Frédéric Druon

On thermal effects in solid-state lasers: The case of ytterbium-doped materials

Abstract A review of theoretical and experimental studies of thermal effects in solid-state lasers is presented, with a special focus on diode-pumped ytterbium-doped materials. A large part of this review provides however general information applicable to any kind of solid-state laser. Our aim here is not to make a list of the techniques that have been used to minimize thermal effects, but instead to give an overview of the theoretical aspects underneath, and give a state-of-the-art of the tools at the disposal of the laser scientist to measure thermal effects. After a presentation of some general properties of Yb-doped materials ( Section 1 ), we address the issue of evaluating the temperature map in Yb-doped laser crystals, both theoretically and experimentally ( Section 2 ). This is the first step before studying the complex problem of thermal lensing ( Section 3 ). We will focus on some newly discussed aspects, like the definition of the thermo-optic coefficient: we will highlight some misleading interpretations of thermal lensing experiments due to the use of the dn/dT parameter in a context where it is not relevant. Section 4 will be devoted to a state-of-the-art of experimental techniques used to measure thermal lensing. Eventually, in Section 5 , we will give some concrete examples in Yb-doped materials, where their peculiarities will be pointed out.

Detection of Single Photoluminescent Diamond Nanoparticles in Cells and Study of the Internalization Pathway

Diamond nanoparticles are promising photoluminescent probes for tracking intracellular processes, due to embedded, perfectly photostable color centers. In this work, the spontaneous internalization of such nanoparticles (diameter 25 nm) in HeLa cancer cells is investigated by confocal microscopy and time-resolved techniques. Nanoparticles are observed inside the cell cytoplasm at the single-particle and single-color-center level, assessed by time-correlation intensity measurements. Improvement of the nanoparticle signal-to-noise ratio inside the cell is achieved using a pulsed-excitation laser and time-resolved detection taking advantage of the long radiative lifetime of the color-center excited state as compared to cell autofluorescence. The internalization pathways are also investigated, with endosomal marking and colocalization analyses. The low colocalization ratio observed proves that nanodiamonds are not trapped in endosomes, a promising result in prospect of drug delivery by these nanoparticles. Low cytotoxicity of these nanoparticles in this cell line is also shown.

47-fs diode-pumped Yb 3+ :CaGdAlO 4 laser

The experimental demonstration of a diode-pumped passively mode-locked femtosecond laser based on an Yb3+:CaGdAlO4 single crystal is reported. The oscillator is directly diode pumped by a high-brightness 5 W fiber coupled laser diode, and pulses are produced by use of a semiconductor saturable-absorber mirror. It permits the production of pulses as short as 47 fs at 1050 nm, which are to our knowledge the shortest laser pulses obtained from an oscillator based on Yb3+-doped bulk materials. The average power is 38 mW, and the repetition rate is 109 MHz.

High power diode-pumped Yb3+:CaF2 femtosecond laser

We report what is believed to be the first demonstration of a high-power passively mode-locked diode-pumped femtosecond laser based on an Yb3+:CaF2 single crystal, directly pumped by a 15-W fiber-coupled laser diode. With a 5-at. % Yb3+ -doped sample and prisms for dispersion compensation we obtained pulses as short as 150 fs, with 880 mW of average power and up to 1.4-W average output power, with a pulse duration of 220 fs, centered at 1049 nm. The laser wavelength could be tuned from 1040 to 1053 nm in the femtosecond regime. Using chirped mirrors for dispersion compensation, the oscillator provided up to 1.74 W of average power, with a pulse duration of 230 fs, corresponding to a pulse energy of 20 nJ and a peak power of 85 kW.

Diode-pumped Yb:GGG laser: comparison with Yb:YAG

Abstract A diode-pumped ytterbium-doped Gd 3 Ga 5 O 12 (Yb:GGG) laser is reported for the first time. We report up to 4.2 W of CW laser power when pumping on the 971 nm absorption band with a 14 W fiber-coupled diode. The easy growth and the high thermal conductivity of Yb:GGG make it a challenger of Yb:YAG for high power applications. The efficiencies of these two crystals have been compared under the same pumping conditions, and turn out to be comparable.

Generation of 90-fs pulses from a mode-locked diode-pumped Yb 3+ :Ca 4 GdO(BO 3 ) 3 laser

A diode-pumped Yb>(3+):Ca(4)GdO(BO>(3))(3) (Yb:GdCOB) laser generating 90-fs pulses at a center wavelength of 1045 nm is demonstrated. This is, to our knowledge, the shortest pulse duration obtained from an ytterbium laser with a crystalline host. This laser is mode locked with a high-finesse semiconductor saturable-absorber mirror and emits 40 mW of average power at a repetition rate of 100 MHz.

Efficient diode-pumped Yb 3+ :Y 2 SiO 5 and Yb 3+ :Lu 2 SiO 5 high-power femtosecond laser operation

We report the mode-locked operation of two new Yb-doped oxyorthosilicates, Y2SiO5 (YSO) and Lu2SiO5 (LSO), that are longitudinally diode pumped. Yb:YSO supplied pulses as short as 122 fs with 410 mW of output power at 1041 nm. More than 2.6 W of average output power, for pulse durations of 198 fs at 1044 nm and 260 fs at 1059 nm for Yb:YSO and Yb:LSO, respectively, were provided. These are, to our best knowledge, the highest values ever obtained and the most efficient mode-locked laser in such a classic fiber-coupled diode-pumping configuration.

High power tunable diode-pumped Yb3+:CaF2 laser

Results of diode-pumped cw laser operation of an Yb3+:CaF2 single crystal are reported for what is to our knowledge the first time. With a 5-at.% Yb3+ -doped sample we obtained 5.8-W output power at 1053 nm for 15 W of incident power at 980 nm. The laser wavelength could be tuned from 1018 to 1072 nm, and a small-signal gain as high as 1.8 was achieved, showing the great potential of Yb3+:CaF2 as an amplifier medium for femtosecond pulses.

Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers

We demonstrate coherent beam combining of two femtosecond fiber chirped-pulse amplifiers seeded by a common oscillator. Using a feedback loop based on an electro-optic phase modulator, an average power of 7.2 W before compression is obtained with a combining efficiency of 90%. The spatial and temporal qualities of the oscillator are retained, with a recombined pulse width of 325 fs. This experiment opens up a way to scale the peak/average power of ultrafast fiber sources.

On Yb:CaF2 and Yb:SrF2 : Review of spectroscopic and thermal properties and their impact on femtosecond and high power laser performance

We present an overview of laser results we obtained with Yb-doped calcium fluoride and its isotype strontium fluoride. In order to study the laser performance in femtosecond and high power regimes, spectral and thermal properties are first discussed including the potential of these crystals at room and cryogenic temperatures. Experimental demonstrations of high-power and ultrashort pulse oscillators and amplifiers are presented and analyzed.