Patent ID: 7198673

Claim:
An apparatus for making low birefringence optical fluoride crystals, comprising: a furnace; one or a plurality of chambers supported inside the furnace for containing at least one optical fluoride crystal, said chambers having a surface for supporting an optical fluoride crystal; and at least one heater disposed external to the one or plurality of chambers, the heater being arranged to apply heat along a shortest path of conduction of the optical fluoride crystal, and a plurality of ports in said furnace for the optional use of temperature probes, gas entry and exit, and the application of vacuum; wherein: (a) said chambers are unsealed, thereby allowing gas communication between an interior of the chamber and an interior of the furnace, and (b) said optical fluoride crystal oriented in either; (i) a facedown orientation on the surface within said chamber or (ii) an edgewise orientation on the surface within said chamber.