Fiber coupler: schematic shows side and front views of optical design for one of the four fiber couplers. The fiber couplers provide all optics and actuators to rotate and stabilize the beam, to split the light of the fringe tracker and science objects, and to optimally couple the light into the single-mode fibers. In addition, the fiber coupler contains several supplementary functions, including a metrology pickup to control the fiber differential delay lines, a retro-reflector to image the fiber input on the acquisition camera, and motorized half-wave plates for co-aligning the polarization.