  We present a practical approach to the numerical optimisation of the guiding
properties of buried microstructured waveguides, which can be fabricated in a
z-cut lithium niobate (LiNbO3) crystal by the method of direct femtosecond
laser inscription. We demonstrate the possibility to extend the spectral range
of low-loss operation of the waveguide into the mid-infrared region beyond
3{\mu}m.
