Page 219 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 213

communications at 1550 nm. They were benefiting from semiconduc-
tor laser diodes that emitted hundreds of milliwatts of optical power,
instead of those used by Julian Stone that emitted only hundreds of
microwatts of optical power. They saw that the amplification of light
by the fiber introduced very little additional noise compared to con-
ventional electronic amplification, but the big benefits are that the op-
tical amplifier does not care what the electronic modulation rate is,
and it does not care what the wavelength is (at least over the range of
wavelengths where erbium shows gain) So you can send simultane-
ously different wavelengths and different bit rates through the same
fiber. The combined signals emerge from the erbium-doped fiber am-
plifier with a gain that is significant (typically 30 dB) and independ-
ent of the modulation rate.
The erbium-doped glass amplifier functions like every other laser.
The luminescence spectrum of erbium in glass is shown in Fig. 9.13.
The gain spectrum is very similar to the luminescence spectrum. It is
easy to see that the gain spectrum is not very flat. This creates the
need for gain equalization, which is performed after the light passes
through the amplifier section. The useful part of the gain is centered
in a 30 nm window around the peak, as indicated in Fig. 9.13. The op-
tical gain associated with erbium luminescence can be used to make a
laser source at 1540 nm by putting the gain region in between two
mirrors. The primary interest for optical communication is not to
make a laser source, but rather a laser amplifier. Amplification by

0.8
0.7
0.6
Amplitude (au) 0.4
0.5

0.3
0.2
0.1
0
1400 1450 1500 1550 1600 1650 1700
Wavelength (nm)

Figure 9.13. Luminescence spectrum of erbium-doped glass. The erbium-doped glass is
incorporated in a single-mode optical fiber having the same dimensions as a transmis-
sion fiber. The luminescence leads to laser amplification, the magnitude of which de-
pends on the length of the gain section. An erbium-doped section several meters long
can produce a gain of 30 dB.

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