Thermally induced depolarization of radiation is the principal limiting factor of using Faraday devices in laser radiation with a high average power. In this work thermally induced depolarization is ...analyzed in a system of two optical elements made of cubic magneto-optical materials with a negative optical anisotropy parameter separated by a quartz rotator. The parameters of the optical scheme of a Faraday isolator (FI) based on single crystals cut in the C orientation, at which effective compensation of thermally induced depolarization is realized, are found. These parameters are universal and do not depend on the material of the magneto-optical element. Analytical expressions for the integral thermally induced depolarization are derived and the contributions of thermally induced linear birefringence and of the temperature dependence of the Verdet constant are analyzed. The parameter that allows assessing the contributions to depolarization of each of the thermal effects and predicting the efficiency of using the compensation scheme for a specific magneto-optical material is introduced. The proposed FI scheme with compensation is investigated for a number of known magneto-optical materials possessing a negative optical anisotropy parameter. The results obtained will significantly reduce thermally induced polarization distortion of radiation, and will allow developing FIs providing a high isolation ratio in powerful laser radiation.
The expressions for the thermal lens of linearly polarized radiation arising in m3m, 432 and <inline-formula> <tex-math notation="LaTeX">\bar {4}3\text{m} </tex-math></inline-formula> cubic crystals ...with arbitrary crystallographic axes orientation in the presence of circular birefringence <inline-formula> <tex-math notation="LaTeX">\delta _{\mathbf {c}} </tex-math></inline-formula> are analyzed. It is shown that the thermal lens may have astigmatism the magnitude of which is proportional to the power of heat generation. The Euler angles determining the crystallographic axes orientation P are found and the conditions for the material constant - optical anisotropy parameter <inline-formula> <tex-math notation="LaTeX">\xi </tex-math></inline-formula> - under which the thermal lens astigmatism in active and magneto-active optical elements can be eliminated, are formulated. It is demonstrated that the orientation P depends only on optical anisotropy parameter <inline-formula> <tex-math notation="LaTeX">\xi </tex-math></inline-formula> of the crystal and on circular birefringence <inline-formula> <tex-math notation="LaTeX">\delta _{\mathbf {c}} </tex-math></inline-formula>.
Faraday isolators for high average power lasers operating at room temperature are surveyed. Three devices on 001 oriented TGG crystals with the most known optical schemes are considered: traditional ...scheme, and schemes with compensation of thermally induced depolarization inside magnetic field and outside magnetic field. We report a unique 30-mm-aperture Faraday isolator with thermally induced depolarization compensation inside magnetic field. It provides 33.5-dB isolation ratio at 1.5-kW average laser power.
The dependence of thermally induced depolarization on crystallographic axis orientation in crystal materials with negative optical anisotropy parameter in the presence of Faraday rotation has been ...analyzed. The orientation at which thermally induced depolarization is minimal has been found. The contributions to thermally induced depolarization of linear birefringence caused by photoelastic effect, of the temperature dependence of Verdet constant, and of the temperature dependence of optical anisotropy parameter are assessed. Three magneto-optical materials with negative optical anisotropy parameter have been compared with a currently widely used TGG crystal from the viewpoint of their prospective application in Faraday devices for powerful laser radiation. It has been demonstrated that these magneto-optical materials are undoubtedly superior to a TGG crystal.
The magneto‐optical ceramics of terbium oxide doped with different rare earth elements (REE) REE:Tb2O3 (REE—Y3+, Lu3+, Sc3+, Ce3+, Dy3+, Ho3+, Yb3+, Nd3+) are successfully fabricated by hot pressing ...using powders obtained by the self‐propagating high‐temperature synthesis (SHS) method. The prepared REE:Tb2O3 ceramics have good optical quality and magneto‐optical properties. The in‐line transmittance of Tb2O3 ceramics doped with REE except Sc3+ is close to undoped terbium oxide, especially in the long‐wavelength region. The Verdet constant of the REE:Tb2O3 samples considered at 633 nm is about 3.3 and 2.6 times higher in absolute value than that of the commercial terbium–gallium garnet (TGG) single crystal and terbium–aluminum garnet (TAG) ceramics, respectively. It is noted that REE (except Yb3+ and Nd3+) doping only slightly increases the Verdet constant of pure Tb2O3 (when the content of terbium ions in the optical element is taken into consideration in the calculation). The dilution of the Tb2O3 matrix with REE in relatively high concentrations leads to a decrease in the absolute value of the Verdet constant.
Rare earth elements in terbium oxide ceramics increase its Verdet constant, normalized to the concentration of terbium ions. However, the Verdet constant absolute value decreases due to the dilution of the Tb2O3 matrix with rare earth elements. Thus, pure terbium oxide has a record Verdet value among the studied materials.
Magnetoactive materials are of considerable current interest, primarily for applications in nonreciprocal Faraday devices used for polarization control, optical isolation, optical switching, and ...modulation. The need for such devices is growing with laser power enhancement. They reduce risk of self-excitation of the amplifiers and optical elements damage and are a handy tool for organizing multipass schemes. However, at high average power of radiation these devices are subject to thermally induced effects that impair their operability and lead to increased losses and to the formation of phase distortions in the transmitted radiation. One of the methods to reduce thermally induced effects is to use in Faraday devices new magnetoactive materials with better thermo-optical properties. This paper is devoted to the study of thermo-optical and magneto-optical characteristics of a unique magnetoactive material-a terbium scandium aluminum garnet (TSAG) crystal. The TSAG has an extraordinary value of optical anisotropy parameter ξ, a Verdet constant 25% higher than the traditionally used terbium gallium garnet crystal and the highest magneto-optical figure-of-merit known in magnetoactive materials at the moment.
Transparent ceramics are at the heart of modern magneto-optical materials providing promising opportunities for Faraday isolators. 1.0 at% RE:Tb
3
Ga
5
O
12
(rare earth (RE) = Pr, Tm, Dy) transparent ...ceramics were successfully prepared by air sintering and sequential HIP technique using the coprecipitated powders as the raw material. All the powders have shown to be a pure cubic terbium gallium garnet (TGG) phase and exhibit good dispersion. Additionally, a change could not be observed in particle shape with the different doped ions. After the two-step sintering, all the obtained ceramics have good optical quality, and the in-line transmittances at 1070 nm are higher than 80%. Moreover, no secondary phase can be detected from the microstructures. However, the pores which remain entrapped in the ceramics can be noted. The Verdet constant of ceramic samples is optimized by RE doping, and the Verdet constant at 632.8 nm is about −143 rad·T
−1
·m
−1
, which is about 5% higher than that of TGG ceramics. Finally, the thermo-optical properties of 1.0 at% RE:TGG transparent ceramics are compared. The annealed TGG ceramic showed the best thermo-optical properties, and the thermally induced depolarization of 1.0 at% Ce:TGG and 1.0 at% Tm:TGG was inferior to that of annealed TGG ceramic.
A cycle of works on manufacturing and studying laser and magnetooptical ceramics with a focus on their thermo-optical characteristics performed by the research team is analyzed. Original results that ...have not been published before such as measurements of the Verdet constant in the Zr:TAG, Re:MgAl2O4, and ZnAl2O4 ceramics are also presented.
As a promising magneto-optical (MO) material applied in Faraday isolators, magneto-optical ceramics possess excellent comprehensive properties and have attracted much attention these years. Herein, ...we review the fabrication and properties of magneto-optical ceramics including garnet, sesquioxide, and A2B2O7 ceramics. Some of the ceramics have been proved to possess applicable performance, while further studies are still needed for most of the magneto-optical ceramics. Aiming at the application for isolators, the research status, existing problems, and development trends of magneto-optical ceramics are shown and discussed in this review.
Samples of TAG ceramics doped with Ce and Pr ions (0, 0.5, 1, 1.5, 2 at.% concentrations) and with Ho ions (0.5 at.%) were studied. The wavelength dependence of the Verdet constant of each sample in ...the 405 nm - 1064 nm range was measured, and analytical approximation was found. The obtained value of the Verdet constant of the studied TAG ceramics did not depend on the dopant concentration and type in the studied range of concentrations. The dependence of thermally induced depolarization on laser radiation power was investigated, and the magneto - optical figure of merit was estimated for each studied sample. It was demonstrated that doping of ceramics can provide a substantial reduction of the magneto-optical figure of merit. The obtained results will be useful for creating Faraday isolators and rotators based on TAG ceramics.
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