Novel Approaches to Eliminating Thermally Induced Birefringence in High Average-Power Laser Amplifiers

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 a phase retarder plate which uses a custom fabricated metasurface to control the polarization, as a countermeasure to the birefringence in common gain media. The metasurface retarder plate has the appearance and dimensions of a typical, laser-grade window. The Phase I effort will evaluate the plate’s tolerance to high laser fluences and high average- powers. To be practical, the plate should have a damage threshold that is above 5 J/cm2 for a 10-ns, near- infrared, laser pulse, and it should have negligible absorption. The theoretical transmission of the proposed plate is better than 97%. Should the project continue to Phase II, the program will demonstrate efficacy in a high average-power, high peak-power, laser amplifier which uses the metasurface phase retarder plate to mitigate birefringence. The successful demonstration of the technology proposed under this Funding Opportunity work will enable the drastic reduction in size, complexity, and cost of high average-power lasers. A broad range of potential applications for such lasers exist, including hadron therapy, isotope production, and inertial fusion energy.