308
8 Digital Optical Fiber Links
binary 1 is mapped into the binary pair 10, and a binary 0 becomes 01. The overhead
for such a code is 50%.
Suitable mBnB codes for high data rates are the 3B4B, 4B5B, 5B6B, and 8B10B
codes. If simplicity of the encoder and decoder circuits is the main criterion, then the
3B4B format is the most convenient code. The 5B6B code is the most advantageous
if bandwidth reduction is the major concern. Various versions of Ethernet use the
3B4B, 4B5B, or 8B10B formats. Section 8.3 discusses more advanced codes for
error detection and correction.
8.1.2 Considerations for Designing Links
In carrying out a link power budget, one approach is to first decide at which wavelength to transmit and then choose components that operate in this region. If the
distance over which the data are to be transmitted is not too far, it may be advantageous to operate in the 770-to-910 nm region or in the T-band. On the other hand, if
the transmission distance is relatively long, it is better to take advantage of the lower
attenuation and lower dispersion that occur in the O-band through U-band region.
Having decided on a wavelength, the next step is to interrelate the system performances of the three major optical link building blocks; that is, the receiver, transmitter, and optical fiber. Normally, the designer chooses the characteristics of two of
these elements and then computes those of the third to see if the system performance
requirements are met. If the components have been overspecified or underspecified,
then iterations in the design may be needed. The procedure followed here is first
to select the photodetector. Then the system designer can choose an optical source
and see how far data can be transmitted over a particular fiber before an amplifier is
needed in the line to boost up the power level of the optical signal.
In choosing a particular photodetector, a main factor is to determine the minimum
optical power that must fall on the photodetector to satisfy the bit-error rate (BER)
requirement at the specified data rate. In making this choice, the designer also needs
to take into account any design cost and complexity constraints. As noted in Chaps. 6
and 7, a pin photodiode receiver is simpler, more stable with changes in temperature,
and less expensive than an avalanche photodiode receiver. In addition, pin photodiode
bias voltages are normally less than 5 V, whereas those of avalanche photodiodes
range from 40 V to several hundred volts. However, the advantages of pin photodiodes
may be overruled by the increased sensitivity of the avalanche photodiode if very
low optical power levels are to be detected.
The system parameters involved in deciding between the use of an LED and
a laser diode are signal dispersion, data rate, transmission distance, and cost. As
shown in Chap. 4, the spectral width of the laser output is much narrower than that
of an LED. This is of importance in the 770-to-910-nm region, where the spectral
width of an LED and the dispersion characteristics of multimode silica fibers limit
the data-rate-distance product to around 150 (Mb/s)·km. For higher values [up to
2500 (Mb/s)·km], a laser must be used at these wavelengths. At wavelengths around
8 Digital Optical Fiber Links
binary 1 is mapped into the binary pair 10, and a binary 0 becomes 01. The overhead
for such a code is 50%.
Suitable mBnB codes for high data rates are the 3B4B, 4B5B, 5B6B, and 8B10B
codes. If simplicity of the encoder and decoder circuits is the main criterion, then the
3B4B format is the most convenient code. The 5B6B code is the most advantageous
if bandwidth reduction is the major concern. Various versions of Ethernet use the
3B4B, 4B5B, or 8B10B formats. Section 8.3 discusses more advanced codes for
error detection and correction.
8.1.2 Considerations for Designing Links
In carrying out a link power budget, one approach is to first decide at which wavelength to transmit and then choose components that operate in this region. If the
distance over which the data are to be transmitted is not too far, it may be advantageous to operate in the 770-to-910 nm region or in the T-band. On the other hand, if
the transmission distance is relatively long, it is better to take advantage of the lower
attenuation and lower dispersion that occur in the O-band through U-band region.
Having decided on a wavelength, the next step is to interrelate the system performances of the three major optical link building blocks; that is, the receiver, transmitter, and optical fiber. Normally, the designer chooses the characteristics of two of
these elements and then computes those of the third to see if the system performance
requirements are met. If the components have been overspecified or underspecified,
then iterations in the design may be needed. The procedure followed here is first
to select the photodetector. Then the system designer can choose an optical source
and see how far data can be transmitted over a particular fiber before an amplifier is
needed in the line to boost up the power level of the optical signal.
In choosing a particular photodetector, a main factor is to determine the minimum
optical power that must fall on the photodetector to satisfy the bit-error rate (BER)
requirement at the specified data rate. In making this choice, the designer also needs
to take into account any design cost and complexity constraints. As noted in Chaps. 6
and 7, a pin photodiode receiver is simpler, more stable with changes in temperature,
and less expensive than an avalanche photodiode receiver. In addition, pin photodiode
bias voltages are normally less than 5 V, whereas those of avalanche photodiodes
range from 40 V to several hundred volts. However, the advantages of pin photodiodes
may be overruled by the increased sensitivity of the avalanche photodiode if very
low optical power levels are to be detected.
The system parameters involved in deciding between the use of an LED and
a laser diode are signal dispersion, data rate, transmission distance, and cost. As
shown in Chap. 4, the spectral width of the laser output is much narrower than that
of an LED. This is of importance in the 770-to-910-nm region, where the spectral
width of an LED and the dispersion characteristics of multimode silica fibers limit
the data-rate-distance product to around 150 (Mb/s)·km. For higher values [up to
2500 (Mb/s)·km], a laser must be used at these wavelengths. At wavelengths around