Phase II Amount
$1,149,999
The development of high average-power lasers is fundamentally challenged by the onset of thermally induced birefringence in typical gain media. This depolarization caused by birefringence is spatially dependent and difficult to correct. The result is a significant loss of energy and beam uniformity. The best-known solutions for this problem involve paired numbers of passes through identically stressed gain media with some means of rotating the beam polarization 90-degrees, wholesale. The subsequent pass will then undo the effects of the prior pass with varying degrees of success. This is not always feasible given the cost and complexity of adding amplifier stages and/or the costs (or even existence) of the optics required to rotate the polarization. Other solutions include changing the gain configuration so that the thermal stresses are along the axis of the gain instead of radially. This translates to not using cylindrical rod amplifiers and forgoing the many design and efficiency benefits associated with rods.The novel solution proposed here is the implementation of phase retarder plates which uses a custom fabricated metasurface to convert linearly polarized light into radially or azimuthally polarized light, and then back again. The thermally induced birefringent axes of YAG and glass-based amplifier rods are in the radial and azimuthal direction. Thus, such a change in polarization would prevent the laser from seeing the birefringence and eliminate the depolarization effect. During the Phase I, Voss Scientific demonstrated fabrication of nanostructures which showed a high degree of polarization control. Depolarization mitigation using radially polarized light was also demonstrated using a lower-power- handling, commercial phase retarder as a surrogate for the metasurface phase retarder. Metasurfaces fabricated using silicon were tested for their laser induced damage threshold (LIDT) which was found to be less than 200 mJ/cm2 for 1-ns pulses at 1064 nm. Alternative materials to silicon were identified and also tested for LIDT which exceeded 1.5 J/cm2. During the Phase II project, the metasurface will be redesigned and optimized using the new materials. Fabrication techniques using silicon should be directly transferable. The goal of the Phase II project is the demonstration of a laser system that uses the metasurface phase retarder.The successful demonstration of the technology proposed will enable the drastic reduction in size, complexity, and cost of high average-power lasers and just as importantly, greatly improve beam quality. Furthermore, mechanism by which the metasurface works to mitigate birefringence works independent of changes in the magnitude of the thermal birefringence in the gain. In principle, the metasurface optics can be scaled to very large apertures, making them uniquely suitable for use in high energy (>100 J) lasers, like those sought by the DOE. A broad range of potential applications for such lasers exist, including hadron therapy, isotope production, and inertial fusion energy.
