First, because the 1550nm band is particularly low in “good quality” quartz fiber, it is “small” when it is as low as a few kilometers. Therefore, as long as the wire is transmitted at a long distance, of course, the resistance is low. In this case, why is the laser not preferred 1550? This is the so-called fiber-optic communication window, so in order to play dota2, lol, and wzry, it is necessary to have a higher and faster network. It is necessary to pull a fiber to the home. No one wants the signal of the server to pass to the home and has been attenuated. Gone.
Second, because the erbium ions doped in quartz have exactly two energy levels in the energy level structure, the photon energy is 1550 nm, so the laser, the gain particle level must be with you. The photon energy matches. In this way, the stimulated radiation and the number of particles can be reversed to 1550, isn’t it?
Third, the more powerful point of this cesium ion in quartz is that the sub-levels of his splits will widen because of a bunch of various atomic physics knowledge, and overlap each other to become energy bands. This is awesome. The energy level difference of the transition is not only corresponding to the 1550. Even the other wavelengths of light with a distance of more than a dozen nanometers are also given together. This wide gain bandwidth is also the multimode oscillation of the cavity. Premise, multimode oscillation is the premise of mode locking, and mode locking is the premise of ultrashort pulse laser. In short, erbium-doped fibers can be used as pulsed lasers. Other ions that do not have energy level broadening or widened are difficult to use as pulsed lasers.
In addition, it is also related to loss. Water loss is unavoidable in glass fiber. Even if all methods are used to remove OH in the preform, it will eventually appear. The loss of OH is very large. After 1600, basically infrared. Absorption is significantly enhanced, so 1550 is the lowest loss window for glass fibers, with fiber impurity loss being the lowest. The precondition for our use of fiber optics is that the Nobel Prize paper says that if the loss is low enough, we can use fiber, and several communication windows are near the lowest loss. Second, Er doped fiber, the energy level is relatively clean, multi phonon absoption and other transitions are also small, SMF is very easy to do, or NL can ruin all after the power is high.
In summary, the erbium-doped fiber-optic mobile phone is the best choice for 1550 pulse lasers!