of the central wavelength can be made zero. In [26], the temperature dependence of
the TM and TE mode of the central wavelength of a polarization dependent AWG
with an attached bimetal plate was measured and is shown in Fig. 3.24. With the
bimetal plate, the TM and TE modes had an average temperature dependence of
less than 0.001 nm/8C and 0.004 nm/8C, compared to 0.013 nm/8C for the con-
ventional AWG. A side effect due to the attachment of the bimetal plate to the
silicon substrate of the AWG is the shift in central wavelength due to the initial
stress, which needs to be accounted for in an AWG layout.
Athermal AWGs of up to 40 channels [27–30] and 100/200 GHz channel
spacings are now commercially available [27–31] with good crosstalk perform-
ance (<30 dB), low polarization-dependent loss (<0.4 dB [27]), low insertion
loss (<2.0 dB for Gaussian type passband, <4.5 dB for semi-flat type passband
[28]), and good temperature stability over a wide operating range (<0.02 nm
from 0 8Cto608C [27]).
3.3.1 Field Assembly and Indoor Connectors
In an FTTH environment, the fiber connection between the outdoor drop
fiber and indoor fiber is usually performed via fusion or mechanical splices. For
mechanical splicing, splicing machines and fixtures are used to align and connect
AWG circuit
Bimetal plate
Silicon substrate
Waveguide layer
Figure 3.23 Schematic diagram of AWG with bimetal plate. (From Ref [26])
108 Optical Technologies in Passive Optical Access Networks
optical fibers, and materials such as transparent adhesives (epoxy resin) and
index-matching gels, provide index matching between the connected fibers.
Nonetheless, mechanical splicing does not facilitate easy disconnection and
reconnection, thereby making fiber fault detection and isolation between the
outdoor and indoor fibers difficult. An outer clasp connector comprising of a
field assembly (FA) connector plug and a FA connector socket was proposed in
[32] by NTT Corporation, whereby connection and disconnection can be easily
achieved. Figure 3.25 (a) shows the structure of a FA connector plug and socket,
consisting of connector ferrules, fiber locks, and clasp parts. Only 25 mm of the
leading ends of the outdoor and indoor fibers need to be inserted into both the
plug and the socket to be locked in place. The FA connector socket is also
compatible with SC connectors. The placement of the connectors in the optical
cabinet at the customer premises is shown in Fig. 3.25 (b), where connections
between the outdoor and indoor fibers no longer require the use of specialized
mechanical or fusion splicing equipment.
20 30
Temperature (C)
Center wavelength (nm)
60 70 80
TM mode, with bimetal plate
TE mode, without bimetal plate
TM mode, without bimetal plate
TE mode, with bimetal plate
Figure 3.24 Center wavelength of AWG as a function of temperature. (From Ref [26])
PON Technologies for Indoor Installation 109
Results from connection loss and return loss trials, which charts the distribu-
tion of the loss in each case are shown in Fig. 3.26 (a) and (b) respectively. The
aim is to have as low a connection loss as possible, and as high a return loss as
possible to minimize the reflected optical power at the connector. In connection
loss trials repeated 500 times using the FA connector plug and socket, an average
loss of 0.26 dB was measured. The maximum loss was 0.69 dB, satisfying the
specification of 0.7 dB or lower for 1:3 mm transmission. In return loss trials
taken over 250 measurements, all return losses were measured to be more than
the specified 40 dB, with the average being 48.8 dB. A novel bendable SC
connector shown in Fig. 3.27 was also developed by NTT to be used indoors,
with good results in splice loss, repeated bending, and heat cycle tests [33]. Its
construction enables the connector to be movable between 0 degrees and 90
degrees to the wall socket, allowing increased tolerance to customer handling,
optical wiring workability, safety, and a good appearance.
In terms of reducing return losses which are essential in networks transporting
analog video services, angled physical contact (APC) connectors which are
normally reserved for long haul telecommunication networks are now being
FA connector plug
FA connector plug
FA connector socket
FA connector socket
Internal fiber
Clasp lock clip
Rotation armFiber insertion slot
Indoor optical fiber cable
Drop optical fiber cable
Clasp parts
Clasp parts
Connector ferrule within a fiber
Aluminum element
Figure 3.25 Field assembly (FA) connector: (a) structure of plug and socket; (b) in optical
cabinet at customer premises. (From Refs [32] and [33])
110 Optical Technologies in Passive Optical Access Networks

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