Makoto Minakata

Precise determination of refractive‐index changes in Ti‐diffused LiNbO3 optical waveguides

A new method for determining refractive‐index changes in Ti‐diffused LiNbO3 optical waveguides is reported. The measurement process was as follows: (1) Diffusion profiles and Ti concentrations in LiNbO3 optical waveguides were measured by an x‐ray microanalyzer. (2) Ti concentrations in the samples were converted into refractive‐index changes using calibration curves which gave the relation between Ti concentrations and refractive‐index changes. The calibration curves were obtained from highly Ti‐diffused LiNbO3 samples, as well as Ti‐doped LiNbO3 single crystals. As an example, the maximum refractive‐index changes of a Ti‐diffused LiNbO3 optical waveguide made at 1000 °C for 10 h were determined to be Δne=1.05×10−2 and Δno=7.7×10−3.

Lattice constant changes and electro‐optic effects in proton‐exchanged LiNbO3 optical waveguides

Lattice constants and electro‐optic constants have been measured on proton‐exchanged LiNbO3 single crystals by means of the x‐ray rocking curve method and the phase modulation technique to examine the relationship between the refractive index and the strain, and the degradation of the electro‐optic effect.

Electro‐optic amplitude modulation using three‐dimensional LiNbO3 waveguide fabricated by TiO2 diffusion

An almost single‐mode waveguiding layer has been fabricated by thermal diffusion of TiO2 into LiNbO3. The refractive‐index change in the layer is 2–3×10−3 for both ne and no, and the propagation loss is 0.7–0.8 dB/cm. The electro‐optic coefficients are found to be nearly equal to those of the bulk crystal. An amplitude modulation has been made with a stripe waveguide 10 μm wide and planar electrodes. The half‐wave voltage is 17 V and the maximum extinction ratio is 12 dB.

Efficient LiNbO3 balanced bridge modulator/switch with an ion‐etched slot

A simple modified balanced bridge modulator or switch is reported. The modulator is comprised of only straight waveguides and an ion‐etched slot used for a conventional phase shifter. The slot excavated at the center of two coupled waveguides solves the coupling of them. 3‐dB coupler monitoring is performed here by a metal loading. The modulator is fabricated by diffusing Ti into z‐cut LiNbO3. Each patterned waveguide is fabricated by diffusing Ti into z‐cut LiNbO3. Each patterned waveguide is 7 μm wide and the spacing is 7 μm. The 3‐dB coupler is 2.9 mm long, and the excavated slot is 15 mm long, 4 μm wide, and 4.5 μm deep. The overall length is 20.8 mm. For the TM‐like fundamental mode, the half‐wave voltage was 2.8 V, the extinction ratio was −15 dB, and the optical insertion loss was as low as 2.6 dB at the 1.15‐μm wavelength.

Growth of high crystalline quality LiNbO3 thin films by a new liquid phase epitaxial technique from a solid-liquid coexisting melt

Abstract LiNbO3 films of higher crystalline quality than underlying LiNbO3 substrates have been successfully grown from a solid-liquid coexisting melt using a new liquid phase epitaxial technique. The dependence of crystallinity and lattice mismatch on film thickness and growth temperature has been investigated in detail through X-ray rocking curve analysis.

Study of boron nitride gate insulators onto InP grown by low‐temperature chemical vapor deposition

Thin films of phosphorus‐doped boron nitride have been grown onto InP, as a new gate insulator, by low‐temperature chemical vapor deposition using the reaction of NH3, B2H6, and PH3. Characteristics of the pyrolytic boron nitride obtained have been investigated by ellipsometry, x‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and conductivity measurement. The results of XPS and AES have shown that a stable and comparatively stoichiometric boron nitride film was successfully obtained, even at low temperatures (300–600 °C), by using the two‐temperature zone technique. The density of interface states for the BN/InP system has been found to be about 1010 cm−2 eV−1 or less around the midgap.

Two‐dimensional distribution of refractive‐index changes in Ti‐diffused LiNbO3 strip waveguides

A method is reported for determining the two‐dimensional distribution of refractive‐index changes in Ti‐diffused LiNbO3 strip waveguides. The measurement process was as follows. Distributions of diffused Ti concentration in LiNbO3 were measured using an x‐ray microanalyzer. Using calibration curves the Ti concentrations were then converted into refractive‐index changes. To obtain high spatial resolution in the two‐dimensional XMA measurement the electron beam position was fixed and the sample was moved at a constant velocity. Lateral sliding of the sample perpendicular to the scanning direction was peformed mechanically, and the measured XMA data were corrected for edge effects. The results obtained for the Y‐cut LiNbO3 waveguide diffused at 1000 °C for 10 h with a 10‐μm‐wide 500‐A‐thick Ti strip showed that lateral diffusion of Ti was marked. The contour with a refractive‐index change of 1×10−3 is of approximate depth 3.5 μm and width 40 μm. The maximum values of Δne and Δn0 were 1×10−2 and 8×10−3, respe...

Characteristics of Zinc Oxide Films on Glass Substrates Deposited by RF-Mode Electron Cyclotron Resonance Sputtering System

There are two types of electron cyclotron resonance (ECR) sputtering systems, DC-mode and RF-mode. In this paper, the properties of zinc oxide (ZnO) film deposited by an RF-mode ECR sputtering system capable of long-term stable deposition are investigated. It is confirmed for the first time that this system is capable of depositing a ZnO film having a sidewall structure without columnar or fibrous grains on an interdigital transducer (IDT)/glass substrate. The ZnO films so deposited were capable of driving the 1.1 GHz fundamental mode in a Rayleigh surface acoustic wave (SAW) without the large propagation loss at high frequencies of conventional ZnO films. Furthermore, ZnO films deposited by this system exhibited 1.7 dB lower insertion loss and a closer agreement between effective electromechanical coupling factors (keff) and the corresponding values calculated with the finite element method (FEM) in comparison with the films deposited by the DC-mode ECR sputtering system.

Single-mode optical-waveguide fiber coupler

A single-mode fiber coupler to the Ti diffused LiNbO(3) strip waveguide has been devised. The influences of three axial displacements and two angular misalignments on the coupling efficiency have been investigated at 6328-A wavelength. The coupler has a special feature wherein coupling degradation caused by fiber displacement after connection can be recovered to the initial state. The total optical insertion loss is 3 dB after fixing the fiber to the LiNbO(3) strip waveguide, which is 4 microm wide and 8 mm long.

Hysteresis free SiO2/InSb metal‐insulator‐semiconductor diodes

Hysteresis free capacitance‐voltage (C‐V) characteristics of SiO2/InSb metal‐insulator‐semiconductor (MIS) diodes are attained by introducing an in situ Br vapor etching technique into the fabrication process. Interface state density is also reduced by using this technique. It is found that following vapor etching, MIS C‐V characteristics are strongly dependent on both the SiO2 film deposition temperature and oxygen flow rate. The optimum condition for SiO2 film deposition to obtain hysteresis free C‐V curves and an interface state density of ≤1×1011 cm−2 eV−1 is determined for InSb MIS diodes. The area of the typical optimum conditions for MIS diodes is located in the oxygen flow rate range of 5–6 cc/min and the deposition temperature range of 200–210 °C. The refractive indices of SiO2 films are 1.44–1.45, the breakdown field strengths are more than 2×106 V/cm, and the dielectric constants for 1 MHz are typically 4.7 at 77 K, respectively.