Tomoaki Tojo

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.

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.