Magnus Engholm

Preventing photodarkening in ytterbium-doped high power fiber lasers; correlation to the UV-transparency of the core glass

Photodarkening experiments are performed on ytterbium-doped silicate glass samples. A strong charge-transfer (CT) absorption band near 230nm in aluminosilicate glass is found to be correlated to the mechanism of induced color center formation. Excitation into the CT-absorption band generates similar color centers as observed in ytterbium-doped fiber lasers under 915nm high power diode pumping. The position of the CT-absorption band is compositional dependent and is shifted to shorter wavelengths in ytterbium doped phosphosilicate glass. Very low levels of photodarkening is observed for the ytterbium doped phosphosilicate glass composition under 915nm high power diode pumping. Possible excitation routes to reach the CT-absorption band under 915nm pumping are discussed.

Strong UV absorption and visible luminescence in ytterbium-doped aluminosilicate glass under UV excitation

A broad visible luminescence band and characteristic IR luminescence of Yb(3+) ions are observed under UV excitation in ytterbium-doped aluminosilicate glass. Samples made under both oxidizing and reducing conditions are analyzed. A strong charge-transfer absorption band in the UV range is observed for glass samples containing ytterbium. Additional absorption bands are observed for the sample made under reducing conditions, which are associated with f-d transitions of divalent ytterbium. The visible luminescence band is attributed to 5d-4f emission from Yb(2+) ions, and the IR luminescence is concluded to originate from a relaxed charge-transfer transition. The findings are important to explain induced optical losses (photodarkening) in high-power fiber lasers.

Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping

We show that the photodarkening resistivity of ytterbium-doped fiber lasers can be greatly improved by cerium codoping. It is suggested that the coexistence of the redox couple Ce(3+)/Ce(4+) in the glass provides means for trapping both hole- and electron-related color centers that are responsible for the induced optical losses in Yb-doped fiber lasers.

Experimental evidence for the formation of divalent ytterbium in the photodarkening process of Yb-doped fiber lasers.

In this work we present experimental evidence that the valence instability of the ytterbium ion play a key role for the observed photodarkening mechanism in Yb-doped fiber lasers. Luminescence and excitation spectroscopy performed on UV irradiated Yb/Al doped silica glass preforms and near-infrared diode pumped photodarkened fibers show a concentration increase of Yb(2+) ions. A concentration decrease in Yb(3+) could also be observed for the UV irradiated preform. The findings contribute to an increased understanding of the kinetic processes related to photodarkening in Yb-doped high power fiber lasers.

Degradation-resistant lasing at 980 nm in a Yb/Ce/Al-doped silica fiber

In this work we measure how photodarkening affects the optical efficiency for three different Yb/Al-doped silica fibers operating at 980 nm, one of which is codoped with cerium. A volume Bragg grating is used for linewidth control and added rejection of amplified spontaneous emission. Several hours of degradation-resistant operation is obtained with the Ce-codoped fiber, while for the Yb/Al doped fibers a large drop in efficiency is observed within the first hour of operation. Our results show that Yb/Ce/Al-doped fibers could be excellent candidates for high-power 980 nm fiber laser sources.

Reduction of photodarkening in Yb/Al-doped fiber lasers

A strong charge-transfer band at UV-wavelengths is found to play a major role for the observed induced optical losses (photodarkening) in ytterbium doped high-power fiber lasers. This is correlated to the valence stability of the ytterbium ion in the silicate glass matrix, which we believe is the origin of photodarkening. We have performed UV-irradiation experiments on ytterbium-doped preform samples and accelerated photodarkening experiments on Yb-doped fibers, by using 915 nm high power diodes. Our results show that photodarkening can be reduced, to low levels, either by preparing the preform glass in a reducing atmosphere or by hydrogen loading the fiber in a pressure chamber at room temperature.

Accurate spin axes and solar system dynamics: climatic variations for the Earth and Mars

Celestial mechanical simulations from a purely classical point of view of the whole solar system, including our Moon and the Mars moons { Phobos and Deimos { are carried out for 2 millions of years before present. Within the classical approximation, the results are derived at a very high level of accuracy. Eects from general relativity for a number of variables are investigated and found to be small. For climatic studies of about 1 Myr, general relativity can safely be ignored. Three dierent and independent integrations schemes are used in order to exclude numerical anomalies. The converged results from all methods are found to be in complete agreement. For verication, a number of properties such as spin axis precession, nutation, and orbit inclination for Earth and Mars have been calculated. Times and positions of equinoxes and solstices are continously monitored. As also observed earlier, the obliquity of the Earth is stabilized by the Moon. On the other hand, the obliquity of Mars shows dramatic variations. Climatic influences due to celestial variables for the Earth and Mars are studied. Instead of using mean insolation as in the usual applications of Milankovitch theory, the present approach focuses on the instantaneous solar radiation power (insolation) at each summer solstice. Solar radiation power is compared to the derivative of the icevolume and these quantities are found to be in excellent agreement. Orbital precessions for the inner planets are studied as well. In the case of Mercury, it is investigated in detail.

The role of charge transfer processes for the induced optical losses in ytterbium doped fiber lasers

In this paper we present how charge transfer processes influences the induced optical losses (photodarkening) in ytterbium doped fiber lasers. The location of the charge transfer absorption band is strongly composition dependent and is correlated to the valence stability of the ytterbium ion in the silicate glass matrix. An improved photodarkening performance can in general be observed for a charge-transfer band shifted to shorter wavelengths, although other routes are also possible to reduce photodarkening. Other parameters that affect the laser performance, such as absorption and emission cross section, must also be considered.

Charge transfer processes and ultraviolet induced absorption in Yb:YAG single crystal laser materials

Charge transfer (CT) transitions and UV induced color centers in Yb:YAG single crystals have been investigated. A simultaneous pair formation of a stable Yb2+ ion and a hole related (O−) color center (hole polaron) are observed through a CT-process. Slightly different types of hole related color centers are formed in Yb:YAG crystals containing small levels of iron impurities. Furthermore, excitation spectroscopy on the UV irradiated Yb:YAG samples could confirm an energy transfer process between Yb3+ and Yb2+ ions. The findings are important for an increased knowledge of the physical loss mechanisms observed in Yb-doped laser materials, such as the nonlinear decay process in Yb:YAG crystals as well as the photodarkening phenomenon in Yb-doped fiber lasers.

Strong excited state absorption (ESA) in Yb-doped fiber lasers

Excited state absorption (ESA) measurements performed on Yb-doped silica bers show the onset of a strong absorption band in the visible range. In this work, we perform experiments to investigate the possibility for ESA to be part of the induced optical losses (photodarkening) observed in Yb-doped ber lasers. Our results indicate that an ESA process, from the 2F5/2 excited state manifold in the Yb3+ ion to the charge-transfer state with absorption bands in the UV range, may constitute a transfer route for pump- and laser photons in the near-infrared range.