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.
For a study of the European Space Agency ESA, Fraunhofer ILT developed and built single-frequency, linearly polarized, power-stabilized highly stable fiber amplifiers as Elegant Breadboard (EBB) with an output power ⪆3 W for the future space-based gravitational wave detector LISA (Laser Interferometer Space Antenna). While the fiber amplifier developed at Fraunhofer ILT has fulfilled most of the optical performance requirements, e.g. the relative intensity noise (RIN), we further optimized our setup to investigate the power scaling to expand the number of potential applications. In this paper, we present the power scaling of our optimized breadboard setup of our single-stage Ytterbium-doped fiber amplifier. Our fiber amplifier is seeded by a commercial Non-Planar Ring Oscillator (NPRO) with a linewidth ⪅10 kHz at a wavelength of 1064 nm. The fiber amplifier is based on cladding-pumped 10 μm-core Ytterbium-doped SM-PM fiber and provides an SBS-free amplification to an output power of more than 15 W while maintaining narrow-linewidth and polarization. Additionally, we investigate the relative intensity noise to analyze the temporal stability of the output power and achieve high power stabilities.
We present the European development of an engineering model Laser Head for LISA. This single box includes a seed laser, an electro-optical phase modulator, a fiber amplifier and all PCBs to operate the Laser Head.
For a study of the European Space Agency ESA, Fraunhofer ILT developed and built single-frequency, linearly polarized, power-stabilized fiber amplifiers as elegant breadboard (EBB) with an output power <3 W for the future space-based gravitational wave detector LISA (Laser Interferometer Space Antenna). The fiber amplifier developed at Fraunhofer ILT in a previous phase has fulfilled most of the optical performance requirements, except the relative intensity noise (RIN) [1]. In this paper, we present our revised and optimized fiber amplifier which now, in addition to the earlier demonstrated parameters, fulfills the LISA RIN requirements. Currently, the engineering model (EM) of the chosen fiber amplifier concept is being implemented by our project partner SpaceTech GmbH. Furthermore, since the Technology Readiness Level (TRL) of the components has to be confirmed for the EM, Fraunhofer ILT conducted 1000hours operational long-term tests of the components installed in a thermal-vacuum chamber and 2-weeks nonoperational tests in a thermal cycling chamber to qualify them for space applications.
Within the context of the E-TEST (Einstein Telescope EMR Site & Technology) project, Fraunhofer ILT develops thulium- and holmium-based seed sources and fiber lasers at app. 2 μm wavelength with highest demands on linewidth and stability for usage in a third-generation gravitational wave detector, the Einstein telescope. To fulfill the requirements, we develop a seed laser and a multi-stage fiber amplifier, consisting of holmium-doped fibers. Within this paper, we present our current laser concept and the first results of our dual-stage holmium-doped fiber amplifier stage. We achieve a low linewidth (< 2 MHz) output power of more than 5 W at a wavelength of 2095 nm. By using our in-house developed fiber laser simulation, we show that the efficiency of our amplifier is currently limited by pair induced quenching and the potential for further power scaling.
Based on our study of a high stability ultra-narrow linewidth fiber amplifier for the gravitational wave detector LISA, we have set up a hybrid fiber and Innoslab amplifier for further power scaling into <100 Watt regime. The fiber amplifier can provide a seed power of up to 8 W at 1064 nm and a linewidth <10 kHz. The booster stage consists of an in-band pumped Innoslab amplifier, which is pumped by stabilized laser diodes at 880 nm. An output power of 437 W has already been demonstrated with almost diffraction-limited beam quality. Further investigations of power stability and modal properties are currently ongoing.
Within the European Space Agency (ESA) activity “Gravitational Wave Observatory Metrology Laser” we designed a laser head to fulfill the LISA laser requirements using a non-NPRO seed laser technology: an external cavity diode laser (ECDL) with resonant optical feedback from an external cavity as master oscillator for further linewidth narrowing. Furthermore, our design features a single-stage fiber amplifier with an amplification factor of about 20 dB. This paper covers the requirements on the laser source for LISA, the design and first results of performance characterization of the laser head breadboard.
For studies of the European Space Agency ESA, Fraunhofer ILT develops and builds narrowband, power-stabilized fundamental mode fiber amplifiers especially for future space-based gravitational-wave detectors, e.g. LISA, and for Earth gravity field missions. In this paper, we present the status of our ongoing work, based on a highly stable fiber amplifier designed for a Next Generation Gravity field Mission (NGGM) pre-study, towards power scaling as well as enhancement of the Technology Readiness Level (TRL). Our amplifier has already demonstrated to meet the requirements for future gravity field missions. It features a design that is free of stimulated Brillouin scattering (SBS) and a feedback loop for power stabilization.
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