What is a semiconductor optical amplifier

What is a semiconductor optical amplifier

A semiconductor optical amplifier is a type of optical amplifier that uses a semiconductor gain medium. It is similar to a laser diode, in which the mirror at the lower end is replaced with a semi-reflective coating. The signal light is transmitted through a semiconductor single-mode waveguide. The transverse dimension of the waveguide is 1-2 micrometers and its length is on the order of 0.5-2mm. The waveguide mode has a significant overlap with the active (amplification) region, which is pumped by the current. The injected current generates a certain carrier concentration in the conduction band, allowing the optical transition of the conduction band to the valence band. The peak gain occurs when the photon energy is slightly greater than the bandgap energy. SOA optical amplifier is typically used in telecommunication systems in the form of pigtails, with an operating wavelength around 1300nm or 1500nm, providing approximately 30dB of gain.

The SOA semiconductor optical amplifier is a PN junction device with a strain quantum well structure. The external forward bias reverses the number of dielectric particles. After the external excitation light enters, stimulated radiation is generated, achieving the amplification of optical signals. All the above three energy transfer processes exist in SOA optical amplifier. The amplification of optical signals is based on stimulated emission. The stimulated absorption and stimulated emission processes exist simultaneously. The stimulated absorption of the pump light can be utilized to accelerate the recovery of carriers, and at the same time, the electric pump can send electrons to a high energy level (conduction band). When spontaneous radiation is amplified, it will form amplified spontaneous radiation noise. ‍SOA optical amplifier is based on semiconductor chips.

Semiconductor chips are composed of compound semiconductors, such as GaAs/AlGaAs, InP/AlGaAs, InP/InGaAsP and InP/InAlGaAs, etc. These are also the materials for making semiconductor lasers. The waveguide design of SOA is the same as or similar to that of lasers. The difference lies in that lasers need to form a resonant cavity around the gain medium to generate and maintain the oscillation of the optical signal. The optical signal will be amplified multiple times in the cavity before being output. In SOA amplifier(what we are discussing here is limited to the traveling wave amplifiers used in most applications), light only needs to pass through the gain medium once, and the backward reflection is minimal. The SOA amplifier structure consists of three areas: Area P, Area I (active layer or node), and Area N. The active layer is usually composed of quantum Wells, which can improve the photoelectric conversion efficiency and reduce the threshold current.

Figure 1 Fiber laser with integrated SOA for generating optical pulses

Applied to channel transfer

SOAs are usually not only applied to amplification: they can also be used in the field of optical fiber communication, applications based on nonlinear processes such as saturation gain or cross-phase polarization, which utilize the variation of carrier concentration in SOA optical amplifier to obtain varying refractive indices. These effects can be applied to channel transfer (wavelength conversion), modulation format conversion, clock recovery, signal regeneration and pattern recognition, etc. in wavelength division multiplexing systems.

With the advancement of optoelectronic integrated circuit technology and the reduction of manufacturing costs, the application fields of SOA semiconductor optical amplifier as basic amplifiers, functional optical devices and subsystem components will continue to expand.