Research On High Power Laser - Heat - Insensitive Harmonic Conversion Technology

High-average power solid-state laser is one of the most important development directions for laser technology. At present, in the regime of fusion energy, national security, industry machining and the basic science research, it has highly significant applications. Due to the lack of suitable laser materials, we can only acquire specific laser wavelength by direct lasing from the solid-state medium. The nonlinear harmonic conversion technology is a typical nonlinear optical process and widely used for extending the optical wavelength from a fixed laser source, to fulfill various practical application requirements. In the mean time, as for the ultra-high-peak power solid-state systems, with the further promotion of the output peak-power, there is more of a requirement for the laser pulse contrast. The nonlinear second-harmonic generation is an effective pulse-cleaning scheme. If it can be appropriately applied in the chirped pulse amplification (CPA), the pulse contrast will be drastically enhanced.In this paper, focusing on the high-average and high-peak power solid-state laser systems, we particularly studied the nonlinear harmonic conversions in these specific operation environments. In order to boost the limited conversion efficiency in the high-average power regime and enhance the pulse contrast for the high-peak power laser pulses, in this paper, we principally carried out the following works:1. Proposed and numerically demonstrated a versatile harmonic conversion scheme capable of supporting temperature-insensitive phase-matching (PM) based on a two-crystal design.There are many obstacles in the way of achieving high conversion efficiency in high-average power regime. One of the most critical is the thermal loads caused by the linear optical absorption in the nonlinear crystal, which will give rise to thermal gradients that lead to a nonuniform distribution of temperature. Since the refractive index is temperature-dependent, thus perfect PM condiction could not be maintained across the beam transversely, resulting in limited conversion efficiency and unsatisfied beam-quality.In this paper, we proposed a versatile PM scheme appropriate for various harmonic generation processes, including the widely-used second-and third-harmonic generations (SHG, THG). The involved two crystals have opposite signs of the first derivation of phase-mismatch to temperature, and are arranged in a cascaded manner. As a result, the temperature-induced phase-mismatch in the first crystal will be well compensated in the second crystal, resulting in a temperature-insensitive PM with larger temperature-acceptance2. Via a series of proof-of-principle experiments, the novel harmonic conversion scheme was experimentally verified.Taking the typical high-power-laser wavelength of1064nm as an example, via a series of brief but accurate experiments, i.e., SHG and THG, the novel PM scheme was fully verified in experiment. For both of these two distinct harmonic generation processes, the proposed two-crystal design showed more temperature-insensitivity, and temperature-acceptance of PM approximately2times larger than that of using a traditional single crystal were observed. Additionally, the potentially deleterious phase shift caused by the dispersion of air, along with its influence on the temperature-insensitive PM, were also discussed. Both of the experiment results and the corresponding numerical simulations clearly showed that the temperature-acceptance was nearly insusceptible to these potentially deleterious phase shifts.3. On the basic of thoroughly studying the temperature distribution within nonlinear crystals, full numerical simulations for various harmonic generation processes were implemented at a typical wavelength of1064nm, under the high-average power operation.Under the high-conversion regime with high incident power, due to the stronger absorption at the shoter wavelength, as well as the natural heat exchange within the crystal, the temperature distribution may significantly vary along both the radial and the propagation directions.To illustrate the potencial applications of proposed PM scheme in the high-average power regime, full numerical simulations, based on the SHG/THG process and the thermal conducting equations, were implemented. In the calculations, we adopted a finite difference method (FDM), and all of the involved crystal parameters, incident intensity, etc., were set according to the actual situation. Taking the widely-used～1μm SHG as an example, for the proposed two-cryatal design, a maximum average power of～2kW, that can be tolerated while still maintaining high (60-70%) conversion efficiency, can be anticipated, which is more than two times higher than that of using a traditional single crystal. 4. Propose a novel version of double-CPA scheme for high-contrast optical pulses generation. Taking the advantage of its high efficiency and significant effect, a SHG stage is adopted to enhance the pulse contrast in this scheme.In principle, the optical parametric chirped-pulse amplification (OPCPA) can be designed for generating ultra-short optical pulses at various wavelengths. In this scheme, a mid-IR OPCPA is adopted as the preamplifier while a SHG stage acts to enhance the pulse contrast as well as to convert the wavelength to near-IR that matches the conventional Nd:glass-or Ti:sapphire-based CPA boosting system. In the proof-of-principle experiments, the generated1054nm femtosecond (fs) pulses showed a measurement-limited pulse contrast higher than109, with a spectral bandwidth larger than20nm. Provided seeding these accquired high-contrast pluses into the successive Nd:glass-based CPA for energy boosting, ultra-intense laser pulses with high-contrast can be anticipated.