The refractive indices of AlxGa1−xAs epitaxial layers (0.176⩽x⩽1) are accurately determined below the band gap to wavelengths, λ<3 μm. The layers are grown on GaAs substrates by molecular beam epitaxy metal organic and chemical vapor deposition with thicknesses ranging from 4 to 10 μm. They form improper waveguide structures with the GaAs substrate. The measurements are based on the excitation of the improper waveguide modes with grating couplers at 23 °C. The refractive indices of the layers are derived from the modal propagation constants in the range of 730 nm<λ<830 nm with an estimated uncertainty of Δn=5×10−4. The temperature coefficient of the refractive index is investigated in the same spectral range. From the effective indices of the TE and TM modes, we derive the strain-induced birefringence and the elasto-optic coefficients. High-resolution x-ray diffraction is used to determine the strain of the layers. The layer compositions are obtained with inductively coupled plasma atomic emission spectroscopy. The measurement range of the refractive index is extended from the direct gap to λ<3 μm by observing the Fabry-Pérot interference fringes of the transmission spectra of isolated layers. The measured values of the refractive index and the elasto-optic coefficient are compared to calculated data based on semiempirical models described in the literature. Published data of the index of refraction on GaAs, AlAs and GaP are analyzed to permit the development of a modified Sellmeier approximation. The experimental data on AlxGa1−xAs can be fitted over the entire composition range 0⩽x⩽1 to provide an accurate analytical description as a function of composition, wavelength, and temperature.
In honour of Prof. Dr. F. ST~CKMANN'S 60th birthdayThe elastic stiffness functions c, , , c, , %3, and c, are determined by ultrasonic velocity measurements as a function of temperature below and above the phase transition a$ T, = 373 K. Their variation below T, is discussed in terms of critical exponents. From their value of the order of 0.6 no unique decision can be made whether the elastic critical behaviour is determined by the crossover exponent for the stress acting as a "modifying field", is due to a tricritical point, or is the consequence of a weakly first-order transition.Mit Hilfe von Schallgeschwindigkeitsmessungen werden die elastischen Steifigkeiten cll, c,,~, cS3 und c, ale Funktion der Temperatur unterhalb und oberhalb des Phaseniibergangs bei T, = = 373 K bestimmt. Ihr Verlauf unterhalb T, wird an Hand von kritischen Exponenten diskutiert. Ihr Wert von der GroBenordnung 0,5 erlaubt nicht zu entscheiden, ob das kritische elastische Verhalten vom Crossover-Exponenten fiir die betreffende elastische Spannung als ,,modifizierendes Feld" bestimmt wird, ob ein trikritischer Punkt vorliegt, oder ob es durch einen schwach diskontinuierlichen Phaseniibergang beeinfldt wird.
Transparent polymer elements, containing both 3d-positioning structures and planar optical elements made by surface structuring, open the way for the mutual passive alignment of optical elements with respect to fibers, detectors and light sources in micro-optical benches with sub-micron precision. A fabrication process is presented for polymer inserts in micro-optical benches, which combines the mechanical precision of the LIGAprocess with the wide variety of optical functions offered by diffractive optical elements (DOEs). For this purpose, metal masters with lens elements made by surface structuring, and frame structures made with deep Xray lithography (DXRL) and electroplating were used in a combined molding tool, and precision micro-optical elements were replicated by injection molding. The fabrication of the different parts of the mold insert and the alignment and fixing schemes for metal plates forming the micro cavity is described in detail.Injection molding experiments have been carried out using polycarbonate (PC), a polymer known for its good optical properties. We discuss the different designs of mold inserts and injection geometries used for the mold, which were chosen in order to control the shrinkage of the molded element, to restrict damages during demolding, and to avoid inhomogeneities in the area of the lenses due to flow anisotropies and seam lines.We report on the characterization of the molded lens components. Injection molded lens structures are compared with hot embossed replicas, and used for the purpose of collimation applications. The imaging properties of these optical elements from single mode fibers onto single mode fibers is discussed.The miniature optical elements are arranged in arrays with 250 jim pitch which make them well suited for applications with fiber ribbons. Various positioning schemes and bench arrangements are under development.
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