322
8 Digital Optical Fiber Links
8.2 Concepts of Link Power Penalties
The analysis in Sect. 8.1 assumed that the optical power falling on the photodetector
is a clearly defined function of time within the statistical nature of the quantum
detection process. In reality, a number of signal impairments that are inherent in
optical fiber transmission systems can degrade the link performance.
When any signal impairments are present in a link, a lower optical power level
arrives at the receiver compared to the ideal reception case. This lower power results
in a reduced signal-to-noise ratio of the link compared to the case when there are no
impairments. Because a reduced SNR leads to a higher BER, a higher signal power
is required at the receiver in order to maintain the same BER as in the ideal case. The
ratio of the reduced received signal power to the ideal received power is known as
the power penalty for that effect and generally is expressed in decibels. If P ideal and
P impair are the received optical powers for the ideal and impaired cases, respectively,
then the power penalty PP x in decibels for impairment condition x is given by
P P x = −10 log
P impair
P ideal
(8.18)
In some cases one can increase the optical power level at the receiver to reduce the
power penalty. For other situations, for example for some nonlinear effects described
in Chap. 12, increasing the power level will have no effect on the power penalty. The
main power penalties are due to chromatic and polarization-mode dispersions, modal
or speckle noise, mode-partition noise, the extinction ratio, wavelength chirp, timing
jitter, optical reflection noise, and nonlinear effects that arise when there is a high
optical power level in a fiber link. Modal noise is present only in multimode links,
but all the other effects can be serious in single-mode links. This section addresses
these performance impairments except nonlinear effects, which Chap. 12 describes.
Additional power penalties due to optical amplifiers and WDM-channel crosstalk
are given in Chaps. 11 and 12, respectively.
8.2.1 Power Penalties from Chromatic Dispersion
Chromatic dispersion originates from the fact that each unique wavelength travels at
a slightly different velocity in a fiber. Thus distinct simultaneously launched wavelengths arrive at different times at the fiber end. Therefore, the range of arrival times
at the fiber end of a spectrum of wavelengths will lead to temporal spreading of
a pulse. As noted in Sect. 3.3, chromatic dispersion is a fixed quantity at a specific
wavelength and is measured in units of ps/(nm km). Figure 3.18 shows the chromatic
dispersion behavior as a function of wavelength for several different standard singlemode fiber types. For example, a G.652 fiber typically has a chromatic dispersion
8 Digital Optical Fiber Links
8.2 Concepts of Link Power Penalties
The analysis in Sect. 8.1 assumed that the optical power falling on the photodetector
is a clearly defined function of time within the statistical nature of the quantum
detection process. In reality, a number of signal impairments that are inherent in
optical fiber transmission systems can degrade the link performance.
When any signal impairments are present in a link, a lower optical power level
arrives at the receiver compared to the ideal reception case. This lower power results
in a reduced signal-to-noise ratio of the link compared to the case when there are no
impairments. Because a reduced SNR leads to a higher BER, a higher signal power
is required at the receiver in order to maintain the same BER as in the ideal case. The
ratio of the reduced received signal power to the ideal received power is known as
the power penalty for that effect and generally is expressed in decibels. If P ideal and
P impair are the received optical powers for the ideal and impaired cases, respectively,
then the power penalty PP x in decibels for impairment condition x is given by
P P x = −10 log
P impair
P ideal
(8.18)
In some cases one can increase the optical power level at the receiver to reduce the
power penalty. For other situations, for example for some nonlinear effects described
in Chap. 12, increasing the power level will have no effect on the power penalty. The
main power penalties are due to chromatic and polarization-mode dispersions, modal
or speckle noise, mode-partition noise, the extinction ratio, wavelength chirp, timing
jitter, optical reflection noise, and nonlinear effects that arise when there is a high
optical power level in a fiber link. Modal noise is present only in multimode links,
but all the other effects can be serious in single-mode links. This section addresses
these performance impairments except nonlinear effects, which Chap. 12 describes.
Additional power penalties due to optical amplifiers and WDM-channel crosstalk
are given in Chaps. 11 and 12, respectively.
8.2.1 Power Penalties from Chromatic Dispersion
Chromatic dispersion originates from the fact that each unique wavelength travels at
a slightly different velocity in a fiber. Thus distinct simultaneously launched wavelengths arrive at different times at the fiber end. Therefore, the range of arrival times
at the fiber end of a spectrum of wavelengths will lead to temporal spreading of
a pulse. As noted in Sect. 3.3, chromatic dispersion is a fixed quantity at a specific
wavelength and is measured in units of ps/(nm km). Figure 3.18 shows the chromatic
dispersion behavior as a function of wavelength for several different standard singlemode fiber types. For example, a G.652 fiber typically has a chromatic dispersion