Abstract: A compact laser diode monitor for measuring concentrations of individual atomic and molecular species in both gaseous and semitransparent fluids, includes a first diode laser having an output for resonantly exciting a first selected electronic transition of a specific atomic or molecular species of a sample, and a second diode laser having an output for resonantly exciting a second selected electronic transition of said specific atomic or molecular species of said sample. A difference between said first and second frequency outputs is equal to a vibrational energy level difference of the specific atomic or molecular species. Further said first and second frequency outputs are simultaneously directed into said sample, with selected momentum vectors in order to cause the selected atomic or molecular species to emit a coherent photon signal in a selected direction having a frequency equal to twice the first frequency output minus the second frequency output.