Naoya Uchida

Calculation of diffraction efficiency in hologram gratings attenuated along the direction perpendicular to the grating vector

A thick hologram grating with modulation that decreases along the direction perpendicular to the grating vector has been analyzed using coupled-wave theory developed by Kogelnik. General solutions of two predominant waves, undiffracted and first-order diffracted waves, are given for transmission and reflection types of hologram gratings. Calculated efficiency of diffraction for some special cases is compared with that for the uniform grating. The influence of the attenuation of grating modulation upon the validity of the theory when applied to the Bragg reflection is also discussed.

Interferometric Method for Measuring Electro-Optic Coefficients in Crystals

An interferometric method for measuring the electro-optic (EO) coefficients has been proposed. The method consists of the relative determination of the coefficient of a sample using a Mach–Zehnder interferometer, by compensating the phase shift due to the EO effect of the sample with that of a standard material. The technique offers a simple and highly sensitive measuring method for the EO coefficient. Values of the coefficients for typical EO crystals have been measured and are found to be in agreement with the previous data and with those obtained by another interferometric method proposed by Fujii and Sakudo.

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.

Fabrication of optical waveguiding layer in LiTaO3 by Cu diffusion

Thermal diffusion and electrodiffusion of Cu into LiTaO3 have been investigated. By an appropriate choice of diffusion conditions, a single‐mode waveguiding layer has been fabricated using the electrodiffusion method. The refractive‐index change at the surface is 5×10−3 for both ne and no and the diffusion depth is about 4 μm. The layer supports only the fundamental modes TE0 and TM0, and exhibits good waveguiding properties.

Active transmission line: light amplification by backward-stimulated Raman scattering in polarization-maintaining optical fiber

The possibility of an active transmission line with a polarization-maintaining optical fiber has been investigated by means of backward-stimulated Raman gain. The fourth Stokes line of the stimulated Raman scattering is placed near 1.3 μm with pumping light of 1.06-μm wavelength, in which Raman gain at 1.30 μm is produced in terms of a strong pump that is due to the third Stokes line at 1.24 μm. A laser diode (InGaAsP/InP) operating at 1.30 μm is used as signal light to meet the Raman gain. As a result, a Raman gain as high as 20 dB and a gain coefficient of 2.0 × 10−12 cm/W have been obtained. It is shown experimentally that it is important to meet exactly the polarization directions between the pump and the signal pulses to obtain a large Raman gain.

Electro‐optic modulation of optical guided wave in LiNbO3 thin film fabricated by EGM method

Experiments on light propagation and electro‐optic modulation have been made for poled LiNbO3 single‐crystal thin films fabricated by the method of epitaxial growth by melting (EGM). The values of the refractive indices are found to be no = 2.200 and ne = 2.184. A phase modulation experiment has been carried out using a Mach‐Zehnder interferometer in the region of 100 Hz‐3 MHz. The electro‐optic coefficients of the film have been determined by the phase‐compensation method, and the values obtained are r33=12 and r13=2.3×10−12 m/V.

Photon probe fault locator for single-mode optical fiber using an acoustooptical light deflector

A new backscattering technique for diagnosing the attenuation characteristics, spatial imperfections with length (fault location), and splice loss in a single-mode optical fiber has been developed by using a TeO 2 acoustooptical light deflector operating at 120 MHz. Due to the small insertion loss and high extinction ratio of the deflector, the dynamic range of the backscattered signal has been increased by at least 10 dB, which corresponds to the extension of 5 km in measurable length for fiber loss of 1 dB/km, compared with the conventional back-scattering technique in which the beam splitter and polarizer-analyzer combination are utilized. Another advantage of this technique is in that the saturation of the amplifier is avoided by arbitrarily cutting off a large power in the early stage of the Rayleigh scattering signal. A single-mode fiber of 19.2 km in length has been examined, and the distance for fault location up to 18.4 km was obtained.

Measurement and analysis on polarization properties of backward Rayleigh scattering for single-mode optical fibers

Fluctuation components on a backward Rayleigh scattered signal measured by the polarization optical time domain reflectometer (POTDR) have been investigated in detail by means of the least squares method and the power spectra. As a result, it is revealed that the fluctuation component for the single-mode optical fibers is attributable to the inherent polarization beat length determined by the difference of the phase velocities between the orthogonal e HE 11 and o HE 11 modes. The power spectrum of the fluctuation is sharply peaked at the Fourier frequency determined by the polarization beat length of approximately 15 m, which is in good agreement with the analysis. The utilization of the power spectrum presents a new method to diagnose the polarization properties along the optical fiber.

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.

Optical loss measurement in graded-index fiber using a dummy fiber

The utility of an auxiliary fiber, called a dummy fiber, is investigated for optical fiber loss measurements. The dummy fiber is spliced and used to excite the test fiber. Excess loss caused by undesirable modes is found to be reduced to less than 0.05 dB by using a 500-m dummy fiber and choosing the test fiber cut length to be 2 m for reference. Loss linearity to the fiber length is examined on 6-km spliced fibers, and satisfactory agreement is obtained between the total loss and the sum of the individual fiber and splice losses.