Waveguides Fabricated in Fused Silica by Germanium Ion Implantation at Varying Doses
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ABSTRACT The implantation of MeV Ge 3+ ions into fused silica has been used to fabricate single mode channel waveguides with a low propagation loss of 0.10-0.15 dB/cm. The loss coefficient, (X,has been measured as a function of ion dose (8 x 1013 to 8 x 1016 ions/cm 2 ) and annealing temperature (250 to 600 °C) at X = 1300 nm. The value of cc for the as-implanted waveguides exhibited a minimum of -1.0 dB/cm at an intermediate range of dose from 8 x 1014 to 8 x 1015 ions/cm 2 . A progressive reduction in ox occurred as the annealing temperature was increased from 300 to 500 'C. Annealing of the implanted waveguides at 500 'C for 1 h has produced an order of magnitude decrease in cc to 0.1 dB/cm at 8 x 1014 ions/cm 2 . At doses which were outside of the intermediate range, the value of ax was > 10 dB/cm. This trend in ox with ion dose has been attributed to the dominance of a residual nuclear component of damage after annealing.
INTRODUCTION The implantation of selected ions into silica glass has been shown to produce a buried layer of increased refractive index (An) [1]. For implantation with protons, a compaction of the structure of the glass in the modified layer has resulted in a typical value of An -+1 % at 300 K [2]. The implanted layer of increased refractive index has been used to define the core region of planar waveguides. For He+ [2], N+ [3-5] and Si2 ÷ [6] ions implanted into silica glass, the reported values of waveguide loss coefficient, ax, have ranged from 0.2 to 6.0 dB/cm, depending on the ion species and the conditions of irradiation. Wang et. al. have shown that a threshold level of dose of N ions was necessary to achieve waveguide propagation in silica glass [4]. The annealing treatment of N implanted waveguides has also been recognized as a critical factor in determining optical characteristics, with a significant reduction in a [3,4] and change in the profile of refractive index [5] at a temperature of - 500 'C. Recently, the implantation of MeV Ge ions into silica glass at 300 K has produced both an increase in refractive index (-1 %) and an enhancement in optical absorption at 244 and 212 nm [7,8,12]. The level of ion-induced optical absorption was subsequently reduced by irradiation of the implanted samples with 249 nm laser light, thereby allowing the direct writing of gratings in a waveguide. The formation of slab [12] and channel [9] waveguides by the MeV implantation of fused silica with Ge ions has also recently been demonstrated. As a result of these developments, the potential exists for the fabrication of planar waveguides with the incorporation of in-core devices. However, the propagation characteristics of the waveguides produced by implantation with MeV Ge ions have not yet been optimized in terms of implant dose and annealing temperature. We demonstrate the fabrication of the first low loss waveguides by Ge implantation of fused silica. Channel type of waveguides were chosen in the present experiments because of the known ability of this geometry to attain a low attenuation loss
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