C.A. Miller

Chalcogenide glass films for the bonding of GaAs optical parametric oscillator elements

There are many applications driving the need for frequency agile solid state laser systems for use in the mid-infrared. Most of these are centred on the development of optical parametric oscillators (OPOs), which exploit the non-linear optical characteristics of non-centrosymmetric materials. In a new approach highlighted in a companion paper, OPO elements are formed by bonding gallium arsenide wafers precoated with RF sputtered films of a quaternary chalcogenide glass. The conditions used for sputtering the glass films are critical in ensuring the realisation of reliable bonds, where the glass is required to be index matched to the GaAs within very close tolerances. Issues such as glass composition, purity, porosity, devitrification and optical absorption are all key factors in determining the success of the approach. This paper describes a summary of some of the results achieved, emphasising the degree of control necessary for both the sputtering process and the preparation of the sputtering targets. Composition changes on sputtering can influence the refractive index of the glass and can easily introduce levels of insertion loss that are unacceptable by the time that stacks containing 50 or more individual phase-matched GaAs elements have been produced. Oxygen-related impurities are also easily introduced from a variety of sources and can degrade performance levels further. Such difficulties have been overcome and a reproducible technique for fabricating glass-bonded GaAs crystals has been developed. Optimised conditions for thermal bonding pairs of glass coated GaAs wafers are also reported.

The effect of GaSe on Ga–La–S glasses

Abstract Gallium–lanthanum–sulphide (GLS) glasses have been investigated as candidate materials for fabrication of IR transmitting optical fibres. Previously, significant oxide additions have been used to increase glass stability and also to allow the tailoring of the RI the produce compatible core/clad pairs. However, oxide additions have a detrimental effect on the IR transparency and suitability for rare-earth doping. A method of extrusion has been proposed that can be used to produce core/clad optical fibre performs. In this work a new method of altering the RI of the GLS glasses is investigated which can be used in conjunction with extrusion to produce optical fibre preforms. Variation of refractive index has been achieved using small additions of GaSe to the standard GLS composition, producing GLSSe glasses, avoiding the need for unfavourable oxide additions.

The application of the mid-infrared spectral region in medical surgery: chalcogenide glass optical fibre for 10.6 μm laser transmission

Infrared-transmitting glass optical fibres are being developed for intended applications in medicine and industry as part of a laser delivery system, giving more flexibility and accuracy of positioning of the laser beam for the User. Chalcogenide glass optical fibre is being designed to transmit light at 10.6 μm, to coincide with the wavelength of the output light from the CO2 laser. In medicine, ablative surgery performed using the CO2 laser causes less damage to surrounding tissue than when using shorter wavelength laser sources. The effect of composition of chalcogenide glasses on optical absorption, across the wavelength range 3 μm to > 15 μm, has been investigated using Fourier transform infrared (FTIR) spectroscopy, for a range of binary, ternary and quaternary glasses, in the form of small bulk glass specimens. Glasses containing germanium tended to exhibit higher glass transformation temperatures but a shorter wavelength multiphonon edge. The optical loss of fibre samples has been measured at 10.6 μm using a high power CO2 laser source and employing the fibre cut-back method. As2Se and Te30As20Se50 fibres (both unclad) exhibited 7.2, and 2.3, dBm-1, respectively. Ge17As18Se65 / Ge17As18Se62S3 core/clad. fibre exhibited an optical loss of 10.3 dBm-1. After the optical loss measurements, fibres were imaged using scanning electron microscopy and it was found that the high power CO2 laser caused damage to the launch end of some fibres. In particular, at the launch-end of Te-As-Se fibres the glass appeared to have undergone partial melting and possibly also suffered some vaporisation.

Large core, single-mode glass-based waveguides for photonic integrated circuits

We previously demonstrated light guiding in fiber-on-glass (FOG) dielectric waveguides using fluoro-tellurite glasses. These waveguides were fabricated by mechanically pressing a fiber onto a polished planar glass substrate of lower refractive index above the glass transition temperatures. However, two handling constraints have been discovered in this approach. In practice, for novel inorganic compound glasses, the minimum dimension of fiber that can be handled is preferably around 30μm. The minimum refractive index difference between the fiber and the substrate that can be reliably achieved at present with these glasses is 0.01. Our simulation results showed that, taken together, these restrictions provide a practical barrier to achieving single-mode FOG operation at telecommunications wavelengths. Here we present simulation and experimental results for a new inorganic glass FOG waveguide that simultaneously meets these handling constraints and achieves mono-mode operation around 1.55 μm. In this new design, a homogeneous glass fiber is partially embedded lengthwise in a substrate of higher refractive index glass; the nonembedded part of the fiber is air clad. Simulation results presented for fluoro-tellurite FOG waveguides confirm the success of the new design in realizing single-mode propagation at 1.55 μm for a fiber diameter of 30 μm and a fibersubstrate refractive index difference of 0.01. The design is robust, with good dimensional fabrication tolerance, but predicted losses are over 6 dBcm-1. A proof-of-principle demonstrator is fabricated using two commercially available multi-component silicate glasses (Schott F2 and F4). This shows multimode waveguiding at 0.633 μm, guidance around a curve, and appears mono-mode at 1.575 μm.