The fundamental idea behind this design is an optical transformation that projects a stationary array onto a rotating array of collimated fiber ends. detection, and processing of high-frequency signals. Consequently, more and more fiber-optic channels are employed in rotary joints. Whenever several analog or digital optical signals need to be transmitted, a contactless multichannel fiber-optic rotary joint is needed. Multiple-channel transmission can be realized either over a single fiber using wavelength division multiplex, as described in [24], or over multiple independent fibers. The authors of [25] present a series of imaginable embodiments for such devices. However, only one of them-the Dove prism rotary joint-has gained a practical relevance for surveillance radars. The basic optical principle of a rotary joint following the latter design is shown in Figure 14(a). The fundamental idea behind this design is an optical transformation that projects a stationary array onto a rotating array of collimated fiber ends. The necessary derotation is provided by a Dove prism driven by a differential gear at half the relative rotating speed of the rotor with respect to the stator. Figure 14(b) shows a possible mechanical solution, with the Dove prism A Collimators B A Differential Gearing 2:1 Ratio (a) (a) Fiber Bundle Fiber Bundle Collimator Array (b) B Collimators Rolling Bearings Collimator Array Differential Gearing Dove Prism (b) (c) (c) Figure 13. A gigabit/Ethernet module: (a) the inside view of the electronics for signal processing and amplification in a submodule that covers a single channel [1]; (b) a complete module covering four such channels (size in mm: Ø165 x Ø16 x 139) [1]; and (c) the core transmission lines of a module with a free inner diameter of 300 mm. 68 Figure 14. A fiber-optic multichannel module with a Dove prism: (a) the basic functional principle, operating by means of two optical beam paths, A and B [26]; (b) a mechanical solution containing a bundle of fibers and a bevel-type gearing; and (c) a complete 20-channel module with the corresponding input and output fibers (fiber type: E9/125; approximate size in mm: Ø60 x 200). (Stator: blue; rotor: red; differential stage: green.) May 2016