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Method of making large-volume CaF2 single crystals for optical elements with an optic axis parallel to the (100)- or (110)-crystal axis and CaF2 single crystal made thereby
Method of making large-volume CaF2 single crystals for optical elements with an optic axis parallel to the (100)- or (110)-crystal axis and CaF2 single crystal made thereby
The method for making single crystals, especially CaF2 crystals, includes tempering, in which the crystal is heated at 18 K/h to a temperature of 1000° C. to 1350° C. and held at this temperature for at least 65 hours with maximum temperature differences within the crystal of 0.2 K. Subsequently the crystal is cooled with a cooling rate of at maximum 0.5 K/h above a limiting temperature between 900° C. to 600° C. and then further below this limiting temperature at maximum 3 K/h. The obtained CaF2 crystals have refractive index uniformity 0.025×10−6 (RMS) in a (111)-, (100)- and/or (110)-direction and a stress birefringence of less than 2.5 nm/cm (PV) and/or a stress birefringence of less than 1 nm/cm (RMS) in the (100)- or (110)-direction. In the (111)-direction the stress birefringence is 0.5 nm/cm (PV) and/or the stress birefringence is 0.15 nm/cm (RMS).
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机译：制备单晶特别是CaF 2 晶体的方法包括回火，其中将晶体以<18 K / h的速度加热到1000°C至1350°C的温度并保持在该温度至少持续65小时，且晶体内的最大温度差小于0.2K。随后，以高于900°C至600°C的极限温度的最大速率为0.5 K / h的速度冷却晶体。然后以最大3 K / h的速度进一步低于此极限温度。所获得的CaF 2 晶体在（111）-，（100）-和/或（110）-中的折射率均匀度<0.025×10 -6 （RMS）方向和在（100）或（110）方向上的应力双折射小于2.5 nm / cm（PV）和/或小于1 nm / cm（RMS）的应力双折射。在（111）方向上，应力双折射<0.5 nm / cm（PV）和/或应力双折射<0.15 nm / cm（RMS）。
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