Toshihiko Yoshino

Fiber-optic Fabry-Perot interferometer and its sensor applications

Fiber-optic Fabry-Perot interferometers using monomode fibers are fabricated and their basic properties of finesse, polarization, and thermal response are studied. Fiber-optic Fabry-Perot interferometers are applied to the sensors of temperature, mechanical vibration, acoustic wave including human voice, ac voltage, and ac and dc magnetic fields. It has been demonstrated that a fiber-optic Fabry-Perot interferometer can simplify the interferometric fiber sensor system and that high measurement sensitivity can be obtained by using a high-finesse and/or long-distance fiber Fabry-Perot interferometer.

Fiber-optic evanescent-wave methane-gas sensor using optical absorption for the 3.392-μm line of a He–Ne laser

Gas sensing using evanescent waves of optical fibers is investigated. Methane gas is detected by means of its strong optical absorption of the 3.392-microm line of a He-Ne laser. A single fiber is used as both a sensor and an optical transmission line. The sensor has a small diameter, ranging from 1.8 to 7microm, made by heating and expanding a part of a step-index silica fiber. An evanescent wave of 5 to 40% of the total propagating power is generated outside the fiber. When a sensor fiber of 1.8-microm diameter and 10-mm length is used, the minimum detectable concentration of methane is less than the lowest explosive limit of 5%.

Laser diode feedback interferometer for stabilization and displacement measurements

Active laser diode interferometers in which the interference signal is fed back to the diode current are investigated for Twyman-Green and self-coupling interferometers. The Twyman-Green interferometer is stabilized with a stabilization factor of more than 100. By using the feedback signal of either type of interferometer, displacement is measured in a linear scale over a dynamic range of 8-9,microm with a precision of 10-60 nm. The feedback signal vs displacement shows hysteresis and multistable behavior, in accordance with theoretical results.

Interferometer for measuring displacement and distance.

A simple interferometer for measuring both relative displacement and absolute distance is fabricated that uses a laser diode. The sign of the displacement is detected by means of a lambda/8 plate, and the distance is measured by a FM radar technique of modulating the laser-diode frequency. Measurement accuracies of 0.02 microm for displacement and 100 microm for distance are obtained over a range of a few meters.

Optimum design of multilayer-medium structures in a magneto-optical readout system

An explicit formula for the normal-incidence polar Kerr magneto-optical coefficient in a general multilayer structure consisting of a magnetic film and multilayers has been derived. The optimum condition of the multilayer-medium structure for maximizing the signal-to-noise ratio in the readout system is analytically derived by means of an equivalent bilayer treatment. For the case of the quarter-wave multilayer structure, numerical examples are given. It is shown that the optimization of both the multilayer structure and the readout optical components can greatly increase the signal-to-noise ratio in the readout system.

Frequency Stabilization of Internal-Mirror He–Ne (λ=633 nm) Lasers Using the Polarization Properties

The laser frequency of an internal-mirror He-Ne (λ=633 nm) laser has been stabilized with a new scheme. The difference in intensity between the oscillating two axial modes with orthogonal linear polarizations is detected by modulating the azimuths of polarization of the modes by means of a highly efficient Faraday rotator. The electric voltage proportional to the difference signal is fed to a d.c. motor which rotates a cooling fan placed near the laser tube. This suppresses the thermal expansion of the cavity length due to the discharge current, consequently the laser frequency is stabilized. The relative frequency stability is obtained as 1.7×10-8 over a period of 1 min or longer.

Compact and highly efficient Faraday rotators using relatively low Verdet constant Faraday materials.

A compact and highly efficient Faraday rotator which can produce a large amount of Faraday rotation despite using a relatively low Verdet constant Faraday material is described. This Faraday rotator consists of a small thin plate with perfectly reflecting surfaces and a small electromagnet or permanent magnet which applies the magnetic field to the plate in the direction of the thickness. A light beam with small diameter travels through the plate with multiple passes. The condition under which the light beam couples with the plate with the least optical loss is described. The effect of a small wedge angle of the plate on Faraday rotation is studied. A practical Faraday rotator for the He–Ne laser beam (λ=633 nm) is constructed by using an FR5 glass plate of 3 mm thickness and a small electromagnet. Faraday rotation of 45° is obtained when the power consumption of the magnetic coil is 1.5 W (2.8 V–0.56 A). The polarization extinction ratio is larger than 250: 1 and the total optical loss is 10%.

Polarization Properties of Internal-Mirror He-Ne Lasers at 6328 Å

The polarization properties of internal-mirror He-Ne lasers at 6328 A, are experimentally and theoretically investigated. The oscillating mode is linearly polarized in most cases. The azimuth of polarization is measured as a function of the orientation angle of one of the reflectors around the axis of the discharge tube. Theoretical calculations of the mode polarization are made on the assumption that the reflectors have anisotropic reflectances. The calculated curves fit fairly well with the experimental results.

Reflection Anisotropy of 6328 A Laser Mirrors

The reflection anisotropy of 6328 A laser mirrors is investigated using an internal mirror 6328 A He–Ne laser with a fused-silica plate inside the cavity. Experimental and theoretical studies are made of: 1) the dependence of the mode polarization of the laser on the mirror orientation about the axis of the discharge tube; 2) the dependence of the mode polarization on the transverse mechanical pressure applied to the plate; and 3) the dependence of the beat frequency of the output light on the axial magnetic field applied to the plate. From these studies, the mirror anisotropy is obtained as 2.3×10-3 of the phase type.

Longitudinal magneto-optical effect in Ni and nickel-rich Ni-Fe films in visible and near infrared regions

Abstract Magneto-optical properties of Ni and Ni-Fe films are studied experimentally for the spectral range 0.4 to 2.2 μ. From the measurement of longitudinal Kerr rotation and of optical constant of these evaporated films, the diagonal and off-diagonal components of the gyroelectric tensor are determined. The magneto-optical resonance is observed at a frequency of about 1 eV.