Passive Optical Networks

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

3.3.2.1 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].

10.00

1.00

Relationship between bend radius (mm) and

average loss (10 turns at 1.55 µm) (dB)

0.10

0.01

0.00

10

Bend loss (dB)

15

(a) (b)

Bend radius (mm)

20

( =50)

= 30 mm

(existing)

= 15 mm

0.25 dB

25

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

3.3.2.2 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

0.8

0.7

0.6

0.5

0.4

Attenuation (dB/km)

Bending loss (dB/turn)

0.3

0.2

0.1

3.0

2.5

7.5-mm radius

Conventional SM fiber

(G.652)

PureAccess

PureAccess-Ultra

2.0

1.5

1.0

0.5

0.0

1,200 1,300

Wavelength (nm)

Wavelen

g

th (nm)

1,310 1,383 1,625

1,400 1,500 1,600 1,700

1,6501,260

1,200 1,300 1,400 1,500 1,600 1,700

1,6501,550

O-band E-band

S-band

Low-water peak and bend-insensitive

PureAccess-Ultra

L-band

C-band U-band

(a)

(b)

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