This project seeks to demonstrate a new approach for implementing tunable mid-infrared (IR) ultrafast lasers at high peak-and average powers, to specifically address the need outlined in Topic 32: Laser Technology R&D for Accelerators, listed under Type III. This category seeks to develop ultrafast lasers in the 2-5 micron mid-infrared (IR) spectral range with energies in the mJ- J range, repetition-rates approaching 1 MHz, sub-50 fs pulse duration, and wallplug efficiency ~20%. Here, we propose a novel and disruptive approach to implement an efficient, compact, and robust ultrafast laser, in a project compatible with an STTR budget but using an approach that can be scaled to meet the Type III criteria. Coherent combination of fiber lasers has become a popular approach for scaling of ultrafast lasers, but suffers from a number of technical shortcomings. Furthermore, that approach is only efficient and implemented to-date for near-IR pulse generation. We propose an alternate approach which integrates beam combining and frequency conversion into the mid-IR. By using multiple fiber lasersoperating in the most energy-efficient, long-pulse regimeto pump a frequency conversion step employing optical parametric chirped pulse amplification (OPCPA), we can make a simple and robust mid-IR laser. This OPCPA scheme is based on chirped pulse amplification (2018 Nobel Prize in Physics), where a low-energy seed pulse is first stretched in time, and sent into an optical amplifier, and subsequently recompressed. The use of periodically poled (PP) nonlinear crystals for the OPCPA step makes it possible to straightforwardly implement multibeam pumping in a way that coherently combines the energy from these beams with no need for interferometric stability. This provides an elegant and compact approach for implementing high-energy mid-IR ultrafast lasers. Moreover, this approach can allow for 70% conversion efficiency in the fiber laser amplification, and 30% in the frequency conversion step, exceeding the efficiency, pulse duration, and tunability characteristics of alternative direct mid-IR amplification schemeswith the potential for scaling to multi-kW output. It is also ideal for high repetition-rate operation in the ~<1 MHz range. This general architecture also addresses many of the technical problems associated with Topic 32d, since the OPCPA process is self-gating without the need for faraday rotators in the mid-IR. The likely limit will be power handling for the periodically poled materialan area of active research where improvements are continuing. In Phase I, we will demonstrate our concept using the mid-IR OPCPA we developed as part of a DARPA-funded PULSE project. This system uses a pump laser that delivers 350 ps pulses with 25 mJ energy at 1 kHz repetition rate. These pulses will be split to simultaneously pump a single OPCPA stage seeded by a 1.5 ?m beam. In parallel, we will design a two-channel fiber laser system delivering 500 ps pulses with up to 1 mJ total energy at a 100 kHz repetition rate. Noncollinear beam combining in a periodically-poled crystal will also be performed and, if time allows, we will experimentally demonstrate the design using the newly developed fiber laser. Phase II will scale to multiple pump beams and higher power. The approach we propose makes use of existing technologies, which guarantees a short path to commercialization based on KMLabs past successes. The mid-IR laser system that will result from this project is ideal for driving secondary sources such as soft X-ray high-harmonic sources, and tabletop ultrafast electron accelerators. These sources in turn have a large societal and technological appeal. For instance, high flux soft X-ray sources are currently in demand in the semiconductor industry for the inspection of photolithograhy masks for the next generation of computer and smart phone chips. Continued advances in nanotechnology will require novel, compact short-wavelength light sources for metrology.
