Sub-Doppler spectral resolution in a resonantly driven four-level coherent medium

Sub-Doppler spectral resolution has been studied in Doppler-broadened multi-level 87Rb atoms coherently coupled by two strong laser fields. Narrow spectral features of absorption or gain are observed in the center or sides of the Doppler-broadened absorption profile. Analytical and numerical calculations based on a four-level N type model are presented to explain the experimental results.