This high-speed fiber-optic polarimeter is used in the HP 8509B polarization analyzer, an instrument that can characterize polarization-mode dispersion problems in long fiber systems (see page 27). Near-infrared light (simulated here) comes in from a single-mode optical fiber (at top). Focused by a lens, the light is split by three beam-splitter cubes and sent to four separate photodetectors. Polarizing optics inserted in the separate beams, including linear polarizers and a Fresnel rhomb, make it possible to measure the polarization state of the optical signal and the total optical power simultaneously.