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Dominik Rabus and Michael Hamacher
Heinrich-Hertz-Institut fuer Nachrichtentechnik
TechOnline
Active and passive ring-resonator devices
are good for wavelength filtering, routing, switching, modulation,
and multiplexing/demultiplexing applications. Ring resonators do
not require facets or gratings for optical feedback, making them
particularly suited for monolithic integration with other
components.
This article examines the characteristic responses of single and
double multimode- interference- (MMI) coupled GaInAsP/InP
micro-ring resonators. Specifically, the article will explore these
resonators configured as racetracks with radii of 100- and 200
microns and a free spectral range (FSR) of 100- and 50-GHz,
respectively (Figure 1).
Figure 1: Double (left) and single (right) ring
resonators with MMI couplers
Resonator Design
The device's layer sequence (from bottom to top) is:
- InP substrate
- GaInAsP (
 gap=1.06 µm, 0.38 µm)
- InP etch stop layer (0.020 µm)
- GaInAsP (gap=1.06 µm, 0.84 µm)
- InP cap (0.2 µm)
The design assures a monomodal propagation of light in the
waveguide and, due to a good confinement in the resonator, very low
bending losses (Figure 2).
Figure 2: The structure (left) and the mode profile
(right) of the straight waveguide, calculated using BPM.
In addition, the waveguide was etched down on the outer side of
the waveguide in the curvatures. The width of the waveguide is 1.8
µm (Figure 3).
Figure 3: The structure of the waveguide in the
curved region (left) and the mode profile (right) for a radius of
100 µm
MMI couplers couple light into the resonator. These couplers are
widely used in photonic integrated circuits; in the ring resonators
we use them as power splitters with a ratio of 50:50 (3 dB). The
coupler has two input and two output ports. The splitting ratio is
mainly defined by the length of the MMI (Figure 4).
Figure 4: The intensity at the two output ports is
shown as a function of the length. The dashed line corresponds to
Output Port 1 and the solid line corresponds to Output Port 2. At a
length of approximately 160 µm, the two curves meet and have
an intensity of 0.5 each.
Fabrication
The ring resonators were fabricated using
standard photolithography and a CH4/H2
reactive ion etching (RIE) technique. SiNx was used as
the etching mask, which also served as the mask for the deep
etching process. A small fraction of oxygen was added to reduce the
formation of polymers during dry etching and to minimize the
sidewall roughness (Figure 5).
Figure 5: The step-by-step fabrication process for
the ring resonators
We added an antireflection coating to the facets of the input
and output waveguides in order to avoid Fabry-Perot resonances in
the straight waveguide section.
Measurement and Simulation
We characterized the ring resonators using
a tapered fiber and an external cavity laser. The characterized
ring resonators are polarization-dependent due to waveguide
asymmetry. This is irrelevant if the devices comprise an active
section and act as a laser. You can eliminate
polarization-dependent filter characteristics with a polarization
diversity architecture.
We performed the measurements reported in Figure 6 for TE
polarization. As designed, an FSR of approximately 0.8 nm (100 GHz)
for the 100 µm ring devices and 0.4 nm (50 GHz) for the 200
µm devices are observed near = 1.55 µm. The insertion losses of the
devices are between 7-8 dB (including coupling losses of
approximately 5 dB). The transmission difference between the
minimum and the off-resonant values for the single micro ring
resonators where R = 100 µm and R = 200 µm is more than
13 dB. The FWHM (full width at half maximum) was approximately 0.14
nm and 0.08 nm, respectively.
Figure 6: The results for the measured and
simulated ring resonators
The single-ring resonators are suitable for laser applications
due to the small FWHM. You must use multiple, cascaded ring
resonators for optical filters. The contrast of the throughput port
and of the drop port of the double micro ring resonator with R=100
µm are around 3.5 dB and 7.5 dB, respectively. The expected
broadening of the FWHM for the throughput port was 0.25 nm and 0.4
nm for the drop port. The low contrast is in agreement with
theoretical calculations; using gain sections can improve the
contrast further.
References
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