Seiji Nishiwaki

Optical head employing a concentric-circular focusing grating coupler

An optical head employing a concentric-circular grating coupler (CGC) and a concentric-circular focusing grating coupler (CFGC) is proposed, and its operating principle and characteristics are reported. Satisfaction with a prerequisite for the head, i.e., the removal of aberrations caused by deviations in wavelength and the effective index, is theoretically achieved by application of the concept of optimization of an annular aperture. With CGC and CFGC fabricated by an electron-beam-writing method, we experimentally confirmed its fundamental characteristics of light input, waveguiding, output, and convergence, with an elliptical focusing spot converging at half-intensity widths of 1.8 and 4.0µm.

Track Center Servo and Radial Tilt Servo System for a Digital Versatile Rewritable Disc(DVD-RAM)

In the push-pull tracking error signal, which is generally used for rewritable discs, detection error occurs due to disc radial tilt. We have developed a track center detection method in which the center of a track is accurately detected using the complementary allocated pit address (CAPA) of a digital versatile rewritable disc (DVD-RAM). The track center servo corrects the tracking deviation caused by the detection error. We have also developed a method for detecting the disc radial tilt based on the fact that the level of the push-pull tracking error signal indicates the amount of disc radial tilt when the track center servo is in operation. The developed servo system was introduced in a 4.7-G byte DVD-RAM disc drive, and it has been confirmed that jitter deterioration during reproduction was reduced to 0.5% or lower in the range of ±0.5 degree of disc radial tilt.

Optical element converting linear polarization into circular-tangential polarization

A new optical element capable of converting linear polarization into circular-tangential polarization (i.e., the electric vector is along the tangential direction to a circle) is proposed and demonstrated. The conversion characteristics of the fabricated element are evaluated, and the magnitude of aberration produced by the element is shown to be insignificant.

Diffraction grating lens array

We have proposed a new type of camera module with a thin structure and distance-detection capability. This camera module has a four-lens-array with diffraction gratings (one for blue, one for red, and two for green). The diffraction gratings on the mold are formed mechanically, and the plastic lens array is fabricated by injection molding. The two green images are compared to detect parallax, and parallax-corrected blue, red and green images are then composed to generate a color image. We have developed new design software and molding technologies for the grating lenses. The depth and period of blazed gratings and the shapes of aspheric lenses are optimized; and blue, red and two green aspheric lenses with gratings are molded as a single four-lens-array. The diffraction gratings on both surfaces of each lens act to improve field curvature and realize wide-angle imaging. However, blazed gratings sometimes cause unnecessary diffraction lights that impede the formation ofhigh-resolution images. We have developed a new method to measure necessary first-order diffraction lights and unnecessary diffraction lights separately. Use of this method allows the relationship between molding conditions and necessary/unnecessary diffraction lights to be shown. Unnecessary diffraction lights can be diminished by employing the optimal molding processes, allowing our grating lenses to be used for image capture.

New camera module with thin structure, high resolution and distance-detection capability

Our new type of camera module has a four-lens-array and an imaging sensor. The imaging sensor is divided to four regions, and these four regions are aligned in one-to-one correspondence with the four lenses. Four color filters are placed over the four imaging regions. First region has a blue filter, second has a red, and the other two have green filters, and two regions with green filters are aligned diagonally. Diffraction gratings are formed on aspheric surfaces of the four lenses, and MTF characteristics of these lenses are improved. The four images taken through the different lenses have parallax, but these parallaxes can be calculated by comparison of the two green images. Pixel shifts of blue, red and green images are realized by rotating the four-lens-array slightly with respect to the imaging sensor. After correcting the parallaxes, the green image, the parallax-corrected blue image and the parallax-corrected red image are composed to generate the resultant color image with high resolution. Distances between objects and the four-lens-array are detected by use of the above parallaxes, and measurement error is less than 2.5% for near objects. With above configuration and functions, our camera module has realized smaller height, higher image resolution and distance-detection capability, and will be applied for cellular phones and automobile vehicles.

Aberrations and convergence characteristics of a concentric-circular focusing grating coupler: analysis

The convergence characteristics of the previously proposed concentric-circular focusing grating coupler (CFGC) are analyzed, and aberration functions for such typical errors as wavelength errors, effective index errors, grating pattern distortion, and CFGC eccentricity are derived for evaluation of their allowances. The analyzed results prove that the Strehl intensity deterioration caused by a wavelength error and an effective index offset can be minimized by optimization of the annular aperture of the CFGC. In the case of Marechals criterion, a wavelength error of ±8.6% at the annular aperture of NA = 0.440-0.607 and an effective index error of ±20.4% at NA = 0.500-0.652 are permissible, and these values are ∼100 times greater than those that are seen in a conventional focusing grating coupler that has a rotationally asymmetrical structure with respect to its optical axis.

Analysis of grating lens aberration

A method that approximates a diffraction formula to the Fourier integral formula by the application of a coordinate transformation is proposed, and it is proved to be a superior and more accurate approximation when it is applied to focusing systems under a nonsine condition. Based on this approximation, the condition that gives the diffraction focus position is derived by the application of two methods. The first is applicable to rotationally symmetrical aberrations such as spherical aberration, and the second is a more general method that utilizes the standard deviation of a wave aberration. These methods are actually applied to a grating lens as a way to study both chromatic aberration characteristics and oblique incidence characteristics.

Fabrication of a concentric-circular focusing grating coupler by a conic-wave-front interference method and light-convergence experiments using the coupler.

An interference method utilizing conic-wave-front light for the fabrication of a concentric-circular and chirped grating is proposed. The design method and fabrication of an interference lens that generates conic-wave-front light are also shown. A focusing element is constructed from a concentric-circular grating coupler with a 0.4-mm diameter and a concentric-circular focusing grating coupler with an annular aperture of 2-mm focal length and 2.0-4.0-mm diameter. Light-convergence experiments using the focusing element were able to obtain a focusing spot of 0.5 µm × 0.7 µm at half-intensity widths for a wavelength of 820 nm in combination with liquid-crystal polarization elements.

Calculations of optical field by fast Fourier transform analysis

A repeated FFT algorithm method is devised and calculation of the field distribution across a plane situated close to and far from the focal plane becomes possible. This overcomes the shortcomings of the fast Fourier transform (FFT) algorithm by which a substantial reduction of the computation time for the field distribution of focused light can be achieved only for the local field distribution adjacent to the focal plane.

Improved light extraction from white organic light-emitting devices using a binary random phase array

We have developed a binary random phase array (BRPA) to improve the light extraction performance of white organic light-emitting devices (WOLEDs). We demonstrated that the scattering of incoming light can be controlled by employing diffraction optics to modify the structural parameters of the BRPA. Applying a BRPA to the substrate of the WOLED leads to enhanced extraction efficiency and suppression of angle-dependent color changes. Our systematic study clarifies the effect of scattering on the light extraction of WOLEDs.