Takeshi Obokata

Compact polarization-independent optical circulator.

This paper introduces a compact polarization-independent optical circulator for communications using multimode optical fibers. This circulator consists of two pairs of rutile prisms used as polarization separators, a YIG Faraday rotator, a ring-shaped magnet, and a quartz halfwave plate. Insertion losses of 2.3 dB including fiber connection losses and isolations of 32 dB at a wavelength of 1.3 epsilonm were obtained using this circulator, and stabilization of the semiconductor laser was confirmed. Details on the design, fabrication, and characteristics of this circulator are presented.

Two‐dimensional microscopic uniformity of resistivity in semi‐insulating GaAs

The two‐dimensional microscopic uniformity of resistivity has been studied using the three‐electrode guard method with 70‐μm resolution as a new microscopic characterization technique for semi‐insulating GaAs. A clear microscopic correlation between resistivity and crystal defects is revealed for the first time by this method. In undoped GaAs, a significant decrease of resistivity greater than one order of magnitude is observed at the vicinity of the walls of dislocation clustered networks and slip lines. Resistivity fluctuation corresponding to rotational growth striation is also observed in undoped GaAs.

Improved Uniformity of Resistivity Distribution in LEC Semi-Insulating GaAs Produced by Annealing

Two-dimensional distribution of resistivity in two-inch diameter crystals of LEC semi-insulating GaAs has been studied by the three-electrode guard method. Results are given in the form of a contour map for the wafers before and after annealing. Very significant fluctuations in resistivity on a 280 µm scale are observed for as-grown crystals, although the average of resistivity corresponds inversely to etch pit densities. The homogeneity of crystal grown at an 18 mm/h pulling rate, about two times faster than usual, is remarkably improved by annealing at 850°C for 20 minutes or longer.

Nonmechanical optical switch for single-mode fibers

This paper introduces a bistable nonmechanical optical switch for single-mode optical fibers. A l-input/2-output device for the l-microm wavelength range, the optical losses of the switch are independent of the polarization state in the input fiber. The insertion losses for the optical signal from the input fiber to the output fiber are 1.4 dB and the crosstalks are -28 dB at 1.3-microm wavelength. Switching is performed by a 20-microsec one-shot pulse with a maximum current of 500 mA. The voltage to switch the optical path is 5 V.

Effect of Water Content of B2O3 Encapsulant on Semi-Insulating LEC GaAs Crystal

Undoped LEC GaAs crystals were grown in PBN crucibles using dry and wet B2O3. The influence of the water content of B2O3 on the melt composition and the electrical properties of the crystal was examined, and it was found that when a highly Ga-rich charge composition was used, crystals grown using wet B2O3 became semi-insulating, while crystals grown using dry B2O3 were p-type conductive. The purity of crystals was investigated through SIMS analysis, LVM IR absorption measurement, and 3He activation analysis.

Inhomogeneity of Resistivity in In-Doped Dislocation-Free Semi-Insulating LEC GaAs

In-doped dislocation-free semi-insulating GaAs crystals were prepared from various melt compositions and measurement of resistivity distributions and infrared absorption at 1 ?m were carried out. Inhomogeneity of resistivity, both along the radial and longitudinal directions, was observed even in some dislocation-free crystals. This inhomogeneity strongly depends on melt composition, and is mainly due to non-uniform distribution of stoichiometry-related defects revealed by infrared absorption measurement.

Bistable magnetooptic switch for multimode optical fiber

A bistable nonmechanical optical switch for multimode optical fiber is presented. It is a 2-input/2-output device for the 1-microm wavelength range. The optical characteristics are independent of the polarization state of incident light. The switch consists of Faraday rotators of thin plates of yttrium-iron-garnet single crystal, electromagnets whose cores are made of semihard magnetic material, halfwave plates of crystal quartz, polarizing prisms of rutile, and lenses. Switching is performed by one-shot 3-V 20-microsec pulses with a maximum current of 500 mA. The 1.5-dB insertion losses and -32-dB cross talk were obtained at a wavelength of 1.3 microm.

Growth and Resistivity Characteristics of Undoped Semi-Insulating GaAs Crystals with Low Dislocation Density

Remarkably low dislocation density of approximately 1000/cm2 (minimum 600/cm2) and uniform distribution were attained in two inch diameter undoped GaAs single crystals grown by an improved LEC technique. Resistivity was revealed to be high enough (1×108 Ωcm) and homogeneity was remarkably improved over that of conventional high dislocation density LEC crystal.

Influence of Melt Composition on Uniformity of Electrical Properties in Semi-Insulating LEC GaAs

Influence of melt composition on uniformity of resistivity and mobility in semi-insulating, undoped GaAs grown by liquid encapsulated Czochralski technique was investigated in detail. Microscopic two-dimensional resistivity distribution was measured using the three-electrode guard method, and Hall mobility was measured by the van der Pauw method. Relatively uniform, four fold symmetric resistivity distribution was observed in crystals grown from stoichiometric or slightly As-rich melt, whereas very significant fluctuations in resistivity were found in crystals grown from Ga-rich or heavily As-rich melt. High and rekatively uniform mobility along both axial and radial directions is obtained at near stoichiometric melt compositions.

High speed pulling of GaAs single crystal using the magnetic field applied LEC technique

Abstract The properties of high pulling rate (20–27 mm/h) LEC GaAs single crystals under a magnetic field are investigated by using the infrared absorption coefficient, α, at a wavelength of 1.0 μm, the microscopic resistivity and the Hall measurement technique. The distributions of α and the microscopic resistivity in these crystals are not as homogenous as those of the conventional 9 mm/h pulling rate crystal with or without magnetic field. After whole ingot annealing at 950°C, however, high speed pulling crystals are drastically changed to very homogeneous in the distributions of α and the resistivity. This is attributed to the increase and the homogeneous distribution of the native deep donor in the crystals.