Faraday Isolators for High Average Power Lasers

Thermo-Optical and Magneto-Optical Characteristics of Terbium Scandium Aluminum Garnet Crystals

“…Knowing the value of α 0 |Q|, from the measurements of thermally induced depolarization we found P/|Q| = 395. We present for comparison analogous values for TGG: P/|Q| = 10 [8]; α 0 P varies from 2.2·10 −6 1/(m·K) to 8.1·10 −6 1/(m·K), from which it follows that due to the closeness of constants P the thermal lenses in TSAG and TGG are close. If we assume, as before, that the absorption of the studied TSAG sample is α 0 = 1.3·10 −3 1/cm, then for equal lengths of the elements, laser beam radii and absorption, the TSAG single crystal with [001] orientation will introduce only a 26% larger lens than the TGG single crystal with [001] orientation.…”

“…Making use of the obtained value of thermal conductivity, α 0 |Q| was assessed to be 5.2·10 −9 1/(m·K). For comparison, in the TGG crystal Q = −17·10 −7 1/K [8], and the absorption coefficient α 0 varies from 1.3·10 −3 1/cm to 4.8· 10 −3 1/cm [7], [8], hence, α 0 |Q| may take on values from 2.2·10 −7 1/(m·K) to 8.1·10 −7 1/(m·K).…”

“…Some magneto-optical glasses have comparable μ * with the TGG crystal, but the Faraday isolators based on these glasses provide a much worse isolation ratio at high average power because of an order of magnitude lower thermal conductivity. According to (8), the magneto-optical figure of merit for the TSAG material μ = 32.1·10 9 rad·W/(T·m) for the wavelength of 1070 nm at room temperature, which is a factor of ∼30 more than μ of the TGG single crystal under the same conditions (1.1·10 9 rad·W/(T·m)) [46] and a factor of ∼320 more than μ of the best known magneto-optical glass (0.1·10 9 rad·W/(T·m)) [8]. At high average power, from the viewpoint of a possible isolation ratio, TSAG is better than TGG as much as TGG is better than magneto-optical glass.…”

“…Today, one of the most popular and well studied magnetoactive materials for Faraday devices used in high average power lasers is terbium gallium garnet (TGG) by virtue of a rather high Verdet constant (35-40 rad/(T·m) at the wavelength of 1 micrometer [8]), relatively high thermal conductivity (4.4-5.3 W/(m·K) [8]) and a possibility to grow high-quality large-size (30 mm) crystals using the Czochralski method [7], or a possibility to make high-quality ceramic elements [9]- [12]. However, other materials with better magneto-optical properties have long been known.…”

Thermo-optical and magneto-optical characteristics of CeF 3 crystal

“…where a is the absorption coefficient and A is the numerical coefficient determined by the laser beam profile and equal to 0.137 for the Gaussian intensity distribution [21]. Please note the difference from the isotropic case.…”