A Wavelength Converter Integrated with a Discretely Tunable Laser ...

4.5 Conclusions 61
4.5 Conclusions
The integration of passive waveguides, SOAs and electro-optic phase modulators in InP enables
the fabrication of advanced photonic integrated circuits. The trade-offs that have to be
made for the integration have been analyzed. It was shown that high doping levels are advantageous
for low electrical resistance in the SOAs, but it also result in high optical loss, especially
in p-type material. Different doping profiles can be realized in the passive waveguides and the
SOAs by employing an epitaxial re-growth step. The composition of the quaternary film layer
shows a trade-off between the fabrication tolerance of the waveguides and the efficiency of
the electro-optic effect in phase modulators. The tolerance and polarization independence ben-
efit from £ ¤ ¤ short nm, while the modulators are best realized ¤¨ using nm. Although
the fabrication tolerance and the polarization independence of the waveguides becomes
less ¤¤ when nm, it is still feasible and polarization independence can be achieved by
using polarization compensating waveguides. A drawback of short is that the optical field
expands into the cladding layers, resulting in higher loss. Deeply-etched waveguides can be
employed together with shallowly-etched guides. Deeply-etched guides have two nice features:
They are polarization independend at a specific waveguide width and due to their high
optical confinement, they allow for radii of curvature down ¤ to .
SOAs can be optimized for either non-linear behaviour or for high saturation output power.
Both type of SOAs can be realized on one wafer. Rougly speaking, non-linear behavior benefits
from narrow and long SOA waveguides, while the oposite is true for the high power SOAs.
This differentiation can simply be realised in the mask layout, thus very flexible. The thickness
of the bulk active layer can also be optimized for non-linear and low threshold in lasers
simultaneously. Simulations showed that the non-linear behavior was optimal for a large range
of thicknesses of the active layer, and the thickness can be decreased downto 120 nm without
compromising the optical conditions for non-linear behavior. Thin active layer are advantages
for a low laser threshold.