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.