Reduction of Optical Absorption in Bi-Substituted Garnet Film by Annealing

LPE film was grown using Bi
₂
O
₃
single flux to realize the high Faraday rotation and low absorption provided by high density Bi-substitution required of material used for 0.8 μm optical isolators. The melt from Bi
₂
O
₃
single flux does not contain bivalent ions such as Pb
²⁺
, so Pt
⁴⁺
taken into the film or oxygen vacancies occurring within the film generate Fe
²⁺
, and increase optical absorption at 0.8 μm. Generation of Fe
²⁺
was prevented by adding bivalent ions, like Ca
²⁺
, in the melt, and optical absorption declined. This paper reports the results of investigations in which in addition to adding bivalent ions, annealing was used successfully to reduce Fe
²⁺
and optical absorption. Investigation of the relation between the effect of oxygen partial pressure during annealing and annealing temperature established that the equilibrium oxygen partial pressure and reaction rate differ with the electrical type of the film.