InP-based polarisation independent wavelength demultiplexers

4.3 Loss reduction 71
On-chip loss [dB]
0
-10
-20
-30
-40
1510 1515 1520 1525 1530
Wavelength [nm]
4.2.4 Conclusions
In this section the method for obtaining polarisation independence by using nonbirefringent
waveguides has been presented. Two waveguide structures were analysed: the embedded
square waveguide and the raised-strip waveguide. The first has the advantage that it is
inherently polarisation independent, due to the identical lateral and transverse index contrast.
For low-contrast waveguides, however, the bending radius becomes unpractically large,
whereas the size of high-contrast waveguides gets into the submicron range, which puts
stringent demands on the lithography. Additionally, an expitaxial regrowth is needed. The
raised-strip waveguides at the other hand only need one single etching step for the production.
Furthermore, they can be made relatively wide and thick, while maintaining their monomode
nature. Also small bending radii can be used (even with a small GaAs-fraction in the guiding
layer) due to the high lateral index contrast. Both structures have low fibre-chip coupling losses
due to the circular mode profile, which gives a good match to the fibre mode. The raised-strip
waveguide, however, has a higher potential for application because compact devices can be
produced with a relatively simple manufacturing process.
4.3 Loss reduction
(a) (b)
A significant part of the phased-array loss occurs at the junction between the array waveguides
and the FPR. At this junction the field in the waveguide section shows a very deep modulation
(figure 2.10a, solid line) caused by the trenches between the array waveguides. Due to the
ripple in the field pattern, a considerable fraction of the power is diffracted into adjacent
orders. As the coupling efficiency is found from the overlap of the sum field of the array
waveguides and the far field (dashed line), filling the zeros between the individual waveguide
modes (figure 2.10b, solid line) allows for an increase of the coupling efficiency. This can be
obtained by the insertion of a shallowly etched transition region (TR) between the deeply
etched array waveguides and the FPR [33]. This method has successfully been applied earlier
to waveguide bends in order to reduce the bending and scattering losses [102].
On-chip loss [dB]
0
-10
-20
-30
-40
1480 1482 1511 1513
Wavelength [nm]
1544 1546
Figure 4.10 Measured wavelength response of all channels (a), and of a single
channel for different orders of operation (b), both for TE (solid lines) and TM
polarisation (dashed lines).