Research-Page of Dominik G. Rabus
Research
Integrated Ring
Resonator Filters in GaInAsP/InP
A key device in
all-optical networks is the optical filter. There are different types of
optical filters, for example, Bragg gratings, thin film filters, arrayed
waveguide gratings (AWGs). An optical filter which
has emerged in the last few years in integrated optics and which I have
designed and investigated on the basis of GaInAsP / InP
is the ring resonator filter.
Ring resonator
filters do not require facets or gratings for optical feedback and are thus
particularly suited for monolithic integration with other components such as
lasers and photodetectors for example.
A required passband
shape of ring resonator filters can be custom designed by the use of multiple
serial or parallel coupled resonators.
The operating
principle of a single ring resonator add-drop filter is shown in the figure
below.
REQUIREMENTS:
The realization of
ring resonators in integrated optics requires low loss waveguides with a strong
confinement to achieve low bending radii on one hand and an easy fiber to chip coupling on the other (here minimum bending radii
of R = 100 µm have been fabricated).
Tuneability is
essential for the system application of optical filters. In the case of
periodic filters, in our case ring resonators, it is important to fit the transmission
curve to the defined channel spacing (e.g. ITU-grid).
The performance of
passive ring resonators for filter applications is limited by internal losses.
The incorporation of a semiconductor optical amplifier (SOA) enables additional
functionality (e. g. switchability) including the
compensation of internal losses.
REALIZED DEVICES:
A photograph of a
fabricated single ring resonator with integrated semiconductor optical
amplifier highlighting the different building blocks of a ring resonator is
shown in the figure below. The width of the waveguide is 1.8 µm. The size of
the chip is approximately 2 mm × 1.5 mm.
Passive single and
double ring resonators in the material system GaInAsP / InP in the form of
racetracks, coupled to a multimode interference (MMI) coupler or a directional
coupler with radii of R = 100 µm – 200 µm and free spectral ranges (FSRs) of 50 GHz and 100 GHz and on–off ratios of more
than 20 dB have been realized.
Single, double and
triple ring resonators with integrated SOAs (length =
100 µm – 800 µm), coupled to two input/output waveguides using directional
couplers with radii of R = 100 µm – 800 µm are demonstrated. The ring losses
are compensated by the SOA and an on–off ratio for the throughput and drop port
of more than 20 dB is realized.
The achieved free
spectral range is 12.5 GHz, 25 GHz and 50 GHz.
The tuning to a
specific wavelength and the resonance matching of the double and triple ring
resonators is demonstrated by using integrated Pt – resistors.
The passband shape
depends sensitively on the relative interactions between all resonators and the
used couplers. In order to achieve a box-like filter response, a general design
rule is developed for engineering definite filter shapes using double and
triple coupled ring resonators.
Photographs of
fabricated ring resonators with integrated SOA are shown in the figure below.
