S.G.Grechin, V.G.Dmitriev, V.A.D'yakov, V.I.Pryalkin
"Anomalously temperature-noncritical birefringence in biaxial optical crystals"
Quantum Electronics (Rus.), 2000, v.30, N1, p.1-2

      Temperature-independent birefringence in a biaxial crystal was predicted theoretically and observed experimentally for the first time. The width of the plot against temperature (the range corresponding to the temperature independence of the birefringence)at a fundamental radiation wavelength of 632,8 nm in a KTP crystal 5,9 mm long was more than 160 oС.