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Minimum Waveguide Separation Between
Input/Output Waveguides (dx): In the second step,
it was necessary to fix the design parameter dx, i.e.
minimum waveguide separation between input/
output waveguides. This parameter has an impact on
the crosstalk between adjacent output channels. To
this purpose, four 20-channel, 50-GHz AWGs with
different output waveguide separations were
designed with AWG-Parameters tool: dx = 2.5 µm, 3
µm, 3.5 µm and 4 µm and simulated applying all three
photonics tools (Seyringer, 2017). The simulations
showed that there is some minimum waveguide
separation dx necessary to keep the crosstalk between
output channels sufficiently low (in our AWG design,
dx = 3.5 µm). At this separation, the output
waveguides are positioned far enough from each
other, to prevent the focusing of the power from
selected channel into the neighbour output
waveguides and vice versa. This implies that by
increasing this value the performance of AWG did
not change much (case of dx = 4 µm). If the
waveguide separation was too small, the crosstalk
strongly increased. This is in particular the case of
waveguide separation dx = 2.5 µm and partially dx =
3 µm, where the AWG spectral response contained
side-lobes inducing high channel crosstalk. Based on
this study we fixed this parameter to dx = 3.5 µm.
We would like to point out that all above
described designs were technologically verified and
confirm the simulated results.
3.2 Design of 40-Channel, 50-GHz
AWG
Based on the previous study we designed 40-channel,
50-GHz AWG. The AWG structure was then
simulated applying WDM PHASAR tool and the
spectral response is shown in Fig. 4. As can be seen,
the transmitted optical signals are well separated from
each other which is confirmed by low background
crosstalk, BX = -42 dB (Fig. 4a). Adjacent channel
crosstalk reached its highest value at the side-lobes in
the middle of the spectrum, AX = 23.8 dB (Fig. 4b).
In addition, the losses are very low (Fig. 4c). There
are nearly no losses in the middle of the
characteristics (the highest peaks) and there is about
1.2 dB loss at the lowest peaks (insertion loss, IL).
This loss is mainly a result of the non-uniformity i.e.
difference between the highest- and the lowest peaks;
also called insertion loss uniformity, ILu = 1.18 dB
(in AWG-Parameters tool labelled as Lu parameter).
From this can be concluded that applying an
optimized design based on the previous study of the
design parameters in order to eliminate losses and
crosstalk, the insertion loss was suppressed nearly to
zero and the channel crosstalk is low, too.
4 DESIGN OF HIGH-CHANNEL
AWGS
Above described optimized design ensures rather
satisfying optical properties of AWGs up to 40
channels. However, the optical spectrometers for
medical applications require much higher AWG
channel counts. We will show that increasing the
number of output channels (parameter Num) brings
some additional design problems to be solved.
4.1 Design of 80-Channel, 50-GHz
AWG
To this purpose, we designed 80-channel, 50-GHz
AWG in which we used the same design parameters,
i.e. dd = 1.2 µm, dx = 3.5 µm; and the parameters Lf
and dL were calculated accordingly (we will call it
DESIGN1). The AWG structure was then simulated
applying WDM PHASAR tool keeping the same
calculation conditions. The simulated spectral
response is shown in Fig. 5. From the characteristics
is evident that the optical signals are much wider
compared to 40-channel, 50-GHz AWG (defined
through the bandwidth, B@5dB, and B@20dB, i.e. a
width of optical signal, measured at a −5 dB, and −20
dB drop from transmission peak), as can be seen in
Figs. 4b and 5b. It causes the increase of insertion loss
by nearly 1 dB (IL = 2.18 dB in Fig. 5c) and
particularly much higher adjacent channel crosstalk,
AX = 11.8 dB (Fig. 5b). Non-uniformity ILu is similar
in both cases, since both AWGs were designed to
have insertion loss uniformity about 1 dB.
Background crosstalk, BX increased from -42 dB (40-
channel AWG) to -38 dB (80-channel AWG, Fig. 5a).
4.2 Optimization of 80-Channel,
50-GHz AWG Design
Since the same design procedure as well as the same
simulation conditions were used in both designs the
question is what is the reason for such widening of the
optical signals causing deterioration of the optical
properties of designed 80-channel, 50-GHz AWG and
how can be this negative effect eliminated.
We have tested various AWG design parameters
and the deep study of the results showed that the
reason for widening of the optical signals is the
crosstalk caused by the coupling between PA
waveguides.
Design and Optimization of High-Channel Si
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