Nicolas Aubry

Yb:YAG single crystal fiber power amplifier for femtosecond sources

We demonstrate a versatile femtosecond power amplifier using a Yb:YAG single crystal fiber operating from 10 kHz to 10 MHz. For a total pump power of 75 W, up to 30 W is generated from the double-pass power amplifier. At a repetition rate of 10 kHz, an output energy of 1 mJ is obtained after recompression. In this configuration, the pulse duration is 380 fs, corresponding to a peak power of 2.2 GW. The M2 beam quality factor is better than 1.1 for investigated parameters.

Passively Q-switched diode-pumped Er:YAG solid-state laser.

We demonstrated laser operation of a passively Q-switched diode-pumped Er:YAG solid-state laser emitting at 1645 or 1617 nm depending on the initial transmission of the Cr:ZnSe saturable absorber. The crystal emitted up to 10 W at 1645 nm and up to 8 W at 1617 nm in CW mode while pumped with 65 W of incident pump power at 1533 nm. In passive Q-switched mode with 40 W of incident power, a Cr:ZnSe saturable absorber with initial transmission of 85% led to 330 μJ pulse energy, 61 ns pulse duration at a repetition rate of 1460 Hz at 1645 nm. An 80% initial transmission Cr:ZnSe sample led to 510 μJ energy pulses, 41 ns pulse duration at a repetition rate of 820 Hz with a central wavelength change from 1645 to 1617 nm. This is the first reported passively Q-switched diode-pumped Er:YAG laser operating on the (4)I(13/2)→(4)I(15/2) transition.

250 W single-crystal fiber Yb:YAG laser.

We demonstrate an Yb:YAG single-crystal fiber laser with 251 W output power in continuous-wave and an optical efficiency of 44%. This performance can be explained by the high overlap between pump and signal beams brought by the pump guiding and by the good thermal management provided by the single-crystal fiber geometry. The oscillator performance with a reflectivity of the output coupler as low as 20% also shows high potential for power amplification.

Direct amplification of ultrashort pulses in μ-pulling-down Yb:YAG single crystal fibers.

We demonstrated that Yb:YAG single crystal fibers have a strong potential for the amplification of femtosecond pulses. Seeded by 230 fs pulses with an average power of 400 mW, the system produced 330 fs pulses with an average power of 12 W. This is the shortest pulse duration ever produced by a Yb:YAG amplifier. The gain in the single crystal fiber reached a value as high as 30 in a simple double pass configuration.

Magic mode switching in Yb:CaGdAlO 4 laser under high pump power

We present unique spatial-mode switching in a cw Yb:CALGO laser when pumped at a multihundred-watts power level. It permits us to automatically stabilize to a TEM(00) mode from a highly spatial multimode regime. This stabilization is achievable thanks to polarization-mode switching allowed by the particular spectroscopic and thermal properties of Yb:CALGO crystal. This atypical and unexpected behavior is studied in detail in this Letter and explained by analysis of the thermo-optical coefficients for CALGO.

Amplification of cylindrically polarized laser beams in single crystal fiber amplifiers

Yb:YAG single crystal fiber (SCF) amplifiers have recently drawn much attention in the field of amplification of ultra-short pulses. In this paper, we report on the use of SCF amplifiers for the amplification of cylindrically polarized laser beams, as such beams offer promising properties for numerous applications. While the amplification of cylindrically polarized beams is challenging with other amplifier designs due to thermally induced depolarization, we demonstrate the amplification of 32 W cylindrically polarized beams to an output power of 100 W. A measured degree of radial polarization after the SCF of about 95% indicates an excellent conservation of polarization.

1617 nm emission control of an Er:YAG laser by a corrugated single-layer resonant grating mirror

A resonant grating mirror (RGM) that combines a single layer planar waveguide and a subwavelength grating is used to simultaneously control the beam quality, the spectral bandwidth, and the polarization state of an Er:YAG laser. This simple device is compared to classical methods using several intracavity components: an etalon for wavelength selection, a thin film polarizer for polarization selection, and an aperture for spatial filtering. It is demonstrated that the RGM provides the same polarization purity, an enhanced spectral filtering, and a significant improvement of the beam quality. In CW operation, the Er:YAG laser with a RGM emits an output power of 1.4 W at 1617 nm with a M2 of 1.4.

Test beam results of a high granularity LuAG fibre calorimeter prototype

The progresses in the micropulling-down technique allow heavy scintillating crystals to be grown directly into a fibre geometry of variable shape, length and diameter. Examples of materials that can be grown with this technique are Lutetium Aluminum Garnets (LuAG, Lu3Al5O12) and Yttrium Aluminum Garnets (YAG, Y3Al5O12). Thanks to the flexibility of this approach, combined with the high density and good radiation hardness of the materials, such a technology represents a powerful tool for the development of future calorimeters. As an important proof of concept of the application of crystal fibres in future experiments, a small calorimeter prototype was built and tested on beam. A grooved brass absorber (dimensions 26cm×7cm×16cm) was instrumented with 64 LuAG fibres, 56 of which were doped with Cerium, while the remaining 8 were undoped. Each fibre was readout individually using 8 eightfold Silicon Photomultiplier arrays, thus providing a highly granular description of the shower development inside the module as well as good tracking capabilities. The module was tested at the Fermilab Test Beam Facility using electrons and pions in the 2–16 GeV energy range. The module performance as well as fibre characterization results from this beam test are presented.

High-power laser with Nd:YAG single-crystal fiber grown by micro-pulling down technique

We designed single-crystal fibers to combine excellent spectroscopic and thermo-mechanical properties of bulk crystals and ability of pump guiding and good heat repartition of doped glass fibers. Such single-crystal fibers of excellent optical quality were grown by the micro-pulling-down technique. A remarkable advantage of this technique is that pump guiding is achieved in the directly grown fiber without additional polishing on the cylinder. We designed 0.2%-Nd doped YAG crystal fibers sample of 50 mm and 1 mm diameter and AR coated on both end faces. It was longitudinally pumped by a fiber-coupled laser diode with a maximum output power of 120 W at 808 nm. Laser emission at 1064 nm was achieved inside a two concave mirrors cavity. We obtained 20 W of laser emission with a M2 quality factor of 6, for an incident pump power of 120 W and a slope efficiency of 18% without any thermal management problems. Besides, a power of 16 W with linearly polarized laser emission has been obtained under the same pump power by introducing a thin plate polarizer in the cavity. An acousto-optical modulator was inserted inside the cavity and 360 kW of peak power with 12 ns pulses at 1 kHz repetition rate were achieved under 60 W of pump power. This work shows real improvements of laser performances in directly grown single crystal fibers. A complete thermal study confirms a good heat management and demonstrates scalability to high average power laser sources.

Oxide crystal-fibers grown by micro-pulling-down technique and applications for lasers and scintillators

We demonstrated growth of YAG, LuAG and CALGO single crystal fibers with doping Nd, Yb, Er, and Ce by the micro-pulling-down technique. Those fibers have applications in high power lasers and scintillating detectors. For laser operation, average power of 65 W energy of 4 mJ and peak power above 7 MW have been demonstrated in various configurations. Those results push the limits of end-pumped bulk crystals in terms of average power and exceed the limits of pulsed fibers lasers in terms of energy. For scintillating applications, high density/high light yield detectors are developed for nuclear science and medical applications.