Page 225 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 219

the semiconductor with an index of refraction of about 3.5 and the op-
tical fiber having an index of refraction of 1.45. In a typical SOA, the
coupling losses are about 2 dB. In the case of an Er-doped fiber, the
losses due to coupling are much less than 1 dB. With further engi-
neering, the coupling loss in SOAs may be further reduced
The second important factor is the result of the excited state life-
time in a semiconductor amplifier. The lifetime of an electron in an
excited state in the conduction band is several nanoseconds. This is
three orders of magnitude less than the excited state lifetime for Er in
glass. The shorter lifetime makes it much harder to reach 100% popu-
lation inversion. The rate of spontaneous recombination will be much
higher in an SOA compared to an Er-glass amplifier. The higher rate
of spontaneous recombination leads to an increase in the ASE noise.
The ASE noise is not increased by 1000 times, fortunately, but there
is a difference of about 1 to 2 dB more ASE noise for the SOA com-
pared to the Er-doped amplifier.
The third important factor is the output power. The gain of Er-
doped fiber amplifiers is comparable to the gain of a SOA, i.e., about
30 dB. In an SOA, this level of gain is achievable only for relatively
low input power, on the order of –20 to –30 dBm, that is, 10 to 1 mi-
crowatts. When the input power is larger, the gain falls off. Typical
output power from a SOA is limited to 15 mW at the present time.
With further engineering, this figure may improve. However, an Er-
doped fiber amplifier can deliver much higher absolute levels of pow-
er. This enables cascading of Er-doped fiber amplifiers for boosting
power in transmission applications.
The short lifetime of the excited state of electrons in a SOA increas-
es the level of ASE noise, as presented above. On the other hand there
is an advantage associated with this situation, and that is that the
gain in an SOA can be switched on and off rapidly. This can be done
electrically by modulating the electrical pumping current. However, it
can also be accomplished optically by coupling an additional optical
beam into the SOA at a different wavelength from the signal and
thereby reducing the gain by depleting the excited-state carrier densi-
ty. This kind of high-speed modulation of the gain is a way to modu-
late one light beam by another. The SOA has a significant advantage
over the Er-doped fiber amplifier because of this functionality. It is an
important element in the implementation of all-optical signal process-
ing, such as switching, wavelength conversion, and all-optical signal
regeneration. Because of its small size, SOA chips are starting to be
incorporated into other optical devices such as filters and modulators
so that there is no net loss in signal power. Of course, there will al-
ways be a degradation in the SNR that accompanies the use of a SOA
as an amplifier.

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