Nonlinear polarization rotation (NPR) in semiconductor optical amplifiers (SOAs) has applications in all-optical signal processing. Modeling NPR in a SOA requires knowledge of the carrier density induced refractive index change. The tensile-strained bulk SOA has attracted much interest due to its relative ease of fabrication and commercial devices are now available. In this letter we determine the polarization dependent refractive index change in such a SOA, with an InGaAsP active region, operating in the 1550 nm region and investigate its dependence on carrier density and wavelength.