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A broadband low-loss optical rotary coupler for single-mode and multi-mode fibers has been developed. It is based on a refractive index-matched liquid in a micro-structured capillary acting as a short ldquoliquid waveguiderdquo to couple optical signals from a rotating to a stationary fiber. In comparison to conventional optical devices using gradient index lenses with a small air gap in-between our fiber joint features extraordinarily low optical coupling loss of less than 0.01 dB which means an improvement by a factor of 450. Furthermore, the rotary fiber coupler exhibits a wide spectral transmission bandwidth of several hundred nanometers around the telecommunication wavelengths in the near-infrared range, a rotational variation loss of better than 0.01 dB, a good long-term stability and temperature robustness. It is well suited for low-loss data transmission especially in all-optical sensor systems where weak analog signals must not be affected by an additional rotation angle dependent modulation.