WAVES AND VIBRATIONS IN INHOMOGENEOUS STRUCTURES ...

Fig. 3. Steady-state magnetic field distribution for the fundamental PBG mode simulated using
2D FDTD. The mode is incident from the bottom-left part of the waveguide. Left: Mode profile
through the generic bends. Right: Mode profile through the topology-optimized bends.
The fabricated PhCWs shown in Fig. 2 have been optically characterized using broadband
light emitting diodes (LEDs) as sources. To cover the full bandwidth of the fabricated
components we used three different LEDs centered around 1310nm, 1414nm, and 1538nm.
Tapered lensed fibers were used to couple light in and out of the ridge waveguides connected
to the PhCWs. Two polarization controllers and a polarizer with an extinction ratio better than
35 dB were used to control the polarization of the light sent into the device under test. The
optical spectra for the transmitted light were recorded with a spectral resolution of 10nm using
an optical spectrum analyzer. To extract the bend loss the transmission spectra have been
normalized to the transmission spectrum for a straight PhCW of the same length. Figure 4
shows the measured bend loss of TE polarized light for the un-optimized generic (red curve)
and the topology-optimized (green curve) 60° bend.
Loss per bend (dB)
12
10
8
6
4
2
Un-optimized
Optimized
0
1250 1300 1350 1400 1450
Wavelength (nm)
Fig. 4. Measured loss per bend for the un-optimized 60º bends (red) and the topologyoptimized
60º bends (green). Both spectra have been normalized to the transmission through
straight PhCWs of the same length to eliminate the coupling and the propagation loss in
straight waveguides. Dotted line marks a bend loss of 1dB.
The topology-optimized bend displays more than 200nm bandwidth with less than 1dB loss
and an average bend loss of 0.43±0.27 dB. In the same wavelength range the un-optimized
bend clearly shows a large bend loss, which only reduces in a narrow range near the cut-off of
the fundamental mode at longer wavelengths. Thus, applying the two-dimensional topology
#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 5920