  We demonstrate how to construct and operate a simple and affordable
experimental apparatus, appropriate for an undergraduate setting, in order to
produce and study coherent effects in atomic vapor and to investigate their
applications for metrology. The apparatus consists of a vertical cavity surface
emitting diode laser (VCSEL) directly current-modulated using a tunable
microwave oscillator to produce multiple optical fields needed for the
observation of the coherent population trapping (CPT). This effect allows very
accurate measurement of the transition frequency between two ground state
hyperfine sublevels (a "clock transition"), that can be used to construct a
CPT-based atomic clock.
