The Diversity Of Fiber Optic Rotary Connectors (Slip Rings)

Abstract

About 50 different fiber optic rotary connectors, or fiber optic slip rings, have been built or described in the literature. These are devices for transmitting fiber optic signals across a rotating interface. The simplest are single channel slip rings where two multimode fibers face each other across a rotary interface. Losses are in the 3-4 dB range, or 1.5-3 dB if lenses are used to inject or collect light across the air gap, and light can be transmitted in either direction. Multiple channel optical slip rings have the advantage of redundancy and transmitting more data, omitting the necessity of multiplexing, while possibly freeing the rotational axis of the slip ring for a microwave, hydraulic, or pneumatic rotary joint. A number of different designs have been advanced which can be grouped into (1) concentric annular fiber bundles, (2) circular waveguides comprising open reflective channels or transparent dielectrics in the form of rings, rods, tubes, or wrapped fiber, (3) derotating intermediate optical components comprising prisms, mirrors, or fiber bundles, and (4) coaxial arrangement of mirrors and lenses rotating synchronously. These devices are primarily for large diameter multimode fiber and are often described for passive or active operation, i.e., they may incorporate active light sources and receivers rather than passively transmitting light supplied by optical fiber. The technical challenge in building a multiple, off-axis fiber optic slip ring is having incoming light available for transmission over 360A‚° of rotation, transmitting the light across the rotational interface with minimum light loss, dispersion, and modulation, then collecting sufficient light on the receiving side for further transmission. Important causes of light loss in optical slip rings are axial and angular misalignment of the two rotating members, Fresnel losses, and lens to fiber misalignment. During operation losses may result from dust; fogging, breakage, aging, elevated temperatures, and radiation. The system designer must carefully consider matching the slip ring to the light modulation method to avoid degrading or distorting signal transmission. An ideal optical slip ring would be passive, off-axis, bidirectional, achromatic, and capable of supporting any size single or multimode fiber.