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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.

Interferometric gravitational wave detectors set very high requirements on their single-frequency laser source in terms of output power and stability. We discuss current approaches to fulfill these requirements using high-power single-frequency fiber amplifiers.

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 %.

A current limit in power scaling of Yb-doped fiber amplifiers is the sudden onset of mode instabilities. We investigated this effect on a single-frequency Yb-doped photonic crystal fiber amplifier with a low mode instability threshold power. By measuring the overlap of the fiber output beam with the fundamental mode of an external cavity to be about 95%, we could exclude significant modal power transfer below a sharp power threshold. Furthermore, we directly measured the frequency resolved intensity noise spectra. No fluctuations in the overall output power were observed, but for the modal content different oscillation regimes were identified.

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.

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.

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.