Apparatus and method employing depolarization to eliminate effect of polarization dependent loss

An apparatus for measuring the IL (insertion loss) and the PDL (polarization dependent loss) of an optical device, includes a first switch adapted to connected to plural light sources on one side and to either a first depolarizer or a polarization controller on the other side thereof. A second switch is connected to both the first depolarizer and the polarization controller on one side and to the device under test on the other side. A detection instrument is connected to the device under test opposite to the second switch. Therefore, a first test path is set up from the light source, the first switch, the first depolarizer, the second switch, the device under test (DUT), to the detection instrument for measuring the pure IL of the DUT without the PDL involved therein, while a second test path is set up from the light source, the polarization controller, the second switch, the DUT, to the detection instrument for measuring the PDL of the DUT.