Design Considerations for Spatial Monomode Operation of InP-based Passive Vertically Coupled Microring Resonators

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1 Design Considerations for Spatial Monomode Operation of InP-based Passive Vertically Coupled Microring Resonators Dimitris Alexandropoulos 1, Alexandros Kapsalis 1, Dimitris Syvridis 1, Ute Troppenz 2, Michael Hamacher 2 and Helmut Heidrich 2 1., Department of Informatics and Telecommunications, University Of Athens, Panepistimiopolis, Ilissia, Athens, GR 15784, Greece2. Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut, Berlin, This work was supported by the European Commission under IST-II-WAPITI (004073).

2 Outline Motivation Introducing microring resonators Vertical coupling scheme motivation and benefits Requirement for monomode operation Design Scheme Rational of the scheme Design maps Proposed Structure Results Discussion and Conclusions

WAPITI s Aims WAPITI - Objectives Ring Laser Features - Development of µring cavity devices of

InGaAsP/InP GaAs active/passive waferbonding technology - Material composition and pattern of

waveguide layer - Vertically coupled to active waveguide layer via compact couplers of few 10 µm

3 WAPITI s Aims WAPITI - Objectives Ring Laser Features - Development of µring cavity devices of compact footprint with high performance and novel functionalities - Implementation using InGaAsP/InP GaAs active/passive waferbonding technology - Material composition and pattern of the passive and active waveguides can be optimised independently - Optical I/O ports in passive waveguide layer - Vertically coupled to active waveguide layer via compact couplers of few 10 µm length - Accurate coupling strength control due to layer thickness tolerances with epitaxial growth accuracy

4 Waferbonding of ring resonator components Wafer bonding of GaInAsP layer stack on GaAs Nanofabricated vertically coupled compact ringresonator devices

Microring resonators Microring resonators ideal for integration Versatility demonstrated (lasers, filters,wavelength converters, logic gates)

active/passive coupling Vertical coupling Ring placed on top of the bus waveguide - wafer bonding Tailoring of the characteristics of the two separately

5 Microring resonators Microring resonators ideal for integration Versatility demonstrated (lasers, filters,wavelength converters, logic gates) Passive/active integration: Lateral coupling bus and ring waveguides on the same wafer level Restrictive in design Additional growth steps for active/passive coupling Vertical coupling Ring placed on top of the bus waveguide - wafer bonding Tailoring of the characteristics of the two separately Advantages for active/passive coupling z Offset x R Need for monomode operation. Higher order modes radiate and contribute to losses. A monomode device is more predictable. y x

6 Design scheme: Rational for suppressing higher order modes Maximum of coupling between modes with same propagation constant Straight/bent waveguide coupler: we propose selective excitation of the fundamental mode of the ring By tailoring the material and geometrical properties of the bus and ring waveguides the effective indices n eff can be leveled Facilitated by the vertical geometry

7 Passive Devices: Proposed Structure The values of ring core thickness and bus width are chosen from the design maps For these values the n eff of the ring and bus is approximately the same

8 Design maps 3.35 Effective index n eff Radius 20 µm 25 µm 30 µm Effective index n eff Bus width 2 µm 3 µm 4 µm Ring core thickness (µm) Bus thickness (µm) TE fundamental mode Ring waveguide width: 1.8 µm TE fundamental mode

9 Results simulation details Simulation performed using FULLWAVE, RSOFTs FDTD simulation package Field launched in fundamental mode (TE polarization at wavelength λ=1.55µm) of the bus and then collected in the ring Field monitor Discretisation of grid is set to λ eff /10 Calculation performed for two offsets, changes intentionally or unintentionally perturb modal properties of structure Wider bus study is motivated by need to relax fabrication uncertainties Bus waveguides will be multimode - use of taper structure to restore monomodal operation Launched field

10 Results Structure 1 R = 20 µm Offset 0.5 µm Bus width 2.0 µm Ring width 1.8 µm Ring core: 0.25 µm Offset 1 µm Field maximum of mode shifts towards outer surface of ring which is typical for ring geometry Monomode operation for both offsets

11 Results Structure 2 R = 20 µm Offset 0.5 µm Offset 1 µm Bus width 3.0 µm Ring width 1.8 µm Ring core: 0.28 µm Monomode operation for 1 µm offset Excitation of higher order modes for 0.5 µm offset

12 Features Simple design strategy: efficient for high offset values Small offset values and relatively wide waveguide dimensions result in multimode operation Offset: additional degree of freedom for coupler design Proposed scheme of selective excitation of the ring fundamental mode

13 Conclusions Vertical coupling flexibility in tailoring the properties of the waveguides independently from one another Waferbonding novel fabrication technology with potential of innovative small footprint devices, but also challenges

14 First realised couplers

Vertically coupled microring resonators using one epitaxial growth step and single-side lithography

Vertically coupled microring resonators using one epitaxial growth step and single-side lithography Óscar García López, 1,3,* Dries Van Thourhout, 2 Daniel Lasaosa, 1 Manuel López-Amo, 1 Roel Baets, 2