Ultra-Low-Noise, Single-Frequency, All-PM Thulium- and Holmium-Doped Fiber Amplifiers at 1950 nm and 2090 nm for Third-Generation Gravitational Wave Detectors

In the context of the E-TEST Interreg project, Fraunhofer ILT develops high stability, narrow linewidth laser seed sources and amplifiers at a wavelength of approx. 2 μm for potential usage in a third-generation gravitational wave detector: the Einstein telescope. While for such an application, there are highest demands on laser parameters such as linewidth, spectral purity, and polarization, especially stability properties such as the relative intensity noise must be optimized. To achieve highest output power stabilities, we develop a multi-stage low-noise holmium-doped fiber amplifier at a wavelength of 2095 nm, which is core-pumped by low-noise thulium-doped fiber lasers at a wavelength of approx. 1950 nm. In this article, we present the concept of the full laser system, and the achieved results for the pre-amplifier section, which perfectly fulfills the E-TEST pre-amplifier requirements. Our pump source, a thulium-doped fiber amplifier, achieves approx. 2 W output power at a wavelength of 1950 nm with 5 MHz linewidth and a polarization extinction ratio of 28 dB. With our holmium-doped fiber amplifier, we demonstrate an output power of approx. 400 mW at a wavelength of approx. 2095 nm with a linewidth of 2 MHz and a polarization extinction ratio of 18 dB. For both currently free-running systems we analyze the relative intensity noise (RIN) and obtained for example RINs of approx. 10 −6 Hz −0.5 at a frequency of 100 Hz, which shows the suitability of the concept to achieve highest stabilities. To evaluate our system, we perform a system performance analysis, where we present the influence of signal and pump wavelength, and signal and pump power on the achievable optical-optical efficiencies.