Fiber-Optic Rotary Joint and Optical Link for RF-Antenna Measurement

A practical optical rotary joint and optical links were prepared for an antenna measurement. The rotational variation loss of the optical rotary joint depends on the mechanical misalignments. To achieve 0.2 dB or less rotational variation loss, we proposed a new structure that has a noncontact interface using two size-mismatched fibers without a lens. The optical links are made of a distributed-feedback laser diode, a PIN-photodetector, and single-mode fiber jumper cords. The proposed rotary joint has very low rotational variation loss. The optical links are sufficiently stable to be applied to the antenna measurement. We verified that the rotary joint and optical links can be adopted in RF antenna test applications.