Integrated-optic Focus Spot Intensity Modulator For Disc Head

Implementation of integrated-optic (IO) pickup heads would result in great improvements in reduction of size, weight, and number of fabrication processes as well as in application flexibility in optical memory systems. Several IO pickups [1-3] have been proposed and demonstrated by the authors for read-only optical discs, magneto-optical discs or optical cards. In this paper, we propose a new IO device modulating focus spot intensity and demonstrate preliminary experimental results for a future application to an IO writing/reading head used in rewritable disc systems. Following the IO pickups previously reported [1-3], the IO modulator is constructed in a slab waveguide on a Si substrate because of its advantages such as excellent flatness, possibility of monolithic integration of photodiodes, compatibility with the IO pickups, low cost, etc. In order to modulate the focus spot intensity for writing data on the disc, electro-optic (EO) modulation is required provided grating components are used in the IO device, since power modulation of laser diode (LD) causes wavelength fluctuation which should be avoided because the gratings have wavelength dispersion. An EO polymer film is used as a guiding layer for the EO modulation because of its fabrication easiness. Although the material is not currently well-chosen and the fabrication process not well-established, we have investigated IO-EO switching devices [4,5] integrating grating couplers and electrodes in a slab waveguide consisting of an EO polymer on a Si substrate. Device configuration A schematic view of the proposed IO focus spot modulator is shown in Fig. 1. The IO device is constructed by integrating a focusing grating coupler (FGC) and poling/modulating electrodes in a slab waveguide on a Si substrate. The waveguide is made of a poled p-NAn-PVA (p-nitroaniline n-polyvinyl alcohol) [6] guiding layer, a polymer (OEBRlOO) upper- and a SiO2 lower-buffer layers, and is sandwiched by planar Au and Au/Cr films (see Fig. 2) which are used as poling electrodes as well as modulating electrodes. The FGC consists of corrugation of Si-N layer deposited on the lower buffer layer and couple a guided wave diverging from a butt-coupled LD to a free-space beam focusing to a point on a disc. The upper electrodes are separated to two parts (interdigital configuration with 4 fingers is shown in Fig. 1). By applying modulation signal to one of the upper electrodes, we can modulate a phase retardation of the guided wave under the one part electrode against the wave under the other part. The intensity distribution of the FGC focus spot along the lateral direction changes according to the phase retardation. The examples are shown in Fig. 2 for the cases of two fingers and three fingers electrodes. As the retardation increases up to 1t, the center peak reduces and other peaks grow. An extinction ratio may be defined by the ratio of the center peak intensity in on-state to the highest peak (no matter which the highest peak is) intensity in off-state. The maximum extinction ratio is 3 dB for two fingers case and becomes higher when the number of fingers is larger.