Optical amplifiers in the field of optical fiber communication
An optical amplifier is a device that amplifies optical signals. In the field of optical fiber communication, it mainly plays the following roles: 1. Enhancing and amplifying optical power. By placing the optical amplifier at the front end of the optical transmitter, the optical power entering the fiber can be increased. 2. Online relay amplification, replacing existing Repeaters in optical fiber communication systems; 3. Preamplification: Before the photodetector at the receiving end, the weak light signal is pre-amplified to enhance the receiving sensitivity.
At present, the Optical amplifiers adopted in Optical fiber communication mainly include the following types: 1. Semiconductor optical amplifier (SOA Optical amplifier)/Semiconductor laser amplifier (SLA Optical amplifier); 2. Rare earth-doped fiber amplifiers, such as bait-doped fiber amplifiers (EDFA Optical amplifier), etc. 3. Nonlinear fiber amplifiers, such as fiber Raman amplifiers, etc. The following is a brief introduction respectively.
1.Semiconductor optical amplifiers: Under different application conditions and with different end face reflectance, semiconductor lasers can produce various types of semiconductor optical amplifiers. If the driving current of the semiconductor laser is lower than its threshold, that is, no laser is generated, at this time, an optical signal is input to one end. As long as the frequency of this optical signal is near the spectral center of the laser, it will be amplified and output from the other end. This kind of semiconductor optical amplifier is called a Fabry-Perrop type optical amplifier (FP-SLA). If the laser is biased above the threshold, the weak single-mode optical signal input from one end, as long as the frequency of this optical signal is within the spectrum of this multimode laser, the optical signal will be amplified and locked to a certain mode. This kind of optical amplifier is called an injaction-locked type amplifier (IL-SLA). If the two ends of a semiconductor laser are mirror-coated or evaporated with a layer of anti-reflection film, making its emissivity very small and unable to form a Fabry-Perrow resonant cavity, when the optical signal passes through the active waveguide layer, it will be amplified while traveling. Therefore, this type of optical amplifier is called a traveling wave type optical amplifier (TW-SLA), and its structure is shown in the following figure. Because the bandwidth of the traveling wave type optical amplifier is three orders of magnitude larger than that of the Fabry-Perot type amplifier, and its 3dB bandwidth can reach 10THz, it can amplify optical signals of various frequencies and is a highly promising optical amplifier.
2. Bait-doped fiber amplifier: It consists of three parts: The first is a doped fiber with a length ranging from several meters to tens of meters. These impurities are mainly rare earth ions, which form the laser activation material; The second is the laser pump source, which provides energy of appropriate wavelengths to excite the doped rare earth ions in order to achieve the amplification of light. The third is the coupler, which enables the pump light and signal light to couple into the doped optical fiber activating material. The working principle of a fiber amplifier is very similar to that of a solid-state laser. It causes a reversed particle number distribution state within the laser-activated material and generates stimulated radiation. To create a stable particle number inversion distribution state, more than two energy levels should be involved in the optical transition, typically three-level and four-level systems, with a continuous supply of energy from a pump source. In order to provide energy effectively, the wavelength of the pump photon should be shorter than that of the laser photon, that is, the energy of the pump photon should be greater than that of the laser photon. Furthermore, the resonant cavity forms a positive feedback, and thus a laser amplifier can be formed.
3. Nonlinear fiber amplifiers: Both nonlinear fiber amplifiers and erbium fiber amplifiers fall under the category of fiber amplifiers. However, the former utilizes the nonlinear effect of quartz fibers, while the latter employs erbium-doped quartz fibers to act on active media. Ordinary quartz optical fibers will generate strong nonlinear effects under the action of strong pump light of appropriate wavelengths, such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and four-wave mixing effects. When the signal is transmitted along the optical fiber along with the pump light, the signal light can be amplified. Thus, they form fiber Raman amplifiers (FRA), Brillouin amplifiers (FBA), and parametric amplifiers, all of which are distributed fiber amplifiers.
Summary: The common development direction of all optical amplifiers is high gain, high output power, and low noise figure.
Post time: May-08-2025