Yoshiro Ohmachi

Temperature Dependence of Elastic, Dielectric, and Piezoelectric Constants in TeO2 Single Crystals

All of the elastic stiffness constants except c13 have been found to decrease almost linearly with temperature between −120° and 120°C. The constant c13 reaches the maximum in the vicinity of 0°C, above which it also decreases with temperature. On the other hand, the effective elastic constant (c11–c12)/2, which corresponds to the exceptionally slow shear wave propagating along [110], increases with temperature. The shear wave propagating in the (001) plane making angle of 35.9° with the X axis has zero temperature coefficient of velocity. Acoustic absorptions for several sound modes vary within 1 dB/cm for both the longitudinal wave (measured at 36 MHz) and shear wave (48 MHz) in the measured temperature range. Both real and imaginary parts of the dielectric constants increase monotonically with temperature, while the piezoelectric constant d14 decreases in the temperature range between −150° and 180°C.

LiNbO 3 TE-TM mode converter using collinear acoustooptic interaction

A highly efficient TE-TM mode conversion device utilizing the acoustooptic effect has been demonstrated. Mode conversion has been made by an acoustic surface wave propagating collinearly with an optical guided wave. The maximum mode conversion, although the fraction was restricted to 70 percent, has been obtained with an acoustic power of only 55 mW (electrical input 0.55 W). Effective interaction length of the present device is limited to 5 mm due to optical and/or acoustic inhomogeneity of the waveguides. For selecting TE and TM waves, a polarizer is constructed monolithically and the extinction ratio is improved by up to 21 dB. Performance characteristics of a tunable optical filter of this type of the mode conversion are also discussed.

Electro‐optic light modulator with branched ridge waveguide

A light modulator based on an electro‐optic phase modulation and an optical interference of Mach‐Zehnder type has been demonstrated. The modulator is constructed monolithically with a branching ridged optical waveguide on TiO2‐diffused LiNbO3. The half‐wave voltage is 19 V and the extinction ratio is 34%.

Acousto‐optical light diffraction in thin films

The diffraction of optical guided waves in thin films through an acousto‐optic effect is treated. The diffraction of the bounded optical waves in thin films is a rather complicated phenomenon as compared with that of the free waves in bulk media. An expression for the diffraction efficiency has been derived using coupled‐wave equations. Acousto‐optical figure of merit and the mode coupling factor applicable to the diffraction in thin films are defined. The TE0 and TM0 waves in an As2S3 film deposited onto a LiNbO3 substrate were diffracted by Rayleigh waves with 93 and 72% efficiency at electrical input powers of 150 and 50 mW, respectively. The present experiment revealed that approximately 5% of the total acoustic power penetrated into the film and took part in the diffraction.

Acoustic Wave Propagation in TeO2 Single Crystal

Acoustic phase velocity surfaces and ray directions in paratellurite, TeO2, have been calculated. Characteristic features of the wave propagation in this highly anisotropic medium have been optically confirmed using the shadow method and Schaefer‐Bergmann diffraction technique. A small misalignment of the wave normal direction has been found to degrade seriously the performance characteristics of acoustic or acousto‐optical devices composed of a highly anisotropic medium such as TeO2. The existence of the transverse mode with a zero temperature coefficient of velocity has been also confirmed by the present calculation and experiment.

Vitreous As2Se3; Investigation of Acousto‐Optical Properties and Application to Infrared Modulator

Acoustic and acousto‐optical measurements have been carried out on vitreous As2Se3. Values of acousto‐optical figures of merit M (=n6p2/ρ v3) and M′ (=n7p2/ρv) for longitudinal acoustic waves are 1090 × 10−18 sec3/g and 1600 × 10−7 cm2 sec/g, respectively, for He–Ne laser radiation of 1.153 μ polarized along the propagation direction of the acoustic wave. These values are the largest among those for known materials which are transparent in the near‐infrared spectral region. Acoustic attenuation has been evaluated as 11 dB/cm at 200 MHz, and the material is considered useful for a modulator element operable up to several hundred MHz. Experimental pulse modulation and transmission of the 1.153‐μ laser radiation have been demonstrated using the As2Se3 modulator at an acoustic carrier frequency of 140 MHz. The transmitted pulse pattern has been displayed by the use of TeO2 light deflector.

Acousto‐optic property of single‐crystal 5PbO·3GeO2

Acousto‐optical figure of merit and acoustic attenuation have been measured for a new ferroelectric material—5PbO·3GeO2. The highest value of M is found to be 22 × 10−18 sec3/g for the longitudinal wave along the X axis. However, a fairly large loss, 2 dB/cm at 100 MHz for this acoustic mode, restricts the acousto‐optical device applications of the crystal in the relatively low‐frequency region.

Phase Tuning in Optical Directional Coupler

A new phase tuning method for an optical directional coupler fabricated in a c-plate LiNbO3 by TiO2 diffusion is presented. The method consists of applying a bias voltage to an additional electrode placed closely outside one of the modulation electrodes. The propagation constants of the two waveguides could be made equal by a 5 V bias voltage without affecting the 8 V modulation voltage.