Mustafa Sefünç

Low-loss sharp bends in polymer waveguides enabled by the introduction of a thin metal layer

Embodying a thin metallic layer underneath the core of a sharply bent polymer waveguide is shown in this work to considerably reduce the total losses of both the quasi-transverse-electric and quasi-transverse-magnetic modes. The computational results show a total loss as low as ~0.02 dB/90° for the quasi-transverse-electric mode for radii between 6 and 13 µm at the wavelength of 1.55 µm, which corresponds to a 10-fold improvement over the purely dielectric counterpart. The radii range exhibiting such low total loss can be tuned by properly selecting the parameters of the structure. For the quasi-transverse-magnetic mode, the metal layer reduces the total losses modestly for radii ranging from 3 to 10 µm. Simulation results for different structural parameters are presented.

Loss compensation in metal-loaded hybrid plasmonic waveguides using Yb 3+ potassium double tungstate gain materials

The compensation of propagation losses of plasmonic nanowaveguides will constitute an important milestone towards the widespread use of these structures as enabling components for highly dense, fast, on-chip nanophotonic circuitry. Rare-earth doped double tungstate gain materials can not only provide elevated modal gain per unit length, but are capable of the amplification of very high rate signals, making them excellent potential candidates for such application. In this paper, a model that permits simulating plasmonic structures in rare-earth doped potassium double tungstates is described. The model is applied to study the achievable net gain in metal-loaded hybrid plasmonic waveguides with different structural parameters.

Low-loss sharp bends in low contrast polymer hybrid metallic waveguides

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.

Design and length optimization of an adiabatic coupler for on-chip vertical integration of rare-earth-doped double tungstate waveguide amplifiers

The integration of rare-earth doped double tungstate waveguide amplifiers onto passive technology platforms enables the on-chip amplification of very high bit rate signals. In this work, a methodology for the optimized design of vertical adiabatic couplers between a passive Si3N4 waveguide and the on-chip amplifier is proposed. The methodology shows high efficiency and tolerance in the adiabatic coupler design. The calculated losses of the adiabatic coupler are as low as 0.5 dB for 0.98 μm and 0.14 dB for 1.55 μm. The length of the taper is quantitatively optimized with an adiabatic transfer.

High index contrast passive potassium double tungstate waveguides

High-refractive-index-contrast potassium double tungstate waveguides have been experimentally demonstrated. A bulk KY(WO4)2 layer was successfully bonded onto a lower refractive index carrier using a UV curable optical adhesive and polished down to the thickness of 2.4 µm. A set of rib waveguides with ~2 μm width and 0.85 μm slab thickness were fabricated on the thin transferred KY(WO4)2 layer by focused-ion-beam milling. The upper-limit of the propagation losses of the fabricated waveguides is estimated to be 1.5 dB/cm at the wavelength of 1.55 μm using the Fabry-Perot method.

Design and fabrication of adiabatic vertical couplers for hybrid integration by flip-chip bonding

Rare-earth ion doped crystalline potassium double tungstates, such as KY(WO4)2, KLu(WO4)2 and KY(WO4)2, 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 Si3N4 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(WO4)2 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 Si3N4 tapers, which is in good quantitative agreement with the simulations.

Towards on-chip high index contrast rare-earth-doped potassium double tungstate amplifiers

The monoclinic double tungstate crystals are good candidates as host materials for rare-earth ions. Thanks to their crystalline nature, clustering of doping ions is avoided. High concentrations of active ions can therefore be achieved, which, together with the large absorption and emission cross sections of the active ions in these materials, leads to compact and efficient on-chip amplifiers and lasers. However, the fabrication of integrated on-chip waveguide amplifiers and lasers integrated with dielectrics or semiconductors is challenging. In this work, we will present our current efforts on the development of on-chip high index contrast waveguide amplifiers in rare-earth ion doped potassium double tungstates.

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

Rare-earth ion doped KY(WO4)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(WO4)2 optical waveguide amplifiers based on rare-earth ions were conventionally fabricated on layers overgrown onto undopedKY(WO4)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(WO4)2, compatible with the integration onto passive photonic platforms. Firstly, a layer of KY(WO4)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, Er3+ or Yb3+, 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.

Fabrication of high-contrast waveguide amplifiers in erbium doped potassium double tungstates

High-contrast waveguides in crystalline potassium double tungstates pave the road towards compact and efficient on-chip amplifiers. In this work, the design and fabrication of erbium doped high contrast potassium double tungstates waveguides will be described.

Study of sharp bends in anisotropic potassium double tungstate waveguides

Rare earth ion doped potassium double tungstate gain materials have recently shown a great promise for the development of waveguide amplifiers and lasers exhibiting excellent performance. To enable the use of this material in larger nanophotonic platforms, sharp bends are required. In this work we study the effect of the anisotropy of potassium double tungstates on the bend losses of high contrast waveguides using three different simulation methods. It is concluded that the existence of this anisotropy has not detrimental effect on the bend losses, therefore opening the door to the utilization of this material for integrated nanophotonics.