Page 122 - Optical Communications Essentials

Photodiodes and Receivers

112 Chapter Seven

Figure 7.4. Photodetector response to an optical pulse show-
ing the 10 to 90 percent rise time and the 90 to 10 percent fall
time.

terms of the time it takes the output signal to rise from 10 percent to 90 percent
of its peak value when the input to the photodiode is turned on instantaneously.
This is shown in Fig. 7.4 and is known as the 10- to 90-percent rise time.
Similarly, the time it takes the output to drop from its 90 percent to its 10 per-
cent value is known as the fall time.
The rise and fall times depend on factors such as how much of the light is
absorbed at a specific wavelength, the width of the intrinsic region, various
photodiode and electronics capacitance values, and various detector and elec-
tronic resistances. As a result, the rise and fall times are not necessarily equal
in a receiver. For example, large capacitance values can cause a long decay tail
to appear in the falling edge of the output pulse, thereby creating long fall
times.

7.1.6. Bandwidth
The response speeds of the photodiode and the electronic components result in
a gradual drop in the output level beyond a certain frequency. The point at
which the output has dropped to 50 percent of its low-frequency value is called
the 3-dB point. At this point only one-half as much signal power is getting
through the detector compared to lower frequencies. The 3-dB point defines the
receiver bandwidth (sometimes referred to as the 3-dB bandwidth), which is the
range of frequencies that a receiver can reproduce in a signal. If the rise and fall
times are equal, the 3-dB bandwidth (in megahertz) can be estimated from the
rise time by the relationship

Bandwidth, MHz = 350 (7.4)
rise time, ns

where the rise time is expressed in units of nanoseconds.

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