Optical pumping effect on modulation transfer spectroscopy of the $$D_1$$ line of $$^{85}$$Rb atoms with non-cycling transition

The [Formula: see text] line of [Formula: see text]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 [Formula: see text] line via optical pumping (OP) of the [Formula: see text] line of [Formula: see text]Rb. Four linear parallel (lin ∥ lin) and linear perpendicular (lin⊥lin), [Formula: see text], and [Formula: see text] polarization configurations were investigated using an OP laser with four different polarizations such as linear horizontal (P), linear vertical (S), [Formula: see text], and [Formula: see text] polarizations. Under the linear perpendicular configuration, the MTS signal for the [Formula: see text] transition of the [Formula: see text] 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 [Formula: see text] configuration in the presence of [Formula: see text]- or [Formula: see text]-polarized OP beam as an example matched well with experimental observations.