Faraday Isolator Research Articles

Influence of germanium concentration on the microstructure and optical transparency of terbium gallium garnet ceramics

AbstractIn recent years, transparent terbium gallium garnet (TGG) ceramics have garnered significant interest for their application in high‐power Faraday isolators. However, challenges in achieving high transparency have led researchers to explore the addition of various sintering aids as a key strategy to enhance the optical quality of TGG ceramics. Through this work, the effect of germanium (Ge) addition on the microstructure and optical transparency of TGG magneto‐optical ceramics was investigated. TGG powders were synthesized by the co‐precipitation method, and the source Ge was Ge ethoxide added through a ball‐milling step. Transparent TGG ceramics were prepared by air pre‐sintering combined with hot isostatic pressing post‐treatment and subsequent annealing. The ceramics containing 200 ppm Ge exhibit optimal transmittance of 81.3% at 1064 nm (a value of theoretical transmittance), the Verdet constant was −133.0 rad·T−1·m−1 at 633 nm. When the addition of Ge reaches 600 ppm, a secondary phase can be observed on the surface of ceramic. Subsequently, TGG ceramics prepared from 1425°C to 1500°C with 200 ppm of Ge were analyzed, which revealed that the optimal pre‐sintering temperature is 1450°C.

Transparent Tb2Ti2O7 ceramics for use in Faraday isolators

Magneto-optical and thermo-optical characteristics of transparent Tb2Ti2O7 ceramics were investigated. The dependence of the index of refraction on the wavelength in the 0.29–2 μm range, the wavelength and temperature dependence of the Verdet constant, as well as the dependence of thermally induced depolarization on laser radiation power were measured. The value of the Verdet constant in Tb2Ti2O7 surpasses that in Tb3Ga5O12 by more than 1.68 times. The thermo-optical characteristic Qeff was estimated to be (1.8–3.7)∙10−8 1/K, which is record small compared to Qeff of the known magneto-optical materials. The small value of Qeff makes Tb2Ti2O7 a highly promising magneto-optical material for Faraday isolators and rotators for high average power lasers.

Multipass Faraday rotators and isolators

Faraday isolators are usually limited to Faraday materials with strong Verdet constants. We present a method to reach the 45° polarization rotation angle needed for optical isolators with materials exhibiting a weak Faraday effect. The Faraday effect is enhanced by passing the incident radiation multiple times through the Faraday medium while the rotation angle accumulates after each pass. Materials having excellent thermo-optical properties in the ultraviolet and mid-infrared range become available for optical isolators. Herriott-type multipass cells offer a simple and compact way to realize the desired propagation length in usual optical materials of standard sizes. A proof-of-principle experiment was carried out, demonstrating polarization rotation of a 532 nm laser beam by an angle of 45° in anti-reflection-coated fused silica surrounded by a standard neodymium ring magnet.

Holmium-doped TbVO4 as a highly potential magneto-optical crystal for Faraday isolator

Tb1-xHoxVO4 (THV) magneto-optical crystal with diameter of 27 mm and length of 40 mm was grown by the Czochralski (Cz) technique for the first time. The structural perfection of THV crystal was studied by the high-resolution X-ray diffraction rocking curve measurement, proving the high crystallinity quality of THV. THV shows high transmittance of greater than 75 % within the wavelength range of 665–1600 nm. The Verdet constants of THV at 532, 633 and 1064 nm are 370, 226 and 56 rad·m−1T−1, 40 %–95 % higher than these of commercial TGG crystal. Additionally, THV exhibits lower absorption coefficient (0.05 cm−1), higher magneto-optical figure of merit (175 deg/dB) and extinction ratio (43 dB) at 1064 nm compared with TGG. Evaluation of thermal performance and laser-induced damage threshold of THV were also made to promote the potential application in high-power optical isolators.

Effect of non-stoichiometric Ga content on the microstructure and optical transparency of terbium gallium garnet ceramics

The use of Faraday isolators in high-power lasers is currently a high-priority research topic and terbium gallium garnet (Tb3Ga5O12, TGG) transparent ceramic is a magneto-optical material with great potential for applications in the visible and near-infrared ranges. In this paper, the effect of chemical composition adjustment on the microstructure and properties of Tb3Ga5(1+x)O12 ceramics (where x is the non-stoichiometric index corresponding to the percentage difference in the amount of Ga used in the synthesis with respect to its content in the stoichiometric compound) was investigated. Powders obtained from starting materials of different stoichiometry were found to have similar morphology. However, the relative densities of the ceramics after pre-sintering in air increase with increasing Ga content. After the HIP post-treatment, the optimal properties with an in-line transmittance of 78.5 % (1.5 mm thick) at 1064 nm, and the Verdet constant of the ceramics with x = 0.8 % reaches −134.1 rad T−1 m−1 at 633 nm.

Omni-polarized Faraday isolator based on non-Hermitian Faraday system

Non-Hermitian systems have recently attracted significant attention in photonics due to the realization that the interplay between gain and loss can lead to entirely new and unexpected features. Here, we propose and demonstrate a non-Hermitian Faraday system capable of non-reciprocal omni-polarizer action at the exceptional point. Notably, both forward and backward propagating light with arbitrary polarization converge to the same polarization state. Leveraging the robustness and non-reciprocity of the non-Hermitian Faraday system, we realize an omni-polarized Faraday isolator that can effectively isolate any polarized light without the need for a polarizer at the incident port of backward propagation. Remarkably, under the given parameter configuration, the isolator achieves a maximum isolation ratio of approximately 100 dB and a minimum isolation ratio of around 45 dB for various polarized light, accompanied by near-zero insertion loss. Furthermore, our research reveals the remarkable tolerance of the non-Hermitian Faraday isolator to nonlinear effects. This unique characteristic allows us to harness nonlinear effects to achieve various optical functions, all while maintaining excellent isolation performance. The proposed non-Hermitian Faraday system paves the way for the realization of magnetically or optically switchable non-reciprocal devices.

Rational design of a Tb3Al4GaO12 magneto-optical crystal for Faraday isolators

Novel magneto-optical crystals are highly demanded to meet the rapid development of high-power laser machinery. The Tb3Al5O12 crystal has been demonstrated to be an excellent magneto-optical crystal, however, only small-sized single crystals can be produced so far due to its incongruent melting behavior. Herein, a Tb3Al4GaO12 crystal with Ga3+ substitution through the TGG structural modulation was designed to improve the bulk single crystal growth. It has been successfully grown by the Czochralski method for the first time. The crystal exhibits superior transmittance of over 80 % and enhanced Verdet constant (∼1.2–1.3 times that of TGG crystal), suggesting a promising application for high-power lasers.

Faraday isolator with compensation depolarization caused by Verdet constant temperature dependence

Faraday isolator with compensation depolarization caused by Verdet constant temperature dependence

Highly transparent Tb3Al5O12 ceramics for kilowatt‐level Faraday isolator

AbstractTb3Al5O12 (TAG) ceramics with high optical quality were fabricated successfully by a solid‐state reaction method combined with hot isostatic pressing (HIP) post‐treatment. The microstructure, optical transmittance, Verdet constant, and thermo‐optical properties were investigated. The obtained TAG ceramics showed a dense microstructure, and the in‐line transmittance reached 82.8% at 1070 nm after laser‐grade polishing. The Verdet constant of TAG ceramics was measured to be 50.9 rad·T1·m−1 at 1070 nm, which is higher than that of commercial Tb3Ga5O12 crystals. The Faraday isolator based on TAG ceramics can provide an isolation ratio of 30 dB with a laser power of more than 1 kW. With excellent optical quality, high Verdet constant, and maximum working power, TAG transparent ceramics are promising for applications of kilowatt‐level Faraday isolators.

A Faraday isolator based on fused silica in a Herriott cell

We demonstrated a multipass Faraday isolator based on fused silica. The polarization rotation angle of 45° was accumulated in a Herriott cell. The concept enables the use of magneto-optic materials with low Verdet constants in Faraday isolators.

Large aperture Faraday isolator based on the SiO2 glass for the high energy laser drivers

Large aperture Faraday isolator based on the SiO₂ glass for the high energy laser drivers

Single-stage Faraday isolator ensuring isolation ratio over 50 dB

A unique compact Faraday isolator has been developed, based on a single-stage magnetic system, operating at wavelengths up to ∼800 nm and providing an isolation coefficient of more than 50 dB. The device was created thanks to the development of a new approach to optimizing permanent magnet systems.

Control over magneto-optical plasmon rotation with Doppler broadening and cross-Kerr non-linearity

We investigate the impacts of cross-Kerr non-linearity and Doppler broadening on the magneto-optical rotation (MOR) induced in surface plasmon polaritons (SPPs) located at the interface of a metalic medium and a 4-level atomic system. In this paper, the external static magnetic field and a weak probe field under the cross-Kerr effect result in the birefringence enhancement of the system. Moreover, the incoherent effect of Doppler broadening can regulate the MOR of SPPs. Notably, the controlled MOR of SPPs can be leveraged for a variety of advantageous implementations in the fields of polarization spectroscopy, optical communication, nanoscale Faraday isolation, accurate metrology, as well as bio/chem sensing technologies.

Fine-grained transparent Dy2O3 ceramics fabricated from precipitated powders without sintering aids

As a promising material for Faraday isolator used in visible and infrared range, Dy2O3 ceramics with the average grain size of ∼800 nm and the in-line transmittance of 79.3% at 2000 nm and 70.0% at 633 nm were successfully fabricated. The Dy2O3 wet-chemically precipitated powders were densified in two steps consisting of vacuum pre-sintering and hot isostatic pressing (HIP) post-treatment. The precipitated precursor was calcined at 1150–1250 °C for 4 h in air to obtain Dy2O3 powders. The influences of calcination temperature on the morphology and sintering behavior of Dy2O3 powders were studied. The optical quality and microstructure of Dy2O3 ceramics with different pre-sintering temperatures were investigated. The Verdet constant of Dy2O3 ceramics at 633 nm is −322.1 ± 3.5 rad T−1 m−1, about 2.4 times that of commercial Tb3Ga5O12 crystals. Transparent Dy2O3 ceramics with high Verdet constant and suitable transparent windows present a prospective application for Faraday rotators.

Faraday isolator for a 100 J/10 Hz pulsed laser.

We report the first-ever, to the best of our knowledge, demonstration of the optical isolation of a kilowatt average power pulsed laser. A Faraday isolator capable of stable protection of the laser amplifier chain delivering 100 J nanosecond laser pulses at the repetition rate of 10 Hz has been developed and successfully tested. The isolator provided an isolation ratio of 30.46 dB in the course of an hour-long testing run at full power without any noticeable decrease due to the thermal effects. This is the first-ever, to the best of our knowledge, demonstration of a nonreciprocal optical device operated with such a powerful high-energy, high-repetition-rate laser beam, opening up the possibilities for this type of laser to be used for a number of industrial and scientific applications.

Fabrication and characterizations of Ho3Sc2Al3O12 magneto‐optical transparent ceramics

AbstractCurrently, Faraday isolators and rotators for high‐power lasers are popular research topics; therefore, finding magneto‐optical materials with excellent Verdet constants and thermal properties is crucial. In this study, novel holmium scandium aluminum garnet (HoSAG) magneto‐optical ceramics were fabricated using vacuum sintering. This study improved the traditional ball milling method and pre‐synthesized powders with great sinterability after secondary ball milling and sieving. Different pre‐synthesis temperatures were found to have significant effects on the particle size and micromorphology of the synthesized powders. HoSAG ceramics with 0.05 wt% MgO sintering aid and held at 1700°C for 30 h reached a transmittance of 76.7% at 1550 nm. Meanwhile, HoSAG transparent ceramics maintain higher transmittance in the infrared region (>1500 nm) than terbium gallium garnet (TGG) crystals, indicating better application prospects. The Verdet constants of HoSAG magneto‐optical ceramics at 405, 532, 808, and 1064 nm were −421.5, −181.2, −65, and −32.3 rad T−1 m−1, respectively, which are slightly less than those of TGG magneto‐optical ceramics. Moreover, the thermal conductivity of HoSAG ceramics at 300.00 K was 4.89 W m−1 K−1, which is comparable to that of the TGG ceramics.

A review on magneto-optical ceramics for Faraday isolators

As a promising magneto-optical 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 A₂B₂O₇ceramics. 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.

Fabrication and performance optimization of novel (Tb0.9975R0.0025)3Sc2Al3O12 (R= Pr, Ho, Dy, Eu) magneto-optical transparent ceramics

Magneto-optical materials with high Verdet constants at high extinction ratios are desirable for the Faraday isolators. However, current terbium-based crystals have encountered several issues, such as low Verdet constants (TGG crystals), difficult growth (TSAG crystals), and low utilization of raw materials, which limit the development of the Faraday isolators. Here, R0.0025TSAG (R= Pr, Ho, Dy, Eu) transparent ceramics are prepared. The ceramics exhibit high optical transparency above 74.0% in the 525–1565 nm range and a high relative density of 98.9%. In particular, the transmittance of R0.0025TSAG ceramics at 1064 nm is approximately 81.6%, 81.6%, 79.8%, and 80.6%, respectively. The Verdet constants of R0.0025TSAG ceramics at the measurement wavelength are greater than those of the TSAG ceramic. The compelling magneto-optical performance of the Dy0.0025TSAG ceramic includes a high Verdet constant of 161.8 ± 3 rad·m−1·T−1 at 635 nm. The Verdet constant of the Dy0.0025TSAG ceramic is still superior to that of TSAG ceramic in the same temperature range. The extinction ratios of the R0.0025TSAG (R= Pr, Ho, Dy) ceramics exceed 40 dB. The figure of merit points out the superior characteristics of R0.0025TSAG ceramics. Moreover, R0.0025TSAG (R= Pr, Ho, Dy) ceramics with a thickness above the centimeter level are successfully prepared. This work indicates that the Dy0.0025TSAG ceramic can promote the miniaturization of magneto-optical isolators and can be applied in high-power lasers.

Thermal lens in magneto-active ZnS, ZnSe and CdSe semiconductor media

Thermal lens in zinc selenide (ZnSe), cadmium selenide (CdSe) and zinc sulfphide (ZnS) magnetooptical media was investigated by phase-shifting interferometry. It was shown that the thermal lens in ZnSe is 10 times weaker and in ZnS and CdSe are 2 and 16 times higher than in popular magnetooptical terbium gallium garnet crystal. The focal shift and thermo-optical characteristics and of magneto-optical figure of merit were determined. The obtained values of absorption coefficient 1.1х10−4 cm−1 allowed determining a record, 4 kW maximum operating power of a ZnSe Faraday isolator.

A perspective on Faraday isolators for advanced lasers

Advances in the development of lasers with high output power raise the problem of increasing the operating power of Faraday isolators, since these devices can be indispensable in a number of applications. Due to the relatively high absorption of laser radiation in magneto-active media that are the basis of isolators, their use in laser systems is limited. In this Perspective, we overview the approaches that can be used to develop Faraday isolators with a high level of operating power and summarize the related achievements made in recent years, which have become possible due to the production of a wide range of new magneto-active media and the study of their properties. The issues and challenges on the path of further progress, as well as promising directions for it, are discussed.