Bundles or bundles of optical fibers are finding increasing applications in various areas of fiber optics, despite the relatively low resolution of such devices, which does not exceed the wavelength λ. One way to solve this problem is to use materials with a high refractive index, which will allow for strong localization of radiation modes in the fiber. This review discusses the use of sapphire fibers with a high refractive index of n>3 for these purposes. They are used as the basis for fiber bundles operating in the terahertz (THz) range and provide visualization with spatial resolution exceeding the Abbe diffraction limit for free space. Bundles of sapphire fibers of various configurations have been manufactured, consisting of arrays of parallel and non-parallel fibers, and theoretical and experimental evaluations of their spatial resolution have been obtained using both analysis of the paired correlation function of disordered fiber packing and THz imaging. In particular, for a bundle consisting of parallel fibers with metallic coating, the resolution varies with aperture with a mean value of 0.53.λ and can reach 0.3 λ in individual areas. For a bundle with diverging fibers with dielectric coating, the resolution is 0.35 λ, which significantly exceeds the Abbe limit. The developed principles can be transferred to any spectral range where materials for fiber optics with a high refractive index are available. Finally, methods for reconstructing THz images of test binary objects obtained using the proposed bundles are described.
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