Coherent/incoherent amplification

Light amplification by stimulated emission of radiation was first demonstrated by Maiman in 1960, the result of a population inversion produced between energy levels of chromium ions in a ruby crystal when irradiated with a xenon flashlamp. Since then population inversions and coherent emission have been generated in literally thousands of substances (neutral and ionized gases, liquids, and solids) using a variety of incoherent excitation techniques (optical pumping, electrical discharges, gas-dynamic flow, electron-beams, chemical reactions, nuclear decay). 

The word laser is an acronym for the expression light amplification by stimulated emission of radiation. The process of stimulated emission, described below, produces light that is coherent, giving it quite different properties from those of light from an incandescent source, which is incoherent. 

Summary Optical parametric oscillators are coherent devices similar to lasers. There are, however, important differences. While lasers can be pumped by incoherent sources, OPOs require coherent pump sources. Often diode laser-pumped solid state lasers are used. While in lasers coherent amplification can last until the inversion in the active medium has fallen below threshold, in OPO s the time dependence of the coherent output is directly coupled to that of the pump laser. Since the pump photon is split into signal and idler photon with u> = u>i, the energy of the output equals that of the input i.e. there is no energy, i.e. heat deposited in the active crystal. The spectral tuning range is by far wider than for tunable lasers. Most OPOs operate in the near infrared but can be tuned from the visible region to the far infrared. 

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