320
8 Digital Optical Fiber Links
Greater transmission distances are possible when a laser diode is used in conjunction with an avalanche photodiode. Consider an AlGaAs laser emitting at 850 nm
with a spectral width of 1 nm that couples 0 dBm (1 mW) into a fiber flylead. The
receiver uses an APD with a sensitivity depicted in Fig. 8.4. The fiber is the same
as described above in this section. In this case, the material-dispersion-limit curve
lies off the graph to the right of the modal-dispersion-limit curve, and the attenuation limit (with an 8-dB system margin) is as shown in Fig. 8.7. The achievable
transmission distances now include those indicated by the shaded area.
8.1.6 Attenuation Limits for SMF Links
For single-mode links there is no modal dispersion. In this case, in addition to
attenuation factors, the repeaterless transmission distance is limited by dispersion
arising from the source spectral width, from polarization mode dispersion, and from
nonlinear effects in the fiber. This section examines the limits that signal attenuation imposes on repeaterless transmission distances. The transmission limits due to
chromatic dispersion and polarization-mode dispersion are described in Sect. 8.2.
Here it is assumed that the optical power launched into the fiber is no more than
0 dBm (1 mW), so that there are negligible nonlinear effects on the optical signals.
Discussions on signal distortion arising from the nonlinear effects resulting from
high optical powers in fibers are given in Chap. 12.
Example 8.5 To illustrate the attenuation-limited repeaterless transmission limits,
consider two single-mode links operating at 1550 nm based on using the pin and APD
receivers described for Fig. 7.11. The component and performance characteristics
for the two links are taken to be as follows:
1. The optical source is a DFB laser that has a fiber-coupled output of 0 dBm at
1550 nm;
2. At 1550 nm the single-mode fiber has a 0.20-dB/km attenuation;
3. Consider the receiver to have a load resistor R L = 200 and let the temperature
be 300 °K;
4. The performances of the two links are measured at a 10
−12 BER, so that a value
of Q = 7 is needed;
5. The InGaAs pin and APD photodiodes each have a responsivity of 0.95 A/W.
The gain of the APD is M = 10 and the noise figure F(M) = 5 dB.
What are the attenuation-limited repeaterless transmission distances?
Solution (a) From the receiver sensitivity curves shown in Fig. 7.11, it can be
deduced that for an InGaAs pin photodiode operating at 1550 nm with a 10
−12
BER, the receiver sensitivity can be approximated by the straight-line equation P R
= 8 log B − 28 dBm, where B is the data rate in Gb/s. To find the attenuation-limited
8 Digital Optical Fiber Links
Greater transmission distances are possible when a laser diode is used in conjunction with an avalanche photodiode. Consider an AlGaAs laser emitting at 850 nm
with a spectral width of 1 nm that couples 0 dBm (1 mW) into a fiber flylead. The
receiver uses an APD with a sensitivity depicted in Fig. 8.4. The fiber is the same
as described above in this section. In this case, the material-dispersion-limit curve
lies off the graph to the right of the modal-dispersion-limit curve, and the attenuation limit (with an 8-dB system margin) is as shown in Fig. 8.7. The achievable
transmission distances now include those indicated by the shaded area.
8.1.6 Attenuation Limits for SMF Links
For single-mode links there is no modal dispersion. In this case, in addition to
attenuation factors, the repeaterless transmission distance is limited by dispersion
arising from the source spectral width, from polarization mode dispersion, and from
nonlinear effects in the fiber. This section examines the limits that signal attenuation imposes on repeaterless transmission distances. The transmission limits due to
chromatic dispersion and polarization-mode dispersion are described in Sect. 8.2.
Here it is assumed that the optical power launched into the fiber is no more than
0 dBm (1 mW), so that there are negligible nonlinear effects on the optical signals.
Discussions on signal distortion arising from the nonlinear effects resulting from
high optical powers in fibers are given in Chap. 12.
Example 8.5 To illustrate the attenuation-limited repeaterless transmission limits,
consider two single-mode links operating at 1550 nm based on using the pin and APD
receivers described for Fig. 7.11. The component and performance characteristics
for the two links are taken to be as follows:
1. The optical source is a DFB laser that has a fiber-coupled output of 0 dBm at
1550 nm;
2. At 1550 nm the single-mode fiber has a 0.20-dB/km attenuation;
3. Consider the receiver to have a load resistor R L = 200 and let the temperature
be 300 °K;
4. The performances of the two links are measured at a 10
−12 BER, so that a value
of Q = 7 is needed;
5. The InGaAs pin and APD photodiodes each have a responsivity of 0.95 A/W.
The gain of the APD is M = 10 and the noise figure F(M) = 5 dB.
What are the attenuation-limited repeaterless transmission distances?
Solution (a) From the receiver sensitivity curves shown in Fig. 7.11, it can be
deduced that for an InGaAs pin photodiode operating at 1550 nm with a 10
−12
BER, the receiver sensitivity can be approximated by the straight-line equation P R
= 8 log B − 28 dBm, where B is the data rate in Gb/s. To find the attenuation-limited