We demonstrate an all-fiber ultrafast ytterbium laser oscillator mode-locked by means of a nonlinear polarization evolution (NPE) method realized in polarization-maintaining (PM) fibers. A sequence of the PM fiber pieces is shown to perform NPE action while maintaining a required temporal overlap of the ordinary and extraordinary pulses propagating through it. We present details of simple numerical simulations showing the advantage of the proposed scheme of segmented PM fibers. The laser utilizing the above mentioned design which generates ultrashort pulses at a 20.54 MHz repetition rate with the dechirped pulse duration around 150 fs and a pulse energy of 0.85 nJ is also presented.