Interferometric characterization of stress birefringence in germanium

We report on the characterization of the refractive index homogeneity in large blanks of Czochralski-grown Germanium, for thermal imaging use. With a phase-measuring Twyman-Green interferometer working at 10.6 micrometers , a map of the index of refraction with an accuracy better than 1 10-5 can be obtained for blanks which do not exhibit high birefringence.In the other case, principal stresses in the disks can be determined through the effect of birefringence on the interferogram, if the stresses are distributed cylinder-symmetrically in the plane of the disk. Relations between stresses, transmittance, and electrical resistivity of the material are observed.