Faraday Devices Research Articles

Magneto-optical and thermo-optical properties of the Ge-Sb-As-S glass

Thermo-optical and magneto-optical properties of high-purity GexAsySbzS100-x-y-z (x = 10–13; y = 20–23; z = 4–6) glasses are studied. Transmission spectra, spectral dependence of the Verdet constant in the 632–1940 nm range, thermally induced depolarization, and thermal lens are investigated. The material parameters responsible for thermally induced depolarization and thermal lens are estimated. The high-purity Ge-As-Sb-S glasses are shown to be promising materials for creating optical elements for Faraday devices operating in high power laser radiation.

Growth and characterization of the KDy3F10 and Na0.38Dy0.62F2.24 fluoride crystals for the Faraday devices

The latest research on magneto-optical materials for Faraday devices has been repeatedly highlighting the benefits of using the crystalline media with the negative value of the optical anisotropy parameter (OAP). The reason is that these media have proven to be very efficient in eliminating the degrading impact of the thermal-stress-induced birefringence which occurs in the Faraday devices that are operated with high average power light. In this paper, we report on the successful growth of OAP-negative fluoride crystals – the KDy3F10 (KDF) and Na0.38Dy0.62F2.24 (NDF) – and on the characterization of the material properties allowing to assess the benefits of using these crystals for the Faraday devices. Unlike the recently commercialized OAP-negative KTb3F10 (KTF) crystal, these crystals exhibit high transparency at the mid-IR wavelengths (>1500 nm), for which there are currently no OAP-negative crystals available. And although the Verdet constant at these wavelengths is not exceptionally high (≤12 rad/Tm in the absolute value) it is sufficient for the practical implementation of a Faraday device based on these crystals using the currently available permanent magnets. In addition, the thermal conductivity of the KDF crystal is advantageously ∼24% higher than that of the state-of-the-art KTF crystal.

Selection of Magneto-Optical Material for a Faraday Isolator Operating in High-Power Laser Radiation

Faraday isolators are the inherent components of complex laser systems. The isolation degree is essentially determined by the effects that occur in its magneto-optical element, so the choice of material from which it is made is very important. The principal approaches to choosing a magneto-optical material for Faraday isolators are addressed. Characteristic features of materials for Faraday devices operating in laser radiation with high average and high peak power are considered. Some trends in magneto-optical ceramics and the advantages and shortcomings of a number of ceramic samples are analyzed. Using the proposed approaches and recommendations will allow to create devices with unique characteristics for any wavelength range for different practical applications.

Faraday effect in CoPt-CaF2 nanogranular films with hard magnetic property for magnetic-field-free faraday devices

Magneto-optical Faraday devices, such as optical isolators, require a structure to apply a magnetic field to the incorporated magneto-optical material. The application of a magnetic field hinders the miniaturization and integration of these devices. Nanogranular films show Faraday rotation angles up to 40 times larger than Bi-YIG in the optical communication band (1,550 nm). Nanogranular films are submicron-to several-micrometer-thick and contribute to the miniaturization and integration of optical devices. Here, we introduce (Co-Pt)–(CaF2) nanogranular films exhibiting a magnetic-field-free magneto-optical Faraday effect, which are Co3Pt alloy that are hard magnets with residual magnetization. These nanogranular films exhibit the Faraday effect without requiring a magnetic field owing to their residual magnetization.

Faraday isolator based on crystalline silicon for 2-µm laser radiation.

The magneto-optical properties of single-crystal silicon were investigated as a function of wavelength and temperature. A bulk free-space traditional Faraday isolator for the radiation with a wavelength of 1940 nm (magnetic field ∼2.8 Т) was implemented. The negative value of the piezo-optical anisotropy ratio of the used material allowed for the development of a Faraday isolator with compensation of thermally induced depolarization without a reciprocal rotator. The potential of single-crystal silicon as a magneto-optical material for Faraday isolators operating at room as well as at cryogenic temperatures in high-power laser radiation was considered. It was shown that single-crystal silicon is highly promising for the development of Faraday devices, including ones for next-generation laser interferometers aimed at detecting gravitational waves.

Spectroscopic and magneto-optical characteristics of erbium doped cerium trifluoride crystal grown by the Bridgman method

In this study, a 2 at% erbium doped cerium trifluoride (Er:CeF3) single crystal was grown using the Bridgman method. The consequent absorption, infrared transmittance, and fluorescence spectra were recorded. The spectroscopic properties of Er:CeF3 crystals were intensively studied based on the Judd–Ofelt (J–O) theory. The Ω2, Ω4, and Ω6 parameters were fitted to be 5.206 × 10−20, 1.805 × 10−20, and 3.06 × 10−20 cm2, respectively. The emission cross-section centered at 1564.6 nm was calculated to be 1.053 × 10−20 cm2, and the corresponding fluorescence lifetime was measured to be 1.675 ms. Additionally, the magneto-optical (MO) performance of the Er:CeF3 crystal was investigated for the first time. The Verdet constants of Er:CeF3 at 532, 632.8, 1064, and 1310 nm are 231.0, 165.4, 43.4, and 24.4 rad·T−1·m−1, respectively, which are significantly higher than those of pure CeF3 and traditional Tb3Ga5O12. The results show that the Er:CeF3 crystal can be applied both to solid-state lasers and Faraday devices.

Compensation for Thermally Induced Depolarization in Magneto-Optical Media Made of Materials With a Negative Optical Anisotropy Parameter

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.

Highly Dy2O3 and Er2O3 doped boron-aluminosilicate glasses for magneto-optical devices operating at 2 µm

Magneto-optical glass materials with large Verdet constants at 2 μm have attracted increasing attention due to the significant advances of novel lasers operating at this wavelength region. In this paper, 13Al2O3-21B2O3-35.4SiO2-(30.6-X)Er2O3-XDy2O3 glasses have been synthesized, and their physical, optical, and magneto-optical properties were studied for making Faraday devices at 2 μm. A Verdet constant of as high as −5.9 rad/(T*m) at 1950 nm was measured with a 13Al2O3-21B2O3-35.4SiO2-30.6Dy2O3 glass. The temperature difference between crystallization and glass transition of 150 °C and the synthesis temperature of below 1500 °С make this glass very promising for making magneto-optical devices for 2 μm applications.

Preparation of (Tb1-xLux)2O3 transparent ceramics by solid solution for magneto-optical application

Novel (Tb1-xLux)2O3 magneto-optical transparent ceramics were prepared through a solid-solution method via vacuum pre-sintering followed by hot isostatic pressing (HIP). The Lu-containing samples were solid-solution phases between Tb2O3 and Lu2O3, which effectively stabilized the phase transition of Tb2O3 during the sintering process. A typical component of (Tb1-xLux)2O3 ceramic with x = 0.5 was selected to study the densification behavior and microstructure evolution in detail. The polished (Tb0.5Lu0.5)2O3 ceramics with a thickness of 3 mm vacuum pre-sintered at 1700 °C under 1.0 × 10−3 Pa combined with HIP post-treatment at 1600 °C showed good magneto-optical property. The Verdet constant measured at 633 nm was -224.33 rad·T−1 m−1, 30% higher than that of Tb3Al5O12 (TAG). However, the in-line transmittance was 73.6% at 633 nm, lower than that of TAG. The (Tb1-xLux)2O3 ceramics will be a promising material for Faraday devices after further improving the optical quality.

Faraday Rotation of Dy2O3, CeF3 and Y3Fe5O12 at the Mid-Infrared Wavelengths

The relatively narrow choice of magneto-active materials that could be used to construct Faraday devices (such as rotators or isolators) for the mid-infrared wavelengths arguably represents a pressing issue that is currently limiting the development of the mid-infrared lasers. Furthermore, the knowledge of the magneto-optical properties of the yet-reported mid-infrared magneto-active materials is usually restricted to a single wavelength only. To address this issue, we have dedicated this work to a comprehensive investigation of the magneto-optical properties of both the emerging (DyO ceramics and CeF crystal) and established (YFeO crystal) mid-infrared magneto-active materials. A broadband radiation source was used in a combination with an advanced polarization-stepping method, enabling an in-depth analysis of the wavelength dependence of the investigated materials’ Faraday rotation. We were able to derive approximate models for the examined dependence, which, as we believe, may be conveniently used for designing the needed mid-infrared Faraday devices for lasers with the emission wavelengths in the 2- spectral region. In the case of YFeO crystal, the derived model may be used as a rough approximation of the material’s saturated Faraday rotation even beyond the 2- wavelengths.

Verdet Constant of Magneto-Active Materials Developed for High-Power Faraday Devices

We review the progress in the investigation of the Verdet constant of new magneto-active materials for the Faraday-effect-based devices used in high-power laser systems. A practical methodology for advanced characterization of the Verdet constant of these materials is presented, providing a useful tool for benchmarking the new materials. The experimental setup used for the characterization is a flexible and robust tool for evaluating the Faraday rotation angle induced in the magneto-active material, from which the Verdet constant is calculated based on the knowledge of the magnetic field and the material sample parameters. A general model for describing the measured Verdet constant data as a function of wavelength and temperature is given. In the final part of this review, we present a brief overview of several magneto-active materials, which have been to-date reported as promising candidates for utilization in the Faraday devices. This overview covers room-temperature investigations of the Verdet constant of several materials, which could be used for the ultraviolet, visible, near-infrared and mid-infrared wavelengths.

Magneto-optic pyrochlore ceramics of Tb2Hf2O7 for Faraday rotator.

In the first demonstration of the Faraday effect in an optical-grade Tb2Hf2O7 (THO) ceramic, its optical properties and the Verdet constant's wavelength dependence were evaluated at room temperature. The transmittance loss of this material was equally low as for the terbium gallium garnet, a well-known magneto-optic material. The Verdet constant at 1064 nm was 50.4 rad/Tm, which was 1.4 times greater than that of the terbium gallium garnet. THO ceramics show great potential for use in Faraday devices in near-infrared laser applications.

Ceramic Magneto-Optical Material

It has been more than 20 years since the 1st Nd:Y3Al5O12 (Nd:YAG) polycrystalline ceramic laser was reported, and nowadays, there are a lot of works for transparent ceramics on scintillators, phosphors, faraday rotators, and optical windows not only for laser gain media. A Faraday rotator is one of the key optics element for the isolation and birefringence compensation of laser amplifiers in high-averagepower laser systems and demands high Verdet constant, size scalability, excellent optical quality, high laser damage threshold, and a high thermal conductivity. We report the optical properties, Faraday effect and Verdet constant of ceramic TGG and TAG. Large diameter ceramic TGG (Tb3Ga5O12) have been used as Faraday devices for high-average laser systems. Advanced ceramic TAG (Tb3Al5O12) is a promising Faraday material for high-average-power lasers system.

Fabrication of magneto-optical waveguides inside transparent silica xerogels containing ferrimagnetic Fe3O4 nanoparticles.

Magneto-optical waveguides with a refractive-index change were successfully fabricated inside silica xerogels containing ferrimagnetic Fe3O4 nanoparticles (NPs) using femtosecond laser processing. Aminopropyltriethoxysilane-derived xerogels were prepared via a sol-gel process with aqueous solutions of Fe3O4 NPs synthesized by coprecipitation. The mass/volume concentration of Fe3O4 NPs in the xerogels was determined by comparing the absorbance of the xerogel with that of an aqueous solution of Fe3O4 NPs. We evaluated Faraday rotation angles for light propagating through waveguide structures in xerogels containing Fe3O4 NPs at mass/volume concentrations of 0.087 and 0.148 mg/cm3 at a wavelength of 488 nm. Ferrimagnetic saturation of the Faraday rotation angle was observed, which is consistent with the magnetization curves measured at room temperature. Magneto-optical waveguides can potentially be used to produce micro-sized Faraday devices, such as optical isolators, high-density magnetic recording devices, and optical sensors, which can be integrated with optical and electronic hybrid circuits.

Faraday rotation in cryogenically cooled dysprosium based (Dy2O3) ceramics

The temperature dependences of a Verdet constant of ceramic samples with the compositions (DyxY0.95−xLa0.05)2O3, x = 0.7, 0.85, 0.9 and their approximations in the temperature range 80–294 K and in the wavelength range 405–1064 nm were obtained with the <10% deviation from the experimental data. Cryogenic cooling leads to increase the Verdet constant value, proportional to inverse temperature, which allows to shorten a magneto–optical element length in cryogenic Faraday isolators. Results obtained are necessary for developing of Faraday devices for Tm3+ and Ho3+ lasers, and for lasers operating in the visible spectral region.

Magneto-optical Faraday effect in dysprosium oxide (Dy2O3) based ceramics obtained by vacuum sintering.

The method of self-propagating high-temperature synthesis of submicron powders with subsequent vacuum sintering was used to produce a series of optical ceramics based on dysprosium oxide (DyxY0.95-xLa0.05)2O3, where x=0.7, 0.85, and 0.9. The influence of ceramics composition on optical transmission of the obtained samples was investigated. The studied materials had several transparency windows in the visible 500-730nm, near-IR 1900-2300nm, and mid-IR 3500-7500nm ranges. The dependence of the Verdet constant on wavelength in the 400-1940nm range was measured for each ceramics composition, and analytical approximations were obtained. When Y and La dopants were used, the Verdet constant depended linearly on the concentration of Dy3+ ions, and its value in the visible wavelength range was about twice that of the analogous value for a terbium gallium garnet crystal. The high transmission and relatively high Verdet constant make the produced materials highly promising for fabricating Faraday devices for Tm3+ and Ho3+ lasers and for lasers emitting in the visible spectrum.

Features of Thermally Induced Depolarization in Magneto-Active Media With Negative Optical Anisotropy Parameter

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.

Cryogenic temperature characteristics of Verdet constant of terbium sesquioxide ceramics

The dependence of the Verdet constant on temperature in the (80–300 К) range for a promising magneto-active material terbium sesquioxide Tb2O3 at the wavelengths of 405–1064 nm is considered. For each of the studied wavelengths, the Verdet constant of the material cooled down to the liquid nitrogen temperature increased by more than a factor of 3.2 as compared to the room temperature value. Similarly to the other paramagnetics, the increase follows the law ∼1/T. Approximations for the temperature dependence of the Verdet constant have been obtained and the value of 1/V·(dV/dT) has been estimated. This information is needed to determine the angle of rotation as well as the variation of the extinction ratio of a Faraday isolator with temperature and extremely important at creation a cryogenic Faraday devices.

Temperature-wavelength dependence of terbium gallium garnet ceramics Verdet constant

A polarization-stepping method for measurement of the Faraday rotation in magneto-active materials has been modified for evaluating the Verdet constant as a function of wavelength and temperature. It has been used for a comparison of wavelength and temperature dependent Verdet constant, dominant electronic transition wavelength, and Curie-Weiss temperature in terbium gallium garnet (TGG) single crystals and TGG ceramics samples. The room-temperature values of Verdet constant varied from 36.6 to 44.2 rad/T.m at 1.064 μm wavelength and from 122.4 to 155.5 rad/T.m at 0.632 μm wavelength for all samples under investigation. These results match very well the values obtained from single wavelength measurement in the literature and show the same measurement error for single crystal and ceramics, respectively. The Curie-Weiss temperature has been evaluated for the first time in TGG ceramics. This method could be used for the detailed characterization of magneto-optical properties of the materials over a wide range of wavelengths and temperatures which is of great importance for the proper design of Faraday devices.

Wavelength dependence of Verdet constant of Tb3+:Y2O3 ceramics

Samples of the magneto-active material—Tb3+:Y2O3 ceramics with Tb3+ ion concentrations of 10%, 20%, 30%, and 100% (Tb2O3)—were prepared and studied. The wavelength dependence of Verdet constant in the 380 nm–1750 nm range was approximated for all investigated ceramic samples and was predicted for a pure Tb2O3 material. Tb2O3 ceramics demonstrates a more than three times higher Verdet constant in comparison with terbium gallium garnet crystal or ceramics. The linear dependence of the Verdet constant on Tb3+ ion concentration in the Tb3+:Y2O3 ceramics was demonstrated. The obtained data will be useful for fabricating magneto-optical elements of Faraday devices based on Tb3+:Y2O3 with arbitrary Tb3+ ion concentration operating at room temperature in the wavelength range of 380 nm–1750 nm.