Page 121 - Optical Communications Essentials

Photodiodes and Receivers

Photodiodes and Receivers 111

1.0
90% InGaAs
Quantum
0.8 efficiencies
70%
Responsivity (A/W) 0.6 Si 50% Ge

0.4

0.2 30%
10%
0
0.7 0.9 1.1 1.3 1.5 1.7
Wavelength ( m)
Figure 7.3. Comparison of the responsivity and quantum
efficiency as a function of wavelength for pin photodiodes
constructed of different materials.

Example Photons of energy hν 1.53 10 19 J are incident on a photodiode which
has a responsivity of 0.65A/W. If the optical power level is 10 µW, then from Eq. (7.3)
the photocurrent generated is
I = R P = (065 A/W)(10 µ W) = 6 5 µ A
.
.
0
p
In most photodiodes the quantum efficiency is independent of the power level
falling on the detector at a given photon energy. Thus the responsivity is a lin-
ear function of the optical power. That is, the photocurrent I p is directly pro-
portional to the optical power P 0 incident on the photodetector. This means that
the responsivity R is constant at a given wavelength. Note, however, that the
quantum efficiency is not a constant at all wavelengths, since it varies accord-
ing to the photon energy. Consequently, the responsivity is a function of the
wavelength and of the photodiode material. For a given material, as the wave-
length of the incident photon becomes longer, the photon energy becomes less
than that required to excite an electron from the valence band to the conduc-
tion band. The responsivity thus falls off rapidly beyond the cutoff wavelength,
as can be seen in Fig. 7.3.

7.1.5. Speed of response
Photodiodes need to have a fast response speed in order to properly interpret
high data rate signals. If the detector output does not track closely the variations
of the incoming optical pulse shape, then the shape of the output pulse will be
distorted. This will affect the link performance since it may introduce errors in
interpreting the optical signal. The detector response speed is measured in

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