Thickness and complex refractive index of each layer in a double hetero structure, consisting of InP and InGaAsP, are obtained by spectroscopic ellipsometry. Accuracy is at least 2% in layer thickness and 0.005 in the refractive index of InP. An equation description of the complex refractive index allows for flexible description of InP-based materials in the wavelength range of 800-1700 nm. Simultaneous determination of thickness and refractive index by non-destructive ellipsometry in the near infra red (NIR) for the layers in a double hetero layer stack requires additional information on the properties of these layers