How does semiconductor optical amplifier achieve amplification?

How does semiconductor optical amplifier achieve amplification?

After the advent of the era of large-capacity optical fiber communication, optical amplification technology has developed rapidly. Optical amplifiers amplify input optical signals based on stimulated radiation or stimulated scattering. According to the working principle, optical amplifiers can be divided into semiconductor optical amplifiers (SOA) and optical fiber amplifiers. Among them, semiconductor optical amplifiers are widely used in optical communication by virtue of the advantages of wide gain band, good integration and wide wavelength range. They are composed of active and passive regions, and the active region is the gain region. When the light signal passes through the active region, it causes the electrons to lose energy and return to the ground state in the form of photons, which have the same wavelength as the light signal, thus amplifying the light signal. The semiconductor optical amplifier converts the semiconductor carrier into the reverse particle by the driving current, amplifies the injected seed light amplitude, and maintains the basic physical characteristics of the injected seed light such as polarization, line width and frequency. With the increase of the working current, the output optical power also increases in a certain functional relationship.

But this growth is not without limits, because semiconductor optical amplifiers have a gain saturation phenomenon. The phenomenon shows that when the input optical power is constant, the gain increases with the increase of the injected carrier concentration, but when the injected carrier concentration is too large, the gain will saturate or even decrease. When the concentration of the injected carrier is constant, the output power increases with the increase of the input power, but when the input optical power is too large, the carrier consumption rate caused by excited radiation is too large, resulting in gain saturation or decline. The reason for the gain saturation phenomenon is the interaction between electrons and photons in the active region material. Whether the photons generated in the gain medium or the external photons, the rate at which the stimulated radiation consumes the carriers is related to the rate at which the carriers replenish to the corresponding energy level in time. In addition to the stimulated radiation, the carrier rate consumed by other factors also changes, which adversely affects gain saturation.

Since the most important function of semiconductor optical amplifiers is linear amplification, mainly to achieve amplification, it can be used as power amplifiers, line amplifiers and preamplifiers in communication systems. At the transmitting end, the semiconductor optical amplifier is used as a power amplifier to enhance the output power at the transmitting end of the system, which can greatly increase the relay distance of the system trunk. In the transmission line, the semiconductor optical amplifier can be used as a linear relay amplifier, so that the transmission regenerative relay distance can be extended again by leaps and bounds. At the receiving end, the semiconductor optical amplifier can be used as a preamplifier, which can greatly improve the sensitivity of the receiver. The gain saturation characteristics of semiconductor optical amplifiers will cause the gain per bit to be related to the previous bit sequence. The pattern effect between small channels can also be called cross-gain modulation effect. This technique uses the statistical average of cross-gain modulation effect between multiple channels and introduces a medium intensity continuous wave in the process to maintain the beam, thus compress the total gain of the amplifier. Then the cross-gain modulation effect between channels is reduced.

Semiconductor optical amplifiers have simple structure, easy integration, and can amplify optical signals of different wavelengths, and are widely used in the integration of various types of lasers. At present, the laser integration technology based on semiconductor optical amplifiers continues to mature, but efforts still need to be made in the following three aspects. One is to reduce the coupling loss with the optical fiber. The main problem of the semiconductor optical amplifier is that the coupling loss with the fiber is large. In order to improve the coupling efficiency, a lens can be added to the coupling system to minimize the reflection loss, improve the symmetry of the beam, and achieve high efficiency coupling. The second is to reduce the polarization sensitivity of semiconductor optical amplifiers. The polarization characteristic mainly refers to the polarization sensitivity of the incident light. If the semiconductor optical amplifier is not specially processed, the effective bandwidth of the gain will be reduced. Quantum well structure can effectively improve the stability of semiconductor optical amplifiers. It is possible to study a simple and superior quantum well structure to reduce the polarization sensitivity of semiconductor optical amplifiers. The third is the optimization of the integrated process. At present, the integration of semiconductor optical amplifiers and lasers is too complicated and cumbersome in technical processing, resulting in a large loss in optical signal transmission and device insertion loss, and the cost is too high. Therefore, we should try to optimize the structure of integrated devices and improve the precision of devices.

In optical communication technology, optical amplification technology is one of the supporting technologies, and semiconductor optical amplifier technology is developing rapidly. At present, the performance of semiconductor optical amplifiers has been greatly improved, especially in the development of new generation optical technologies such as wavelength division multiplexing or optical switching modes. With the development of the information industry, the optical amplification technology suitable for different bands and different applications will be introduced, and the development and research of new technologies will inevitably make the semiconductor optical amplifier technology continue to develop and prosper.