# Verdet Constant of Magneto-Active Materials Developed for High-Power Faraday Devices
## 4 Conclusions  
In this review, we deal with the development of magneto-active materials for Faraday-effect-based magneto-optical devices for high-power lasers. The major task to be solved in the area of high-power FDs is the compensation and reduction of the absorption-induced thermal effects which seriously deteriorate the performance of these devices. In the Section 1.3, we mention several methods on how to mitigate the thermal effects:  
* Optical layout for compensation of the thermally-induced polarization and wavefront distortions
* Enhancement of the applied magnetic field
* Design of optimal geometry and cooling of the magneto-optical elements
* Development of new magneto-active materials \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & \(F\) [rad/(Tm)] & \(\lambda_{0}\) [nm] & \(|V|\) **at 1550 nm** & \(|V|\) **at 1940 nm** & **Reference** & **Additional References** \\ \hline YIG & - & - & 304 & - & [158] & [65; 152; 153; 154; 155; 156; 157] \\ EuF\({}_{2}\) & 231.5 & 436 & 17.6 & 12.3 & [159] & [42] \\ (Dy\({}_{0.9}\)Y\({}_{0.05}\)La\({}_{0.05}\))\({}_{2}\)O\({}_{3}\) & - & - & 13.8 & [129] & [130] \\ Dy\({}_{2}\)O\({}_{3}\) & - \({}^{\circ}\) & - & 28.9 & 19.2 & [48] & - \\ CeF\({}_{3}\) & 260.8 & 375.1 & 16.2 & \(\sim\)8 & [46] & [60; 61; 134; 135] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: A brief overview of magneto-active materials for the NIR-MIR FDs (\(\lambda>\) 1100 nm). The Verdet constant dispersion \(V(\lambda)\) is given by the single-transition model in Equation (13) with the specified \((F,\lambda_{0})\)-parameters; the \(G\)-parameter is equal to 0 for all materials. The listed \(V\)-values are the absolute values of the Verdet constant, specified in rad\(/(\text{Tm})\). For the YIG crystal, the saturated Verdet constant is specified in rad\(/\text{Tm}\). \({}^{\circ}\) A function different from the model in Equation (13) for the Verdet constant dispersion may be found in the listed reference. The desirable properties of a magneto-active material for high-power FD are: possession of high Verdet constant and transparency, high thermal conductivity and low value of thermo-optical constants \(Q\) and \(P\) (evaluating the thermally induced polarization and wavefront distortions). In the following Section 2, we describe an advanced method for characterization of the Verdet constant dependence on wavelength and temperature. The presented experimental setup is an easily-adjustable and robust tool for the measurement of the Faraday rotation angle induced in the magneto-active material. The Verdet constant is calculated based on the knowledge of the magnetic field and the material sample parameters. Due to the utilization of a broadband source, the wavelength dependence of the Verdet constant may be studied at a higher level of detail. For the purpose of this analysis, a general model function for describing the Verdet constant as a function of wavelength and temperature is introduced. In the final part Section 3, we review the room-temperature investigations of the Verdet constant for several magneto-active materials developed for the high-power FDs. The overview covers several materials, which could be used for the UV, VIS, NIR and MIR wavelengths. For the UV wavelengths, the fluorides CeF\({}_{3}\), PrF\({}_{3}\) and LiREF\({}_{4}\) (RE = Tb, Dy, Ho, Er and Yb) represent promising materials for the high-power FDs: they possess high Verdet constant and local windows of high transparency. Nevertheless, reports on the construction of UV FDs based on these materials are still rare. The by far most intensively studied magneto-active materials for FDs are those for the VIS-NIR region \(400<\lambda<1100\) nm. These studies often aim at achieving the higher value of the Verdet constant as compared with the TGG crystal--an established material for this wavelength range, which, however, has insufficient properties to be employed in the great number of the newly developed high-power laser systems.