Page 250 - Optical Communications Essentials

Performance Measures

240 Chapter Fourteen

Figure 14.5. Biasing conditions of a laser diode and its response to analog signal modulation.

Here P t is optical power level at the bias current point, and the modulation
index m is given by

Ppeak
m (14.11)
P t
where P peak is defined in Fig. 14.5. Typical values of m for analog applications
range from 0.25 to 0.50.
In an analog system the main noise sources are due to thermal noise, shot
noise in the photodetector, and relative intensity noise (RIN) from the laser. RIN
arises from random intensity fluctuations within a semiconductor laser, which
produce optical intensity noise. These fluctuations could arise from temperature
variations or from spontaneous emission contained in the laser output. RIN is
measured in decibels per hertz with typical values ranging from 135 to
150dB/Hz. Further details are given below.

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