Here, we present very low pulse energies (below 200 nJ) at very high speed (over 100 kHz) LIBS experiment by employing our recently developed 25 ps, 14W, 2.8 GHz intra-burst repetition rate Yb-doped fiber laser system. To carry out the LIBS experiments, the laser beam is directed into a Galvo scanner and focused through a 56 mm long F-theta lens onto different materials, including steel, copper, aluminum, and silicon. Our investigation of LIBS encompassed various parameters, including varying pulse energies, the number of intra-burst pulses, and burst repetition rates. For instance, with a burst repetition rate of 100 kHz with 83 ns burst width (232 intra-burst pulses), we observed the threshold pulse energy stands at approximately 26 nJ for LIBS experiment on steel. Furthermore, at about 200 nJ, it is enough to keep a high signal-to-noise ratio. To the best of our knowledge, this is the first report on LIBS experiment by GHz-range laser operating in the burst mode regime.
We introduce a single-mode Erbium-doped master oscillator power amplifier (MOPA) fiber laser, capable of directly generating 100 fs pulses at 2.2 W and 120 fs pulses at 4.5 W of average power. This laser operates at a pulse repetition rate of 1.2 GHz with a repetition rate multiplier, at the central wavelength around 1550 nm. The laser system comprises a passively mode-locked oscillator with a repetition rate of 77.6 MHz and an average power of 16.3 mW followed by a repetition rate multiplier and a cladding-pumped co-doped Er-Yb fiber laser. 100 fs long pulses, as the shortest pulse duration, was directly achieved at an output power of 2.2 W. In this experiment, the pulse dynamic at different output powers has been studied and verified by simulation. This developed system is employed in micromachining and sub-surface silicon processing at low pulse energy at a GHz-range repetition rate.
Three dimensional (3D) particle tracking technique has gained significant attention in recent years due to its ability to provide accurate and reliable data on the motion of particles, including micro and nanoscale particles. Here, we present a novel technique based on chromatic aberration that achieves 3D tracking using two cameras. Due to chromatic aberration of the lens, the axial position of the particle is mapped onto the lateral position in the image plane at a specific color, allowing us to use a diffraction grating to determine the lateral position on the color spectrum and thus the axial position of the particle. We also perform experiments on a 6.24 μm polystyrene particle in water, and collect data. We finally implement our method in Python and demonstrate that it performs within the tolerable error range and processes images and makes 3D coordinate predictions at a speed of 1.5 kHz.
Here, we demonstrate a new configuration of intracavity optical tweezers based on a ring cavity fiber laser. In this scheme, we placed the optical trapping system inside the Yb:doped fiber laser cavity operating with backward pumping. We use two counter-propagating inversely correlated beams, a pump at 976 nm from top to bottom and a signal at 1030 nm in the opposite direction. They are focused on the sample with a ultra-low numerical aperture (NA=0.088) aspheric lenses. Using this approach, counter-propagating intracavity optical tweezers (IOT), we are capable of 3D optical trapping of 1.98 µm-diameter polystyrene particles. Using such a low NA lens reduces the laser intensity on the trapped particle compared to the standard intracavity optical tweezers. The total average power on the particle is 885 µW, which corresponds to the average intensity of 21.2 µW µm−2
Intracavity optical tweezers are a powerful tool to trap microparticles in water using the nonlinear feedback effect produced by the particle motion when it is trapped inside the laser cavity. In such systems two configurations are possible: a single-beam configuration and counterpropagating one. A removable isolator allows to switch between these configurations by suppressing one of the beams. Trapping a particle in the counterpropagating configuration, the measure of the optical power shows a feedback effect for each beam, that is present also when the two beams are misaligned and the trapped particle periodically jumps between them.
In this work, we investigate the effects of noise on real-time focal distance control for laser material processing by generating the images of a sample at different focal lengths using Fourier optics and then designing, training, and testing a deep learning model in order to detect the focal distances from the simulated images with varying standard deviations of added noise. We simulate both input noise, such as noise due to surface roughness, and output noise, such as detection camera noise, by adding zero-mean Gaussian noise to the source wave and the simulated image, respectively, for different focal distances. We then train a convolutional neural network combined with a Gaussian process classifier to predict focus distances of noisy images together with confidence ratings for the predictions.
This work explores a real-time and high precision focus finding for the ultrafast laser material processing for a different types of materials. Focus detection is essential for laser machining because an unfocused beam cannot affect the material and, at worst, a destructive effect. Here, we compare CNN and non-CNN-based approaches to focus detection, ultimately proposing a robust CNN model that can achieve high performance when only trained on a portion of the dataset. We use an ordinary lens (11 mm focal length, 0.25 NA) and a CMOS camera. Our robust CNN model achieved a focus prediction accuracy of 95% when identifying focus distances in {-150, -140,...,0,...,150} µm, each step is about 7% of the Rayleigh length, and a high processing speed of 1000+ Hz on a CPU
We demonstrate a novel intracavity optical trapping approach based on a self-adjusted counter propagation beam. In this configuration, the trapping part is placed within a backward pumping fiber ring cavity. The particle in this setup is trapped by two counter propagation beams including pump, from top to bottom and signal from bottom to top. This trapping scheme benefits from the cavity design in which the individual beams are adjusted depending on the displacement of the trapped particle. When the particle is in the center of the trap and turns the laser signal off, the pump then automatically turns on and this keeps the particle in the trap position. We believe this approach can find its applications in the trapping of submicron particles where the previous intracavity OTs were not able to trap, especially biological samples that are subjected to photo-damage due to the intensity of laser light.
In standard optical tweezers optical forces arise from the interaction of a tightly focused laser beam with a microscopic particle. The particle is always outside the laser cavity and the incoming beam is not affected by the particle position. Here we describe an optical trapping scheme inside the cavity of a fiber laser where the laser operation is nonlinearly influenced by the displacement of trapped particle and there is a coupling between laser operation to the motion of the trapped particle and this can dramatically enhances optical tweezers action and gives rise to nonlinear feedback forces. This scheme operates using an aspheric lens at low numerical aperture (NA=0.125), NIR wavelength (λ = 1030 nm), and very low average power which results in about two orders of magnitude reduction in exposure to laser intensity compared to standard optical tweezers. Ultra-low intensity at our wavelength can grant a safe, temperature-controlled environment, away from surfaces for microfuidics manipulation of biosamples that are sensitive to light intensity. As the main advantage of our approach and highly relevant application, we observed that we can trap single yeast cells at a very low power, corresponding to an intensity of 0.036 mW μm-2, that is more than a tenfold less intensity than standard techniques reported in the literature.
We propose a novel approach for trapping micron-sized particles and living cells based on optical feedback. This approach can be implemented at low numerical aperture (NA=0.5, 20X) and long working distance. In this configuration, an optical tweezers is constructed inside a ring cavity fiber laser and the optical feedback in the ring cavity is controlled by the light scattered from a trapped particle. In particular, once the particle is trapped, the laser operation, optical feedback and intracavity power are affected by the particle motion. We demonstrate that using this configuration is possible to stably hold micron-sized particles and single living cells in the focal spot of the laser beam. The calibration of the optical forces is achieved by tracking the Brownian motion of a trapped particle or cell and analysing its position distribution.
The thermally induced stress in pulsed pump solid state lasers with super-Gaussian profile has been investigated. An
analytical expression for the thermal stress is introduced. We consider the heat deposited in the crystal due to the pump.
The temperature distribution in the crystal has been calculated by solving the non-steady state heat conduction equation.
A Ti: Sapphire crystal is assumed pumped by a pulse laser. All the stress components have been obtained and discussed
in details. The results show that the non-homogenous temperature distribution is induced by the thermal stress in the
crystal.
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