The effects of pump pulse fluence on the output energy and amplified spontaneous emission of a femtosecond regenerative amplifier

In this paper, the effects of pump pulse fluence on the output energy and amplified spontaneous emission (ASE) of a femtosecond regenerative amplifier are investigated. One can easily enhance the output energy of laser amplifiers by increasing their pump fluence. This in turn can increase the ASE and reduce the performance of amplifiers in terms of output beam quality, beam stability, etc. This effect would eventually lead to what is called ‘temporal intensity contrast deterioration’. In this work, it is shown that an optimum state of the pump pulse fluence can indeed optimize the amount of the output energy from a regenerative amplifier without much reducing the performance of the amplifier due to the higher ASE. Temporal gain characteristics were employed to achieve this optimum value for a better design, performance, and maintenance of femtosecond laser amplifiers. The results of the current study can be effectively used in designing a wide range of regenerative amplifiers for femtosecond pulses.

A Diode Pumped Middle Infrared Laser Based on Ho: GdVO4 Crystal

Ho: GdVO4 crystal is the host material for the production of laser working in the middle infrared range. In this contribution, the characteristic parameters of the Ho: GdVO4 crystal were measured, and the material was used as a gain medium to build a diode-pumped laser for the first time, to reach a laser output at 2047.9 nm. The output beam quality factor M2 was measured to be 1.4 and 1.3 in x-direction and y-direction, respectively. In addition, the influence of the transmittance of the output mirror on the generation of laser was obtained through exploration. The results showed that the laser wavelength blue-shifted as the output transmittance increased.

Observation of spatial nonlinear self-cleaning in a few-mode step-index fiber for special distributions of initial excited modes

In this paper, we experimentally demonstrate that a nonlinear Kerr effect in suitable coupling conditions can introduce a spatially self-cleaned output beam for a few-mode step-index fiber. The impact of the distribution of the initial excited modes on spatial beam self-cleaning has been demonstrated. It is also shown experimentally that for specific initial conditions, the output spatial pattern of the pulsed laser can be reshaped into the LP11 mode due to nonlinear coupling among the propagating modes. Self-cleaning into LP11 mode required higher input powers with respect to the power threshold for LP01 mode self-cleaning. Our experimental results are in agreement with the results of numerical calculations.

Effect of Collimator Elements on the Beam Spectrum and KERMA In Gamma Irradiator

In the development of low-medium energy photon calibration facilities we have simulated several types of gamma irradiator collimator materials with ISO 4037-1 design connected to the output beam spectrum and the resulting kerma. Four types of collimator material, namely Al, Fe, Pb, and WCu have been simulated with gamma radiation sources 241Am, 57Co, 137Cs, and 60Co. Simulations were carried out using the Monte Carlo method with the PHITS computer program. Based on the comparison of air kerma produced, collimators made from Al are suitable for gamma sources 241Am, Fe material for gamma sources 57Co, and Pb material for sources 137Cs and 60Co.

Purcell Effect and Beaming of Emission in Hybrid AlGaAs Nanowires with GaAs Quantum Dots

Control of directionality of emissions is an important task for the realization of novel nanophotonic devices based on nanowires. Most of the existing approaches providing high directionality of the light emitted from nanowires are based on the utilization of the tapered shape of nanowires, serving as nanoantenna coupling with the light waveguided in nanowire and the directional output beam. Here we report the beaming of the emitted light with wavelength near 800 nm by naturally formed core-shell AlGaAs NW with multiply GaAs quantum dots (QDs) diameter 30 nm and height 10 nm, while the diameter of NW 130 nm, what does not support efficient emission into waveguided modes, including the mode HE11. Experimental measurements show that intensity of emission for directions in the vicinity of the axis of NW is about two orders of magnitude higher than for perpendicular directions. The developed theoretical approach allowed us to calculate the probability of spontaneous emission for various directions and into waveguided modes and showed that highly directional radiation can be provided by the intrinsic emission properties of cylindrical NW. Our results suggest that for the small diameter of NW, directional emissions are associated with an TM0 leaky mode (when electric field oriented in axial direction) and therefore manifests in an existence of axial electric dipole transitions in quantum dots.

An adaptive approach to machine learning for compact particle accelerators

Machine learning (ML) tools are able to learn relationships between the inputs and outputs of large complex systems directly from data. However, for time-varying systems, the predictive capabilities of ML tools degrade if the systems are no longer accurately represented by the data with which the ML models were trained. For complex systems, re-training is only possible if the changes are slow relative to the rate at which large numbers of new input-output training data can be non-invasively recorded. In this work, we present an approach to deep learning for time-varying systems that does not require re-training, but uses instead an adaptive feedback in the architecture of deep convolutional neural networks (CNN). The feedback is based only on available system output measurements and is applied in the encoded low-dimensional dense layers of the encoder-decoder CNNs. First, we develop an inverse model of a complex accelerator system to map output beam measurements to input beam distributions, while both the accelerator components and the unknown input beam distribution vary rapidly with time. We then demonstrate our method on experimental measurements of the input and output beam distributions of the HiRES ultra-fast electron diffraction (UED) beam line at Lawrence Berkeley National Laboratory, and showcase its ability for automatic tracking of the time varying photocathode quantum efficiency map. Our method can be successfully used to aid both physics and ML-based surrogate online models to provide non-invasive beam diagnostics.

Squared Focal Intensity Distributions for Applications in Laser Material Processing

Tailored intensity profiles within the focal spot of the laser beam offer great potential for a well-defined control of the interaction process between laser radiation and material, and thus for improving the processing results. The present paper discusses a novel refractive beam-shaping element that provides different squared intensity distributions converted from the Gaussian output beam of the utilized femtosecond (fs) laser. Using the examples of surface structuring of stainless-steel on the micro- and nano-scale, the suitability of the beam-shaping element for fs-laser material processing with a conventional f-Theta lens is demonstrated. In this context, it was shown that the experimental structuring results are in good agreement with beam profile measurements and numerical simulations of the beam-shaping unit. In addition, the experimental results reveal the improvement of laser processing in terms of a significantly reduced processing time during surface nano-structuring and the possibility to control the ablation geometry during the fabrication of micro-channels.

Propagation of Vortex Hermite-Cosh-Gaussian Beams in a Gradient-Index Medium

Based on the Huygens–Fresnel integral, the propagation equation for a vortex Hermite-cosh-Gaussian beams (vHChGB) in gradient-index medium (GIM) is derived. From the obtained expression, the evolution of the intensity and the phase distributions of a vHChGB through a GIM are numerically demonstrated as a function of the gradient-index parameter β under the change of incident beam parameters. The results show that the characteristics of the output beam evolve periodically versus the propagation distance, and the period of evolution slows down when β is increased. Furthermore, it is demonstrated that the self-repeating properties of the intensity pattern and the phase distribution for the propagated vHChGB are altered by the incident beam parameters. The results obtained may be beneficial for applications in fiber communications and beam shaping.