On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160)  Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

/pdf/the-observation-of-gravitational-waves-from-a-binary-black-58srpupgg9.pdf

The Observation of Gravitational Waves from a Binary Black Hole Merger

In this paper, we summarize the fundamental properties and review the latest developments in high power fiber lasers. The review is focused primarily on the most common fiber laser configurations and the associated cladding pumping issues. Special attention is placed on pump combination techniques and the parameters that affect the brightness enhancement observed in single-mode and multimode high power fiber lasers. The review includes the major limitations imposed by fiber nonlinearities and other parasitic effects, such as optical damage, transverse modal instabilities and photodarkening. Finally, the paper summarizes the power evolution in continuous-wave and pulsed ytterbium-doped fiber lasers and their impact on industrial applications.

High Power Fiber Lasers: A Review

This work presents a review on the effect of transverse mode instability in high-power fiber laser systems and the corresponding investigations led worldwide over the past decade. This paper includes a description of the experimental observations and the physical origin of this effect, as well as some of the proposed mitigation strategies.

Transverse mode instability

We present a simple theoretical model of transverse mode instability in high-power rare-earth doped fiber amplifiers. The model shows that efficient power transfer between the fundamental and higher-order modes of the fiber can be induced by a nonlinear interaction mediated through the thermo-optic effect, leading to transverse mode instability. The temporal and spectral characteristics of the instability dynamics are investigated, and it is shown that the instability can be seeded by both quantum noise and signal intensity noise, while pure phase noise of the signal does not induce instability. It is also shown that the presence of a small harmonic amplitude modulation of the signal can lead to generation of higher harmonics in the output intensity when operating near the instability threshold.

/pdf/theoretical-analysis-of-mode-instability-in-high-power-fiber-eqze33oheo.pdf

Theoretical analysis of mode instability in high-power fiber amplifiers

We present comprehensive theoretical study of mode instability with a universal model for the first time, which includes quantitative study on the various influence factors and mitigation methods. With large numerical results, the influence of various fiber and system parameters has been studied to summarize mitigation methods. The mode instability threshold behavior of multiwavelength pumping configurations has been studied for the first time, and the influence of pump wavelength shifting in the tandem pumping technique and amplitude modulation have also been given for the first time. The mitigation methods are discussed with review of the latest development in experimental results. The drawbacks and possible overcome measures are also discussed. By properly combining the mitigation methods, an approach to achieve beyond 10-kW diode pumped fiber laser with near diffraction limited beam quality has been proposed, which provides a creative way to overcome the mode instability limitation on diode pumped fiber lasers, and demonstrates the effectiveness of the mitigation methods.

Comprehensive Theoretical Study of Mode Instability in High-Power Fiber Lasers by Employing a Universal Model and Its Implications

At the beginning of 2016, the first direct detection of a gravitational wave by a laser-based Michelson interferometer has been reported. So far, the number of confirmed detections has raised to three and will certainly increase further. Thereby, mankind has an amazing new tool to observe and study the universe, namely by gravitational wave astronomy. However, in order to develop their full potential, a more sensitive generation of earth-bound gravitational wave detectors will be required in the future. Current detectors are operating with solid-state laser systems at 1064 nm but higher output power levels and longer wavelengths will be required by next-generation detectors. The requirements with respect to the laser sources, e.g., on the beam quality and the linewidth, are very challenging and unique. In recent years, it has been demonstrated that single-frequency fiber amplifiers are a very attractive concept as next-generation gravitational wave detectors laser sources. This paper reviews, highlights, and summarizes the corresponding research results, in particular, regarding power scaling, noise properties, and coherent beam combination.

Single-Frequency Fiber Amplifiers for Next-Generation Gravitational Wave Detectors

We experimentally investigated the influence of amplified spontaneous emission within the Brillouin gain bandwidth on the Brillouin scattering of a single-frequency signal. The experiments were performed for the case of artificial ASE injected in backward direction into a passive fiber, as well as in forward direction of a low-power fiber amplifier. A significant influence could be observed, when the ASE was counter-propagating to the signal. Injecting 160.6 nW of ASE within the Brillouin gain bandwidth led to a decrease of about 3 dB of the SBS-threshold of an approximately 335 m long passive fiber from about 80 mW to less than 40 mW. At a fixed signal power of 81 mW the backscattered power and the power in the Brillouin scattered Stokes maximum increased by a factor of 19.

Impact of amplified spontaneous emission on Brillouin scattering of a single-frequency signal

Summary form only given. Applications in gravitational wave detectors [1] require high power lasers with approximately 200 W of single-frequency output in linearly polarized TEM00-mode. Single-frequency signals have been amplified to more than 500 W in master oscillator power amplifier (MOPA) configurations, employing photonic crystal fibers (PCF) [2] as well as chirally coupled core (CCC) fibers [3]. For a PCF amplifier system, a fundamental Gaussian mode power of 203 W was demonstrated in linear polarization [4]. In this contribution we report for the first time measurements of the overlap of high power CCC fiber amplifiers output beam with the TEM00-mode of a scanning non-confocal ring cavity [5].The seed signal for our experiments had a linewidth of about 1 kHz at 1064 nm and a maximum power level of about 11 W. Nominal core and modefield diameters of the tested active fibers were 37 μm and about 30 μm, respectively. The fiber was available in standard polymer-coated double-clad geometry (413 μm) and with an airclad (260 μm inner and 440 μm outer cladding). However, due to the non-optimum cladding shape (round clad) of the polymer-coated fibers in this batch approximately 50 % of the coupled pump light remained unabsorbed. Therefore, here we concentrate on results obtained with air-clad fibers. TEM00 mode content (%)Fig. 1(a) shows the output power versus absorbed pump power for 2.2 m of an air-clad version of this fiber, as well as the corresponding fractional TEM00-mode content. This fiber had a nominal absorption of 9 dB/m and was mounted on a cold-plate with a coiling diameter of 30 cm. With increasing pump power, the fundamental mode content first increased from 95.6 % at 19 W of amplifier output to 97 % at output powers levels between 49 W and 78 W. Concurrently, the PER increased from 17.8 dB to more than 20 dB. At an amplifier output of 103 W, about 98 W of power could be extracted in the linearly polarized TEM00-mode. The corresponding modescan is shown in Fig. 1(b). Increasing the pump power led to a decreased beam quality and a higher sensitivity of the modal content to the seed alignment. Power scaling was limited by the onset of modal instabilities [6] to 230 W at the amplifier output, at which point the highest TEM00-mode power reached 186 W, corresponding to a fractional fundamental mode content of 80.7 %.

Fundamental Gaussian mode content measurements on active large core CCC fibers

We experimentally investigated the influence of amplified spontaneous emission within the Brillouin gain bandwidth on the Brillouin scattered part of a single-frequency signal. A significant influence could be seen in a passive fiber.

Investigations on the Impact of Amplified Spontaneous Emission on Brillouin Scattering of a Single-Frequency Signal

High-power single-frequency lasers using nonplanar ring-oscillator (NPRO) seed sources amplified to about 200 W have found application in gravitational wave detectors [1], in which only the TEM 00 -mode content can be used. Using photonic crystal fibers (PCF) in a master oscillator power amplifier (MOPA) configuration seems to be a promising way to achieve several 100 W of single-frequency output power at 1 µm wavelength [2], but the TEM 00 -mode content has only been measured of a system with a limited output power of 148 W [3]. In this contribution, we report on mode-content measurements on a PCF MOPA system suitable for further power scaling.

Single-frequency Yb-doped PCF MOPA with 294 W of output power for gravitational wave detection

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