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.
Excitation
Multiple-prism grating laser oscillators can be excited either electrically, as in the case of gas lasers and semiconductor lasers, or optically, as in the case of crystalline lasers and organic dye lasers. In the case of optical excitation it is often necessary to match the polarization of the excitation laser to the polarization preference of the multiple-prism grating oscillator. This can be done using a polarization rotator thus improving the laser conversion efficiency.
Linewidth performance
The multiple-prism dispersion theory is applied to design these beam expanders either in additive configuration, thus adding or subtracting their dispersion to the dispersion of the grating, or in compensating configuration thus allowing the diffraction grating to control the tuning characteristics of the laser cavity. Under those conditions, that is, zero dispersion from the multiple-prism beam expander, the single-pass laser linewidth is given by where is the beam divergence and M is the beam magnification provided by the beam expander that multiplies the angular dispersion provided by the diffraction grating. In the case of multiple-prism beam expanders this factor can be as high as 100-200. When the dispersion of the multiple-prism expander is not equal to zero, then the single-pass linewidth is given by where the first differential refers to the angular dispersion from the grating and the second differential refers to the overall dispersion from the multiple-prism beam expander. Optimized solid-state multiple-prism grating laser oscillators have been shown, by Duarte, to generate pulsed single-longitudinal-mode emission limited only by Heisenberg's uncertainty principle. The laser linewidth in these experiments is reported as ≈ 350 MHz in pulses ~ 3 ns wide, at power levels in the kW regime.
Applications
Applications of these tunable narrow-linewidth lasers include:
