Kiyoshi Yokomori

Dielectric surface-relief gratings with high diffraction efficiency

Diffraction properties of dielectric surface-relief gratings were investigated by solving Maxwell’s equations numerically using the differential method. The diffraction efficiency of a grating with a groove depth about twice as deep as the grating period is comparable with the efficiency of a volume phase grating. Dielectric surface-relief gratings interferometrically recorded in photoresist can possess a high diffraction efficiency of up to 94% (throughput efficiency 85%). Calculated results were also found in good agreement with the experimental data.

Simultaneous measurement of refractive index and thickness of thin film by polarized reflectances

We have developed an innovation of the polarized reflectances measurement technique for thickness and index (PRETTI) method, PRETTI-S, which is a simple and accurate technique to obtain the refractive index n and thickness d of a thin film by using S-polarized and P-polarized reflectances measured at oblique angles of incidence. In the PRETTI-S method, the n and d are determined by using only S-polarized reflectances. Therefore, the measurement and numerical procedure to extract the n and d are simpler than the conventional PRETTI method. As an example, measurement of a single-layer film (SiO(2)/Si) is carried out and excellent confirmation is obtained.

Focusing waveguide mirror with a tapered edge

We propose a focusing waveguide mirror with a shallow tapered edge in a slab waveguide and demonstrate it by planar processes and wet etching. A light beam 2 mm wide is focused to 6.5 µm full width at half-maximum at a wavelength of 0.6328 µm. A reflectivity of higher than 90% at the tapered edge is obtained. The inclination ratio of the tapered mirror edge is 1:90. Fairly good correspondence between measured and calculated focused spot sizes is obtained.

Integrated optical pickup with highly sensitive servo signal detection

We proposed a novel structure for an integrated optical pickup based on monolithic integration of waveguide mirrors and photodetectors in a waveguide on a Si substrate. The detection characteristics of focusing and tracking error signals have been demonstrated to be sufficient for practical applications.

Integrated waveguide device for magneto-optical disk signal detection and its fabrication techniques

We propose an integrated optical polarization detection device for magneto-optical disk pickups. The device has a prism coupler and a TE/TM mode splitter which is monolithically integrated on a silicon substrate with p-i-n photodiodes and two optical waveguides with different effective indices of refraction. These two waveguides are connected at two tapered transition regions. This paper describes the device fabrication techniques and the performance of each element integrated in the device. The optical waveguide TE/TM mode splitter is shown to have wide tolerance for wavelength change, fabrication error, or deviation from designed incident angle.

TE/TM mode splitter with waveguide photodetectors

We propose a novel TE/TM mode splitter for an integrated optical detection device for magneto-optical disk pickup. The new mode splitter is monolithically integrated on a Si substrate with pin-photodiodes. This device is composed of optical waveguides A and B with effective indices of refraction NA and NB, where NA > NB. These two waveguides are connected by a tapered transition region. The light wave guided in waveguide A is incident on the tapered transition region at an angle of incidence greater than the critical angle for the TE0 mode and smaller than the critical angle for the TM0 mode. The TE0 mode is totally reflected and the TM0 mode is totally transmitted at that region so that this device works as a TE/TM mode splitter. We fabricated the mode splitters tapered transition region using a novel wet etching process, then examined the TE/TM mode splitting operation using a GaAlAs laser diode ((lambda) equals 0.788 micrometers ). When both TE0 and TM0 modes were exited in waveguide A, the extinction ratios of the TE0 and TM0 modes were measured with the integrated photodiodes. The extinction ratios were -25.4 dB for the TE0 mode and -23.3 dB for the TM0 mode. These values are sufficient for practical application. Furthermore, the polarization detection function was experimentally confirmed using a Faraday rotator.

Refractive-index and thickness measurements for an anisotropic film by S- and P-polarized reflectances

A new technique is proposed for determining the refractive index and thickness of anisotropic films. Suppose the film is aligned in the X-Y plane, and that the plane of incidence includes the X-axis. From the reflectances of S- and P- polarized light, the principal refractive indices nx, ny, and nz can be obtained along with the film thickness d. First, ny and d can be derived from the S-polarized reflectance, since the S-polarization is parallel to the Y- axis. It is also possible to derive nx in the same way. The P-polarization is in the X-Z plane and Fresnel equations for P-polarization are expressed by nx, nz, and d. Therefore, nz can be determined using the P-polarized reflectance and the values already calculated for nx and d. Trial measurements are made on a Sol-Gel derived LiNbO3 film on an Al2O3 substrate, using a system with a reflectance measurement accuracy of the order of 0.01%. The values obtained for ny (equals nx), nz and d are 1.980, 2.056, and 107.5 nm, respectively. According to the results of this simulation, the accuracy of the obtained values can be estimated as < 0.3%.