Study of power limitation of AlGaInN LEDs in pulse regime at high current

LEDs operating under high pulsed current are of a great interest for different applications, in particular, for VLC (LiFi) systems and laser pumping. Current dependences of the efficiency and emission spectra as well as the rise and fall times of high-power blue LEDs were investigated under extremely high pulse current density up to 7 kA/cm2 and pulse duration from 100 ns to 3 μs. Analysis of the pulse behaviour of the LEDs reveals that the main droop in the efficiency and change in spectra occur up to the current densities ~ 1 kA/cm2 and seems to be non-thermal.

Optimal Photon Energy Transfer on Titanium Targets for Laser Thrusters

Using two infrared pulsed lasers systems: a picosecond solid-state Nd:YAG laser with tunable repetition rate (400 kHz - 1MHz) working in a burst mode of multi-pulse train and a femtosecond Ti:Sapphire laser amplifier with tunable pulse duration inthe range of tens of femtoseconds up to tens of picoseconds, working in single-shot mode (TEWALASS facility from CETAL-NILPRP), we have investigated the optimal laser parameters for kinetic energy transfer to a titanium target for laser-thrustapplications. In the single-pulse regime, we controlled the power density by changing both duration and pulse energy. Inthe multi-pulse regime, the train’s number of pulses (burst length), and the pulse energy variation were investigated. Heatpropagation and photon reflection-based models were used to simulate obtained experimental results. In the single-pulseregime, optimal kinetic energy transfer was obtained for power densities of about 500 times the ablation threshold correspondingto the specific laser pulse duration. In multi-pulse regimes, the optimal number of pulses per train increases with the trainfrequency and decreases with the pulse power density. An ideal energy transfer efficiency resulting from our experiments andsimulations is close to around 0.02%.

Conventional Soliton and Noise-Like Pulse Generated in an Er-Doped Fiber Laser with Carbon Nanotube Saturable Absorbers

Conventional soliton (CS) and noise-like pulse (NLP) are two different kinds of pulse regimes in ultrafast fiber lasers, which have many intense applications. In this article, we experimentally demonstrate that the pulse regime of an Er-doped fiber laser could be converted between conventional soliton and noise-like pulse by using fast response saturable absorbers (SA) made from different layers of single-wall carbon nanotubes (CNT). For the monolayer (ML) single-wall CNT-SA, CS with pulse duration of 439 fs at 1560 nm is achieved while for the bilayer (BL) single-wall CNT, NLP at 1560 nm with a 1.75 ps spike and a 98 ps pedestal is obtained. The transition mechanism from CS to NLP is investigated by analyzing the optical characteristics of ML and BL single-wall CNT. The further theoretical simulation illustrates that CNT-SA enables the switching between CS and NLP in anomalous dispersion regime in Er-doped fiber lasers.

Measurement of ultrashort laser ablation of four metals (Al, Cu, Ni, W) in the single-pulse regime

We provide measurements of the ablation of four post-transition and transition metals [aluminum (Al), copper (Cu), nickel (Ni) and tungsten (W)] irradiated by single 800 nm laser pulses, in ultrashort regime from 100 femtosecond (fs) pulse duration down to 15 fs covering a temporal range little explored as yet. For each metal and pulse duration tested, we measured its ablation characteristics (depth and diameter) as a function of incident energy allowing us to determine its laser-induced ablation threshold and ablation rate in a single-shot regime. For all the metals studied, we observed a constant ablation threshold fluence as a function of pulse duration extending this scaling law to pulse duration of few-optical-cycles. We provide evidence of the interest of adjusting the incident fluence to maximize the energy specific ablation depth but also of the absence of any peculiar advantage related to the use of extremely short-pulse duration for ablation purposes. Those informative and detailed ablation data have been obtained in the single-pulse regime and in air ambiance. They can serve as rewarding feedback for further establishing smart strategy for femtosecond laser micromachining and laser damage handling of metallic and metal-based components as well as for enhancing accuracy of modeling of fs laser interaction with metals in ultrashort regime.