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• High Power Laser Science and Engineering
• Vol. 4, Issue 3, 03000e32 (2016)
Patrick Rambo, Jens Schwarz, Mark Kimmel, and John L. Porter
Author Affiliations
• Sandia National Laboratories, PO Box 5800, MS 1197, Albuquerque, NM 87185, USA
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Abstract

We have developed high damage threshold filters to modify the spatial profile of a high energy laser beam. The filters are formed by laser ablation of a transmissive window. The ablation sites constitute scattering centers which can be filtered in a subsequent spatial filter. By creating the filters in dielectric materials, we see an increased laser-induced damage threshold from previous filters created using ‘metal on glass’ lithography.

1 Introduction

Many laser applications, including high energy laser development, call for the generation of a high-order super-Gaussian (‘flat top’) beam profile. For high energy laser systems, where flat-top beams are desired to optimize energy extraction while minimizing spatial clipping[1], certain common methods do not apply. Refractive beam shapers that use custom aspheric lenses are not used due to undesired potential ghost reflections and chromatic aberrations. Similarly, beam integrators (like relay imaged lenslets) and diffractive optics have a poor depth of focus and coherent artifacts which make them undesirable in a laser chain (although they may be used at a single plane such as a target plane). Rather, to generate flat-top beams, most high energy lasers around the world rely upon apodization methods, which involve the spatial filtering of a modulated near-field beam. Such modulation can be in amplitude or phase and can be generated by reflective, transmissive or scattering techniques. Most of these large laser systems apodize the beam early in the laser chain where the beam fluence is low, often using precise serrated-edge apodizers that are fabricated via ‘metal on glass’ lithography techniques[2] or via laser cutting of thick foils. The ‘metal on glass’ method can be extended via continuous material deposition or a dithered binary deposition[3]. Using these dithered or continuous ‘metal on glass’ methods, amplitude filters can be created and placed in the same low-fluence region to allow the compensation of gain profile nonuniformities in the main amplifiers[37]. However, there are situations where such filters and apodizers are desired in a higher fluence region of the beam. Such is the case in some high energy petawatt lasers which employ optical parametric chirped pulse amplification (OPCPA) for a first stage amplifier and traditional Nd:Phosphate Glass for subsequent amplifiers[6, 8]. Rather than sending the OPCPA output directly into subsequent Nd:Phosphate Glass amplifiers, it is advisable to apodize and/or amplitude filter after the nonlinear gain process, allowing one to have better defined parameters at that position. Unfortunately, we have empirically observed ‘metal on glass’ apodizers placed in the chirped output of an OPCPA system damage at a modest $0.2~\text{J}~\text{cm}^{-2}$ or less average fluence. One could attempt to optimize the metal material choice for higher damage threshold but the resulting damage threshold would still be inferior to dielectric materials.

Copy Citation Text
Patrick Rambo, Jens Schwarz, Mark Kimmel, John L. Porter. Development of high damage threshold laser-machined apodizers and gain filters for laser applications[J]. High Power Laser Science and Engineering, 2016, 4(3): 03000e32