Fiber Optical Amplifier is a kind of optical amplifier using optical fiber as gain medium. Typically, the gain medium is fiber doped with rare earth ions, such as erbium (EDFA, Erbium-Doped Fiber Amplifier), neodymium, ytterbium (YDFA), praseodymium and thulium. These active dopants are pumped (provided with energy) by light from a laser, such as a fiber-coupled diode laser; in most cases, the pump light and the amplified signal light travel simultaneously in the fiber core. A typical fiber laser is a Raman amplifier (see figure below).
Figure 1: Schematic diagram of a simple erbium-doped fiber amplifier. Two laser diodes (LDs) provide pump energy to the erbium-doped fiber, which can amplify light at wavelengths around 1550 nm. Two ponytail-style Faraday isolators isolate back-reflected light, thus eliminating its effect on the device.
Initially, fiber amplifiers were mainly used for long-distance fiber-optic communication, in which signal light needs to be periodically amplified. A typical situation is to use an erbium-doped fiber laser, and the power of the signal light in the 1500nm spectral region is moderate. Subsequently, fiber amplifiers were used in other important fields. High power fiber amplifiers are used for laser material processing. This amplifier usually uses ytterbium-doped double-clad fiber, and the spectral region of the signal light is 1030-1100nm. The output optical power can reach several kilowatts.
Due to the small mode area and long fiber length, a high gain of tens of dB can be obtained under the action of the pump light of medium power, that is to say, a high gain efficiency (especially for low power) can be obtained. device). The maximum gain is usually limited by ASE. The fiber has a large surface-to-volume ratio and stable single-mode transmission, so good output power can be achieved, and the output light is a diffraction-limited beam, especially when using double-clad fibers. However, high power fiber amplifiers typically do not have very high gain in the last stage, in part due to power efficiency factors; an amplifier chain is then required so that the preamp provides most of the gain and the last stage gives high power output.
The gain saturation of fiber amplifiers is quite different from that of semiconductor optical amplifiers (SOAs). Due to the small transition cross section and high saturation energy, it can usually reach several tens of mJ in erbium-doped communication fiber amplifiers, and hundreds of mJ in ytterbium-doped amplifiers with large mode areas. Therefore, a lot of energy (sometimes several mJ) can be stored in the fiber amplifier and then extracted by a short pulse. Only when the output pulse energy is higher than the saturation energy, the pulse distortion caused by saturation is serious. If you amplify the laser produced by a mode-locked laser, the saturation gain is the same as amplifying a CW laser at the same power.
These saturation characteristics are very important for fiber optic communications because any intersymbol crosstalk, which occurs in semiconductor optical amplifiers, is avoided.
Fiber amplifiers usually work in the strong saturation region. In this way, the maximum output can be obtained, and the effect of slight changes in the pump light on the signal output optical power will be reduced.
The maximum gain usually depends on the amplified spontaneous emission, not the pump optical power. It manifests itself when the gain exceeds 40dB. High-gain amplifiers also need to eliminate parasitic reflections, which can generate parasitic laser oscillations and even damage the fiber, so optical isolators are usually added at the input and output.
ASE provides a fundamental limit on amplifier noise performance. In low-loss four-level amplifiers, the excess noise can reach the theoretical limit, that is, the noise figure is 3dB at high gain, which is larger than the noise in the usual lossy quasi-three-level gain medium. ASE and excess noise are generally larger in backward pumped lasers.
The pump light source also introduces some noise. These noises directly affect the gain and signal output power, but have no effect when the noise frequency is much larger than the inverse of the upper energy state lifetime. (Laser-active ions are similar to energy storage, reducing the effects of high-frequency power fluctuations.) Changes in pump power also cause temperature changes, which then translate into phase errors.
ASE itself can be used as a superradiant light source with low temporal coherence, which is needed in optical coherent imaging. A superradiant light source is similar to a high gain fiber laser.