Optical amplifier

Optical Amplifier

Optical amplifier is a subsystem product that can amplify optical signals in optical fiber communication systems. The principle of the optical amplifier is basically based on the stimulated radiation of the laser, which realizes the amplification effect by converting the energy of the pump light into the energy of the signal light.

Optical amplifiers have profoundly changed the status quo of the optical fiber communications industry since their commercialization in the 1990s. Prior to this, the amplification of the transmission signal was to achieve photoelectric conversion and electro-optical conversion, that is, O/E/O conversion. With the optical amplifier, the optical signal can be amplified directly. The successful development and industrialization of optical amplifiers is a very important achievement in optical fiber communication technology, which has greatly promoted the development of optical multiplexing technology, optical soliton communication and all-optical networks.

There are two main types of optical amplifiers, semiconductor amplifiers and optical fiber amplifiers. Semiconductor amplifiers are divided into resonant type and traveling wave type; fiber amplifiers are divided into rare earth-doped fiber amplifiers and nonlinear optical amplifiers. Non-linear optical amplifiers are divided into Raman (SRA) and Brillouin (SBA) fiber amplifiers.

Semiconductor optical amplifier

Generally refers to a traveling wave optical amplifier, and its working principle is similar to that of a semiconductor laser. Its working bandwidth is very wide, but the gain range is slightly smaller, making it more difficult to manufacture. Although this optical amplifier has been practical, the output is small.

Optical fiber amplifier

It is to dope rare earth ions (such as erbium, praseodymium, thulium, etc.) in the optical fiber as laser active materials. The gain bandwidth of each dopant is different. The gain band of erbium-doped fiber amplifier is wide, covering the S, C, and L bands; the gain band of thulium-doped fiber amplifier is S-band; the gain band of praseodymium-doped fiber amplifier is around 1310nm.

The composition of Erbium-doped Optical Fiber Amplifier (EDFA) basically includes erbium-doped fiber, pump laser, and optical combiner. Based on different applications, erbium-doped fiber amplifiers have developed many different structures.

The amplification principle of EDFA is similar to the principle of laser generation. The energy difference between the meta-stable state and ground state of the rare earth element Er(3+) doped in the optical fiber is equivalent to the energy of a 1550nm photon.

When the pump light energy of the appropriate wavelength (980nm or 1480nm) is absorbed, the electrons will transition from the ground state to the exciting state with a higher energy level, and then release a small amount of energy to transfer to a more stable metastable state. When the light source is sufficient, the electrons of erbium ions will undergo population reverse, that is, the number of high-level metastable state electrons is greater than the number of low-level ground state electrons. When a proper light signal passes through, the metastable state electrons will be affected. The excitation radiation effect emits a large number of photons of the same wavelength, but because of the vibration energy level, the wavelength is not single but a range, with a typical value of 1530~1570nm.

Raman optical amplifier

The Raman optical amplifier is an optical amplifier made by using the Raman scattering effect, that is, after a high-power laser is injected into the fiber, the nonlinear effect of Raman scattering will occur. In the process of continuous scattering, the energy is transferred to the signal light, so that the signal light is amplified. Therefore, it is not difficult to understand that Raman amplification is a distributed amplification process, that is, gradually amplification along the entire line. Its working bandwidth can be said to be very wide, almost unlimited. This optical amplifier has begun to be commercialized, but it is quite expensive.