A research team in the Key Laboratory of High Power Laser and Physics, Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences proposed a dynamic scheme with a large dynamic range to compensate chromatic aberration of femtosecond petawatt laser systems accurately and effectively. This result was published in Optics Express.

The peak power of ultrashort laser systems has grown dramatically in the range of 10²¹–10²² W/cm² owing to the rapid development in the chirped pulse amplification (CPA) and optical parametric CPA (OPCPA) techniques. Such an intense laser provides a powerful tool for the experimental study of laser–matter interactions in the relativistic regime. A major problem hindering the performance of high peak-power ultrashort laser systems is associated with chromatic aberration that is introduced by lens-based beam expanders, which could significantly reduce output intensity by spatially and temporally distorting the beam profile.

Traditionally, chromatic aberration can be avoided or compensated by using all-reflective beam expanders or applying pre-compensators. Many types of compensation schemes have been proposed and utilized by introducing opposite chromatic aberration: a diffractive chromatic corrector—made from a Fresnel lens imprinted on one surface of a negative lens—for the PETEL and OMEGA EP laser systems, a combination of a biconcave lens and aspherical mirror at the Texas Petawatt Laser facility, an Offner triplet imaging system combined with a negative lens in the MTW-OPAL system at the Laboratory for Laser Energetics (LLE). However, these above methods all suffer from a common insufficiency—the degree of freedom to continuously control the value of chromatic aberration within a sufficiently large dynamic range. As such, the accurate and complete compensation of chromatic aberration of an entire laser system is difficult.

The newly-proposed scheme consists of a positive lens, a group of negative lenses, spherical mirror, and vacuum chamber. Compared with other techniques, the proposed scheme is capable of providing a large dynamic range of chromatic aberration, making it possible to accurately correct the chromatic aberration without affecting the beam profile.

Additionally, the scheme has the advantages of double-pass and beam switching configurations; consequently, it is able to correct the chromatic aberration of the whole laser system, even for a small input beam size. Furthermore, the proposed scheme is low-cost and can be removed and aligned with ease without changing the systematical optical paths.

The performance of the proposed pre-compensation scheme was verified experimentally by designing a tailored compensator for the SG-II 5PW laser system. The focusing performance of the SG-II 5PW laser system was improved dramatically following the application of the pre-compensator and the focal spot size (FWHM) was reduced noticeably, which was close to the diffractive limited spot size.

The pre-compensation scheme is capable to be applied in high-power laser systems as a pre-compensator for it provides the flexibility to correct chromatic aberration accurately and effectively.

This work was supported by the National Natural Science Foundation of China (NSFC) and the International Partnership Program of Chinese Academy Of Sciences.

Fig. 1. Schematic image of the chromatic aberration pre-compensation scheme (Image by SIOM)

 Fig. 2. Focal spot before and after applying the pre-compensator(Image by SIOM)