What is Multiple-prism grating laser oscillator?

Multiple-prism grating laser oscillators, or MPG laser oscillators, use multiple-prism beam expansion to illuminate a diffraction grating mounted either in Littrow configuration or grazing-incidence configuration. Originally, these narrow-linewidth tunable dispersive oscillators were introduced as multiple-prism Littrow grating oscillators, or hybrid multiple-prism near-grazing-incidence grating cavities, in organic dye lasers. However, these designs were quickly adopted for other types of lasers such as gas lasers, diode lasers, and more recently fiber lasers.