Page 252 - Optical Communications Essentials

Performance Measures

242 Chapter Fourteen

If the laser has a high RIN value so that its associated noise dominates over
the other noise terms, then the carrier-to-noise ratio becomes

1 / 2 (mM) 2
CNR limit 3 (14.17)
RIN(B e )
which is a constant. In this case the performance cannot be improved unless the
modulation index is increased.
As an example of the limiting conditions, consider a link with a laser trans-
mitter and a pin photodiode receiver having the following characteristics:

Transmitter Receiver
m 0.5 R 0.6A/W
RIN 143dB/Hz B 10MHz
P 0dBm R eq 750Ω
F t 3dB

where P is the optical power coupled into the fiber. To see the effects of the dif-
ferent noise terms on the carrier-to-noise ratio, Fig. 14.6 shows a plot of CNR
as a function of the optical power level at the receiver. Note that in this plot the

Figure 14.6. CNR as a function of optical power level at the receiver. In this case,
RIN dominates at high powers, shot noise gives a 1-dB drop in CNR for each
1-dB power decrease at intermediate levels, and receiver thermal noise yields a
2-dB CNR rolloff per 1-dB drop in received power at low light levels.

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