Page 14 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Photodetection Basics

Photodetection Basics 7

. mA mW (1.3)
r ideal = 0 807l mm
Hence, for a fixed incident power, the limiting performance of our detection
systems is usually better at a longer wavelength. At longer wavelengths you
simply have more photons arriving in the measurement period than at shorter
wavelengths. This also explains why optical communication systems seem
always to need microwatts of optical power, while your FM radio receiver
operating at about 100MHz gives a respectable signal-to-noise ratio for a few
femtowatts (say, 1mV in a 75-W antenna). In each joule of radio photons there
are five million times more photons than in a visible optical joule. We will return
to this important point in Chap. 5, when dealing with detection noise.
The quantity r(l) is usually given in the photodetector manufacturers’ liter-
ature. Figure 1.5 shows typical curves for some real photodiodes. The straight
line is the ideal 0.807l mm result for a detector with 100 percent quantum effi-
ciency, and it can be seen that the responsivity of real silicon diodes typically
approaches within about 30 percent of the ideal from about 0.4mm to 1mm. The
ratio of actual responsivity to ideal responsivity is called the quantum efficiency
(h):

r l ()
h = (1.4)
r ideal l ()
Departures from the 0.807l mm unit quantum efficiency straight line for silicon
diodes seen in Fig. 1.5 occur as a result of several effects. The rapid fall-off in
sensitivity at wavelengths above approximately l g ª 1.1mm wavelength for
silicon is caused by the increasing transparency of the silicon crystal at those
wavelengths. Photons with energy less than the material bandgap energy E g

1.2
1.1
InGaAs
1.0 100% Quantum (std.) InGaAs
Responsivity (A/W) 0.8 Efficiency Si Ge (long)
0.9
0.7
0.6
0.5
0.4
0.3
0.2
0.1
GaP GaAsP
0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
Wavelength (mm)
Figure 1.5 Photodiodes of different semiconductor materials show sensitiv-
ity in different wavelength regions, limited at long wavelength by their
energy gap. 100 percent quantum efficiency means that one photon produces
one hole-electron pair.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.