Rare-earth ion doped crystalline potassium double tungstates, such as KY(WO<sub>4</sub>)<sub>2</sub>, KLu(WO<sub>4</sub>)<sub>2</sub> and KY(WO<sub>4</sub>)<sub>2</sub>, exhibit many properties that make them promising candidates for the realization of lasers and amplifiers in integrated photonics. One of the key challenges for the hybrid integration of different photonic platforms remains the design and fabrication of low-loss and fabrication tolerant couplers for transferring light between different waveguides. In this paper, adiabatic vertical couplers realized by flip-chip bonding of polymer waveguides to Si<sub>3</sub>N<sub>4</sub> devices are designed, fabricated and tested. An efficient design flow combining 2D and 3D simulations was proposed and its validity was demonstrated. The vertical couplers will ultimately be used for the integration of erbium doped KY(WO<sub>4</sub>)<sub>2</sub> waveguides with passive platforms. The designed couplers exhibit less than 0.5 dB losses at adiabatic angles and below 1 dB loss for ±0.5 μm lateral misalignment. The fabricated vertical couplers show less than 1dB losses in average for different adiabatic angles of Si<sub>3</sub>N<sub>4</sub> tapers, which is in good quantitative agreement with the simulations.
Surface plasmons polaritons have drawn significant attention in recent years not only thanks to their capability of confining the field in the dielectric/metal interface, but also thanks to their potential to produce highly efficient thermooptical or electro-optical devices such as modulators and switches due to the presence of the metal layer amidst the electromagnetic field. However, the high confinement comes at the cost of high propagation losses due to the metal’s highly absorptive nature at visible and near-IR wavelengths. In order for plasmonic devices to find a widespread use in integrated optics, an advantage over dielectric waveguides needs to be found that justifies their utilization. In this work, we present an application in which metallic waveguides perform better than their dielectric counterparts. By adding a thin metallic layer underneath the waveguide core, the total bend losses (dB/90° are reduced with respect to the bend losses of the equivalent dielectric structure without the metallic layer for a range of radii from 35 µm down to 1 µm. The results show a dramatic reduction of total bend losses in TE-polarization with values as low as 0.02 dB/90° bend for radii between 6 and 13 µm. The mechanism for the reduction of bend losses is the shielding action of the metal layer, which prevents the field to leak into the substrate. In this paper, both detailed theoretical calculations as well as experimental results for SU-8 channel waveguides will be presented.
Rare-earth ion doped KY(WO<sub>4</sub>)<sub>2</sub> 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<sub>4</sub>)<sub>2</sub> optical waveguide amplifiers based on rare-earth ions were conventionally
fabricated on layers overgrown onto undopedKY(WO<sub>4</sub>)<sub>2</sub> 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<sub>4</sub>)<sub>2</sub>, compatible with the integration onto passive photonic platforms. Firstly, a layer of
KY(WO<sub>4</sub>)<sub>2</sub> 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<sup>3+</sup> or Yb<sup>3+</sup>, 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.