In this paper, modeling and optimization of a highly birefringent microstructured optical fiber with an anisotropic structure of a lamellar core is analyzed. The core consists of a linear stack of a high refractive index lead oxide glass F2 and a low refractive index borosilicate glass NC21A, which contributes to the anisotropy of two orthogonal polarizations of the fundamental mode propagating in the fiber. It is shown, that an appropriate choice of thickness and width of the layers constituting the core structure, enables reducing the dispersion of birefringence of the considered modes, in a wide spectral range. It is further investigated how a sub-wavelength defect, in form of a low refractive index glass introduced in the middle of the core, influences fiber’s birefringence. We show for the first time, that nanodefect introduced into a lamellar core structure further reduces dispersion of birefringence in the fiber over one octave range. An average birefringence of 1.95×10−3 with variation below 5 % is achieved in 800–2,000 nm bandwidth.