Coupling field dependent quantum interference effects in a $\Lambda$-system of $^{87}Rb$ atom

A $\Lambda$-system in $D_2$ line transition of the $^{87}Rb$ atom has been investigated for quantum interference effects for different configurations of coupling field. With a travelling wave coupling field (co-propagating with probe field), the probe beam shows the earlier known electromagnetically induced transparency (EIT) effect at the resonance. With a standing wave coupling field (which is co- and counter-propagating with probe beam), the probe EIT gets transformed into an electromagnetically induced absorption (EIA). A variation in coupling beam power has shown that, at high coupling power, EIA signals have much larger amplitude and slope than those of EIT signals with and without applied magnetic field. These EIA signals can be useful for tight laser frequency locking and optical switching.