1. Shane P. DUGGAN1,2*, Padraic E. MORRISSEY1,2, Frank H. PETERS1,2
Taper Design for Vertical Coupling
between Isolated Active and
Passive Waveguides
3. Future Work
As the device in this work consists of 2 WGs atop each other a p-i-
n structure typically adopted cannot be used. Either pinip or nipin
must be chosen to supply current to both WGs. Due to n-type
carrier mobility and low loss, as well as a reluctance to alter the
more difficult MZM that will be constructed into the upper WG, a
pinip structure has been selected. The lower nip active guide must
be tested to ensure it can be grown and processed correctly,
pending the success of which the full device can be grown and
tested.
1. Tyndall National Institute
Integrated Photonics Group
2. University College Cork
Physics Department
Figure 1: Waveguide architecture for the designed up-down WG device
with a passive region above and active QWs beneath the lower ridge. The
parameters outlined determine the mode shape within the WG.
Figure 2: A surface depicting the overlaps between the mode in the up-
down WG device for different values of the upper ridgewidth.
upper ridgewidth (𝜇𝑚)
upperridgewidth(𝜇𝑚)
Normalised Power Normalised Power
Upper WG
Lower WG Lower WG
Upper WG
Figure 3: A BPM simulation of light entering the upper passive WG and
transitioning down to the active WG before transitioning back up again
for (a) a 2 WG structure and (b) a 3 WG structure. The power in the upper
and lower WGs is monitored next to each mode transition.
(a) (b)
The overlap between the modes in devices with different upper
ridgewidths were calculated to predict the taper that could be
used to provide the least loss, Fig.2. A linear taper was found to
satisfy this requirement.
The effective index was calculated and BPM simulations were
carried out to determine the ridgewidth at which a transition
between the WGs started. Various taper lengths to different end
width combinations were simulated to select that which gave the
most efficient power transfer. Fig.3(a) shows a BPM simulation of
the best device.
In order to increase the coupling efficiency between the WGs an
additional intermediate transparent WG is introduced. This
increases the coupling resulting in >200% power output compared
to a 2WG device as seen in Fig.3(b).
Figure 4: The mode in mid-transition between the upper passive and
lower active waveguides in (a) a 2WG device and (b) a 3Wg device.
(a) (b)
1. Monolithic Integration of Active and
Passive Photonic Devices
Existing methods of monolithic photonic integration require
selective growth or regrowth to laterally couple active and passive
regions. This results in reflections as the mode in the waveguide
abruptly transitions from one material to another, and matching
the thicknesses of each growth can be difficult. The work presented
here couples two waveguides vertically, with a passive WG on top
and an active WG below. This gives device fabricators easier access
to the upper passive region where an MZM may be constructed. To
optimize each device they must be isolated away from the
transition point, and this makes coupling between the WGs
challenging.
2. Device design and BPM simulations
The mode solutions of the device structure of Fig. 1 were calculated
in order to select dimensions that would separately allow for
confinement of the mode in each WG, while ensuring isolation of
the passive WG from the active so there will be no losses when the
light is confined in the passive guide.
3.1 3.2 3.3 3.4 3.5 3.6 3.7
n