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

56 4. Integration of waveguide components on InP
are also larger for low bandgap wavelengths, because the mode extends into the substrate and
radiates away more easily.
The polarization dispersion of the guides, as a function of the composition and the
thickness of the film layer, is shown in Fig. 4.12a. It can be seen that the polarization dispersion
becomes smaller for waveguides with a long band-gap wavelength and a thick film
layer, because the optical mode profile becomes more symmetrical. Dispersion almost disappears
for §©¤ . Polarization dispersion can be compensated by inserting waveguides
in the optical path that have an inverted polarization dispersion [84]. The length of dispersion
compensation waveguide that is needed can expressed using the dispersion compensation
factor , which is defined as
¦ ¦
where the ¤ indices and denote the normal and the dispersion compensating waveguide,
respectively. Using Eq. 4.12 we find that a long waveguide of type 1 can be dispersion
compensated by adding a waveguide of length of type 2 to it. Inverted polarization
dispersion can be realized by wet-chemically etching away the top part of the cladding up to
200 nm above the film using an etch-stop layer [84]. The dispersion compensation factors are
shown in Fig. 4.12b. It can be seen that compensation is easiest accomplished for small ,
but it is feasible for the whole domain of the plot.
(4.12)
The absolute wavelength control is treated in Figure 4.13. It shows, as a function of and
, the tolerances in the width , the etch
depth , the film layer
thickness and the
composition of the film layer that result ¤ in nm.
Figure 4.13a shows the tolerance of the waveguide width . It improves for smaller and
larger , because both result in a wider optical field and for a wider field, for which a change in
the waveguide width is relatively smaller. The overall variation in over the whole contour
plot is only 50%. The maximum tolerance is nm, which is still smaller than the
typical variation in the fabrication process of 200 nm.
Figure 4.13b shows the tolerance of the etch
depth into the film layer. The tolerance is
better than the typical variation in the etch depth in the fabrication process ¨ of nm. The
highest tolerance occurs for waveguides with a relatively low confinement in the film layer.
Figure 4.13c shows that the tolerance is less than 1%, which
of the film layer thickness
is lower than the deviation in of 5% that can be expected during fabrication, including the
wet-etch step that defines the amplifiers and the passive waveguides (Chapter 5). The offset in
the layer thickness of the epitaxy is 2%, with a non uniformity in the inner 40 mm of the wafer
of only 1%. The tolerance of the film layer thickness is highest for small , because in that
case the field expends and a relatively small part is confined in the film layer.
Figure 4.13d shows that the tolerance of the composition of the film layer is larger than
nm, which is much larger than the typical variation in the fabricated film layer composition
¨
£¥¤ of nm, as determined by photoluminescence measurements.
It should be emphasized that the relative wavelength control between two wavelengths in the
1550 nm range is much better than the absolute control, because the variation in effective index