8.1 Basic Optical Fiber Links
321
repeaterless transmission distance L pin , use Eq. (8.2) with a combined connector loss
plus system margin of 3 dB, so that
L pin = (P S − P R − 3 dB)/α = (0 dBm − 8 log B + 28 dB − 3 dB)/α
= (−8 log B + 25)/0.2
= −40 log B + 125
(b) Similarly, from the receiver sensitivity curves shown in Fig. 7.11, for the
InGaAs APD the receiver sensitivity can be approximated by the straight-line equation P R = 5 log B − 38 dBm, where B is the data rate in Gb/s. Again, use Eq. (8.2) with
a combined connector loss plus system margin of 3 dB, so that the attenuation-limited
repeaterless transmission distance L APD when using an APD is
L APD = (P S − P R − 3 dB)/α = (0 dBm − 5 log B + 38 dB − 3 dB)/α
= (−5 log B + 35)/0.2 = −25 log B + 175
The results for the attenuation-limited repeaterless transmission distances L pin
and L APD are plotted in Fig. 8.8.
1
1
10
100
10
100
1000
Data rate (Gb/s)
Distance (km)
L APD = - 25 log B + 175
L pin = - 40 log B + 125
Fig. 8.8 Transmission-distance limits as a function of data rate for 1550-nm laser diodes with 0
dBm fiber-coupled power, InGaAs pin and avalanche photodiodes, and a single-mode fiber with a
0.2-dB/km attenuation