FTTx PON Networks: Fiber and Related Issues

Optical fiber is a key component of the passive optical network (PON), be it a point-to-point (P2P) or a point-to-multipoint (P2MP) architecture. The general critical issues related to fiber can be divided into two categories:

• Optical
  • Absorption/attenuation
  • Chromatic dispersion and polarization mode dispersion (CD and PMD)
  • Non-linear effects
• Mechanical
  • Bending
  • Connection/joint

Optical Issues

The fiber of choice for PONs is the singlemode dispersion-unshifted fiber, based on ITU-T Recommendation G.652 or IEC 60793-2-50 Ed 2. Figure 1 illustrates the typical geometry of this fiber type.

Figure 1. Typical geometry of singlemode dispersion-unshifted fiber used in a PON

Two types of G.652 fiber are available for PONs, depending on whether or not the 1383 nm water peak is present. It is important to note that there are no chromatic dispersion (CD) and polarization mode dispersion (PMD) impairments for PONs (CD may become an issue only when FP lasers are used at very high bit rate).

However, non-linear effects (NLEs) can become a problem in PONs using high-power video signals at 1550 nm. These NLEs are due to changes in the fiber’s dielectric properties (index of refraction) or stimulated scattering when using a very high electric field; i.e., the higher the optical power levels (at constant surface area), the higher the electric field. Nonetheless, when a certain input intensity is reached, the output intensity will follow a non-linear curve, thus no longer increasing linearly. This particular input intensity is called the non-linearity threshold. The field intensity for a given power level is greater with a smaller fiber core, and longer fiber lengths decrease the threshold for NLE.

The table below shows how NLEs affect PONs. The most detrimental effect comes from the Brillouin stimulated scattering, which is due to the very high power used in overlaid analog video transmission. Digital IP video should dramatically decrease this effect.

SPM = Self-phase modulation
XPM = Cross-phase modulation
4WM = Four-wave mixing
BB = Broadband

Mechanical Issues

As for mechanical aspects, they too can potentially affect the performance of a PON, The following is a list of common mechanical issues to be aware of:

• Bending
  • macrobending
  • microbending
• Discontinuities/gaps/voids
• Misalignments/mismatches
• Cracks/angular or straight breaks
• Dirt in connections
• Fiber melt/fusion splicing

Macrobending (or simply bending) refers to excessive fiber curvature that causes loss of light (see Figure 2). When the fiber is bent too much, the angle of total internal reflection between the fiber core and the cladding is no longer met; and since the angle of total internal reflection is what ensures the effective propagation of light inside the core, macrobending can be a major issue as it will reduce the optical energy of the signal. Longer wavelengths (such as 1625 nm and 1650 nm) are more sensitive to macrobending than shorter wavelengths are.

Figure 2. Macrobending in optical fiber

Microbending, on the other hand, is a microscopic bend or bump in the fiber core (shown in Figure 3) and is a possible cause of polarization mode dispersion (PMD) in optical fiber.

Other causes of disruptions include discontinuities, gaps, voids, misalignments, mismatches, angular faults, cracks and dirt, and they typically occur during the connection of two fibers.

Figure 2. Microbending in optical fiber

All of the above cases can affect the signal traveling through an optical fiber and they mostly occur when human intervention is required (e.g., when joining a fiber).

In addition, if extremely high optical power is used localized laser heating may occur and potentially generate a thermal shockwave back to source. At ~1 m/s the wave produce  effects balanced between thermal diffusion and light absorption: thermal lensing caused by the periodic bubbling effect in the fused region (as shown in Figure 3.29). Local fiber fusion has been observed at power levels from 0.5 to 3 MW/cm².

Fiber Cable Types Used in P2MP PONs

There are three types of cable used in the P2MP PON:

(a)        Feeder Cable (from the CO to the splitter distribution hub)

The feeder cable usually has a loose-tube (all-dielectric) design and is the recommended cable type for most PON applications (armor cable is recommended if the cable is directly buried). Figure 3.29 illustrates a possible feeder cable design.

The ANSI/ICEA2 S-87-640-1999, Fiber-Optic Outside Plant Com­munications Cable standard defines the requirements for a general-purpose outdoor cable for both the feeder and the distribution cables.

(b)        Distribution Cable (from the splitter to the drop enclosures)
The distribution cable typically follows the specifications defined for the feeder cable, even though it supports fewer fibers per cable. The distribution cable maybe a ribbon fiber type.

(c)        Drop Cable (from the drop enclosures to the premises)
TIA Sub-Committee FO-4.2 Optical Fibers and Cables, has designated ICEA S-110-717-2003, Optical Drop Cables standard as the drop cable specification suitable for FTTP (TIA 472F000).

For more on FTTx PON technology, please refer to the FTTx PON Technology and Testing book or our pocket-sized FTTx PON Reference Guide.