Miniature Pedestal Research Articles

Technologies for tunable gamma-ray lenses

The tunable gamma-ray lens has turned out to be a promising alternative to the classical fixed-energy Laue-lenses discussed in the past. We describe here our development work on a miniature pedestal with one-axis tilt adjustment. We also outline our design for an optical system, capable of monitoring the alignment of the many crystals needed. An added benefit of the tunable crystal pedestal is that it relieves both the demands for high precision in the crystal mounting and the stringent requirements for long-term stability of the support platform on which the crystals are mounted. Moreover, mounting the individual crystals on separate pedestals facilitates the use of double layers of crystals. Keywords: Gamma-ray astronomy, Telescope technology, Laue lenses

Microstructure and properties of single-crystal rare-earth oxide fibers

Single-crystal Ln2O3 (Ln = Sm, Eu, Gd) fibers are grown by the laser-heated miniature pedestal technique. The relationship between the microstructure of fibers and their plasticity is analyzed using scanning electron microscopy, energy-dispersive microanalysis, electron backscatter diffraction, and X-ray methods. It is shown that the distinction between ductile and brittle fibers is determined basically by the difference in their microstructure. The fracture of samarium oxide fibers is associated with the formation of oxycarbonate phases on their surface. The most promising are the fibers of gadolinium oxide, having the most perfect microstructure, moisture and a carbon dioxide resistance, and a wide range of transparency.

Birefringent and nonlinear optical assessment of single crystal lithium niobate fibres

a-axis fibres of lithium niobate produced by laser heated miniature pedestal growth can possess transverse and longitudinal birefringence variations due to faceted growth. When used for second harmonic generation, these characteristics cause conversion efficiencies to be two to three orders of magnitude smaller than predicted and the phasematchable lengths and apertures to be short. Second harmonic generation and associated birefringence mapping are shown to be sensitive indicators of their uniformity.

Growth and evaluation of lanthanoids orthoniobates single crystals processed by a miniature pedestal growth technique

AbstractOptimized conditions for the growth of lanthanoids orthoniobates (LnNbO4, Ln = lanthanide elements) single crystal minirods by a floating zone technique were investigated. Adequate atmospheres and pulling to feeding speed ratios to grow these materials were determined. Emphasis is given to the study of LaNbO4 because of their more favorable growth conditions and crystalline quality. This material can be efficiently doped with rare earth elements such as erbium. It grows with high crystallinity and its preferential growth direction is [1 1 0]. A preliminary evaluation of optical properties of Er3+‐doped LaNbO4 single crystal under the Judd‐Ofelt formalism indicates spectral parameters Ωt close and even larger than for Er3+ ions in YVO4. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single-crystal ruby fiber temperature sensor

Single-crystal ruby fibers with nominal diameters in the range from 80 to 500 μm and lengths up to 170 mm have been produced using the laser heated miniature pedestal growth technique. These fibers were tested for use as the basis of fiber optic temperature sensors. Characterization of the fiber fluorescence properties and comparison to bulk samples together with their high melting point indicate their potential for fluorescence-based sensing at extremely elevated temperatures. Good geometrical matching with current fiber optic technology and the use of phase-locked detection techniques also allow high resolution measurements to be obtained.

Optical fibre polariser based on GRIN single crystalfibre

A novel optical fibre polariser based on a GRIN crystal fibre and suitable for in-line use with singlemode optical fibre is described and demonstrated in a proof of principle experiment. Using fibres of /spl alpha/-axis lithium niobate, grown by the laser heated miniature pedestal growth method and heat treated to produce radial extraordinary refractive index gradients, one plane of polarisation is coupled through the system while the orthogonal is rejected. Extinction ratios of up to 1200 (31 dB) have been obtained at 632.8 nm.

GRIN single crystal fibres

Fibres of a-axis lithium niobate, grown by the laser heated miniature pedestal growth method, were heat treated to produce radial extraordinary refractive index gradients by out-diffusion of lithium oxide. Information on the resulting refractive index profiles was obtained by both longitudinal refraction and Fresnel reflection from an end face. Potential applications of GRIN crystal fibres are discussed.

Modeling the time-dependent growth of single-crystal fibers

A model is developed to simulate time-dependent behavior in laser-heated miniature pedestal growth (LHPG) of single-crystal fibers. This process is meniscus controlled since the diameter of the growing fiber changes in response to variations between the equilibrium and the instantaneous contact angle at the melt/solidifying-solid interface. One of the operational problems associated with the LHPG technique is controlling diameter fluctuations in the growing fiber due to process perturbations. Smooth source feed and fiber pull rates, steady laser-heat input, and suppression of mechanical vibrations are necessary to achieve a constant fiber diameter. We derive a system of time-dependent, one-dimensional equations of motion and heat transfer as the leading order equations in an asymptotic expansion based upon the melt slenderness ratio (fiber radius/melt height). This system is solved numerically to predict the thermal profiles, mean positions of the melting and solidifying fronts, melt/gas interface, melt volume, and fiber diameter. System response to perturbations in the pull rates, thermal environment, and gravitational field is investigated.

Instability in the growth of Ti: Al 2O 3 single-crystal fibres

Observation of melt instability during the growth of Ti: Al 2O 3 fibres by the laser-heated miniature pedestal growth technique is reported. This instability takes the form of oscillations of the melt temperature and free surface, with a frequency of 0.68 Hz. The onset of oscillation has been found to depend on the Ti concentration [Ti] and its source is attributed to oscillatory convention, likely to be Marangoni in nature, arising from changes in the melt surface tension properties due to the presence of Ti. Under typical growth conditions an upper limit of [Ti] has been found for stable growth. This has been calculated to be 5.8±0.5 wt %.

Uniaxial crystalline fibres: optical methods for determining their physical characteristics

An optical method based on birefringence is described, which can be used to monitor the longitudinal variation in the diameter and nature of the cross section of uniaxial crystalline fibres produced by the laser-heated miniature pedestal growth process. Modification of the technique enables the crystallographic direction to be confirmed relative to the fibre's axis of symmetry.

Dopant incorporation in single-crystal fibre growth by the laser-heated miniature pedestal growth technique

A theoretical model of single-crystal fibre growth by laser-heated miniature pedestal growth has been developed to account for the transfer of dopant from the source rod to the growing fibre. This model predicts the time evolution of the melt dopant concentration when the source rod dopant concentration is axially non-uniform. From this the axial distribution of dopant in the final grown fibre may be predicted. This model has been experimentally confirmed and an estimate of the evaporation time constant of Ti in the growth of Ti:Al 2O 3 fibres made.