Page 217 - Fiber Optic Communications Fund

198 Fiber Optic Communications

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

0.4
30%

0.2
10%

0
0.7 0.9 1.1 1.3 1.5 1.7
Wavelength (μm)

Figure 5.7 Schematic representation of responsivity vs. wavelength for three common absorption materials and various
QEs from 10% to 90%.

Example 5.2
In a GaAs photodiode, if the quantum efficiency = 0.9, band-gap energy E = 1.42 eV, and operating
g
(free-space) wavelength = 1.1 μm, calculate (a) the responsivity R and (b) the cutoff wavelength .
co
Solution:
(a) From Eq. (5.18), we have
(μm)
0
R = A/W
1.24
0.9 × 1.1
= A/W = 0.9A/W.
1.24
(b) The cutoff wavelength is given by Eq. (5.4),
1.2
= μm
co
E (eV)
g
1.2
= μm = 0.873 μm.
1.42