Novel High Peak- and Average-Power Ultrafast Lasers using Parametric Beam Combination of Fiber Lasers

Ultrafast lasers with high peak power are important for current-and-future applications of DOE relevance such as laser-based accelerators and coherent x-ray sources. However, these applications will require lasers with both high peak and high average power to be broadly usedŚand this goal is a longstanding challenge for the laser community. A laser reliably delivering Terawatt-scale peak power at high ~100 kHz repetition rateŚwith kilowatt average powerŚ would enable applications ranging from industrial material processing to driving the abovementioned secondary sources. Ultrafast Yb-fiber lasers have proven excellent at delivering high average power, but the small cross-sectional area of the fibers limits the peak power from a single fiber. Multiple fiber laser channels operating in parallel can generate high per-pulse energy, but coherent combination of their outputs is a challenge, with systems complexity increasing exponentially with power, requiring extremely tight spatial, spectral and temporal control of each amplification channel. In this project, we seek to demonstrate an alternative approach for beam combination that uses optical parametric amplification (OPA) to both frequency-shift the laser wavelength, and to allow for coherent beam combining. Many applications, such as high harmonic generation and laser wakefield acceleration, can benefit from shifting the 1 µm wavelength of Yb-fiber lasers farther into the infrared, here to ~1.5 µm. By using fiber lasers to pump a parametric amplifierŚwhich shifts the laser frequencyŚwith multiple pump beams all amplifying a single signal beam, we can effectively combine the energy from multiple pumps in a way that is very robust compared with the aforementioned coherent beam combination technique. In Phase I of this project, we demonstrated the basic concept of beam combining OPA using a bulk solid-state amplifier, and initiated the development of an ultrafast high power fiber laser system. In Phase II, we developed a unique pump laser consisting of three parallel Yb:YAG rod-type fiber amplifier each delivering ultrashort 270 fs duration optical pulses at 100 W average power and 1 MHz repetition rate. In addition, we have successfully demonstrated parametric amplification of 1.5 µm wavelength optical pulses with a spectral bandwidth broad enough to generate a few tens-of-femtosecond pulse duration, and are on track to demonstrate parametric beam combination of two full-power 100 W channels. In Phase IIb we plan to expand the capabilities and scale our concept. First, we will further develop the system OPA by (i) testing and experimentally comparing several nonlinear crystals to find the best solution for high average power, (ii) investigating OPA outside the plane of incidence to accommodate multiple pump beams in a single amplifier and (iii) demonstrate the wavelength tunability of our concept. Second, we will ruggedize our current pump laser, and scale it toward higher single-pulse energies by lowering the repetition rate (from 1 MHz down to 100-500 kHz) while maintaining the same average power and thus increasing peak power. In combination, these Phase IIa improvements will allow us to validate the concept for first applications such as generating soft X-ray high harmonics at high flux. Summary for congress: Lasers that provide simultaneously femtosecond duration pulses at high-energy and high average power are in demand for DOE and industrial applications including high added-value manufacturing. In this project we propose to develop a novel method to combine multiple ultrafast high- power laser amplifiers, enabling power/energy scaling.