Optical fiber bundles enable diffraction-free transmission of two-dimensional optical field distributions between the bundle input and output ends, which is highly beneficial in many applications in remote sensing and imaging. The spatial resolution of fiber bundles is mostly limited by the mode field diameter of an individual fiber. When using common low-refractive-index fibers, the resolution is typically limited by the wavelength of light ($\sim \lambda$), while using high-refractive-index fibers allows pushing it into a subwavelength range ($\simeq 0.3\lambda$–$0.5\lambda$), as was recently reported for sapphire fiber bundles in the terahertz (THz) range. Despite advanced resolution, such bundles require complicated image readout aided by depixelization techniques based on computational imaging. The analytical models of image formation in such systems are largely absent, which hampers the development of such bundles, as well as efficient processing of collected images. In this paper, we develop a semi-analytical model based on the methods of Fourier optics and sampling theorem to answer the fundamental questions related to image quality and resolution of such bundle-based imaging systems within the modulation transfer function (MTF) formalism. Thus, the developed model is then applied to analyze the THz image formation by the tapered sapphire fiber bundles. The inferred MTFs agree with the experiments. We believe that our findings should be instrumental for designing and applications of optical fiber bundles.

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• Near-field optics
• Photonics
• Optical fibers
• Imaging & optical processing
• Super-resolution techniques
• Terahertz techniques