Passive Optical Networks

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 PON TECHNOLOGIES FOR INDOOR

INSTALLATION

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

ε

ε

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.

1554.6

1554.4

1554.2

1554.0

1553.8

1553.6

20 30

Temperature (C)

5040

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

Cap

Cap

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

(a)

(b)

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