The \(D_1\) line of \(^{85}\) Rb exhibits non-cycling transitions; therefore, only weak modulation transfer spectroscopy (MTS) signals could be obtained. In this study, however, strong MTS spectra were obtained for the \(D_1\) line via optical pumping (OP) of the \(D_2\) line of \(^{85}\) Rb. Four linear parallel (lin \(\parallel \) lin) and linear perpendicular (lin \(\perp \) lin), \(\sigma ^+ - \sigma ^+\) , and \(\sigma ^+ - \sigma ^-\) polarization configurations were investigated using an OP laser with four different polarizations such as linear horizontal (P), linear vertical (S), \(\sigma ^+\) , and \(\sigma ^-\) polarizations. Under the linear perpendicular configuration, the MTS signal for the \(F_g=3 \rightarrow F_e=2\) transition of the \(D_1\) line exhibited approximately a 41-fold increase in the signal slope in the presence of a linearly-polarized OP beam, compared with the signal slope obtained without OP. The calculated MTS spectra for the \(\sigma ^+ \sigma ^-\) configuration in the presence of \(\sigma ^+\) - or \(\sigma ^-\) -polarized OP beam as an example matched well with experimental observations.
