Compact 10 TW laser to generate multi-filament arrays Benjamin Webb, Joshua Bradford, Khan Lim, Nathan Bodnar, Andreas Vaupel, Erik Mckee, Matthieu Baudelet, Magali Durand, Lawrence Shah and Martin Richardson Townes Laser Institute, CREOL, The College of Optics and Photonics, University of Central Florida, 4000 Central Florida Blvd. Orlando, FL 32816, USA Email: bmwebb@creol.ucf.edu Abstract: The design and construction of a compact 10 TW Ti:sapphire CPA system for the generation of filament arrays is presented. The design and implementation challenges are discussed, in particular the optimization of beam quality. OCIS code: (140.7090) Ultrafast lasers; (190.7110) Ultrafast Nonlinear Optics Recently the construction of a 10 TW Ti:sapphire laser suitable for multi-filament research has been completed. The design was motivated by the goal of generating as many as 100 filaments in air. Arrays of filaments are interesting for microwave guiding and laser machining. In order to control the temporal and spatial relationship among the filaments, we need a high quality and stable beam profile so that phase plates may be used to manipulate the beam. An ultrashort pulse laser filament can be formed when the peak power exceeds the self-focusing critical power, 3 GW for air, given by   = 3.77  0 2 8 0  2 , for a Gaussian profile [1]. Typically the energy in a single filament is limited since the maximum irradiance in a single filament is clamped to ~10 13 W/cm 2 , while a surrounding lower intensity energy reservoir sustains the filament in some cases for hundreds of meters [2,3]. If more energy is available than can be contained in a single filament, then local self-focusing can initiate the formation of multiple filaments. The arrangement of multiple filaments is highly sensitive to initial beam quality, as can be seen in Figures 1b and 1c. Here we report on the Multi-Terawatt Femtosecond Laser (MTFL) facility, which has been carefully optimized to produce multi-TW peak power and nearly Gaussian beam quality to support research on ultrashort pulse laser filamentation. This system consists of an oscillator, stretcher, Acousto-Optic Programmable Dispersive Filter (AOPDF), three amplifier stages, and two compressors. The system utilizes the oscillator and regenerative amplifier from a previous Chirped Pulse Amplification (CPA) laser. In upgrading the system, the stretched pulse duration was increased to 450 ps in order to efficiently extract energy from the two additional amplifiers. The two additional amplifiers use bowtie configurations where the beams multi-pass a single amplifier crystal at small angles with respect to each other. The first 6-pass preamplifier is pumped by ~500 mJ at 532 nm and outputs ~140 mJ, while the final 3-pass amplifier is pumped by ~2 J at 532 nm and outputs 790 mJ at 800 nm with an energy stability of ~1% rms over 500 shots. In order to preserve the high quality seed profile from the regenerative amplifier, it is imperative for the pump energy absorbed by the Ti:sapphire crystals to be evenly distributed, since these amplifiers operate above saturation. This is accomplished by minimizing the multi-pass angles, relay imaging the flat-top pump profiles to the amplifier, and pumping the crystals from both ends. Each amplifier is seeded with a beam diameter smaller than the pumped region, which is carefully set to diverge through the amplifier so that the accumulated thermal lens from each pass leaves the beam at the output collimated, saturated and overlapped well with the pumped region. The beam profile after the preamplifier and through the compressor is very good for energy outputs up to 80 mJ (Fig. 1a), however the compressed output at 470 mJ exhibits a ring structure associated with clipping on several apertures in the final amplifier (Fig. 1b). We are working to scale these apertures in order to produce a profile more similar to the 80 mJ compressed profile (Fig. 1a). Gain narrowing, which reduces the amplified bandwidth, limiting the shortest possible compressed pulse duration, was mitigated by the implementation of a Dazzler (AOPDF) by Fastlite. The output bandwidth was maximized via software-controlled feedback between the bandwidth measured after amplification and the filtered spectrum set by the Dazzler before amplification. The Dazzler can also independently adjust 2 nd , 3 rd , and 4 th order dispersion so that compressed pulse duration can be minimized by a separate feedback loop between the Dazzler and a Wizzler by Fastlite which measures pulse duration after the compressor. The minimum output pulse duration is 38 fs.