We investigate Tm-doped double-clad fibers drawn from fused silica preforms with inserted Tm:YAG laser crystal rods. Based on the Molten-Core-Method the drawn crystal-derived fibers show typical amorphous properties covering a Tm concentration range from 0.2 to 0.84 mol% Tm2O3. They are studied in terms of their suitability of for multi-Watt level fiber lasers and compared to a Tm-laser fiber conventionally fabricated by Modified Chemical Vapor Deposition and solution doping. For the crystal-derived fibers, we demonstrate up to 4 W output power around 2 μm emission wavelength and a slope efficiency of 47 %, which are to date the highest achieved values for 790 nm pumping.
Fiber amplifiers with a robust monolithic seed coupling and very high peak power in a near diffraction-limited beam are increasingly demanded by many industrial applications in laser materials processing. A tapered all-solid rod-type fiber amplifier scheme is proposed. The principle of this approach is the use of a local adiabatic taper to provide a monolithic signal path and selectively excite the fundamental mode in highly multimode fiber. A large mode area fiber is used to scale up the peak power and suppress the nonlinear effects. The powder-sintering technology (REPUSIL) was employed to achieve rod-type fibers with excellent refractive index homogeneity.
In this work, we present a double-clad Yb3+-doped rod-type REPUSIL fiber with a core/clad diameter of 45µm/190µm and a core/clad numerical aperture of 0.09/0.19.This developed fiber has a relatively low Al3+-concentration to reduce diffusion during tapering and an optimized outer cladding material to reduce the taper process temperature. Finally it is demonstrated that the diffusion phenomenon is successfully eliminated and the near-diffraction limited beam quality during the amplification process is maintained. First experiments with improved Yb3+-doped rod-type amplifiers delivered 2ns pulses with peak powers of 210kW for the non-tapered rod and 140kW for the tapered rod (limited by facet damage). For the tapered fiber, the beam quality was between 1.3 and 1.7, significantly improved compared to the beam quality of the non-tapered fiber (M2 = 3.3 ~ 4.5). Future work will concentrate on adopting endcaps to protect the fiber facets from damage while scaling up the peak power.
We report on the fluorine incorporation in powder based materials for the fabrication of Al and Al/Yb co-doped silica glasses. The achieved maximum Fluorine concentration of 1.55 mol% SiF4 corresponds to a refractive index decrease of -8 x 10-3. Simultaneously, the Tg of the doped material is reduced by about 200 K compared to pure silica. Moreover, the fluorine doping is also eminently suitable for the direct refractive index adjustment in active doped silica glass materials (e.g. Al/Yb or Al/Tm). The index matching with pure silica is possible to date up to 2.7 mol% Al2O3 and 0.1 mol% Yb2O3. The additional influence on the blue shift of the UV transmission will also be discussed.
We report on the development of large-core Yb-doped fibers with up to 100 μm core diameter and present first experimental results for high peak power amplification. The material for core and pump cladding was fabricated by Powder Sinter Technology. Using a high Al concentration we achieved a numerical aperture (NA) of 0.21 of the pump cladding and a core NA below 0.1. The rod-type fiber exhibits high pump absorption. Using a 0.55 m short fiber sample as the main amplifier in a 3-stage ns pulsed fiber Master Oscillator Power Amplifier system we achieved 3 ns output pulses with 360 kW peak power and 2 mJ pulse energy. We observed suppressed Stimulated Raman Scattering with respect to the signal pulses, which offers the possibility of further power scaling of such fiber amplifier systems.
We demonstrate a distributed measurement technique to observe temperature changes along pumped Yb-doped fibers. This technique is based on an array of fiber Bragg gratings acting as a temperature sensor line. The Bragg gratings are inscribed directly into the Yb-doped fiber core using high-intensity ultrashort laser pulses and an interferometric setup. We studied the temperature evolution in differently co-doped Yb fibers during optical pumping and identified different effects contributing to the observed temperature increase. We found that preloading of fibers with hydrogen supports the formation of Yb2+ during UV irradiation and has a large impact on fiber temperature during pumping. The proposed technique can be applied to investigate the homogeneity of pump absorption in active fibers and to support spatially resolved photodarkening measurements.
The active core diameter in silica preforms can be significantly increased by the deposition of ytterbium (Yb) and the most important codopant aluminum (Al) in the gas phase through the high-temperature evaporation of an Yb chelate compound and Al chloride in the Modified Chemical Vapor Deposition (MCVD) process. Here, we report on systematic investigations of the incorporation of Yb and Al into silica by gas phase doping technique. Preforms and fibers were prepared in a wide range of Yb and Al concentrations. The samples were characterized concerning the radial distribution of the refractive index and dopant concentrations, the efficiency of the deposition, and the absorption and emission properties in the NIR region. First laser experiments have demonstrated a slope efficiency of 80%, which is comparable to fibers made by MCVD/ solution doping and powder sinter technology.
We demonstrate three-level laser operation at 976 nm of a large-core Yb-doped aluminosilicate fiber, which is fabricated by powder-sinter technology and shows a very homogeneous refractive index profile. The investigated fiber has a core diameter of 126 μm and a numerical aperture of 0.18, well-matched to standard fiber coupled pump diodes. The core composition has been optimized to reduce photodarkening effects. Multimode and single mode operation with multiple Watts output power is presented for this fiber making it useful for the realization of high brightness fiber coupled pump sources.
We demonstrate a spatially distributed measurement technique to observe temperature changes along a pumped Ybdoped fiber. The technique is based on an array of fiber Bragg gratings acting as temperature sensor line. The Bragg gratings are inscribed directly into the actively doped fiber core using high intensity laser pulses and an interferometric setup. We studied the temperature evolution in differently doped fibers under pumping conditions and find different effects contributing to the observed temperature increase. To avoid an additional heating caused by pump-induced photodarkening, we measure the spatially resolved temperature profile during pumping along a photodarkening-reduced, cerium co-doped fiber.
We report on SiO2-Al2O3-La2O3 glasses – with and without Yb2O3 – suitable for nonlinear and fiber laser applications. We also present successful supercontinuum generation and fiber laser operation around 1060 nm in step-index fibers. We have optimized the glass compositions in terms of thermal and optical requirements for both a high La2O3 (24 mol%) and Yb2O3(6 mol%) concentration. The aluminum concentration was adjusted to about 21 mol% Al2O3 to increase the solubility of lanthanum and ytterbium in the glass beyond possible MCVD based techniques. The glasses have been characterized by dilatometrical methods to find transition temperatures from 860 to 880°C and thermal expansion coefficients between 4.1 and 7.0 × 10-6 K-1. Structured step index fibers with a SiO2-Al2O3-La2O3 core and silica cladding have been realized showing a fiber loss minimum of about 500 dB/km at 1200 nm wavelength. The chromatic dispersion could be adjusted to shift the zero dispersion wavelength (ZDW) close to the pump wavelength of 1550 nm in a supercontinuum generation setup. First fiber laser experiments show an efficiency of about 41 % with a remarkably reduced photodarkening compared to MCVD based fibers.
XLMA fibers based on Yb-doped bulk silica possess an excellent refractive index and doping level homogeneity [1]. To
achieve the highest optical-to-optical efficiency and long-term operation without degradation we simulated the effect of the brightness conversion factor of different core dopant compositions of such XLMA fibers. We also investigated the beam quality of a multi-kW single XLMA fiber laser system and its long-term stability. The current state-of-the-art
XLMA single fiber laser has 5 kW maximum output power and a degradation rate of about 0.5 % / 500 h at 4 kW
measured over a period of 1700 h. Several application tests demonstrate the excellent performance of the XLMA fiber
laser.
We report on the inscription of FBGs in rare earth doped optical fibers, the reduction of inherent absorption effects in the
FBGs and the FBG-based temperature measurement within the core of actively doped fiber samples during core
pumping. Besides a temperature increase due to the quantum defect of Yb-ions a change in temperature during pumping
was observed and fits qualitatively well to the parallel measured photodarkening evolution.
The quality of Yb-doped fused bulk silica produced by sintering of Yb-doped fused silica granulates has improved
greatly in the past five years [1 - 4]. In particular, the refractive index and doping level homogeneity of such materials
are excellent and we achieved excellent background fiber attenuation of the active core material down to about 20 dB/km
at 1200 nm. The improvement of the Yb-doped fused bulk silica has enabled the development of multi-kW fiber laser
systems based on a single extra large multimode laser fiber (XLMA fiber).
When a single active fiber is used in combination with the XLMA multimode fiber of 1200 μm diameter simple and
robust high power fiber laser setups without complex fiber coupling and fiber combiner systems become possible. In this
papper, we will discuss in detail the development of the core material based on Yb-doped bulk silica and the
characterization of Yb-doped fibers with different core compositions.
We will also report on the excellent performance of a 4 kW fiber laser based on a single XLMA-fiber and show the first
experimental welding results of steel sheets achieved with such a laser.
Fiber Bragg gratings (FBGs) are attractive as reflector elements in fully integrated all-fiber laser systems. Furthermore,
FBGs made with femtosecond laser technology allow to reduce splice connections in the fiber, since
no special photosensitive fibers are required. Fiber Bragg grating inscription with deep ultraviolet femtosecond
laser (267 nm) and two beam interferometry allows to target germanium-free and non-photosensitive fibers
while maintaining versatility in the choice of the output wavelength of the fiber laser. This concept offers the
potential of gratings with high spatial resolution, great flexibility and good homogeneity and complements the
methods of point-by-point inscription at 800 nm or of phase-mask inscription with 400 nm femtosecond laser
exposure. We report on the application of the interferometric fiber Bragg grating inscription technology to build
a grating-stabilized fiber laser with high beam purity. Output powers up 160 W have been achieved.
Sintering of Yb-doped fused silica granulates is a well established technique developed by the IPHT and Heraeus
Quarzglas and it produces very homogeneous rare earth doped bulk silica core rods for fiber laser applications. By using
a newly developed laser induced deflection (LID) technique we are able to pre-characterize directly the material
absorption properties of the bulk material prior to the laser fiber production. The bulk absorption results measured by
LID are without scattering effects and they are typically in good agreement with the total attenuation measured in the
fibers. We achieved a fiber background loss of 20 dB/km. Furthermore, we present detailed studies of the refractive
index homogeneity of the Yb-doped bulk materials and laser fibers to show the unique features of the Yb-doped bulk
silica.
Multimode double cladding laser fibers with an extra large mode area XLMA fiber design (core diameter up to 100 μm)
have been produced from the Yb-doped bulk silica rods by two different techniques. One is a classical jacketing method;
the other employs the stacking of un-doped, Yb- and F-doped rods and F-doped tubes.
Different fiber types have been tested in different fiber laser setups. The influence of the fiber end cap properties on the
fiber laser focus shift is discussed in detail. We have achieved fiber laser output powers up to 1.925 kW, limited only by
the pump power. We also investigated the long term laser stability at different power levels.
Up to now, the role of divalent ytterbium ion has been controversially discussed in the literature concerning its
influence on the photodarkening of ytterbium doped high power laser fibers. In general, however, the experimental
findings are relatively sparse and some discussions are based more on speculations than on examined facts.
Here we report on systematical investigations concerning the formation of Yb2+ during the fabrication process of
preforms and fibers. By Modified Chemical Vapor Deposition, fibers with different codopants (additional to the active
ytterbium doping) have been prepared in a well-defined manner, regarding process parameters and glass composition.
The comprehensive characterization of the samples involves the ytterbium absorption in the NIR, the UV absorption
and UV excited emission. The typical spectral features in the UV and visible range have been analysed and correlated
with the presence of Yb2+. The amount of formed divalent ytterbium ions shows a strong dependence on the process
route and varies remarkably with the kind and concentration of the codopants. Photodarkening tests have been
accomplished in order to correlate the power stability with the Yb2+ content. Moreover, the formation of Yb2+ during
the process of UV radiation darkening was investigated.
We report on the characterization of photodarkening (PD) kinetics at Yb-doped fiber samples, inducing the PD loss by
core-pumping at 975 nm with respect to different fiber temperatures in the range of 77 to 770 K. The thermal
dependency of important PD parameters is presented. Additionally, we introduce a phenomenological model to include
thermal and recovery effects in the description of the PD loss evolution and to improve the understanding of the PD
process.
At the Photonics West 2008 we presented our rare earth doped fused bulk silica for fiber laser applications [1]. This
approach overcame the typical geometrical limitations of other well known production methods for rare earth doped
silica materials. Our unique production technique is based on the sintering of Yb-doped granulates of high-purity SiO2
particles. We have processed our Yb-doped bulk silica rods into ultra large mode area (XLMA) multi-mode double
cladding laser fibers with an active core diameter in the range of 40 μm to 100 μm (depending on the core doping level).
In the XLMA fiber the active core is surrounded by a so-called 2D- or 4D-shaped pure silica pump cladding (with
diameter between 850 μm and 900 μm) and an F-doped outer silica cladding with an outer diameter of 1000 μm.
We have investigated the refractive index and the intrinsic stress profiles of different XLMA laser fibers and their
preforms to visualize interface effects. The fiber cross section designs, the quality of all interfaces and the material
composition are important factors for the laser fiber performance. The laser properties of these fibers have been
investigated in detail. In addition, the preparation of the fiber end-face is important to reduce heat effects and we have
developed concepts to mitigate such thermal load at the fiber end face.
We have developed a production process for rare earth doped bulk silica to fulfill the demand of such material for fiber
laser applications. In contrast to the standard techniques such as a combination of MCVD (modified chemical vapor
deposition) and solution doping, our novel technology is based on a granulate process that enables novel ultra large mode
area fiber designs (XLMA) with active core diameters above 100 μm as well as larger batch sizes. Several Yb-doped
fibers with two different fiber designs were manufactured and successfully tested in both side- and end-pumped fiber
laser setups. Both fiber designs have been compared to similar MCVD fibers. The influence of the material composition
on the photodarkening properties has been investigated.
In the last years, photodarkening in ytterbium doped silica based laser fibers turned out to be a critical factor for high
power laser action. Several investigations have been carried out in order to characterize the time dependent increase of
the fiber loss and to understand and model the complex optical phenomenon. Despite of progress in this field, there is
still a lack of data concerning the detailed influence of fiber composition and preparation process parameters as well as
concerning the role of atomic defects in the core glass.
Here we report on investigations about the photodarkening in dependence on the glass composition of the fiber laser
core. By MCVD, fibers with different codopants (additional to the active ytterbium doping) have been prepared in a
well-defined manner, regarding process parameters and glass composition, and comprehensively characterized. In
addition to the photodarkening measurements, further optical properties have been measured on the fibers and fiber
performs, which are related to the photodarkening effect: intensity and spectral behaviour of the Yb3+ absorption and
emission in the NIR, cooperative visible fluorescence, UV absorption and UV excited visible emission. The concentration
of codopants which are commonly used for active and passive lightguide fibers (aluminium, germanium, phosphorus)
was systematically varied and correlated with the optical properties.
Ytterbium-doped high-power fiber lasers with high beam quality are promising devices for a variety of applications.
Extreme power load and complicated fiber structures make great demands on material properties and preparation
technology. Recently, with the further increase of output power, the problem of photodarkening has been identified as a
critical issue for fiber laser devices. Detailed knowledge of optical properties of materials and fibers are needed for the
successful development of laser fibers, aimed at increasing efficiency and power.
It is well known that the properties of silica based rare earth doped fibers can be influenced and remarkably improved
by the incorporation of further dopants. Here, the influence of combined aluminium-phosphorus codoping on the optical
properties of Yb doped laser fibers was investigated. Preforms and fibers were prepared by MCVD and solution doping
both with phosphorus and aluminium excess. The samples were characterized concerning the radial distribution of
refractive index and dopant concentrations and the absorption and emission properties in the UV/VIS/NIR region. The
observed spectral effects and active properties (laser efficiency, photodarkening) were correlated with changes in the
fiber composition. It could be shown, that the combined doping leads to effects which deviate from a simple additivity
and which can be beneficially utilized for the improvement of the laser fiber performance.
We investigated the influence of the atmosphere during preform collapsing (Cl2/O2, He, reducing gases as CO and H2)
on the photodarkening process in ytterbium doped silica fibers. The measurements were performed for a probe
wavelength of 633nm in-situ during cladding pumping at 915 nm in dependence on the Yb inversion. The equilibrium
values of the core excess loss were found to be remarkably lower in the fibers from preforms collapsed with the
treatment of He or reducing gases, whereas the photodarkening rate constants are rather similar. Subsequent
measurements of the fluorescence properties (pump wavelength 915nm or 976 nm) were carried out at the
photodarkened fibers and compared to earlier results obtained at fibers as drawn.
Laser experiments with the different fibers at low Yb inversion (no photodarkening) show a decrease of the laser slope
efficiency in parallel to the degree of reduction of the doped core glasses. For the He treatment, an optimum of lowered
photodarkening loss and reasonable laser efficiency can be obtained.
The doping of silica yields additional degrees of freedom to vary the optical parameters of index guided and band gap
controlled microstructured optical fibers (MOFs). Aside from the widely investigated rare earth doped microstructured
fibers for lasers also the integration of conventionally doped structural elements with passive functions into MOFs allows
to enhance effectively the optical performance of such fibers.
We report on progress in preparation of microstructured fibers with air holes and solid structural elements composed of
germanium and fluorine doped silica materials. The microstructured fibers were prepared by stack-and-draw technology.
The starting materials are preform rods and tubes with graded dopant concentration prepared by MCVD and sintering
technology. They were elongated to millimeter dimensions before packaging to final MOF preforms. We prepared MOFs
with both holey core and holey cladding. The microstructuring of the holey cladding is achieved with fluorine doped
capillaries.
Several applications have been investigated. The high photosensitivity of germanium-silica MOFs makes possible the
inscription of Bragg gratings with high efficiency. In fiber evanescent field sensors, such microstructured fibers improve
the overlap between the propagating light field and the analyte and allow therefore an increased sensitivity e.g. for gas
sensing with optical fibers. Solid MOFs with multiple cores in a highly precise array arrangement can been investigated
as a model system for the study of nonlinear dynamics in discrete optics.
Important progress in the development of rare earth doped high power fiber lasers was possible by large-mode-area
(LMA) fibers with increased core diameters and reduced core apertures as low as 0.05. In this way, the excellent beam
quality is maintained, but the power density can be reduced below critical values despite of very high output powers
beyond 1 kW. Sophisticated concepts had to be developed in order to maintain the low NA in the case of high doping,
e.g. the codoping by index-decreasing components as boron or fluorine.
Here we report on the progress in the preparation of microstructured LMA laser fibers, the core area of which is
composed of parts with high doping and parts with refractive index lower than the silica pump cladding. In contrast to
the direct codoping, in this way the atomic environment of the active atoms can be tailored and optimized independently
on the mean refractive index of the core. The preparation was carried out by stacking different rods in a multistep
process, leading to cores with up to more than hundred single elements. Both for ytterbium and erbium/ytterbium doped
fibers, good optical properties concerning basic attenuation and rare earth fluorescence could be reached by introducing
additional purification steps. Different fiber structures were characterized concerning mode field distribution, pump
power absorption and laser behavior.
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