Laser cooling by over 7†K in Yb-doped ZBLAN fibers with high-power pumping at atmospheric pressure.

Anti-Stokes fluorescence (ASF) cooling has been demonstrated to be a viable method for balancing the waste heat produced in gain materials. In addition, radiation-balanced fiber lasers and amplifiers at atmospheric pressure have recently been developed. Here, we evaluate the cooling characteristics in a long section of a Yb-doped ZBLAN fiber with high pump power. The fiber has a 200-µm-diameter core and is doped with 3 wt. % Yb3+. As indicated by a thermal camera, cooling by over 7 K below ambient temperature was achieved by core pumping at 1030 nm. The temperature drop distribution at multiple measurement points in the fiber was evaluated with a maximum pump power of tens of watts. The results demonstrate the excellent ASF cooling performance of Yb-doped ZBLAN fibers. This study has great significance for the development of high-power radiation-balanced fiber lasers.

Full-aperture random polarization smoothing for a low-coherence laser facility.

Two new random polarization smoothing methods using full-aperture elements are proposed on low-coherence lasers, one using birefringent wedge and one using flat birefringent plate. By designing the crystal axis direction and wedge angle of the birefringent plates, the methods can selectively introduce time delay and spatial displacement, so as to obtain fast random evolution of transient polarization by utilizing low spatiotemporal coherence of the laser focal field. Both methods avoid the near field discontinuity and can be used under high fluence. The method using birefringent wedge can slightly improve focal spot uniformity, and the method using flat birefringent plate can obtain non-polarization with DOP lower than 2%. Theoretical studies show that the resulting focal polarization evolves rapidly on sub-picosecond timescales and rapidly covers the entire Poincaré sphere. The method using birefringent wedge is achieved in experiment. The results show that the degree of polarization of the focal spot is reduced from 1 to 0.27, which proves the effectiveness of the full-aperture random polarization smoothing. The full-aperture random polarization smoothing can generate a focal field very close to unpolarized thermal light, which is expected to suppress the laser plasmas instability.

Fast time-evolving random polarization beam smoothing for laser-driven inertial confinement fusion.

We propose a random polarization smoothing method for low-coherence laser to obtain focal spot with random polarization that evolves rapidly in sub-picosecond timescales. Random polarization smoothing is realized by a half-aperture wave plate with sufficient thickness. The degree of polarization and polarization evolution of the focal spot are studied theoretically. The calculation results show that random polarization smoothing can make the polarization of focal spot evolve rapidly and randomly in time and space. Experimentally, the polarization of the focal spot of low-coherence laser with random polarization smoothing is measured by a single-shot polarimeter. The measurement results show that the degree of polarization of the focal spot is reduced to 0.22 on average, which proves the effectiveness of random polarization smoothing. The random polarization smoothing technique on low-coherence laser is expected to reduce the laser plasmas instability through its multi-dimensional random evolution properties.

High-power, low-coherence laser driver facility.

We report the first (to the best of our knowledge) high-power, low-coherence Nd:glass laser delivering kilojoule pulses with a coherent time of 249 fs and a bandwidth of 13 nm, achieving the 63%-efficiency second-harmonic conversion of the large-aperture low-coherence pulse and good beam smoothing effect. It provides a new type of laser driver for laser plasma interaction and high energy density physics research.

Experiment and theory of beam smoothing using induced spatial incoherence with a lens array.

The smoothing effect of induced spatial incoherence combined with a lens array on a large-bandwidth and short-coherence-time laser is reported. A theoretical model based on statistical optics is developed to describe the spatial and temporal characteristics of the focal spot. Theoretical simulation is consistent with the experimental results. A method was proposed to remove or reduce the residual interference fringes of the experimental focal spot, and both the simulation and analysis show that this method does not affect the smoothing speed of the focal spot.

High-efficiency second-harmonic generation of low-temporal-coherent light pulse.

The nonlinear frequency conversion of low-temporal-coherent light holds a variety of applications and has attracted considerable interest. However, its physical mechanism remains relatively unexplored, and the conversion efficiency and bandwidth are extremely insufficient. Here, considering the instantaneous broadband characteristics, we establish a model of second-harmonic generation (SHG) of a low-temporal-coherent pulse and reveal its differences from the coherent conditions. It is found that the second-harmonic spectrum distribution is proportional to the self-convolution of that of a fundamental wave. Because of this, we propose a method for realizing low-temporal-coherent SHG with high efficiency and broad bandwidth, and experimentally demonstrate a conversion efficiency up to 70% with a bandwidth of 3.1 THz (2.9 nm centered at 528 nm). To the best of our knowledge, this is the highest efficiency and broadest bandwidth of low-temporal-coherent SHG to date. Our research opens the door for the study of low-coherent nonlinear optical processes.

Beam smoothing by a diffraction-weakened lens array combining with induced spatial incoherence.

The smoothing scheme combining a diffraction-weakened lens array with the induced spatial incoherence method is proposed and demonstrated to be an efficient smoothing scheme for broadband laser systems. In our simulation, the RMS illumination nonuniformity of the target spot is reduced to 2% after sufficient smoothing time. The temporal characteristics and spatial power spectral density of the scheme are theoretically analyzed. When the incident light has intensity fluctuations, the uniformity of the target spot is stable, which means a robust smoothing scheme, and which predicts practical applications to the smoothing of broadband laser systems.

Investigation of phase effects of coherent beam combining for large-aperture ultrashort ultrahigh intensity laser systems.

Large-aperture ultrashort ultrahigh intensity laser systems are able to achieve unprecedented super-high peak power. However, output power from a single laser channel is not high enough for some important applications and it is difficult to improve output power from a single laser channel significantly in the near future. Coherent beam combining is a promising method which combines many laser channels to obtain much higher peak power than a single channel. In this work, phase effects of coherent beam combining for large-aperture ultrashort laser systems are investigated theoretically. A series of numerical simulations are presented to obtain the requirements of spatial phase for specific goals and the changing trends of requirements for different pulse durations and number of channels. The influence of wavefront distortion on coherent beam combining is also discussed. Some advice is proposed for improving the performance of combining. In total, this work could help to design a practical large-aperture ultrashort ultrahigh intensity laser system in the future.