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 M<sup>2</sup> < 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. <p> </p>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. <p> </p>Laserline GmbH and Fraunhofer Institute for Laser Technology joined their forces<sup>1</sup> 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. <p> </p>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 M<sup>2</sup> 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<sup>-3</sup> Hz to 10 Hz and a polarization extinction ratio of >30 dB.
Diode lasers are frequently used for numerous applications demanding high cw average power in the kW region and
comparably low brightness. These applications include polymer welding, transformation hardening of metals and
medical applications. Compared to solid state lasers, diode lasers can not be efficiently q-switched due to their low
upper state life time. Therefore diode lasers are usually not suited for applications requiring high peak power like
marking and coating removal. To overcome this problem, we have developed a novel electrically pulsed diode laser
source. If the pulses are comparably short in the region of a few hundred nanoseconds, diode lasers can be operated with
a current five to ten times higher than the maximum cw current. This so called super-pulse mode of operation broadens
the field of applications of high power diode lasers towards applications usually reserved for q-switched solid state
lasers. To benefit from the improved brightness delivered by the super-pulsed diode lasers for materials processing, a
state of the art beam forming optics is required. In this paper, we will demonstrate the design of a super-pulsed diode
laser source consisting of four diode laser bars coupled into a 100 μm NA=0.2 optical fiber. This module is designed for
an output power of 500 W. To select diode laser bars appropriate for the super-pulse mode of operation, different diode
laser bars have been tested with peak currents up to ten times higher than the rated cw current. Material processing
results with super-pulsed diode lasers will be presented.
To realize a completely monolithic, pulsed, fiber laser without free space elements we describe a gain-switched
fiber laser pumped with a pulsed diode laser at about 965 nm with more than 30 μJ in a 200 ns pulse. For
best beam quality we use a single mode fiber with a 6 μm core diameter. We report lasing of an Yb-doped
double-clad fiber at 1080 nm and a pulse energy of about 8 μJ with a variable repetition rate from 1 - 50 kHz.
The experimental results are compared with the data of a time resolved simulation and basic analytically derived