WAVES AND VIBRATIONS IN INHOMOGENEOUS STRUCTURES ...

optimization method has dramatically boosted the performance of the 60° bend and opened
for a practical implementation of the bend without the need for delicately tuning a narrow
operational bandwidth of the bend to the bandwidth of the rest of the PhC component. The
high transmission bandwidth of the bend is obtained for the fundamental even mode in the
PBG of the PhCW. Due to the fundamental properties of the SOI PhCW [7, 8] this bandwidth
is above the silica-line but in the PBG and cannot be attributed to the optimization method, as
this is a purely two-dimensional algorithm.
Loss per bend (dB)
3
2
1
0
Topology-Optimized PhCW 60 0 bend
Experimental
3D FDTD
1250 1300 1350 1400 1450 1500
Wavelength (nm)
Fig. 5. Experimental bend loss (green) compared to 3D FDTD calculated bend loss (blue). The
3D FDTD curve has been shifted 1.2% in absolute wavelength.
Figure 5 shows a detailed comparison between the experimental and calculated loss of the
optimized bend and a good agreement is found. The negative theoretical propagation losses
are due to numerical artifacts when calculating near zero losses. The 3D FDTD spectrum has
been shifted 1.2% in absolute wavelength and slightly undershoots the experimental values.
These deviations are partly due to uncertainties in the experimental hole diameters, but more
importantly due to the limited grid resolution in the calculations [13].
4. Conclusion
We have optimized the performance of a 60° planar photonic crystal waveguide bend using a
two-dimensional inverse design strategy called topology optimization. The design was
fabricated in silicon-on-insulator material and we experimentally obtained a record-high 1-dB
transmission bandwidth exceeding 200nm for the TE polarization. The experimental results
agree well with 3D finite-difference-time-domain simulations. The broadband topologyoptimized
60° waveguide bend solves an important issue in designing planar photonic crystal
components and opens for the realization of a wide range of ultra-compact, low-loss, and
broadband optical devices.
Acknowledgments
This work was supported in part by the Danish Technical Research Council through the
research programs ‘Planar Integrated PBG Elements’ (PIPE) and ‘Designing bandgap
materials and structures with optimized dynamic properties’.
#5520 - $15.00 US Received 19 October 2004; revised 12 November 2004; accepted 15 November 2004
(C) 2004 OSA 29 November 2004 / Vol. 12, No. 24 / OPTICS EXPRESS 5921