Page 121 - Fiber Optic Communications Fund

102 Fiber Optic Communications

where int is the coefficient of internal loss due to scattering and other loss mechanisms in the gain medium.
The gain and attenuation occur simultaneously in the gain medium. So, we have
(z)= (0) exp (g z), (3.34)
net
where g = g − is the coefficient of the net gain.
net int
A laser is an oscillator operating at optical frequencies. Just like an electronic oscillator, the optical oscillator
(laser) has three main components: (i) amplifier, (ii) feedback, and (iii) power supply, as shown in Fig. 3.9.
The atomic system we have discussed before can act as a gain medium and the light is amplified by stimulated
emission. The feedback is provided by placing the gain medium between two mirrors, as shown in Fig. 3.10.
The optical or electrical pumps required to achieve population inversion are the power supply.
Consider the optical wave propagating in the Fabry–Perot (FP) cavity shown in Fig. 3.10. Let (0) be the
optical intensity at A. After passing through the gain medium, the intensity is (0) exp (g L), where L is the
net
length of the cavity. The light wave is reflected by the mirror at B, whose reflectivity is R . This means that
2
the reflected intensity at B is R (0) exp (g L). The reflected field passes through the gain medium again and
2
net
is reflected by the mirror at A with reflectivity R . The optical intensity after a round trip is (see Fig. 3.11)
1
(0)R R exp [2(g − )L]. (3.35)
1 2 int
The condition for laser oscillation is that the optical intensity after one round trip should be the same as the
incident intensity (0). Otherwise, after several round trips, the optical intensity in the cavity would be too

Feedback

Amplifier

Power Supply

Figure 3.9 The structure of an optical oscillator (laser) or electronic oscillator.

R 1 R 2

Gain medium
A B

z = 0 z = L
Figure 3.10 The Fabry–Perot cavity formed by mirrors.