inherent chip material loss, and connector loss. The targeted areas for improve-
ment of insertion loss in PLC-based splitters have been in reducing connector
losses, and improving fiber array and splitting ratio nonuniformity [1]. The
connector loss can be improved from 0.5 dB to 0.15 dB through using high-
quality ferrules and an excellent polishing method. With advances in manufac-
turing processes of the fiber array block and PLC chip, insertion losses from
fiber array nonuniformity and splitting-ratio nonuniformity can be reduced from
0.7 dB to 0.4 dB, and 1.8 dB to 1.0 dB, respectively [2, 4]. Collectively, the excess
insertion losses of PLC-based splitters are currently 1–1.5 dB above the ideal
theoretical splitting loss with a nonuniformity within 2 dB over the specified
range of operating wavelengths from 1250 nm to 1625 nm.
3.2.2 Arrayed Waveguide Gratings
3.2.2.1 Introduction
As briefly discussed in Sect. 3.1, the WDM technology has been considered as
one of the most graceful upgrade paths beyond power-splitting PONs to support
more users at higher bandwidths. For an upgrade in the outside plant, the power
splitter in the remote node of a power-splitting PON is replaced with an AWG.
In a 1N configuration, an AWG serves as a wavelength router or a demulti-
plexer because a composite WDM signal launched into the input port is separ-
ated into individual channels by the device [7–10]. Due to reciprocity, the AWG
can be used as a multiplexer in the reverse direction by upstream signals in a
WDM-PON. AWGs based on the silica-on-silicon technology are most com-
monly used for their low propagation loss (<0.05 dB/cm) and high fiber coup-
ling efficiency (losses in the order of 0.1 dB). AWGs based on the InP
semiconductor technology are selected when small footprints and integration
with other functions on a single chip are required, but at the expense of relatively
high adjacent wavelength crosstalk (e.g. 4-channel AWG with a footprint of
230 330 mm
2
and 12 dB crosstalk [11]).
The schematic layout of a 1N AWG is shown in Fig. 3.3 [7]. The free-
propagating regions or slabs, act as a lens whilst the waveguide array acts as a
grating. The difference in length between two neighboring waveguide arms is
constant. The beam of an incoming WDM signal, consisting of multiple channels
of different wavelengths at a constant channel spacing, will become divergent in
the first free-propagating region and will be directed into each waveguide. In
each of the waveguides, the WDM signal experiences a different phase shift
because of the different lengths of waveguides. The net result is that each
wavelength channel will be focused on a specific output port after propagating
through the second free-propagating region, thus forming a demultiplexer.
Optical Technologies in Passive Outside Plant 93
However, the requirements of an AWG used in a PON differ from that of a
conventional WDM system. Aside from very low crosstalk level between adja-
cent channels to provide high channel isolation, an AWG that is located in the
remote note of a PON must be completely passive for cost reduction, exhibit
low-loss for extended reach, and offer reliable operation with stable character-
istics over a wide range of temperatures.
An important parameter in the AWG is the free spectral range (FSR) which
defines the wavelength periodicity of fixed width [12]. In turn, a cyclic AWG is
one in which wavelengths that emerge at a particular output port is spaced by an
integer of the FSR [12]. This cyclic property enables the downstream and
upstream wavelengths assigned to a particular ONU to enter and exit through
the same AWG port. The cyclic property also plays an important role in an NN
AWG which is used in WDM-PON schemes that facilitate colorless ONUs and
wavelength reuse (Sect. 3.4 and Chap. 2, Sect. 2.10), in addition to WDM-PON
protection schemes (refer to Chap. 6). An NN AWG has N inputs and N outputs,
and differs from the 1N configuration by incorporating an additional N1
waveguides at the input (Fig. 3.4). The way in which a signal propagates through
an NN AWG at each input waveguide is identical to that described in the
preceding paragraph for the 1N configuration. Figure 3.5 illustrates the cyclic
property of an NN AWG with N ¼ 5 [12]. In the example, signals on five different
wavelengths are incident on each of the five input ports. The wavelengths,
l
1
,l
2
, ...l
5
, incident on input Port 1 are distributed amongst output Ports b, a,
e, d, and c, respectively. The wavelengths, l
1
,l
2
, ...l
5
, if incident on input Port 2
will however be cyclically rotated and distributed amongst output Ports a, e, d, c,
Input
waveguide
Free-propagation regions
2
1
3
Output
waveguides
Waveguide array
Figure 3.3 Schematic illustration of arrayed waveguide grating comprising two free-
propagating regions and a waveguide array. (From Ref [7])
94 Optical Technologies in Passive Optical Access Networks

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