Anders Harpøth

Topology optimization and fabrication of photonic crystal structures

Topology optimization is used to design a planar photonic crystal waveguide component resulting in significantly enhanced functionality. Exceptional transmission through a photonic crystal waveguide Z-bend is obtained using this inverse design strategy. The design has been realized in a silicon-on-insulator based photonic crystal waveguide. A large low loss bandwidth of more than 200 nm for the bandgap polarization is experimentally confirmed.

Ultralow-loss 3-dB photonic crystal waveguide splitter

A photonic crystal waveguide splitter that exhibits ultralow-loss 3-dB splitting for TE-polarized light is fabricated in silicon-on-insulator material by use of deep UV lithography. The high performance is achieved by use of a Y junction, which is designed to ensure single-mode operation, and low-loss 60 degrees bends. Zero-loss 3-dB output is experimentally obtained in the range 1560-1585 nm. Results from three-dimensional finite-difference time-domain modeling with no adjustable parameters are found to be in excellent agreement with the experimental results.

Efficient propagation of TM polarized light in photonic crystal components exhibiting band gaps for TE polarized light

We have investigated the properties of TM polarized light in planar photonic crystal waveguide structures, which exhibit photonic band gaps for TE polarized light. Straight and bent photonic crystal waveguides and couplers have been fabricated in silicon-on-insulator material and modelled using a 3D finite-difference-time-domain method. The simulated spectra are in excellent agreement with the experimental results, which show a propagation loss as low as 2.5+/-4 dB/mm around 1525 nm and bend losses at 2.9+/-0.2 dB for TM polarized light. We demonstrate a high coupling for TM polarized light in a simple photonic crystal coupler with a size of ~ 20 m x 20 m. These promising features may open for the realization of ultra-compact photonic crystal components, which are easily integrated in optical communication networks.

Broadband topology-optimized photonic crystal components for both TE and TM polarizations

Several planar photonic crystal components topology-optimized for TE-polarized light, including 60 masculine bends, Y-splitters, and 90 masculine bends, have been characterized for the TM polarization. The experimental results are confirmed by finite-difference time-domain calculations. The surprising efficiency for TM-polarized light is found and paves the way for photonic crystal components suitable for both polarizations.

Characterization of UV written waveguides with luminescence microscopy

Luminescence microscopy is used to measure the refractive index profile and molecular defect distribution of UV written waveguides with a spatial resolution of ~0.4 ?m and high signal-to-noise ratio. The measurements reveal complex waveguide formation dynamics with significant topological changes in the core profile. In addition, it is observed that the waveguide formation process requires several milliseconds of UV exposure before starting.

Empirical model for the waveguiding properties of directly UV-written waveguides

We present an empirical model for the waveguiding properties of directly UV-written planar waveguides in silica-on-silicon. The waveguides are described by a rectangular core step-index profile, in which model parameters are found by comparison of the measured waveguide width and effective index with modal field calculations. The model is used as input for beam propagation method calculations to design UV-written optical components. Subsequent fabrication of such components showed a good agreement with the model predictions. Using the model will reduce the number of iterations and thereby the development time of new optical devices.

Topology-optimized and dispersion-tailored photonic crystal slow-light devices

We review our work done for topology optimization of passive photonic crystal component parts for broadband and wavelength dependent operations. We show examples of low-loss topology-optimized bends and splitters optimized for broadband transmission and demonstrate the applicability of topology optimization for designing slow-light and/or wavelength selective component parts. We also present how the dispersion of light in the slow-light regime of photonic crystal waveguides can be tailored to obtain filter functionalities in passive devices and/or to obtain semi-slow light having a group velocity in the range ~(c0/15 - c0/100); vanishing, positive, or negative group velocity dispersion (GVD); and low-loss propagation in a practical ~5-15 nm bandwidth.

The role of local heating in the formation process of UV written optical waveguides

A behavior is reported where the index change process used for UV writing of integrated optical waveguides in deuterium loaded Ge:SiO2 glass can become unstable and suddenly switch off or on. It is shown that such discontinuities are associated with abrupt changes in the amount of absorbed UV power. We suggest that these events are controlled by a coupling between UV absorption, local heating and the D2-GeO2 reaction rate. From our findings we predict, and confirm experimentally, that strong waveguides can not be fabricated under normal UV writing conditions in thin core layers with a low initial UV absorption. Our findings show that an improved understanding of the waveguide formation process and future process development requires that thermal effects are taken into account.

Luminescence microscopy of UV written waveguides

We present a characterization method based on the luminescent properties of UV written waveguides to obtain the UV induced index-distribution, verified by comparison with measurements performed with the refracted near field technique.

Design and fabrication of SOI-based photonic crystal components

We present examples of ultra-compact photonic crystal components realized in silicon-on-insulator material. We have fabricated several different types of photonic crystal waveguide components displaying high transmission features. This includes 60/spl deg/ and 120/spl deg/ bends, different types of couplers, and splitters. Recently, we have designed and fabricated components with more than 200 nm bandwidths. Design strategies to enhance the performance include systematic variation of design parameters using finite-difference time-domain simulations and inverse design methods such as topology optimization.