Microstructure fibers (MFs) with a specially designed form birefringence are shown to be ideally suited for the creation of highly efficient frequency-tunable sources of short pulses in the visible range as a result of nonlinear-optical spectral transformation of unamplified femtosecond Ti∶sapphire laser pulses. The modes of doublet polarized along the fast and small axes of the elliptical core of the fiber have different propagation constants. Controlled by a half waveplate, femtosecond pulses of 820 nm pump radiation polarized along the fast and slow axes of the elliptical core of the microstructure fiber can generate intense blue-shifted lines centered at 490 and 510 nm respectively, observed as bright blue and green emission at the output end of the microstructure fiber. The dependence factors, such as length, power, are also compared and analyzed experimentally.