Sub-PW Ultra-fast CO₂ Laser

Starting from the ATF’s existing picosecond terawatt CO₂ laser, we will establish new regimes of laser operation with the ultimate goal of attaining a peak power in the hundreds of terawatts at 10 µm.

This advance cannot be just a straightforward scaling of the laser amplifiers; we must implement a host of innovative techniques. First, we are replacing the entire picosecond pulse-generation apparatus, including the CO₂ laser oscillator, pre-amplifier, and optical switches, with a commercial, all-solid-state integrated unit comprising a Ti:Sapphire laser oscillator, an amplifier, and an optical parametric amplifier (OPA).This system will provide 350 fs (FWHM), 40 µJ pulses - a considerable improvement over the parameters of the current system (5 ps, 0.1 µJ). The shorter injection pulse correspondingly will entail shorter output pulse. However, due to spectral modulation in the molecular-gas active medium, direct amplification of a femtosecond CO₂ pulse might be problematic, even when we consider expanding the bandwidth using enriched mixtures with isotopes ₁₈O and ₁₃C. According to our simulations, the gain bandwidth will limit the pulse’s duration at 1.5-2 ps. We note that shortening the pulse’s duration from the present 5 ps should nearly proportionally increase the energy extracted from the amplifier, or quadratically raise peak power.

However, when an ultra-shot laser pulse acquires high intensity in propagating through the amplifier medium, undesirable nonlinear effects in optical elements and in the gas medium come to play that may cause uncontrollable stretching and distortions of the output laser-beam. We will lessen such nonlinear effects by incorporating a stretching/compression technique similar to that used in conventional femtosecond Ti-Sapphire laser systems. This approach, not yet realized for molecular gas lasers, can raise the amount of energy extracted from an amplifier by an order-of-magnitude. Therefore, stretching the 1-ps pulse to ~200 ps before amplification, and subsequently recompressing it to 1.5-2 ps will be very crucial in reaching our goal of approaching sub-PW peak power.

The prospect for shortening the CO₂ laser’s pulse length to femtoseconds (few laser cycles) still exists. We will explore this possibility via new approaches to a picosecond pulse-chirping and compression inside or outside the laser amplifier. For the external compression, we will consider passing the laser beam through a pipe filled with xenon gas that has a high nonlinear refractive index, while keeping the laser’s intensity and the gas pressure below the ionization threshold. Then, Kerr effect-induced phase self-modulation in xenon dominates the spectral transformation of the laser pulse. Preliminary theoretical analysis shows that we can convert a quasi-linear frequency chirp, induced by xenon, into efficient pulse-compression in a dispersive element by nearly an order-of-magnitude without appreciable energy loss. A grating pair or a properly selected IR optical window may serve for such compression and should provide us with a few-cycle laser pulse.

Overall, via pulse compression and improved energy extraction from laser amplifiers, we anticipate an increase of over two orders- of- magnitude in the laser peak power up to 100 TW.