Researchers from the Shanghai Institute of Optics and Fine Mechanics (SIOM) proposed a novel design of double chirped pulse amplification (DCPA) laser systems implementing a combination of negatively and positively chirped pulse amplification (NPCPA), which is the first time to the best of our knowledge. The relevant result is recently published on Optics Express.

High-order dispersion, especially the fourth-order dispersion (FOD), is a bottleneck of pulse duration and temporal profile in traditional DCPA laser systems. AOPDF and grism pair are generally utilized to compensate the FOD in laser systems.

However, due to the large energy loss of seed pulse introduced by AOPDF and grism pair, the FOD compensation in DCPA lasers is inevitably accompanied by pulse temporal contrast degradation. At the same time, DCPA scheme is primarily developed for pulse temporal contrast enhancement.

In traditional DCPA laser systems, the two CPA stages are both employing positively chirped pulse amplification (PCPA). For a PCPA stage, both second- and third-order dispersion (GVD, TOD) can be compensated by optimizing the incident angle and grating pair distance of compressor.

Nevertheless, the FOD is impossible to be entirely canceled out, and the residual FOD is generally positive. Hence, the positive residual FOD accumulated by the two PCPA stages in traditional DCPA laser systems is usually very large, which will lengthen the duration and degrade the temporal profile of output pulses. As a result, additional components, such as AOPDF and grism pair, should be applied for the FOD compensation in traditional DCPA laser systems.

Different from traditional DCPA laser systems, the NPCPA laser systems are based on a combination of negatively chirped pulse amplification (NCPA) and PCPA. Here, the NCPA laser system is innovatively adopted as the first CPA stage. It is worth noting that the residual FOD in NCPA laser systems is usually negative, which is contrary to that in PCPA laser systems.

In addition, the residual amount of negative FOD is variable by altering the parameters of the stretcher and compressor in NCPA laser systems. Then, after carefully optimizing the design of the stretcher and compressor in the NCPA stage, it is potential to achieve a good result of FOD control in NPCPA laser systems based on the counteraction of the residual FOD in two CPA stages. The numerical calculations for two different design examples show that near FTL pulse duration around 20fs can be achieved in laser systems with peak power of hundreds-TW to multi-PW level.

Without using any additional dispersion compensation elements, the NPCPA scheme can sufficiently cancel out the GVD, TOD, and especially FOD simultaneously, just by optimizing the parameters of the stretcher and compressor in NCPA stage. Moreover, the NPCPA scheme can avoid the temporal contrast degradation induced by seed energy loss in the presence of extra dispersion compensation components.