High speed, high power 2D beam steering for mitigation of optomechanical parametric instability in gravitational wave detectors.

In this paper, we propose what we believe to be a novel strategy to control optomechanical parametric instability (PI) in gravitational wave (GW) detectors based on radiation pressure. The fast deflection of a high-power beam is the key element of our approach. We built a 2D deflection system based on a pair of acousto-optic modulators (AOMs) that combines high rapidity and a large scan range. As a fast frequency switching configurable AOM driver, we used a Universal Software Radio Peripheral (USRP) combined with a high-performance personal computer (PC). In this way, we demonstrate a 2D beam steering system with flat efficiency over the whole scan range and with a transition time of 50 ns between two arbitrary consecutive deflection positions for a beam power of 3.6 W.

Close look on cubic Tm:KY3F10 crystal for highly efficient lasing on the 3H4 → 3H5 transition.

We report on Czochralski growth, detailed ground- and excited-state absorption and emission spectroscopy and highly-efficient mid-infrared (∼2.3 µm) laser operation of a cubic potassium yttrium fluoride crystal, Tm:KY3F10. The peak stimulated-emission cross-section for the 3H4 → 3H5 transition is 0.34×10-20 cm2 at 2345 nm with an emission bandwidth exceeding 50 nm. The excited-state absorption spectra for the 3F4 → 3F2,3 and 3F4 → 3H4 transitions are measured and the cross-relaxation is quantified. A continuous-wave 5 at.% Tm:KY3F10 laser generated 0.84 W at 2331-2346 nm by pumping at 773 nm, with a record-high slope efficiency of 47.7% (versus the incident pump power) owing to the efficient action of energy-transfer upconversion leading to a pump quantum efficiency approaching 2. The first Tm:KY3F10 laser with ESA-assisted upconversion pumping (at 1048 nm) is also demonstrated. Due to its broadband emission properties, Tm:KY3F10 is promising for ultrashort pulse generation at ∼2.3-2.4 µm.

Thulium laser at ∼2.3 µm based on upconversion pumping.

We report on novel upconversion (UC) pumping schemes for 2.3 µm thulium (Tm) lasers (the H43→H53 transition) based on a photon avalanche mechanism populating the intermediate metastable level (F43) acting as an effective ground state. The proposed pump wavelengths are ∼1 and ∼1.5 µm, each one corresponding to a resonant excited-state absorption transition F43→F2,33 and F43→H43, respectively. UC pumping at 1040, 1055, and 1451 nm of 2.3 µm Tm:LiYF4 lasers is demonstrated. In the former case, the laser generates 102 mW at 2302 nm with a slope efficiency of 14.6% (versus the incident pump power). The laser dynamics is studied. UC pumping is promising for reaching high efficiencies in 2.3 µm Tm lasers.

In-band pumping of Tm:LiYF4 channel waveguide: a power scaling strategy for ∼2 µm waveguide lasers.

We report on a novel power scaling strategy for thulium waveguide (WG) lasers relying on in-band pumping by high-brightness Raman fiber lasers (RFLs) and the use of liquid-phase-epitaxy-grown fluoride crystalline thin films for better thermal management. Thulium channel WGs are produced by microstructuring the Tm3+:LiYF4/LiYF4 epitaxies via diamond-saw dicing. They are pumped by a RFL based on an erbium master oscillator power amplifier and a GeO2-doped silica fiber and emit polarized output at 1679 nm. A CW in-band-pumped (H63→F43) Tm3+:LiYF4 WG laser generates up to 2.05 W of a linearly polarized single-transverse-mode output at 1881 nm with a slope efficiency of 78.3% and a laser threshold of only 12 mW (versus the absorbed pump power).

Laser operation of highly-doped Tm:LiYF4 epitaxies: towards thin-disk lasers.

Quasi-continuous-wave laser operation of 20 at.% Tm:LiYF4 thin films (84-240 µm) grown by Liquid Phase Epitaxy (LPE) on undoped LiYF4 substrates is achieved. The 240 µm-thick Tm:LiYF4 active layer pumped at 793 nm with a simple double-pass scheme generated 152 mW (average power) at 1.91 µm with a slope efficiency of 34.4% with respect to the absorbed pump power. A model of highly-doped Tm:LiYF4 lasers accounting for cross-relaxation, energy-transfer upconversion and energy migration is developed showing good agreement with the experiment. The pump quantum efficiency for Tm3+ ions is discussed and the energy-transfer parameters are derived. These results show that LPE-grown Tm:LiYF4 thin films are promising for ~1.9 µm thin-disk lasers.

Ytterbium calcium fluoride waveguide laser.

Calcium fluoride is a well-known material for optical components. It is also suited for doping with rare-earth ions, e.g., ytterbium ones. Yb:CaF2 is an efficient gain medium for high-power and ultrashort-pulse bulk lasers around 1 µm. We report on the first Yb:CaF2 planar waveguide laser. High-optical-quality single-crystalline waveguiding Yb:CaF2 thin films are grown on bulk CaF2 substrates by Liquid Phase Epitaxy. The spectroscopic study indicates the predominant coordination of isolated Yb3+ ions in trigonal oxygen-assisted sites, C3v(T2). The optical gain in Yb:CaF2 waveguide is demonstrated. A 1.4 at.% Yb:CaF2 planar waveguide laser generated 114 mW at 1037 nm with a slope efficiency of 12.9%. Yb:CaF2 films are promising for power-scalable waveguide mode-locked lasers and amplifiers.

Watt-level Tm:LiYF4 channel waveguide laser produced by diamond saw dicing.

Low-loss surface channel waveguides with a cross-section of 30 × 30 µm2 are produced by diamond saw dicing of 6.2 at.% Tm3+, 3.5 at.% Gd3+:LiYF4 films grown by liquid phase epitaxy (LPE) on (001)-oriented bulk undoped LiYF4 substrates. Pumped by a Ti:Sapphire laser at 783 nm, a continuous-wave Tm:LiYF4 waveguide laser generated 1.30 W at 1880 nm (for π-polarization) with a slope efficiency of 80% with respect to the absorbed pump power. The laser threshold was at 80 mW. The waveguide morphology was studied revealing low roughness (3 ± 2 µm) as expressed by the propagation losses of <0.3 dB/cm. A combination of LPE and diamond saw dicing is a promising technology for multi-watt single-mode channel waveguide lasers and amplifiers.

Tm,Ho:LiYF4 planar waveguide laser at 2.05 µm.

The first holmium fluoride waveguide laser, to the best of our knowledge, is reported using a 25-µm-thick Gd3+-ion-modified 4.5 at. % Tm3+, 0.5 at. % Ho3+-codoped LiYF4 active layer grown by liquid phase epitaxy on (001)-oriented LiYF4 substrate. Pumped by a Ti:sapphire laser at 797.2 nm, the planar waveguide laser generates 81 mW of continuous-wave (CW) output at ∼2051 nm with a slope efficiency of 24%. Power scaling up to 186 mW at 2051 nm and 2065 nm in quasi-CW regime is demonstrated. The parameters of the Tm3+↔Ho3+ energy transfer are determined. Tm,Ho:LiYF4/LiYF4 epitaxies are promising for waveguide lasers and amplifiers at above 2 µm.

2.3 µm Tm3+:YLF mode-locked laser.

A passively mode-locked Tm:YLF laser emitting at 2.3 µm is reported for the first time, to the best of our knowledge. The continuous-wave stable mode-locking operation is obtained with a semiconductor saturable absorber mirror at a repetition rate of 100 MHz. The average output power is 165 mW for a pulse duration of 94 ps.

Design and properties of a coherent amplifying network laser.

The coherent amplifying network laser is based on an array of thousands of active laser fibers coherently combined to generate high peak-power pulses at a high repetition rate. To achieve such a massive network, new combination architectures are presented here. They are based on implementing a spherical array of amplifying fibers, thus removing the need for transport fibers from the initial scheme. These designs present an advantage in terms of scalability leading to significant reduction of the temporal fluctuations compared to those of a conventional high peak-power laser. Noise evolution with fiber number is calculated using a perturbative analysis of each channel parameters (phase, signal intensity, beam profile).