InP-based polarisation independent wavelength demultiplexers

1.3 Phased-array demultiplexers 5
The future WDM communication network will consist of OCCs, ADMs and wavelength
routers, as depicted schematically in figure 1.6. By deploying more ADMs, the network can
handle an increasing number of users, which is an important advantage of such a network.
Another advantage is the enhanced reliability: a defect node can be “bypassed” by changing
the connectivity scheme. However, this strategy fails if the node serves a large number of users,
in which case other strategies are required such as e.g. dual homing on two nodes.
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1.3 Phased-array demultiplexers
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Figure 1.6 WDM communication network (A = ADM, R = Router, O = OCC).
It is obvious that wavelength (de)multiplexers are key components in WDM systems. Many
principles have been introduced and reported for the manufacture of multiplexers and
demultiplexers. Commercially available components are based on fibre-optic or micro-optic
techniques [106,107] and, recently, also fully integrated devices. Since the early nineties,
research on integrated-optic (de)multiplexers has increasingly been focused on grating-based
and phased-array (PHASAR) based devices (also called Arrayed-Waveguide Gratings) [165].
Both are imaging devices, i.e. they image the field of an input waveguide onto an array of
output waveguides in a dispersive way. In grating-based devices, a vertically etched reflection
grating provides the focusing and dispersive properties required for demultiplexing. In phasedarray
based devices these properties are provided by an array of waveguides, the length of
which has been carefully chosen to obtain the required imaging and dispersive properties. As
phased-array based devices are manufactured according to conventional waveguide technology
and do not require the vertical etching step needed in grating-based devices, they appear to be
more robust and production tolerant.
Phased-array demultiplexers were introduced in 1988 by Smit [116]. The first devices
operating at short wavelengths were reported by Vellekoop and Smit [161-163,117]. Takahashi
et al. reported the first devices operating in the long wavelength window [151,152]. Dragone
extended the phased-array concept from 1 × N to N ×
N devices - the so-called wavelength
routers [44,45] which play an important role in multiwavelength network applications. The
broad acceptance of the phased-array concept can be seen from the bargraph in figure 1.7. In
this graph the development of the phased-array demultiplexer is depicted by the number of
publications on single devices (including PHASAR-based system experiments, which started
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