356
8 Digital Optical Fiber Links
the system response speed significantly are the transmitter rise time, the material
dispersion of the fiber, the modal dispersion of the fiber, and the receiver rise time.
The basic link power budget and the system rise-time analysis assume that the
optical power falling on the photodetector is a clearly defined function of time within
the statistical nature of the quantum detection process. However, in an actual link
various signal impairments can reduce the power of the optical signal arriving at the
receiver from the ideal case, which is known as a power penalty for that effect. The
main power penalties are due to dispersions, modal or speckle noise, mode-partition
noise, the extinction ratio, wavelength chirp, timing jitter, optical reflection noise,
and nonlinear effects. Modal noise is present only in multimode links, but all the
other effects can be serious in single- mode links.
To control errors and to improve the reliability of a communication line, first it is
necessary to be able to detect the errors and then either to correct them or retransmit
the information. Error detection methods encode the information stream to have a
specific pattern. Error correction may be done by the use of redundancy in the data
stream. With this method, extra bits are introduced into the raw data stream at the
transmitter on a regular and logical basis and are extracted at the receiver. These
digits themselves convey no information but allow the receiver to detect and correct
a certain percentage of errors in the information-bearing bits. This method is called
forward error correction (FEC).
In a basic optical receiver the light intensity of the optical source is modulated
linearly with respect to the input electrical signal voltage. This method is known as
intensity modulation with direct detection (IM/DD). Optical communication systems
that use homodyne or heterodyne detection instead of IMDD are called coherent
optical communication systems, because their implementation depends on phase
coherence of the optical carrier. In coherent detection techniques the light is treated
as a carrier medium that can be amplitude-, frequency-, or phase-modulated similar
to the methods used in microwave radio systems. Coherent detection techniques
enable a higher spectral efficiency and greater tolerance to dispersion effects than
direct detection methods.
Problems
8.1 A 3B4B code converts blocks of 3 bits to blocks of 4 bits according to the rules
given in Table 8.6. When there are two or more consecutive blocks of three
zeros, the coded binary blocks 0010 and 1101 are used alternately. Similarly,
the coded blocks 1011 and 0100 are used alternately for consecutive blocks
of three ones.
(a) Using these translation rules, find the coded bit stream for the data input
010001111111101000000001111110.
(b) What is the maximum number of consecutive identical bits in the coded
pattern?
8.2 A 4B5B code has 2
4
= 16 four-bit data characters. The code maps these
characters into the 5-bit sequences listed in Table 8.7. Using this information,
encode the following bit stream: 010111010010111010100111.
8 Digital Optical Fiber Links
the system response speed significantly are the transmitter rise time, the material
dispersion of the fiber, the modal dispersion of the fiber, and the receiver rise time.
The basic link power budget and the system rise-time analysis assume that the
optical power falling on the photodetector is a clearly defined function of time within
the statistical nature of the quantum detection process. However, in an actual link
various signal impairments can reduce the power of the optical signal arriving at the
receiver from the ideal case, which is known as a power penalty for that effect. The
main power penalties are due to dispersions, modal or speckle noise, mode-partition
noise, the extinction ratio, wavelength chirp, timing jitter, optical reflection noise,
and nonlinear effects. Modal noise is present only in multimode links, but all the
other effects can be serious in single- mode links.
To control errors and to improve the reliability of a communication line, first it is
necessary to be able to detect the errors and then either to correct them or retransmit
the information. Error detection methods encode the information stream to have a
specific pattern. Error correction may be done by the use of redundancy in the data
stream. With this method, extra bits are introduced into the raw data stream at the
transmitter on a regular and logical basis and are extracted at the receiver. These
digits themselves convey no information but allow the receiver to detect and correct
a certain percentage of errors in the information-bearing bits. This method is called
forward error correction (FEC).
In a basic optical receiver the light intensity of the optical source is modulated
linearly with respect to the input electrical signal voltage. This method is known as
intensity modulation with direct detection (IM/DD). Optical communication systems
that use homodyne or heterodyne detection instead of IMDD are called coherent
optical communication systems, because their implementation depends on phase
coherence of the optical carrier. In coherent detection techniques the light is treated
as a carrier medium that can be amplitude-, frequency-, or phase-modulated similar
to the methods used in microwave radio systems. Coherent detection techniques
enable a higher spectral efficiency and greater tolerance to dispersion effects than
direct detection methods.
Problems
8.1 A 3B4B code converts blocks of 3 bits to blocks of 4 bits according to the rules
given in Table 8.6. When there are two or more consecutive blocks of three
zeros, the coded binary blocks 0010 and 1101 are used alternately. Similarly,
the coded blocks 1011 and 0100 are used alternately for consecutive blocks
of three ones.
(a) Using these translation rules, find the coded bit stream for the data input
010001111111101000000001111110.
(b) What is the maximum number of consecutive identical bits in the coded
pattern?
8.2 A 4B5B code has 2
4
= 16 four-bit data characters. The code maps these
characters into the 5-bit sequences listed in Table 8.7. Using this information,
encode the following bit stream: 010111010010111010100111.