Abstract

The sixteen isomorphs of potassium dihydrogen phosphate (KDP) are very useful in state-of-the-art nonlinear optical devices. Proper design of these crystals in high power laser applications requires knowledge of their thermal characteristics. The thermooptic coefficients α_n= n⁻¹dn/dT and the thermal expansion coefficients α_L= L⁻¹dL/dT were measured for thirteen of these isomorphs (KDP, KD^*P, ADP, AD^*P, CDA, CD^*A, ADA, AD^*A, RDP, RD^*P, RDA, RD^*A, KDA). A double interferometric technique using Fizeau and Twyman- Green interferometers was developed to measure these thermal coefficients. As a sample was heated, fringes from each interferometer shifted as a result of the changing optical path length in the sample. Even though both fringe patterns were functions of α_L and α_n, the two patterns changed with temperature at different rates. α_L and α_n were determined from the measurement of these two fringe rates by the simultaneous solution of the two equations that describe the phase shifts for the two interferometers. The dispersion in α_n was measured for all thirteen isomorphs between 441 and 1152 nm. Data for the KDP crystal were fit to a theoretical prediction based on a two-resonance Sellmeier fit of the refractive index.

© 1985 Optical Society of America