Page 203 - Introduction to Information Optics

188 3. Communication with Optics

Thus,

p = /j (-)— - = 5.5 mW represents energy of photons/second -> power.
\ej A

Example 3.12. A semiconductor laser has a heterostructure. If the cavity
length L = 0.2 mm and the refractive index n l = 3.5, the spectral gain width of
the material is AA 9 = 5 nm. In addition, n 2 — 3.4 and A = 1.5/mi. Calculate
(a) The number of the longitudinal mode for this laser.
(b) The maximum thickness, d, to maintain the single transversal mode.
(c) If the first-order Bragg reflection happens, what is the required period
of the distributed feedback Bragg grating?

Solve:
(a) The spectral space between the adjacent longitudinal modes is

2
A
A/I, — = 1.6 nm.
s
2n,L
Thus, the number of longitudinal mode N L is

A/L 5 nm
Nr — —p = = 3 modes.
Ax ff 1.6 nm
1.5 um
2
2 v/3.5 - 3.4'

(c) Since m = 1 we have 2« } A = A B. Thus,

2«, 2-3.5

3.3.2. OPTICAL RECEIVERS FOR FIBER-OPTIC COMMUNICATIONS
In fiber-optic communication applications, an optical receiver is a device
that converts input light signals into electronic signals. There are many types
of optical receivers (also called photodetectors). However, the most widely used
ones for fiber-optic communication are semiconductor optical receivers, includ-
ing the PIN photodetector and the Avalanche photodetector.