Abstract
1. Introduction
The development of high-intensity femtosecond Ti:sapphire chirped pulse amplification (CPA) lasers has made it possible to realize ultra-intense and ultra-fast lasers with a focused intensity exceeding in small-scale laboratories[
Contrast improvement in Ti:sapphire CPA laser systems can be achieved through pulse cleaning technique development. One solution is to place a pulse cleaner after the final compressor stage, such as a plasma mirror[
In this work, with a double-CPA scheme, a pulse cleaning technique based on NOPA and SHG processes is applied to improve the contrast in our TW-level Ti:sapphire CPA laser system. Further, temporal contrast enhancement with a pulse cleaner based on XPW generation is also experimentally investigated. Based on both of the pulse cleaners, the temporal contrast of the TW-level Ti:sapphire CPA laser can be promoted efficiently. Finally, the application prospects of the two different pulse cleaning techniques in ultra-high contrast laser systems are comparably analysed.
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2. High-contrast Ti:sapphire CPA laser with pulse cleaner based on NOPA and SHG processes
A simplified layout of a high-intensity femtosecond Ti: sapphire laser system with a double-CPA scheme is shown in Figure
A pulse cleaner based on NOPA and SHG processes has been reported and applied to a TW-level Ti:sapphire CPA laser in our previous work[
The second CPA system, operating at 10 Hz, consists of a stretcher, a regenerative amplifier, two stages of multi-pass amplifiers, and a pulse compressor in a vacuum chamber. After being stretched, the seed pulses are amplified to 0.75 J before entering the compressor. The laser pulse duration is then compressed to 55 fs (FWHM) by the compressor. The compressed pulse energy is about 380 mJ, corresponding to a peak power of about 7 TW[
The temporal contrast of the high-contrast TW-level Ti:sapphire CPA laser and that of our original TW-level Ti:sapphire CPA laser are measured with a scanning third-order cross-correlator (Amplitude Technologies, Sequia). Figure
3. High-contrast Ti:sapphire CPA laser with pulse cleaner based on XPW generation
XPW generation is a four-wave mixing process: the XPW-generated wave has the same wavelength as the input pulse and a cubic dependence on the intensity. Meanwhile, the polarization of the XPW-generated wave is crossed with the input pulse. Consequently, it can be discriminated from the input pulse or the low-intensity noise via two cross polarizers, and therefore can be used to improve the temporal contrast of the input pulse. In this work, a pulse cleaner based on XPW generation is also designed and applied to improve the temporal contrast of our Ti:sapphire CPA laser[
The XPW setup is composed of a neutral attenuator, two crossed Glan laser polarizers, one focusing lens and one collimating focus mirror (both with a 1.7 mm focal length), and two BaF2 crystals (crystallographic orientation [011], 2 mm thickness). After passing through the attenuator, the energy of the injected initial pulse decreases to . The focus of the lens is placed into a small vacuum chamber to avoid nonlinear effects in air. Taking into account the stability, after optimizing the positions and the rotation of the nonlinear crystals, and the chirp of the initial pulse, a cleaned seed pulse with energy of is obtained, corresponding to a conversion efficiency of 13.3 %. The cleaned seed pulse is injected into the TW-level Ti:sapphire CPA laser, and an output pulse with temporal contrast of is achieved (as shown in Figure
4. Application prospects of the two different pulse cleaning techniques in an ultra-high contrast laser system
XPW generation has become a well-known pulse cleaning technique; it has been widely investigated and applied to improve temporal pulse contrast in recent years. However, a pulse cleaner based on NOPA and SHG processes is a novel method, and this is the first time it has been applied to a TW-level Ti:sapphire CPA laser. Our high-contrast Ti:sapphire laser system with a pulse cleaner based on NOPA and SHG processes has shown good performance during daily operation. Recently, based on this high-contrast TW-level Ti:sapphire CPA laser, a proton generation experiment with nanometre film targets has been carried out. The experimental results also demonstrate the enhancement of the contrast. The purpose of this work is to improve the temporal contrast in Ti:sapphire CPA lasers and to search for a pulse cleaning technique which can support ultra-high contrast lasers for ultra-high focused intensity in the near future. As the temporal contrast of a Ti:sapphire CPA laser is largely determined by the contrast and energy of the seed pulse[
First, XPW generation is a third-order nonlinear process; the temporal contrast of the XPW-generated wave should reach a very high level in theory. Assume that the contrast of the initial pulse is ; then the contrast of the XPW-generated wave should be , where is the conversion efficiency of the XPW generation. However, the contrast of a pulse cleaner based on XPW generation is actually limited by the extinction of the two crossed polarizers, and contrast enhancement is usually limited at 4–5 orders of magnitude. NOPA and SHG are second-order nonlinear processes; the contrast of a pulse cleaner based on NOPA and SHG processes can be estimated as in theory, where is the gain of the NOPA process and is the conversion efficiency of the SHG process. The contrast enhancement of this pulse cleaner is not limited by any optical element. Thus, the contrast enhancement is at least 7 orders of magnitude for a initial pulse contrast of . Therefore, the temporal contrast of a pulse cleaner based on NOPA and SHG processes can be expected to reach a higher level than that of a pulse cleaner based on XPW generation.
Second, to achieve a third-order nonlinear process, BaF2 crystals should be placed near the focus to reach the adequate peak intensity for XPW generation. To avoid damage, XPW generation works with only a limited input energy, of the order of 1 mJ order[
Compared to a pulse cleaner based on NOPA and SHG processes, a pulse cleaner based on XPW generation also has its own advantages: simple setup, low cost, and broadened spectrum via a third-order nonlinear process. Therefore, it may be a good choice for developing Ti:sapphire CPA lasers with temporal contrast of –. However, due to the limited contrast enhancement and output energy, if the contrast of the first CPA stage is not sufficiently high, it will be difficult for a pulse cleaner based on XPW generation to support contrast better than in a double-CPA scheme. The pulse cleaner based on NOPA and SHG processes has the potential advantage of producing a seed pulse with higher energy and higher contrast. It can be applied to develop an ultra-high contrast laser system for ultra-high focused intensity in the future.
5. Conclusion
Two different nonlinear temporal pulse cleaning techniques (NOPA with SHG processes and XPW generation) are applied to improve the temporal contrast of a high-intensity femtosecond Ti:sapphire laser system with a double-CPA scheme. Although contrast enhancement can be achieved based on both of the two pulse cleaning techniques, there still exists an optimized choice for different contrast enhancement requirements of different laser systems. A pulse cleaner based on XPW generation is sufficient to support the development of a high-intensity femtosecond Ti:sapphire CPA laser with temporal contrast of generally. But if higher contrast is required, a pulse cleaner based on NOPA and SHG processes may have more advantages. Further, a combination of different pulse cleaning techniques is also regarded as a solution for the higher contrast requirements of higher intensity femtosecond laser systems.
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