Overview of Yb doped fiber

Doped fiber is a kind of special fiber that doped trace rare earth elements (such as erbium, ytterbium, etc.) into the quartz glass matrix of conventional transmission fiber. It is also an active fiber. Therefore, it can be said that doped fiber is a special fiber with active characteristics composed of dopants and host fiber matrix. The purpose of doping rare earth elements is to facilitate the transformation of passive transmission optical fiber into active fiber with amplification capability. It can be seen from this that the new characteristics of this fiber - laser characteristics, optical amplification characteristics, magneto-optical characteristics, etc. are closely related to the type, properties, concentration and distribution of doped rare earth elements (ions).

Advantages of Rare Earth Elements and Yb3+Doped Ion Laser Materials

The fiber doped with rare earth elements can become a laser medium, so it is necessary to know some information about rare earth elements and their ions in order to better understand the performance of doped fiber. Rare earth elements, namely lanthanides in the periodic table. At present, there are a total of 15 species, which occupy the penultimate row in the periodic table. The first is the element lanthanum (La, atomic number 57), and the last is lutetium (Lu, atomic number 71). From the perspective of atomic structure, all rare earth elements have the same outer electron shell structure, i.e. 5S5P6S form, which belongs to the full shell structure. The difference between the lanthanide elements is only the number of electrons in the 4f shell, so it can be said that their optical properties only depend on the number of electrons in the 4f shell. It is known that the ions of rare earth elements mostly appear in trivalent form, and they all escape two 6s electrons and one 4f electron. As the remaining 4f electrons are shielded by the electron shell, some of their optical properties (such as fluorescence characteristics and absorption characteristics) are not easily affected by the external field, that is, they have good stability.

In 1878, Swiss chemists Jean Charles and G. de Marignac discovered a new rare earth element in the "erbium". In memory of the discovery place of yttrium ore, the small village near Stockholm named Yteerby, this new element was named Ytterbium, the element symbol was Yb, and the Chinese translation name was "Ytterbium" - a special Chinese character for this element. Although ytterbium ranks behind thulium in lanthanide elements, its crustal abundance reaches 3.3 ppm, which is not only higher than terbium, holmium, thulium and lutetium, but also higher than europium (2.2 ppm). Ytterbium, as a heavy rare earth element, has limited available resources and expensive products, which limits its application research. With the emergence of high and new technologies such as optical fiber communication and laser, ytterbium has gradually found a stage of application. Yb3+doped silicate glass material has attracted extensive attention of material scientists and engineering physics scientists all over the world, and has become a hotspot and important development direction in the current laser material research. It is considered to be one of the better laser working materials in the new generation of inertial confinement he fusion collar.

The advantages of Yb3+doped laser materials over other rare earth ions are:

 (1) Yb3+ion absorption band can effectively couple with ZnlnAs pump source in the wavelength range of 800~1100nm. At the same time, its absorption band is wide, and the absorption cross section changes slowly in the short wavelength range (less than 970nm), which is very beneficial for laser pumping with output wavelength vulnerable to environmental temperature and narrow emission band, That is, it is not necessary to strictly control the temperature to obtain the laser output with matching wavelength.

(2) Yb3+energy level structure is simple, it only contains two multiple states, so there is no excited state absorption at the pump wavelength and the signal wavelength. The light conversion efficiency is very high, and the large energy level spacing also eliminates the occurrence of non radiative relaxation and concentration quenching.

(3) The pump wavelength is very close to the laser output wavelength, and the quantum efficiency is high (up to 90%).

(4) The heat load in the material is low (less than 11%). It is only one third of the same material doped with Nd3+.

(5) The fluorescence life is long, generally more than three times that of the same Nd3+doped material, which is conducive to energy storage. These advantages of Yb3+doped laser materials have far-reaching significance for the development of laser technology. In traditional solid-state lasers, the gain medium is long rod, and the heat flow direction is perpendicular to the laser beam direction, which easily leads to thermal lens effect and temperature rise, leading to the deterioration of laser performance and the reduction of laser efficiency. Especially for the three-level laser system, the thermal effect is more prominent due to the high pump power required. Because Yb3+doping concentration can be very high, the thermal load in the material is low, and even under high pump power density, the temperature change in the material is small, so the thermal stress and thermal distortion in the gain medium are greatly reduced.