|
At present, Faraday isolators (FIs) working with high power laser radiation mainly use terbium gallium garnet (TGG) crystals as media for magneto-optical elements. High optical quality TGG crystals are commercially available, however it has several disadvantages: a large thermally induced lens and relatively high level of thermally induced depolarization. Therefore, at present, there are attempts to create a magnetoactive media that would replace TGG: the most promising media are TSAG, KTF and NTF crystals and also TAG ceramics. We propose to use cerium fluoride (CeF3) crystal, which is uniaxial, but superior to TGG in thermo-optical characteristics.
CeF3 crystal is known for a long time, as found in nature in the composition of various minerals (fluorite, tisonite). More recently, it began to be used as a fast, high radiation hardness scintillator. Cerium Fluoride is a good scintillation crystal with high density and short decay time. Its high Verdet constant and paramagnetic nature of rotation have been known from the 30s of the 20th century. Another important advantage from the viewpoint of using this material in high-power laser systems is a possibility of producing large-aperture (up to ~10 cm) optical elements from CeF3. In this respect CeF3 surpasses most of the magneto-active crystal media. For example, the largest aperture of a TGG single crystal with quality fit for producing an FI is only 40 mm. The CeF3 crystal is transparent in a wide range of visible and near IR wavelengths (300-2500 nm) and has a high Verdet constant in this area whereas the TGG crystal becomes optically nontransparent starting from ~1.4 mm. At radiation wavelength of λ=1 µm, the Verdet constants of TGG and CeF3 are almost equal (37 and 36.7 rad/T/m respectively), which ensures an equal length of crystals, and they can be replaced one by one in FI.
We measured the thermally induced depolarization and thermal lens in this crystal. When the radiation power approaches 1 kW, the dependence of thermally induced depolarization on laser power becomes quadratic and almost coincides to the analogous dependence for the TGG crystal with absorption coefficient of 1.5*10-3 cm-1. However the thermal lens induced in CeF3 has a 6.5 lower optical strength for the same laser beam parameters.
We created a Faraday isolator on CeF3 using a magnetic system with a field strength of 2.8 kOe. The crystal length was 8 mm. Up to a power of 700 W, the degree of isolation was no worse than 30 dB; it was practically independent of laser power and was determined by the quality of the crystal. It should be noted that the uniaxial nature of the crystal imposes restrictions on the divergence of the laser beam, which should not exceed 8 mrad to maintain the degree of isolation.
The mentioned advantages together with the feasibility of producing large-aperture elements allow us to conclude that the CeF3 crystal is promising for developing FIs for high-power laser systems.
|
