Narsingh B. Singh

High-performance acousto-optic materials - Hg2Cl2 and PbBr2

New results achieved with recently grown Hg2Cl2 and PbBr2 crystals are described. With an optimized crystal growth technique Hg2Cl2 crystals were grown that show a significantly reduced acoustic attenuation compared to prior crystals, from 13.4 to 8 dB/μs-GHz2. These crystals allow the development of Hg2Cl2 Bragg cells with time-bandwidth product figures in the 5100 to 6900 range, frequency operation as high as that for Te02, and resolution about 25% higher than TeO2 for similar crystal lengths. PbBr2 crystals were also grown that exhibit a large figure of merit (M2 550) with an attenuation coefficient of 12 dB/μs-GHz2. This material may be the choice for infrared devices where large diffraction efficiencies are needed.

Characterization of directionally solidified lead chloride

Abstract A complete analysis has been carried out on directionally solidified lead(II) chloride material. Purification by directional freezing consistently produced high purity material suitable for subsequent growth of single crystals. It was observed that silicon, magnesium, halogens, sulfur and phosphorous were the hardest impurities to remove from the supplied material. Direct photographic observations of the solid-liquid interface were taken at several values of G / v ratios (denoting the temperature gradient and the translation velocity, respectively) to study the morphology of the interface and optimize the growth conditions. The solid-liquid interface morphology varied from a smooth convex shape to dendritic as the G / v ratio decreased. Single crystals subsequently grown from the material purified by the present method showed no optical distortion, exhibited a transmission range from 0.30 to 20 μm, and displayed extremely low beam scattering.

Fabrication and characterization of mercurous chloride acoustooptic devices

Mercurous chloride crystals, recently synthesized in large good optical quality boules by vapor deposition, have been characterized as to their unusual acoustooptic properties. This material offers the potential for superior performance Bragg cells for signal processing and tunable filters. We have developed fabrication techniques to overcome severe problems associated with its high chemical reactivity, designed transducer structures, and built and tested a number of devices.

Identification and control of a multizone crystal growth furnace

Abstract This paper presents an intelligent adaptive control system for the control of a solid-liquid interface of a crystal while it is growing via directional solidification inside a multizone transparent furnace. The task of the process controller is to establish a user-specified axial temperature profile and to maintain a desirable interface shape. Both single-input-single-output and multi-input-multi-output adaptive pole placement algorithms have been used to control the temperature. We also describe an intelligent measurement system to assess the shape of the crystal while it is growing inside a multizone transparent furnace. A color video imaging system observes the crystal in real time, and determines the position and the shape of the interface. This information is used to evaluate the crystal growth rate, and to analyze the effects of translational velocity and temperature profiles on the shape of the interface. Creation of this knowledge base is the first step to incorporate image processing into furnace control.

Application of stereo imaging for recognition of crystal surface shapes

Abstract The shape of an interface while a crystal is growing inside a crystal growth furnace is an important measurement to assess the crystal quality on-line. If the crystal surface can be visualized by video or X-ray imaging, the interface shape can be determined by the stereo image processing techniques. This paper presents a methodology to determine the shape of the solid-melt interface by stereo based imaging techniques whiles the crystal is growing inside a transparent furnace. The methodology generates a qualitative shape information provided that a good pair of stereo images of the interface can be captured. For a quantitative shape description, i.e., for the determination of interface points with respect to a fixed coordinate frame, both cameras must be calibrated. This paper illustrates the methodology for qualitative shape determination of lead bromide crystal interface.

Preparation and purification of nitrobenzenes for nonlinear optical devices

Abstract ⋅Dinitrobenzenes and ⋅nitroaniline have been purified by a combination of distillation and repeated directional freezing. Distillation followed by zone refining was found to be most suitable for the large production of crystal growth source material. Crystals grown from purified source material showed significant reduction in the optical scattering.

High-performance acousto-optic materials: Hg2Cl2 and PbBr2

Some important results recently achieved with in-house grown high-quality, high-performance Hg2Cl2 and PbBr2 are described. Using an optimized crystal growth technique we have grown Hg2Cl2 crystals that show a significantly reduced acoustic attenuation from 13.4 to 8 dB/microsecond(s) -GHz2. These crystals allow the development of Hg2Cl2 Bragg cells with TBWP figures in the 5,100 - 6,900 range, frequency operation as high as TeO2 and resolution about 123% higher than TeO2 for practically similar crystal lengths. We have also grown for the first time long, high-quality PbBr2 crystals which exhibit a large figure of merit (M2 equals 550) with a modest attenuation figure of 12 dB/microsecond(s) -GHz2. This material may be the choice for devices where large diffraction efficiencies are needed.