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The interconnection of fiber-optic transmission lines to rotating devices requires the development of a continuously rotatable, low-loss, bidirectional optical rotary joint. This paper discusses the physical properties and optical performance of a rotary joint that will accommodate a single optical fiber with graded index profile and 50-micrometer core size. The device was developed for the Naval Research Laboratory by ITT, Electro-Optical Products Division to mechanically decouple optical fibers exiting from a rotating cable-winch system. For this specific application, a rotary joint can be used at each end of the winch axle, thereby allowing access to a total of two optical fibers. Since many uses of a rotary joint require the transfer of both electrical and optical power, the optical joint was incorporated into the housing of a 7-band, 10 Ampere (each) electrical slip-ring assembly provided by Electro-Tec Corporation. mhe nrecision bearings in the slip-ring housing provided a mechanical runout of less than .013 mm and an angular variation no greater than 0.2 degrees. Beam expansion and collimation techniques were used to reduce the dependence of optical loss on longitudinal and transverse offset at the rotational interface. Two quarter-pitch SELFOC rod lenses of 1 mm diameter, obtained from NSG, America, were used to achieve the beam expansion and collimation. The optical leads to the rotary joint were in the form of an unterminated single-fiber cable of approximately one meter length having a KEVLAR aramid sheath and a polyurethane overjacket. The maximum optical insertion loss at any rotational position was 1.3 dB at 830 nm and less than 1.7 dB at 1300 nm. The variation of insertion loss caused by device rotation was less than 0.25 dB at both wavelengths. At the conclusion of an endurance test of 310,000 revolutions at a 30 RPM rate, no degradation in performance could be observed.
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