Bundles of high-refractive-index optical fibers for THz-imaging with subwavelength resolution (review)

Bunches or bundles of optical fibers find more and more applications in various fields of fiber optics, despite the relatively low resolution of such devices, which does not exceed the wavelength $\lambda$. One way to cope with this challenge is to use materials with a high refractive index, which will make it possible to achieve strong localization of radiation modes in the fiber. This review describes the use of sapphire fibers with a high refractive index $n>3$ for these purposes. They are used as the basis for fiber bundles operating in the terahertz (THz) range and provide imaging with a spatial resolution that exceeds the Abbe diffraction limit for free space. Bundles of sapphire fibers of various configurations are fabricated, consisting of arrays of parallel and non-parallel fibers, and their spatial resolution is estimated theoretically and experimentally using both the analysis of the pair correlation function of disordered fiber packing and the THz-imaging. In particular, for a bundle consisting of parallel metal-coated fibers, the resolution varies along the aperture with an average value of 0.53 $\lambda$, and in some areas it can achieve 0.3 $\lambda$. In the case of tapered fiber bundle with dielectric coating the resolution is 0.35 $\lambda$, which is much higher than 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.