  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.
