In nearly all pulsed solid-state laser amplifiers, gain media, such as laser slabs, are pumped by pulses of light emitted by flashlamps or laser diodes. When active ions embedded in the laser gain medium absorb pump light, they transition from the quiescent ground state to an excited state.
When sufficient numbers of ions have been pumped into the excited state and the slab has sufficient gain and stored energy, the laser pulse to be amplified is propagated through the slabs. Amplification occurs through a stimulated emission process in which excited-state ions give up their energy to the laser pulse while the pulse's directional, polarization and phase characteristics are preserved.
To achieve high overall laser efficiency, both high pumping efficiency and high extraction efficiency are required. For efficient pumping, it is desirable to use gain media with long storage lifetime. For efficient extraction, it is (usually) desirable to use gain media with low saturation fluence (several times lower than the damage fluence). Unfortunately, gain media that have both long storage lifetime and low saturation fluence simultaneously are desired but do not currently exist. Gain media with long fluorescence lifetime tend to have high saturation fluence while gain media with low saturation fluence tend to have short fluorescence lifetime.
LLNL researchers have invented a method for scaling the average power of high-energy solid-state lasers to high values of average output power while maintaining high efficiency. This method combines the gas-cooled-slab amplifier architecture with a pattern of amplifier pumping and extraction that is new to high-energy pulsed lasers, in which pumping is continuous and in which only a small fraction of the energy stored in the amplifier is extracted on any one pulse. Efficient operation is achieved by propagating many pulses through the amplifier during each period equal to the fluorescence decay time of the gain medium, so that the preponderance of the energy cycled through the upper laser level decays through extraction by the amplified pulses rather than through fluorescence decay.
A major advantage of LLNL's method over the previous state of the art is that efficient energy extraction can be achieved at operating fluences that are lower than the damage fluence, even when the saturation fluence is many times greater than the damage fluence. This is significantly different from previous high-energy pulsed laser designs, for which it has been important to use gain media having saturation fluence values that are lower than damage thresholds, so that the preponderance of the stored energy can be extracted on each shot without causing laser damage.
Another advantage of LLNL's method is that it makes possible, for the first time in high-energy laser systems, efficient energy extraction from gain media that have exceptionally broad gain spectra but that also have high values of saturation fluence. This makes possible femtosecond-class chirped-pulse amplification (CPA) lasers that are simpler (i.e. one less laser stage) and more efficient than the titanium-doped sapphire lasers or optical parametric chirped pulse amplification (OPCPA) lasers that have been used to generate fs-class pulses up to the present.
LLNL's innovative method to scale high-energy pulsed solid-state lasers to high average power has uses in high-energy, high-average power lasers used for laser accelerator systems, defense applications, medical applications, and generation of secondary sources (electrons, protons, x-rays and gamma rays) for scientific applications.
LLNL has filed for patent protection on this technology; LLNL internal case number (IL-13200).