# Semiconductor Optical Amplifiers (SOA)

This is a continuation from the previous tutorial - light-emitting diodes (LEDs).

An amplifier requires an optical gain for stimulated amplification of an optical signal, but it does not need a resonant cavity. Thus, a semiconductor optical amplifier (SOA), also called a semiconductor laser amplifier, can be made by simply eliminating the optical feedback mechanism of a semiconductor laser.

For a solitary SOA as shown in Figure 13-29, the end facets have to be antireflection coated. Meanwhile, no other feedback mechanism, such as a distributed feedback grating, is incorporated into the device structure.

In theory, the output coupling loss of an amplifier is infinitely large so that it has an infinitely high laser threshold and thus never oscillates no matter how hard it is pumped. In practice, there is always some residual optical feedback in an SOA, but it is small enough not to cause laser oscillation in the operating conditions of the amplifier.

Indeed, a semiconductor laser can be used as an amplifier when it is biased below threshold but above transparency. An SOA is normally pumped with current injection. It has to be pumped to reach population inversion in its active region because an optical gain is required for its operation.

The general characteristics of laser amplifiers described in the laser amplifiers tutorial apply to SOAs as well and need not be repeated here.

Similarly to fiber amplifiers, SOAs can also be used as power amplifiers, optical repeaters, or optical preamplifiers in optical communication systems, as shown in Figure 10-18 [refer to the rare-earth ion-doped fiber amplifiers tutorial].

Compared to fiber amplifiers, however, SOAs have certain disadvantages and advantages in these applications because of the differences in physical properties between semiconductors and rare-earth ion-doped fibers.

A few of these differences are apparent.

For any optical signal emitted by a semiconductor laser at any wavelength, it is always possible to design a matching SOA by simply using the same material and the same structures as the laser. This convenience is not always available to fiber amplifiers.

An SOA can be electrically pumped directly, whereas a fiber amplifier needs to be optically pumped.

SOAs are compatible with other semiconductor devices, including semiconductor lasers, waveguides, detectors, and other semiconductor electronic devices. Therefore, they can be directly integrated into optoelectronic circuits on the chip level when they are used as power amplifiers or preamplifiers.

However, when used in a fiber link, an SOA has a relatively large insertion loss. In contrast, fiber amplifiers have physical compatibility with fiber transmission lines and are ideally suited for all-optical repeater applications.

As discussed in the rare-earth ion-doped fiber amplifiers tutorial, fiber amplifiers have the advantage of being polarization insensitive. Because of the polarization-dependent characteristics of a semiconductor waveguide, the optical gain of an SOA normally depends on the polarization of the incoming signal if no special effort has been made to accommodate a polarization-independent design.

Besides these externally apparent differences, there are other significant differences between SOAs and fiber amplifiers due to fundamental differences between semiconductor and fiber gain media.

Among the most important parameters of an optical gain medium are the gain bandwidth $$\Delta\nu_\text{g}$$, the emission cross section $$\sigma_\text{e}$$, or the gain cross section $$\sigma$$ for a semiconductor, and the fluorescence lifetime $$\tau_2$$, or the carrier lifetime $$\tau_\text{s}$$ for a semiconductor.

The gain bandwidth of an SOA, which is on the order of 10-20 THz, is grater than that of a rare-earth ion-doped fiber amplifier, which is on the order of several terahertz. Both are quite broad, however. Therefore, both semiconductor and fiber amplifiers are capable of amplifying optical signals of broad spectral widths, such as those in the form of ultrashort optical pulses, or optical signals of multiple wavelengths, such as those in a wavelength-division multiplexed system.

The other two parameters are very different for SOAs and fiber amplifiers, however.

For an SOA, $$\sigma$$ is in the range of $$1$$ to $$5\times10^{-20}\text{ m}^2$$ and $$\tau_\text{s}$$ is on the order of 500 ps to 5 ns, whereas for a fiber amplifier, $$\sigma_\text{e}$$ is in the range of $$1$$ to $$5\times10^{-25}\text{ m}^2$$ and $$\tau_2$$ is on the order of 100 μs to 10 ms, depending on the particular rare-earth ions.

According to these numbers, an SOA has a gain cross section about five orders of magnitude larger and a gain relaxation time about six orders of magnitude shorter than those of a fiber amplifier.

A large gain cross section means that the gain changes significantly in response to changes in the population inversion. In such a high-gain medium, the optical gain is easily saturated by the presence of stimulated emission because the saturation intensity of a gain medium is inversely proportional to the emission cross section.

For this reason, a large gain cross section is disadvantageous for the application of an SOA as a linear amplifier. It can cause distortion in a given optical signal due to self saturation by the signal, interference between successive bits or symbols in the same channel, and crosstalk among different channels in a multichannel system due to cross saturation.

The short carrier lifetime of an SOA is also a disadvantage in terms of its susceptibility to high-frequency noise. A carrier lifetime of 1 ns allows the gain of the amplifier to respond to noise or fluctuation at frequencies up to 1 GHz from the electrical pump source or from the optical signal. Consequently, an SOA tends to be less linear but noisier than a fiber amplifier.

The next tutorial covers the topic of semiconductor lasers