considered for the access network with video overlay (refer to Chap. 2) [34, 35].
The return loss of APCs can approach 65 dB. While general physical contact
(PC) connectors have a flat fiber end face, an APC is designed to have a fiber end
face at a standardized angle of 8 degrees as shown in Fig. 3.28, so that any
reflected light is directed into the fiber cladding. The usage of APC connectors
with very low return loss is an advantage in newly deployed PONs, where not all
of the distribution fibers are connected to ONUs. An otherwise unterminated PC
connector would result in Fresnel reflections as high as 14 dB, necessitating a
proper terminator for each unused connector [34].
3.3.2 Fiber for Indoor Installations Bend-Insensitive Single Mode Optical Fiber
Optical fiber cables with bending losses lower than conventional optical fibers
are highly suited for indoor fiber installation due to increased bending loss
tolerance. Conventional standard single mode optical fiber requires a bending
radius of at least 30 mm during installation to meet the standard specification of
less than 0.25 dB/10 turns. In [32], a bend-insensitive silica-based fiber was
developed where studies showed that early adoption and deployment of the
bent-resistant fiber can achieve cost levels similar to existing fiber cables.
Figure 3.28 Cross-sectional view of angled physical contact (APC) connector. The 8-degree
fiber end face directs reflected light into the cladding. (From Ref [34])
112 Optical Technologies in Passive Optical Access Networks
The fiber was also backward-compatible with existing single mode fibers
(SMFs). Figure 3.29 (a) charts the average bending loss in dB measured for 10
turns as a function of bend radius. As expected, the smaller the bending radius
the higher the bending loss. As discussed previously, a bending radius of 30 mm
or more is required to achieve the standard bend loss of 0.25 dB/10 turns for
conventional standard cable; but the bend-insensitive fiber is shown to exhibit
improved bend characteristics that meet the specifications with a bending radius
of only 15 mm. Figure 3.29 (b) compares the standard fiber and the bend-
insensitive fiber with the latter showing a cleaner installation with bent sections
that are less prone to accidental snagging by customers [32].
Bend-insensitive optical fibers are now commercially available specifically for
indoor FTTH installation. For example, the bend-insensitive fiber manufactured
by StockerYale, Inc. is an SMF with a moderately higher numerical aperture
than conventional SMF to achieve an allowable bending radius of 10 mm
at the specific wavelengths of 1550 nm, 1310 nm, and 780 nm [36]. The bend-
insensitive SMF, PureAccess-Ultra, manufactured by Sumimoto Electric has an
allowable bending radius of 7 mm. An additional feature of this fiber is that it is
also a low-water-peak fiber that offers excellent attenuation stability across 1310–
1626 nm. Using hydrogen aging, the water-peak absorption at around 1383 nm is
substantially reduced, as shown in Fig. 3.30 (a), which plots the attenuation
properties of the fiber across the range of wavelengths from 1225 nm to
1675 nm [37]. Figure 3.30 (b) compares the wavelength dependence of bending
losses at 7.5 mm radius between the conventional SMF, PureAccess-Ultra, and its
predecessor PureAccess (allowable bending loss of 15 mm) fibers, with the
PureAccess-Ultra showing the least dependency [37].
Relationship between bend radius (mm) and
average loss (10 turns at 1.55 µm) (dB)
Bend loss (dB)
(a) (b)
Bend radius (mm)
( =50)
= 30 mm
= 15 mm
0.25 dB
Improved cable
Minima Maxima
Figure 3.29 (a) Bend loss characteristics of bend resistant fiber; (b) comparison of conventional
optical fiber with minimum bending radius of 30 mm and bend-resistant with minimum bending
radius of 15 mm. (From Ref [32])
PON Technologies for Indoor Installation 113 Hole-Assisted Fiber-Based Optical Curl Cord
Another type of indoor fiber that has been developed specifically for FTTH
usage is the optical curl cord [33, 37–40], which is shown in Fig. 3.31. The cord
can be stretched several times in length and retracted like a telephone cord
without affecting the optical properties [39]. This enables flexible and reliable
Attenuation (dB/km)
Bending loss (dB/turn)
7.5-mm radius
Conventional SM fiber
1,200 1,300
Wavelength (nm)
th (nm)
1,310 1,383 1,625
1,400 1,500 1,600 1,700
1,200 1,300 1,400 1,500 1,600 1,700
O-band E-band
Low-water peak and bend-insensitive
C-band U-band
Figure 3.30 (a) Typical spectral attenuation of PureAccess-Ultra; (b) bending loss properties as
a function of wavelength. (From Ref [37])
114 Optical Technologies in Passive Optical Access Networks

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