Patent ID: 7968074

Claim:
A method of making a low-stress, large-volume crystal with predetermined dimensions defined by a diameter and a height of the crystal, said crystal having a low birefringence and a uniform refractive index, said method comprising the steps of: a) growing an alkali fluoride, alkaline earth fluoride or a mixed fluoride crystal with larger dimensions than said predetermined dimensions from a melt; b) cooling the crystal with the larger dimensions obtained in step a); c) tempering the crystal with the larger dimensions obtained In step a); and d) after the cooling of step b) and the tempering of step c), removing edge regions of said crystal with the larger dimensions so that a diameter reduction and a height reduction of at least five percent occurs respectively and thus said crystal has said predetermined dimensions defined by said diameter and said height; wherein no further tempering takes place after said removing of said edge regions; and whereby said alkali fluoride, alkaline earth fluoride or mixed fluoride crystal has an RMS value of refractive index uniformity (Δn) that is better than 0.025×10 −6 and a stress birefringence of less than 0.3 nm/cm.