A polarization transformer in combination with a polarization analyzer is well suited for the generation of any wanted polarization state. We describe the setup, calibration, and applications of an in-line fiber optic polarization detector (IPD) built with a fiber optic coupler of 99% coupling ratio. At the low power output port we use standard beamsplitters and polarizers to generate four different polarization dependent signals. For calibration, the polarization is measured at the high-power output port with an oft-line polarimeter (OPD). This enables the determination of the system matrix, which represents the relation between the polarization at the high-power port and the four signals at the low-power port. With this system matrix, a continuous and accurate determination of the polarization at the high-power output port is possible.