High index contrast potassium double tungstate waveguides towards efficient rare-earth ion amplification on-chip

摘要

Rare-earth ion doped KY(WO_4)_2 amplifiers are proposed to be a good candidate for many future applications by benefiting from the excellent gain characteristics of rare-earth ions, namely high bit rate amplification (＞Tbps) with low noise figure (＜5-6 dB). However, KY(WO_4)_2 optical waveguide amplifiers based on rare-earth ions were conventionally fabricated on layers overgrown onto undoped KY(WO_4)_2 substrates. Such amplifiers exhibit a refractive index contrast between the doped and undoped layer of typically ＜0.02, leading to large devices not suited for the high degree of integration required in photonic applications. Furthermore, the large mode diameter in the waveguide core requires high pump input powers to fully invert the material. In this study, we experimentally demonstrate high index contrast waveguides in crystalline KY(WO_4)_2, compatible with the integration onto passive photonic platforms. Firstly, a layer of KY(WO_4)_2 is transferred onto a silicon dioxide substrate using bonding with UV curable optical adhesive. A subsequent polishing step permits precise control of the transferred layer thickness, which defines the height of the waveguides. Small-footprint (in the order of few microns) high index contrast waveguides were patterned using focused ion beam milling. When doped with rare-earth ions, for instance, Er~(3+) or Yb~(3+), such high contrast waveguides will lead to very efficient amplifiers, in which the active material can be efficiently pumped by a confined mode with very good overlap with the signal mode. Consequently, lower pump power will be required to obtain same amount of gain from the amplifier leading to power efficient devices.