Atomic absorption spectroscopy with tunable diode lasers has been performed for monitoring and study of the physical vapor deposition process. The combination of the wavelength-modulation spectroscopy with diode lasers and the balanced detection scheme guarantees the high sensitivity and reliability of the system. Direct measurements of atomic flux in e-beam evaporated yttrium and barium, which are components in YBCO superconducting thin films, have been demonstrated. The measured velocities show that the e-beam evaporated atoms are in a non-thermal-equilibrium state, dependent on source conditions, implying that the flux measurement rather than a simple density measurement for rate control is necessary. Comparison with quartz crystal monitors shows that the present scheme, employing two laser beams counterpropagating at an angle to the substrate surface for measuring directly the velocity component normal to the substrate surface, can provide a pressure-independent flux measurement. In yttrium, which has an additional significantly populated metastable level, results show that pressure-independent flux measurement requires measurements at both the ground state and the metastable levels. Efforts have also been made to extend the accessible wavelengths of diode lasers to the UV region by using nonlinear optical frequency doubling techniques for other technologically important elements.