  Understanding the impact of the cladding tube structure on the overall
guiding performance is crucial for designing single-mode, wide-band, and ultra
low-loss nested hollow-core anti-resonant fiber (HC-ARF). Here we thoroughly
investigate on how the propagation loss is affected by the nested elements when
their geometry is realistic (i.e., non-ideal). Interestingly, it was found that
the size rather than the shape of the nested elements, have a dominant role in
the final loss performance of the HC-ARFs. We identify a unique 'V-shape'
pattern for suppression of higher-order modes loss by optimizing free design
parameters of HC-ARF. We find that a 5-tube nested HC-ARF has wider
transmission window and better single-mode operation than 6-tube HC-ARF. We
show that the propagation loss can be significantly improved by using
anisotropic nested anti-resonant tubes elongated in the radial direction. Our
simulations indicate that with this novel fiber design, a propagation loss as
low as 0.11 dB/km at 1.55 $\mu$m can be achieved. Our results provide design
insights towards fully exploiting single-mode, wide-band, and ultra low-loss
HC-ARF. In addition, the extraordinary optical properties of the proposed fiber
can be beneficial for several applications such as future optical communication
system, high energy light transport, extreme non-nonlinear optics and beyond.
