Page 126 - Fiber Optic Communications Fund

Lasers 107

−1
The internal loss is given in dB/cm. To convert this into cm , consider a length of 1 cm. The loss over a
length of 1 cm is 50 dB,
P out = P exp (− .1cm).
in
int
P out
10 log =−50 dB,
10
P
in
10 log e − int .1cm =− int × 1cm × 10 log e =−50 dB,
10
10
50 −1 −1
−1
int (cm )= cm = 11.51 cm .
4.3429
The distance between mirrors, L = 0.05 cm:
R = R = 0.3,
1
2
1 1
−1
mir (cm )= ln
2 × 0.05 0.3 2
−1
= 24.07 cm ,
and
g = +
int mir
=(11.51 + 24.07) cm −1
−1
= 35.58 cm .

Example 3.4
In a gain medium, under steady-state conditions, the mean power is 20 mW. The area perpendicular to the
2
direction of light propagation is 100 μm . The refraction index of the gain medium is 3.2. Calculate the energy
density.
Solution:
The optical intensity  is power per unit area perpendicular to the direction of light propagation,
P 20 × 10 −3 8 2
 = = = 2 × 10 W/m .
A 100 × 10 −12
The relation between optical intensity and energy density is given by Eq. (3.50),
I = u
uc
= .
n
The energy density is
nI 3.2 × 2 × 10 8 3
u = = = 2.13 J/m .
c 3 × 10 8