Yoshinori Ohta

Antireflection effect in ultrahigh spatial-frequency holographic relief gratings

An interpretation model for low reflectivity in ultrahigh spatial-frequency holographic relief gratings is proposed. The model is based on the concept that the grating effective index, caused by grating ultrahigh spatial frequency, is graded in the depth direction and forms an antireflective constitution similar to the multilayer coating. Numerical results show that a sinusoidal grating is antireflective over wide groove depth, wavelength and incident angle ranges, and a grating with nearly triangular section, having a circle arc index distribution, has a very low reflectivity, >10(-4)%. Reflectivity vs groove depth, obtained experimentally for a holographically recorded photoresist grating, agrees fairly well with the numerical results.

Low-loss broad-band LiNbO 3 guided-wave phase modulators using titanium/magnesium double diffusion method

We report a new fiber-Ti:LiNbO 3 waveguide coupling loss reduction method and its application to modulators for phase shift keying (PSK) coherent systems. The titanium/magnesium double diffusion method, which symmetrizes the waveguide depth index profile and reduces the coupling loss by Mg diffusion after Ti-diffused waveguide formation, is proposed. A fiber-to-fiber total insertion loss of 0.5 dB for a 1-cm-long waveguide has been obtained using this method. The method was successfully applied to phase modulators by using the tapered MgO thickness diffusion which reduces the insertion loss without increasing the drive voltage. Ti:LiNbO 3 guided-wave phase modulators with 1.1-dB total insertion loss, 6.2- and 8.5-V modulation voltage at \lambda = 1.3 and 1.55 μm, respectively, and 4-GHZ bandwidth, have been obtained. A PSK heterodyne differential detection experiment at 1.2 Gbit/s was carried out successfully using one of the phase modulators described above.

A new Faraday rotator using a thick Gd:YIG film grown by liquid-phase epitaxy and its applications to an optical isolator and optical switch

A new Faraday rotator using a thick garnet film grown by liquid-phase epitaxy (LPE) has been proposed and film growth technology for the rotator has been investigated. The new Faraday rotator had good features of very low cost and small size, due to high productivity of the LPE-grown film and low magnetic saturation field, respectively. By using the new Faraday rotator, an optical isolator and magnetooptic switch for single-mode fiber systems have been developed. The optical isolator featured 0.8-dB insertion loss and 25-dB isolation at 1.3-μm wavelength. The magneto-optic 1 × 2 switch was independent of light polarization and featured 1.3-1.7-dB insertion loss, -25-dB crosstalk, and 30-μs switching time at 1.3-μm wavelength. Minimum switching voltage was ±5 V. Magneto-optic devices using the new Faraday rotator is practical for use in1.2-1.7-\mum wavelength fiber-optic systems, because of good optical properties, compactness, and low cost.

Measurement of Knife-Edge Responses of a Schwarzschild X-Ray Objective

The spatial resolution of a Schwarzschild objective was evaluated at the wavelength of 135 A by means of an apparatus constructed using the knife-edge method which is suitable to the measurement of higher resolution. The knife-edge response of the objective was measured with the apparatus. Then the results were compared with the theoretical responses calculated using ray tracing. It is found that the measured responses of the objective are almost the same as the theoretical ones. We conclude that the objective has the spatial resolution of 0.2 µm.

Phase-Matched Electro-Optic Light Modulator

A new type of light modulator, in which incident light polarization is converted to perpendicular polarization by electro-optic effect of r51, was studied, using an a-cut LiTaO3 with a Li/Ta1.2 composition. A spatially periodic electric field for phase-matching between two crossed polarizations is generated by a pair of interdigital electrodes. The electrode period is changed along the light path so that phase-match condition can be satisfied over a wide temperature range. Birefringence inhomogeneity along the light path was found to contribute to wider operation temperature ranges. An extinction ratio of above 30 dB and 73% conversion with an applied voltage of 180 V were obtained in the modulator. Measured temperature characteristics were almost coincident with calculated values.

Sum-peak Method and Angular Correlation Correction for a 60Co Source by a Ge(Tl) Detector

The sum-peak method was applied for a measurement of radioactivity of a 60 Co source by an 80 cm3 Ge(Tl) detector. A correction factor for the angular correlation effect between two cascade γ-rays from the 60 Co source was determined to be about 9% in the present experimental setup by a Monte Carlo simulation.

Nonreciprocal Beam Transmission in Slanted Fiber Bragg Grating with Axially Asymmetric Cladding

A novel slanted fiber Bragg grating with axially asymmetric cladding structures with different refractive indices is proposed. Calculation results successfully show its nonreciprocal guided wave propagation equivalent to that of optical isolators and high unidirectional transmissibility.

Y3Fe5O12 epitaxial growth by CVD and its characterization

A remarkable improvement of the chemical vapor deposition technique for Y3Fe5O12 (YIG) epitaxial growth is attained. The improved reactor markedly extends the YIG film thickness uniformity range. The YCl3-FeCl2-O2-N2 gas system, in which N2 is the carrier gas, makes it easy to suppress defects and to grow good quality YIG films. The film thickness distribution is less than ±4% for four 3 inch diameter wafers which are grown at the same time. The formation of defects, such as Fe2O3 and YIG film surface roughness, depends on the source material transport ratio R=YCl3/FeCl2. In the range of 0.35<R (mol ratio)<0.7, a defect-free smooth YIG film is grown reproducibly. The guided-wave optical loss of He-Ne laser light at 1.15 μm wavelength is 2.8 dB/cm for a YIG film grown at R=0.53. This value is reduced down to 1.0 dB/cm by annealing the film in an oxiding ambient. The source of the optical loss is attributed to Fe2+-Fe3+ ion pairs in the point-defect structure model of {Y3+3}[Fe3+2−yFe2+ y−xY3+x](Fe3+3) O12−(y−x)2VO(y−x)2, in which VO is an oxygen vacancy.

Optical Add/Drop Multiplexers Using Slanted Fiber Bragg Gratings

Novel optical add/drop multiplexers (OADMs) using slanted fiber Bragg gratings (S-FBGs) are proposed. The S-FBGs couple the guided mode to radiation modes, and they also couple specific radiation modes to the guided mode. A set of S-FBGs located near each other function as OADMs. Output light beams from an optical fiber couple to the other fiber by the slanted gratings. The proposed OADM does not require optical circulators that are components of conventional multiplexers using FBGs. Low-cost and small-size optical filters may be constructed. Calculation results successfully show the possibilities of OADMs.

Optical add/drop multiplexers using slanted fiber Bragg gratings

Novel optical add / drop multiplexers (OADMs) using slanted fiber Bragg gratings (S-FBGs) are proposed. The S-FBGs couple the guided-mode to radiation modes, and they also couple specific radiation modes to the guided mode. A set of the S-FBGs located nearby with each other has functions of OADMs. The proposed OADM requires no optical circulators which are components of usual multiplexers using FBGs. Low-cost and small size optical filters may be constructed. Calculations results successfully show the possibilities of OADMs.