We present our current results on the fabrication of arbitrary shaped fiber tapers on our tapering rig using a CO2-laser as heat source. Single mode excitation of multimode fibers as well as changing the fiber geometry in an LPG-like fashion is presented. It is shown that this setup allows for reproducible fabrication of single-mode excitation tapers to extract the fundamental mode (M<sup>2 </sup>< 1.1) from a 30 μm core having an NA of 0.09.
The Gravity Recovery and Climate Experiment Follow-On (GRACE FO) is a space borne mission to map variations in the earth’s gravity field with an even greater accuracy than the first GRACE mission. GRACE FO is a collaborative project of NASA (USA) and GFZ (Germany) scheduled for launch in 2017. On GRACE the gravity field is reconstructed from a measurement of the distance variation between two satellites following each other in 200 km distance by use of a microwave ranging instrument. On GRACE FO a laser ranging interferometer (LRI) is added as a demonstrator in addition to the microwave. Moving from microwave range to optical wavelengths provides an improvement in distance measurement noise from some μm/√Hz to 80 nm/√Hz down to 0.01 Hz frequency. The criteria on the beam delivery system are demanding, in particular with respect to laser beam quality, wave front deviation and pointing as well as thermal and mechanical stability. Conventionally such a system can be manufactured with at least two special mounted lenses or an aspheric lens aligned with respect to the fiber end. However, the alignment of this optical system must be maintained throughout the mission, including the critical launch phase and a wide temperature range in orbit, leading to high alignment effort and athermal design requirements. The monolithic fiber-collimator presented here provides excellent optical and thermal and mechanical performance. It is a part of the LRI and located on the Optical Bench Assembly (OBA) which has already been described in [1, 3].
The generation of high power in active fiber application and the transmission of high laser power via fiber cables both require protection from misdirected laser light. The following paper presents a new approach to removing this unwanted part of light. The deposition of fused silica material on the fiber cladding applied with CO<sub>2</sub> laser processes constitutes a robust cladding light stripper suitable for high power levels. The CO<sub>2</sub> laser processes are easy to apply, obviate the need for any dangerous liquids and promise greater mechanical stability in handling and assembly.
The application of photonic crystal fibers (PCF), especially in high power fiber laser systems, requires special
preparation technologies with some significant differences compared to standard fibers. Features, like air-clad structures,
highly rare-earth doped cores with low NA and stress applying parts of the PCFs, require additional steps in fiber
preparation and innovative splicing technologies to gain optical properties. Here we discuss a contamination- free carbon
dioxide laser splicing device, which is used for defined air-clad collapsing and end cap splicing to get a stable and sealed
fiber end face with preserved high beam quality and additional functionality. The special design of the computer-controlled
laser splicing process provides a versatile tool with high reproducibility for joining different geometries with
an adjustable well-balanced heat distribution. A wide range of PCFs with different diameters, air-clad structures and
doped materials up to ~2 mm have been spliced. For selected PCF-end cap splices cleave or polishing requirements as
well as results on beam quality, tensile strength and further splicing features are presented.
We present an all-fiber side pump combiner for high power fiber lasers and amplifiers. It consists of a capillary with
decreasing wall thickness fused around the active fiber in a way that it becomes an additional cladding layer of it. The
pump fibers are spliced at the edge of the capillary with the thickest wall. This provides enough room to allocate around
12 pump fibers (100μm diameter) in a standard 400μm double-clad fiber.
This side pump combiner offers several advantages such as the fact that it does not interrupt the active fiber core at any
point, thus allowing for truly monolithic all fiber lasers and amplifiers. Additionally, the pump light is coupled into the
active double-clad fiber all along the combiner's body (~ 1-2 cm long), which avoids the concentration of heat in a very
small area, resulting in high pump power handling capabilities.
If the taper angle of the capillary wall is low enough a high coupling efficiency (< 95%) is possible. Using this structure
we have achieved a maximum combined pump power of 86 Watt from 7 pump diodes. This, we believe, is the highest
combined pump power reported so far from a single lumped side pump combiner.
High power fiber laser assemblies require monolithic joining technologies for low loss, mechanically stable and reliable
spliced component interconnection. In contrast to conventional heat sources for splicing a carbon dioxide laser heats
optical fibers and end caps only by radiation. The advantages of laser heating, e.g. precisely defined areas of laser impact
and high process purity, meet the goals for high power applications. Requirements and challenges like tensile strength,
centricity and reproducibility while using the splicing technology for a production line will be shown on behalf of a
special developed CO<sub>2</sub> laser splicing device, splicing experiments and respective results.
Laser based joining technologies for optical assemblies overcome the limitations of standard fixation technologies such
as adhesive or wafer level bonding. By applying the laser energy locally and for a limited time these technologies enable
for higher stability of optical joints as well as additional functionality. Working without intermediate layers laser splicing
creates highly stable transparent joints that are suitable for the transmission of high power, e.g. in fiber laser assemblies.
In contrast, laser beam soldering of optical components as an alternative with a metallic intermediate layer is non-transparent,
but creates flexible and stress-compensating joints as well as thermal and electrical interconnects.