Narrow-linewidth and low-noise holmium-doped fiber amplifier at 2095 nm with compact packaging

We here present our holmium-doped fiber amplifier developed for research on topics on a future third-generation gravitational wave detector for the ETpathfinder research facility. For lasers to be used in gravitational wave detectors, there are not only highest demands on typical laser parameters, such as linewidth, spectral purity, and polarization, but especially on the Relative Intensity Noise (RIN) properties. Our developed laser system consists of a compactly packaged holmium-doped fiber amplifier, which is pumped by a thulium-doped fiber amplifier. By setting up both fiber amplifiers in the same, thermally stabilized and compactly engineered laser housing, we aim to achieve highest output power stabilities. With our current setup, we achieve an output power of app. 450 mW with linear polarization and a low linewidth of app. 2 MHz at a wavelength of 2095 nm. To analyze the RIN, we use our in-house developed measurement setup. We present the achieved RIN results in the frequency range from 10-3 to 104 Hz, and for example at a frequency of 100 Hz, we achieve a RIN of app. 10-6 1/Hz0.5, which shows the suitability of our concept to achieve highest power stabilities.