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.
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.
For the implementation of novel fiber laser concepts, such as extra-large mode area (X-LMA) fiber lasers or multi-core fiber lasers alternative manufacturing processes for highly-doped silica glasses and the laser fibers fabricated from it are required. For efficient laser operation a high absorption of pump power in the active fiber core is a necessary condition. To increase the pump light absorption the fiber development aimed at the preparation of laser-active and adapted passive single-large core fibers up to multi-core structures with 7 large cores showing broken circular fiber symmetry. The optimization of the optical fibers which will be shown in detail is based on the combination of several innovative manufacturing methods such as the powder sintering technology (REPUSIL), the preform preparation by stack-and-draw technique and the fiber drawing process. The described procedure is particularly suitable to produce multifilament glass preforms resp. laser fibers with large cores in which the radial and lateral indices of refraction can be adjusted homogeneously and reproducibly. Due to the realized increase of the laser-active core volume in these fibers the pump light absorption could be considerably increased and the resulting shorter fiber length allows the use of fibers with a moderate attenuation. The results concerning the characterization of materials science and the optical aspects e. g. the dopant concentration distributions and related refractive index profiles as well attenuation and pump absorption spectra will be presented.
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.
The photodarkening resistivity of ytterbium doped alumosilicate fibers can be remarkably improved by cerium codoping. Here we report on systematical investigations of the influence of cerium on an optimized fiber design. Fibers with different ytterbium, aluminium and cerium contents have been prepared both under reducing and oxydizing conditions and characterized concerning refractive index, absorption and emission from UV to NIR. Typical spectral features in the UV and visible range have been analysed with respect to the ratio of Ce3+ / Ce4+. Photodarkening tests have been accomplished in order to correlate the power stability with the Ce content and valency state.
The accuracy of the recently presented1 equivalent step index approximation of multifilament core fibers is analyzed in terms of the effective refractive index, mode field area and bending losses of the fundamental mode. A modified Vparameter for this class of fibers as well as a single-mode condition is proposed. By comparison with a full-vectorial finite element method it is shown that the relative deviation of the effective refractive index and the mode field area are in the magnitude of 1 %. No significant decrease of bending losses is found for multifilament core fibers.
The major challenge in the development of monolithic kW class CW fiber lasers is the efficient conversion of pump
photons into a high brightness laser beam under the constraints of heat management, long term stability and
nonlinearities. This article reviews the interaction of some fiber related aspects as e.g. fiber core composition,
photodarkening and modality, as well as their influence on system complexity and power scalability. Recent work on
active fibers, pump couplers, mode field adaptors and other fiber-optic components will be presented.
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
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.
The time-dependence of photodarkening (PD) in Yb-doped fibers is commonly fitted with a stretched exponential
function to determine typical parameters of the process. But, the experimental conditions to obtain consistent results
from the comparison of PD for different pump powers, various concentrations of the dopants, or fibers from different
manufacturers are not adequately regarded up to now. We discuss the requirements concerning the measurement method
as well as the impact of the initial state of the fiber under test.
Further on, the experimental results of PD characteristics are discussed in the framework of a kinetic model of the
observed processes. The discussion of the measurable PD loss is expanded by introducing the concept of the "PD state"
that defines the PD loss as a weighted mean and takes effect on the further evolution of the process. In this way, the basis
of fiber characterization and the understanding of the PD kinetics will be improved.
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.
In the last years, the performance of ytterbium doped high power silica fiber lasers has been remarkably improved.
Increasing attention has been paid to the role of material composition, impurities and atomic defects, because the extreme
power load and the complicated fiber structure make great demands on material properties and preparation technology.
Recently, with the further growth of output power, the problem of photodarkening has been identified as a critical issue
for fiber laser devices.
Here we report on developments aimed at increasing efficiency and power stability of ytterbium doped laser fibers. The
optical properties of preforms and fibers made by MCVD and codoped with high phosphorus contents were investigated
for a wide range of the ytterbium concentration. Moreover, the influence of the collapsing atmosphere on the properties
was studied. The observed spectral effects, photodarkening and laser efficiency were correlated with changes in fiber
composition and process conditions. It could be shown that phosphorus codoping leads to remarkably different and
advantageous fiber properties compared with the usual aluminium codoping.
In the last years rare earth doped double-clad fibers have been developed to high-power laser sources. Important progress was possible by increasing numerical aperture of the pump cladding and decreasing numerical aperture of the laser core. The high NA of the pump cladding enables the acceptance of large pump intensities whereas the low NA of the laser core makes possible to increase the core diameter and to decrease the laser power density retaining high beam quality. Here, actual challanges are discussed and possibilities are demonstrated to use microstructures for improved fiber designs which are realized by stacking and drawing of capillaries and rods. The rare earth doped parts are prepared by modified chemical vapor deposition and solution doping, but other routes of powder technology are also studied. Concerning the currently most important laser and amplifier types - Yb doped at 1.1 μm wavelength and Er/Yb doped at 1.55 μm wavelength -, the question of a high pump aperture is similar, but the limitations concerning a low core aperture are fairly different, because an efficient Er/Yb laser demands high phosphorus co-doping which naturally increases the core NA. The applied microstructures comprise "holey" fiber cross sections in form of "air clads" for the pump light and multiple hole ring structures for laser core and inner cladding. Moreover, microstructured cores made from solid parts yield new possibilities and parameters to compensate the high refractive index of the active material and to optimize the large mode area design.
We have doped the core of Ytterbium (Yb) laser fibers additionally with Neodymium (Nd) to exploit wavelength-multiplexed high-power pump systems, which are commercially available. By pumping such a Nd:Yb-codoped fiber with the 808/940/978 nm-diode system, we could demonstrate CW output powers of more than 1 kW with high laser slope efficiency. In order to get a better understanding of this laser medium, we studied the fluorescence and the laser behavior of Nd:Yb fibers with different rare earth concentrations in comparison to a fiber doped solely with Nd. A theoretical model for the calculation of the fluorescence decay curve and spectrum as well as the laser characteristic and wavelength was developed, that takes the energy transfer process from Nd to Yb ions into account. Comparing the experimental and theoretical results, the behavior of the Nd:Yb high power fiber laser is understood as a collective emission of both ion types within the same wavelength region. These investigations contribute to the optimization of high power fiber lasers under the viewpoint of thermal load.
Rare earth doped silica based fiber lasers and amplifiers with very high output power and excellent beam quality are efficient devices for a variety of applications in industry, science and medicine. During the last years, important progress was possible by new design concepts but also by carefully tailoring the material properties. Aspects of material and technology development concerning the interaction of different dopants and co-dopants will be discussed in the following. Our discussion concentrates on the optical properties of the laser fiber as refractive index, absorption and emission of the rare earths and especially on phenomena concerning the background loss of the fibers. We have found a strong rare earth specific loss component which remarkably depends on the kind and the ratio of the co-dopants. The relations of this background loss to material composition and fabrication technology are demonstrated and discussed on an empirical base.
A new type of multi-clad rare-earth doped silica fiber was designed, prepared and tested for the power scaling of high power fiber lasers in the 1 .1 tm wavelength region. By means of a dedicated laboratory setup a maximum output power of more than 1 .300 watts with excellent spectral and beam behavior was achieved. The fundamental investigation of the energy transfer processes and of the fluorescence lifetimes of different Nd:Yb co-doped has been studied.
Such fiber-lasers were tested in the laboratory at several materials (plastics, metals, glass) in the fields of material processing and micro-marking, respectively.
It is well known that the efficiency of silica based Er/Yb fiber lasers and amplifiers can be greatly enhanced by a high level of phosphorus co-doping. The interaction of phosphorus with the rare earths and with the further co-dopant aluminium during the preparation process leads to peculiar effects concerning chemistry of dopant incorporation, diffusion behaviour and microscopic glass structure. Until now however, it has been disregarded that these interactions give rise to strong radial and even axial concentration profiles in preform and fiber (for the codopants phosphorus and aluminium, but also for the rare earth components erbium and ytterbium) with remarkable consequences for the laser efficiency. Here, we have studied the incorporation mechanism and the diffusion behaviour (diffusion coefficients in dependence on temperature and concentrations) by X-ray microprobe analysis concerning the dopant interaction effects. Moreover, we have investigated both fluorescence intensities and lifetimes, radially and axially resolved in the fiber preforms. The results were correlated with the properties of double clad fiber lasers made from these preforms. First attempts have been undertaken to interpret and control the incorporation mechanism on the basis of model calculations regarding phosphorus diffusion and chemistry.
Praseodymium doped arsenic sulfide single mode fibers were prepared and investigated with regard to their suitability for 1.3 micrometers amplification. Although these fibers have relatively low basic losses of about 2 dB m-1 at pump (1.02micrometers ) and signal wavelength, till now no signal amplification could be measured because of a strong attenuation induced by the continuous wave (cw) pump light. In order to investigate the more detailed optical behavior, we have carried out time resolved measurements with short pump pulses of 1-5 ms at a repetition rate of 1 Hz on a 2 m fiber length. The evaluation of the time dependent signal intensity at the fiber end in comparison to the fluorescence intensity without signal has shown the fast development of a transient absorption within 0.5-1 s. Moreover, an inner gain could be clearly detected which increased with the pump power (0.75-75 mW). The gain coefficient related to the input pump intensity has been determined to be 0.5 dB- mW-1.