefringence and cross talk in simple analytical terms. Extensive computer simulations have been performed to verify and extend the theoretical results of this paper; the results of some of these simulations are also presented.     

Er-Yb femtosecond ring fiber oscillator with 1.1-W average power and GHz repetition rates

We report an all-fiber passively mode-locked femtosecond laser oscillator based on the heavily doped Er-Yb phosphate-glass active fiber. Only 20 cm of the gain fiber is sufficient to produce as much as 1.1 W of average output power at 1.5 /spl mu/m directly from the oscillator. The laser can be harmonically mode-locked at repetition rates ranging from 1.7 to 7.2 GHz by adjusting the polarization bias in the cavity. The pulsewidth varies from 300 to 570 fs at the lowest and the highest repetition rate, respectively, and the maximum peak pulse power exceeds 1 kW.     

Tapered fiber bundles for combining high-power diode lasers

Tapered fiber bundles are often used to combine the output power of several semiconductor lasers into a multimode optical fiber for the purpose of pumping fiber lasers and amplifiers. It is generally recognized that the brightness of such combiners does not exceed the brightness of the individual input fibers. We report that the brightness of the tapered fibers (and fiber bundles) depends on both the taper ratio and the mode-filling properties of the beams launched into the individual fibers. Brightness, therefore, can be increased by selection of sources that fill a small fraction of the input fiber's modal capacity. As proof of concept, we present the results of measurements on tapered fiber-bundle combiners having a low-output étendue. Under low mode-filling conditions per input multimode fiber (i.e., fraction of filled modes < or =0.29), we report brightness enhancements of 8.0 dB for 19 x 1 bundles, 6.7 dB for 7 x 1 bundles, and 4.0 dB for 3 x 1 combiners. Our measured coupling efficiency variations of approximately 1%-2% among the various fibers in a given bundle confirm the uniformity and quality of the fabricated devices.     

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.     

Design of a compact photonic-crystal-based polarizing beam splitter

A compact polarizing beam splitter based on a photonic crystal (PC) directional coupler with a triangular lattice of air holes is designed and simulated. In the employed PC structure, transverse-electric (TE) light is confined with the photonic bandgap effect, while transverse-magnetic (TM) light is guided through an index-like effect. Due to the different guiding mechanisms, TM and TE light have strikingly different beat lengths, which is utilized to separate the two polarizations in a directional coupler no longer than 24.2 /spl mu/m. The two-dimensional finite-difference time-domain method of computation is used to evaluate the device performance. The extinction ratios are found to be around 20 dB for both TE and TM polarized light.     

Vector diffraction and polarization effects in an optical disk system

The track pitch of current optical disks is comparable with the wavelength of the laser source. In this domain of the pitch-to-wavelength ratio, the complex-diffraction amplitudes are different for different incident polarization states, and the validity of the scalar diffraction theory is questionable. Furthermore, the use of multilayer coatings and high-numerical-aperture beams in modern optical disk technology inevitably entails the excitation of surface waves, which can disturb the baseball pattern significantly. To describe the interaction of a focused beam with a grooved multilayer system fully, it is necessary to have a rigorous vector theory. We use a rigorous vector theory to model the diffraction of light at the optical disk. We present the simulation and the experimental results and demonstrate the ability of this approach to predict or model accurately all essential features of beam-disk interaction, including the polarization effects and the excitation of surface waves.     

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.