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 EKOLAS consortium, which is part of the BMBF-funded EffiLAS (Efficient high-performance laser beam sources) research initiative, we are developing fiber Bragg gratings (FBG) directly written into multimode fibers. Fiber lasers are an established beam source for high-power materials processing due to their high efficiency and high average output power at high beam quality. By using FBG as fiber-integrated output mirrors, which is state-of-the-art in singlemode fiber lasers, we aim to reduce the complexity and increase robustness and reliability of multimode fiber resonators. Therefore, we are investigating the use of FBG as outcoupling mirrors in multimode high-power multimode fiber lasers. As a first step, we directly write an FBG into an active extra-large mode area (XLMA) fiber with <100 μm core and use the FBG as low reflective outcoupling mirror for the fiber resonator with simultaneous frequency stabilization. The setup delivers an output power of more than 800 W at 1077 nm. The output power of the system was limited by the pump laser setup and not by the FBG or its temperature. The FBG is passively cooled and the measured temperature of the fiber at the grating is below 130 °C at 800 W output power. As the next step, we set up an active XLMA-fiber (core <100 μm) with an FBG as outcoupling mirror into a laser resonator with water cooling of the resonator fiber and optimized pump coupling. This setup delivers an output power of more than 8 kW at 1077 nm without failure of the FBG.
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.
In this paper, we present our current work towards a highly efficient XLMA (extra-large mode area) fiber-based laser, which is being performed in the EKOLAS consortium within the BMBF-funded EffiLAS (efficient high-performance laser beam sources) research alliance. To this end, the complete manufacturing process chain of the XLMA fiber was reviewed and optimized. The work started with the material composition of the active XLMA preform with the goal of improving the purity and thus the background loss. A successfully implemented fluorine co-doping process allows refractive index adjustment of the active core material which improves the beam quality of the laser fibers without changing the concentration of active ions in the glass composition. The preform is subjected to a screening in which possible scatter centers, e.g. bubbles, inclusions or contaminants, are mapped and categorized, in order to identify defects, which could lead to a failure in the drawn fiber, already at an early production stage. The subsequent fiber drawing is monitored for scattering using the emissions from the heated preform as well as for inhomogeneities of the dopants using a phase measurement technique. Finally, the fiber is tested for residual impurities and background losses using a multi-mode OTDR to ensure that the fibers are free of any defects.
Within the EKOLAS consortium, which is part of the BmBF-funded Effilas (Efficient high-performance laser beam sources) research alliance, we are developing Fiber Bragg Gratings (FBGs) written in extra-large mode area (XLMA) fibers. By using FBGs as fiber integrated output mirrors, which is state-of-the-art in single-mode fiber lasers, we aim to reduce complexity and increase robustness and reliability of multi-mode fiber resonators. To this end, we are investigating the use of FBGs as outcoupling mirrors with a reflectivity below 10 % in XLMA high-power multi-mode fiber lasers. As a first step, FBGs are written into a passive 105/125 μm multi-mode fiber. We present their application for frequency stabilization of a resonator based on XLMA fibers, and tested the FBGs up to an output power of 150 W at 1075 nm without failure. As the next step, transition from passive 105/125 μm fibers to active XLMA fibers is currently being investigated. For FBG inscription, we use a phase mask and an ultra-fast laser system (100 fs, 800 nm). The setup is adjustable in three translation and three rotation axes. Additionally it features a modular mechanical design for fast and flexible interchanging of fiber mounts, phase mask and process optics. The FBGs are pre-characterized in transmission with a white-light source and an optical spectrum analyzer.
In this paper we present a simple approach to achieving nanosecond pulses from a directly q-switched high-power resonator based on extra-large mode area (XLMA) fibers with a beam quality factor M2 < 15. An average output power of > 500 W has been demonstrated for repetition frequencies between 50-100 kHz. The resonator consists of a single fiber q-switched with soldered Pockels-cells which exhibit a very high contrast ratio leading to output pulses down to about 10 ns and peak powers up to > 250 kW at 1064 nm wavelength.
By using this design instead of a fiber MOPA setup, a cost-effective and less complex system could be implemented.
In surface processing applications the correlation of laser power to processing speed demands a further enhancement of the performance of short-pulsed laser sources with respect to the investment costs. The frequently applied concept of master oscillator power amplifier relies on a complex structure, parts of which are highly sensitive to back reflected amplified radiation. Aiming for a simpler, robust source using only a single ytterbium doped XLMA fiber in a q-switched resonator appears as promising design approach eliminating the need for subsequent amplification. This concept requires a high power-tolerant resonator which is provided by the multikilowatt laser platform of Laserline including directly water-cooled active fiber thermal management.
Laserline GmbH and Fraunhofer Institute for Laser Technology joined their forces1 to upgrade standard high power laser sources for short-pulsed operation exceeding 1 kW of average power. Therefor a compact, modular qswitch has been developed.
In this paper the implementation of a polarization independent q-switch into an off-the-shelf multi-kilowatt diodepumped continuous wave fiber source is shown. In this early step of implementation we demonstrated more than 1000 W of average power at pulse lengths below 50 ns FWHM and 7.5 mJ pulse energy. The M2 corresponds to 9.5. Reliability of the system is demonstrated based on measurements including temperature and stability records. We investigated the variation possibilities concerning pulse parameters and shape as well as upcoming challenges in power up-scaling.
With GRACE (launched 2002) and GOCE (launched 2009) two very successful missions to measure earth’s gravity field have been in orbit, both leading to a large number of publications. For a potential Next Generation Gravity Mission (NGGM) from ESA a satellite-to-satellite tracking (SST) scheme, similar to GRACE is under discussion, with a laser ranging interferometer instead of a Ka-Band link to enable much lower measurement noise. Of key importance for such a laser interferometer is a single frequency laser source with a linewidth <10 kHz and extremely low frequency noise down to 40 Hz / √Hz in the measurement frequency band of 0.1 mHz to 1 Hz, which is about one order of magnitude more demanding than LISA. On GRACE FO a laser ranging interferometer (LRI) will fly as a demonstrator. The LRI is a joint development between USA (JPL,NASA) and Germany(GFZ,DLR). In this collaboration the JPL contributions are the instrument electronics, the reference cavity and the single frequency laser, while STI as the German industry prime is responsible for the optical bench and the retroreflector. In preparation of NGGM an all European instrument development is the goal.
ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission and the American-German Gravity Recovery and Climate Experiment (GRACE) mission map the Earth’s gravity field and deliver valuable data for climate research.
Spaceborne lidar (light detection and ranging) systems have a large potential to become powerful instruments in the field of atmospheric research. Obviously, they have to be in operation for about three years without any maintenance like readjusting. Furthermore, they have to withstand strong temperature cycles typically in the range of -30 to +50 °C as well as mechanical shocks and vibrations, especially during launch. Additionally, the avoidance of any organic material inside the laser box is required, particularly in UV lasers. For atmospheric research pulses of about several 10 mJ at repetition rates of several 10 Hz are required in many cases. Those parameters are typically addressed by DPSSL that comprise components like: laser crystals, nonlinear crystals in pockels cells, faraday isolators and frequency converters, passive fibers, diode lasers and of course a lot of mirrors and lenses. In particular, some components have strong requirements regarding their tilt stability that is often in the 10 μrad range. In most of the cases components and packages that are used for industrial lasers do not fulfil all those requirements. Thus, the packaging of all these key components has been developed to meet those specifications only making use of metal and ceramics beside the optical component itself. All joints between the optical component and the laser baseplate are soldered or screwed. No clamps or adhesives are used. Most of the critical properties like tilting after temperature cycling have been proven in several tests. Currently, these components are used to build up first prototypes for spaceborne systems.
In scope of the ESA funded “High stability Laser” activity, a single-mode and single-frequency fiber power amplifier with 500 mW output power at 1064 nm wavelength has been developed. It is part of an elegant breadboard (EBB) which consists additionally of an ultra-stable Fabry-Perot reference for frequency stabilization. The monolithic fiber amplifier is seeded by a non-planar ring oscillator (NPRO) with a linewidth below 10 kHz. The amplifier is stabilized in power via pump diode modulation and achieves a RIN performance of < 0.01/sqrt(Hz) in the range from 10-3 Hz to 10 Hz and a polarization extinction ratio of >30 dB.
We present a laser drilling technology eminently suitable for structuring of solid glass preforms for microstructured optical fibers (MOF). This technology allows fiber designs that can not be easily adressed by stack and draw technology. As an example, we present a four ring hexagonal hole structure drilled in a silica rod over a length of 80 mm at ILT. The fiber drawn from this preform was used for absorption measurements and fiber Bragg grating inscription experiments at IPHT. Geometrical aspects are compared to those of a MOF with a similar structure made by the stack and draw technology.
High power fiber lasers deliver multiple kW laser power with a diffraction limited beam quality. A drawback for
some applications is the arbitrary polarization. We report on experimental and theoretical results of kW class
cw fiber lasers with linear polarization. A comparison of different concepts for generation of polarized high power
output will be presented. The most feasible design for kW class power scaling will be selected. In order to find
an empirical formula for calculating bend loss, additional measurements are carried out and are then compared
to the theoretical results.