Photoelastic constants of germanium

The photoelastic constants of Ge have been measured at 3.39 and at 10.6 μm. Our stress-birefringence data join smoothly to earlier stress-birefringence data at other wavelengths. The acousto-optic figure of merit M2, computed from our data, agrees well with M2 determined for commercial acousto-optic devices. However, our data disagree with some earlier measurements of M2, the elasto-optic constants pij, and the change of refractive index with hydrostatic pressure dn/dP.