Page 188 - Optical Communications Essentials

Optical Amplifiers

178 Chapter Eleven

where P in and P out are the input and output powers, respectively, of the optical
signal being amplified. The gain generally is measured under small-signal con-
ditions (with P out 0dBm) and is expressed in decibel units as
G dB P out dBm P in dBm (11.2)

where the input and output powers are given in dBm units.

11.3. Semiconductor Optical Amplifiers
Semiconductor optical amplifiers (SOAs) are based on the same technology as
laser diodes. In fact, an SOA is essentially an InGaAsP laser that is operating
below its threshold point. The attractiveness of this is that SOAs can operate in
every fiber wavelength band extending from 1280nm in the O-band to 1650nm
in the U-band. Furthermore, since they are based on standard semiconductor
technology, they can be integrated easily on the same substrate as other optical
devices and circuits (e.g., couplers, optical isolators, and receiver circuits).
Compared to a DFA, the SOA consumes less power, is constructed with fewer
components, and can be housed compactly in a standard 14-pin butterfly pack-
age. SOAs have a more rapid gain response (on the order of 1 to 100ps), which
enables them to be used for switching and signal processing. Additional appli-
cations include optical signal conversion from one wavelength to another with-
out having the signal enter the electrical domain.
Despite these advantages, the limitation of an SOA is that its rapid carrier
response causes the gain at a particular wavelength to fluctuate with the signal
rate for bit rates up to several gigabits per second. Since the gain at other wave-
lengths also fluctuates, this gives rise to crosstalk effects when a broad spec-
trum of wavelengths must be amplified. As a result, the SOA is not highly
suitable for WDM applications.

11.3.1. SOA construction
Figure 11.3 shows a simple diagram of an SOA. Indium phosphide (InP) is used
in an SOA for the substrate, and the active material consists of InGaAsP layers.
Analogous to the construction of a laser diode, the gain wavelength can be
selected between approximately 1100 and 1700nm by varying the composition
of the active InGaAsP material.
As mentioned earlier, an SOA is essentially a semiconductor laser, but with-
out feedback from its input and output ports. Because of this feature it also is
called a traveling-wave (TW) amplifier. This means that in contrast to a laser
where the optical signal makes many passes through the lasing cavity, in the
SOA the significant difference is that the optical signal travels through the device
only once. During this passage the signal gains energy and emerges intensified
at the other end of the device. Thus, since an SOA does not have the optical
feedback mechanism that is necessary for lasing to take place, it can boost
incoming signal levels but cannot generate a coherent optical output by itself.

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