The Fiber Coupler subsystem of the future VLTI instrument GRAVITY

We present the Fiber Coupler subsystem of the future VLTI instrument GRAVITY. GRAVITY is specifically designed to deliver micro-arcsecond astrometry and deep interferometric imaging. The Fiber Coupler is designed to feed the light from a science and a reference object into single-mode fibers. The Fiber Coupler consists of four independent units. The units de-rotate the FoV. A motorized half-wave plate allows rotating the liner polarization axis. Each unit provides actuators for fast piston actuation, tip-tilt correction and pupil stabilization for one of the beams from four VLT telescopes. The actuators are operated in closed-loop. Together with a dedicated Laser Guiding System, this allows to stabilize the beams and maximize the coherently coupled light. The fast piston actuator provides the crucial fringe tracking capability at a bandwidth of >220Hz. A special roof prism design allows to either split the FoV or to serve as a 50/50 beam splitter without changing the optical path. This offers the possibility of on-axis as well as off-axis fringe tracking. The optical train consists solely of mirrors, which ensures an achromatic behavior and maximum throughput. The sophisticated optical design compensates for aberrations which are introduced by off-axis parabolic mirrors. This allows to achieve Strehl ratios of >95% across the FoV.