Infrared and Raman spectra of the PLA alpha-crystal were analyzed. Detailed analysis is only possible by using highly oriented electrospun fiber mat. Infrared dichroic analysis of large two-dimensional PLA crystals was also conducted. By combining the two dichroic analyses, accurate band assignments for an alpha-crystal were achieved for the first time. This analysis is only consistent with a 10/3 helical chain conformation, as opposed to the more simplistic 3/1 helix generally used. Infrared and Raman activities of the PLA 10/3 single-chain helix were derived and correlated to the orthorhombic alpha-crystal. The orthorhombic structure, previously suggested from X-ray studies, was confirmed by vibrational analysis. Poorly understood spectral features, such as the unexpectedly large number of carbonyl stretching vibrations, are attributed to crystal field splitting. An additional Raman component has its origin due to Fermi resonance interactions.