Kazutoshi Onozawa

High-power and high-temperature operation of Mg-doped AlGaInP-based red laser diodes

This paper reports on high-power and high-temperature operation of an AlGaInP-based high-power red laser diode with magnesium (Mg)-doped quaternary-alloy cladding layer. The use of Mg dopant with small diffusion coefficient enables abrupt doping profiles as well as high carrier concentrations when compared to conventional zinc (Zn) dopant. It was also found that the metal-organic vapor phase epitaxial (MOVPE) growth of Mg-doped quaternary AlGaInP alloy is not affected by so-called reactor memory effects, while unintentional incorporation of Mg is observed in GaAs after the growth of Mg-doped GaAs layers. The higher carrier concentration in the p-type cladding layer enhanced carrier confinement in the active layer so that device performance at high temperature is improved. The abrupt doping profile suppressing dopant diffusion into the active layer eliminates the nonradiative recombination in the active layer resulting in higher external quantum efficiency. The characteristic temperature of the Mg-doped red laser with a lasing wavelength of 659 nm is as high as 167 K while the Zn-doped laser exhibits a temperature of 127 K. High kink-free output power of 150 mW is achieved at 75/spl deg/C.

A MOS Image Sensor With a Digital-Microlens

We have developed a MOS image sensor with digital-microlenses (DMLs), each of which has an effective refractive index realized by variation of the subwavelength separations between the concentric SiO2 ring walls. The effective refractive index profiles are optimized for the location of each pixel. The light-collection efficiency of the image sensor is twice as high as that of a conventional image sensor because of the enhanced light acceptance in the periphery. A 2.2-mum pitch 3-megapixel MOS image sensor based on the DML technology exhibited excellent uniformity of the light-collection efficiency across the image area, even for light with a very large incident angle, i.e., over 45deg. The DML promises new levels of performance of image sensors.

Dual-Wavelength High-Power Laser Diodes Fabricated by Selective Fluidic Self-Assembly

We have developed dual-wavelength high-power laser diodes (LDs) with a hybrid integrated solution, in which a 660 nm (red) LD chip and a 780 nm (infrared) LD chip are mounted side-by-side on the same substrate. To achieve the required mounting position accuracy, i.e., the distance between two emitting points is limited to 110±3 µm, we have developed a novel mounting technique called selective fluidic self-assembly (S-FSA). In this technique, we used new self-locking structures of guest-host pairs. Bumps formed on the bonding surfaces of the LD chips are used as guests. A substrate with recesses whose shapes correspond to those of the bumps is used as the host. By forming bumps aligned along the waveguide and assigning a rotationally asymmetric shape for the bumps, an accurate and selective mounting is realized. Dual-wavelength LDs fabricated by this technique demonstrated the required positioning accuracy and high-power operation.

A MOS Image Sensor with Microlenses Built by Sub-Wavelength Patterning

A MOS image sensor has digital-microlenses implemented by sub-wavelength patterning of concentric SiO2 ring walls. The sensitivity at the periphery of the imager is 3000e-/1x-s. In comparison, the sensitivity at the periphery of a conventional imager is 1300e/1x-s. Thus, extremely uniform brightness throughout the reproduced image is demonstrated even with an angle of incidence > 45deg.

A 3D vision 2.1Mpixel image sensor for single-lens camera systems

We present a CMOS image sensor that enables a compact 3-dimensional (3D) vision camera system comprising a single set of the sensor and a camera lens. In order to make binocular parallax, which is essential for 3D imaging, the input pupil of the camera lens is presumed to consist of the right-eye and the left-eye domains, where the pixels exclusively receiving light beams from the right-eye domain and those from the left-eye domain, are arranged alternately. In addition, the sensor features an on-chip lenticular lens to split the incident light from the two directions and a Digital Micro Lens [1,2] to focus the split light beams onto the dedicated pixels without significant crosstalk. The fabricated 3D image sensor enables not only successful stereovision imaging in color with sufficiently high sensitivity, but also accurate calculation of distance.

Multiocular image sensor with on-chip beam-splitter and inner meta-micro-lens for single-main-lens stereo camera

We developed multiocular 1/3-inch 2.75-μm-pixel-size 2.1M- pixel image sensors by co-design of both on-chip beam-splitter and 100-nm-width 800-nm-depth patterned inner meta-micro-lens for single-main-lens stereo camera systems. A camera with the multiocular image sensor can capture horizontally one-dimensional light filed by both the on-chip beam-splitter horizontally dividing ray according to incident angle, and the inner meta-micro-lens collecting the divided ray into pixel with small optical loss. Cross-talks between adjacent light field images of a fabricated binocular image sensor and of a quad-ocular image sensor are as low as 6% and 7% respectively. With the selection of two images from one-dimensional light filed images, a selective baseline for stereo vision is realized to view close objects with single-main-lens. In addition, by adding multiple light field images with different ratios, baseline distance can be tuned within an aperture of a main lens. We suggest the electrically selective or tunable baseline stereo vision to reduce 3D fatigue of viewers.

Integrated Catadioptric Pickup with Ferrofluidic Cooling Structure

We have developed a novel integrated catadioptric pickup with a ferrofluidic cooling structure for digital versatile discs (DVDs). To miniaturize the optical system, we made the catadioptric system on a moving head. The catadioptric system consists of a holographic catadioptric lens (HCL), a reflective mirror, a laser diode (LD), and a photodiode IC (PDIC). The HCL has a holographic surface, an aspherical mirror and two aspherical surfaces. This system realized the integration of all optical components into the moving head. The height of the optical system was 8.6 mm including the working distance. To realize efficient heat transfer without sacrificing the motion of the moving head, we developed a cooling structure in which gaps between magnets and coils are filled with ferrofluid. The thermal characteristics were as good as those of conventional optical pickups, proving that the ferrofluidic cooling structure has superior cooling performance. The ferrofluid did not sacrifice the motion of the moving head because of its fluidity.

A novel integrated catadioptric pick-up with ferrofluid cooling structure

We proposed the integrated optical pick-up with a catadioptric system which has a super resolution effect and with ferrofluid cooling structure. All of optical elements such as an objective lens, a laser diode and a photodetector are integrated into the moving part of the actuator to miniaturize the pick-up. Laser beam is double reflected between first reflecting region on top of the objective lens and second reflecting region on a reflecting mirror placed under the objective lens to miniaturize the optical system. The moving part having the laser diode and the photodetector needs high cooling performance to realize the optical system. We developed the cooling structure with ferrofluid held between a magnet and a coil of the actuator. Ferrofluid works as a cooling path to conduct the heat in the moving part towards external parts. We achieved the results as described below. Temperature of the laser diode is approximately equivalent to conventional pick-up against the heat of about 200mW generated in the moving part. Thermal resistance of 120 degree/Watt is available for practical use. The cooling structure leads the results of optical characteristics. As a super resolution effect, spot size of the integrated optical pick-up with wavelength of 660nm and a numerical aperture (NA) of 0.55 is equivalent to spot size of conventional pick-ups with wavelength of 660nm and a NA of 0.65. Focal and tracking error signals for servo control are available for practical use. The cooling performance is enough for realizing the integrated optical pick-up.