The two interferometers of the Laser Interferometry Gravitaional-wave Observatory (LIGO) recently detected gravitational waves from the mergers of binary black hole systems. Accurate calibration of the output of these detectors was crucial for the observation of these events, and the extraction of parameters of the sources. The principal tools used to calibrate the responses of the second-generation (Advanced) LIGO detectors to gravitational waves are systems based on radiation pressure and referred to as Photon Calibrators. These systems, which were completely redesigned for Advanced LIGO, include several significant upgrades that enable them to meet the calibration requirements of second-generation gravitational wave detectors in the new era of gravitational-wave astronomy. We report on the design, implementation, and operation of these Advanced LIGO Photon Calibrators that are currently providing fiducial displacements on the order of 10 −18 m/ √ Hz with accuracy and precision of better than 1 %.

We investigated the effect of rare earth impurities on the cooling efficiency of Yb³⁺:LiYF₄ (Yb:YLF). The refrigeration performance of two single crystals, doped with 5%-at. Yb and with identical history but with different amount of contaminations, have been compared by measuring the cooling efficiency curves. Spectroscopic and elemental analyses of the samples have been carried out to identify the contaminants, to quantify their concentrations and to understand their effect on the cooling efficiencies. A model of energy transfer processes between Yb and other rare earth ions is suggested, identifying Erbium and Holmium as elements that produce a detrimental effect on the cooling performance.

We investigated the polarization of continuous-wave laser emission in the orange region, at 607 nm, and in the red region, at 639 nm and 643 nm, from a Pr:BaY2F8 (Pr:BYF) crystal, pumped by a 445 nm laser diode. We achieved linearly polarized emission along two optic axes of the crystal by changing its orientation with respect to the pump. Simultaneous emission of two orthogonal linear polarizations was observed in the orange region, at the same wavelength, and in the red region, with concurrent emission from the two separate lines.

We report on, to the best of our knowledge, the first growth, spectroscopy, and laser operation of monocrystalline Ba(Y_0.8Lu_0.2)₂F₈ doped with 1.25 at.% of Pr³⁺ ions (Pr:BYLF). We studied the spectroscopic properties of this material, particularly in comparison with its isomorph Pr³⁺: BaY₂F₈ (Pr:BYF). Under diode-pumped excitation, we achieved continuous-wave laser operation in the orange, red, and deep red regions in the three possible crystal orientations. We obtained considerably improved laser performances and remarkably low passive losses, in comparison with Pr:BYF crystals and other fluorides, indicating that Pr:BYLF is an excellent host material in the visible region.

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