F. S. Chen

Optically Induced Change of Refractive Indices in LiNbO3 and LiTaO3

Local changes of indices of refraction observed in poled single crystals of ferroelectric LiNbO3 and LiTaO3 when illuminated by focused light have been studied. The extraordinary index of refraction was observed to decrease as much as 10−3 with a focused Ar laser light of 20 mW intensity as λ=0.488 μ, while the change of ordinary index was much smaller. The effect is attributed to the drifting of photoexcited electrons out of the illuminated region followed by their retrapping near the beam periphery. The space‐charge field between these retrapped electrons and the positive ionized centers in the illuminated region causes the observed change of refractive indices via the electro‐optic effect of the samples.

Light Modulation and Beam Deflection with Potassium Tantalate‐Niobate Crystals

The dielectric and electro‐optic properties of KTaxNb1−xO3(KTN) are discussed from the point of view of the materials usefulness in light modulators and beam deflectors. It is shown that baseband light modulators with 200‐ to 300‐Mc/sec bandwidths and analog deflectors with 200 to 300 resolvable spots are within the practical capabilities of this material.

Electromagnetic Propagation through Materials Possessing Both Faraday Rotation and Birefringence: Experiments with Ytterbium Orthoferrite

Electromagnetic wave propagation through materials that possess both Faraday rotation and birefringence is analyzed. A matrix equation is developed which relates the amplitude and relative phase of the electric vectors between any two points along the propagation direction. It is shown that the presence of birefringence can drastically affect the behavior of wave propagation and that it is considerably different from pure Faraday rotation. Methods of measuring the material parameters are also described. Criteria for viewing domains in this type of material are established. It is shown that the thickness of the sample plays a great role in determining the contrast between domains and at some thicknesses no contrast at all can be obtained. It is also shown that the method using elliptical analyzers gives greater contrast over the plane analyzers. Photographs of domain patterns in a wedge of ytterbium orthoferrite are presented and they verify the calculated results.

Lithium Tantalate Light Modulators

Light intensity modulators have been developed using single‐domain lithium tantalate as the electro‐optic material. A broadband transistor amplifier which can develop 0.2‐W output power drives the modulator sample which presents a capacitive load of 5 pF. Approximately 80% modulation is achieved from dc to 220 Mc/sec, when the light is made to traverse the sample twice. The modulation bandwidth is limited by the transistor amplifier. Very little acoustic ringing is observed when the modulator is used as a fast light switch.

Failure mode analysis of planar zinc‐diffused In0.53Ga0.47As p‐i‐n photodiodes

In order to understand the irreversible failure mechanisms of planar InGaAs p‐i‐n photodiodes, 32 devices from 19 different wafers that shorted during aging were first examined in the scanning electron microscope. Included were devices that failed during long term aging (>103 h) as well as those that failed during short term aging (<102 h) at higher reverse bias. With a few exceptions, the diodes failed as a result of a single localized leakage source located at the perimeter of the p‐n junction. Three types of leakage sources were found: (a) a microplasma, (b) a microplasma associated with a region of high recombination rate, and (c) a microplasma associated with a thermally damaged region. Analysis of ∼40 devices before and after aging shows that leakage paths found after aging result from microplasmas initially present in the device. Defect analysis shows that neither threading dislocations nor misfit dislocations are generally responsible for these microplasmas. Analysis of the processing shows that t...

A lithium niobate light modulator for fiber optical communications

An electrooptic intensity modulator using lithium niobate has been developed for applications in binary fiber optical digital communications at the wavelength of 1.06 µm. We have shown that many shortcomings generally associated with electro-optic modulators can be surmounted. The modulator was driven by a compact transistor amplifier, temperature dependence of the static birefringence was minimized, and the optical bias was made adjustable by a dc voltage superposed on the signal. The modulator has been operated at 70-Mb/s pulse rate and 100-percent modulation, its extinction ratio is better than 40 to 1 and the optical insertion loss is about 1 dB.

Optical memory planes using LiNbO 3 and LiTaO 3

Optical memory planes are proposed in which writing and nondestructive reading would be performed optically using a laser-induced inhomogeneity of refractive indices which has been observed in poled single crystals of LiNbO 3 and LiTaO 3 . Writing times of the order of 0.5 millisecond using a 0.4-watt Ar laser with a bit density in excess of 106per square inch appear feasible.

Light Modulation and Beam Deflection with Potassium Tantalate-Niobate Crystals

The dielectric and electro-optic properties of KTaxNb1-xO3(KTN) are discussed from the point of view of the materials usefulness in light modulators and beam deflectors. It is shown that baseband light modulators with 200-to 300-Mc/sec bandwidths and analog deflectors with 200 to 300 resolvable spots are within the practical capabilities of this material.