Highly efficient and low-noise amplification of ultrashort pulses through quasi-parametric amplification

Since the earliest demonstration of chirped-pulse amplification (CPA) and optical parametric chirped-pulse amplification (OPCPA), femtosecond lasers have been able to deliver ultra-high peak powers of up to ten petawatts (PW), paving the way for compact particle accelerators and X-ray sources.

To further increase peak powers, laser amplification schemes with both high conversion efficiency and wide bandwidth are needed. However, CPA laser amplifiers suffer from a relatively narrow gain bandwidth, while OPCPAs suffer from relatively low signal efficiency or pump depletion due to inverse conversion.

In a new article published in Light: Science & Applicationsa team of scientists led by Professor Liejia Qian from the Key Laboratory for Laser Plasmas (MOE), School of Physics and Astronomy, Shanghai Jiao Tong University, China, and collaborators have demonstrated an ultra-high-efficiency, low-noise scheme using quasi-parametric chirped -Pulse Amplification (QPCPA), a variation of OPCPA by dissipating the idler with strong crystal absorption.

The idler power dissipation hinders the inverse conversion effect, enabling QPCPA performance with high efficiency, wide bandwidth, and robustness to phase mismatch. They experimentally demonstrated 56% energy efficiency for an 810 nm signal converted by a 532 nm pump, or equivalently 85% pump depletion. Such record high depletion strongly suppressed superfluorescence parametric noise (PSF) in QPCPA to only ~10^-6 relative to the amplified signal energy.

In their experiment, an 8 cm Sm:YCOB crystal with the orientation for maximized nonlinear coefficient was used, which was transparent to both pump and signal but opaque to the idler. Under a pumping intensity of 3 GW cm⁻²the highest signal efficiency of 56% was achieved at a seed intensity of ~7 MW cm⁻²which corresponds to a pump exhaustion of 85%.

The demonstrated depletion of the QPCPA pump was approximately 2.5 times that of OPCPA. The strong pump depletion through efficient signal amplification significantly suppressed the generation of PSF noise. Within the largest signal output of ~65 mJ, the measured PSF noise energy was as low as ~10 μJ. Pulse contrast after compression should be as high as ~10⁹.

Prof. Ma, the first author, explained why they called such a method “quasi-parametric” amplification: “The QPCPA method is very interesting. In the saturated gain regime, its efficiency increases with seed intensity without inverse conversion, quite similar to ‘non-parametric’ laser gain. However, in the small-signal gain regime, it inherits all the parametric behaviors of OPCPA. The QPCPA combines the benefits of parametric and non-parametric processes.”

“Because the reverse conversion effect is completely impeded, the QPCPA is also robust to phase mismatch. This means that QPCPA is insensitive to variations in pump beam alignment and ambient temperature. This benefits QPCPA’s high repetition rate operation.” he added.

“With its very large efficiency-bandwidth product, the QPCPA scheme, based on a large Sm:YCOB crystal, can support up to 50 PW peak power using the same pump energy as current ten-petawatt laser systems, QPCPA may be a suitable candidate to push ultra-intensity lasers beyond the current ten petawatt limit,” said Prof. Ma.