Defect production in silica fibers doped with Tm3+

Progress and Summary of Photodarkening in Rare Earth Doped Fiber

In this paper, we summarize the research on photodarkening in optical fibers. The causes of photodarkening in fiber, the influence of photodarkening on fiber laser, the experimental device of photodarkening, and the mathematical model used to study the phenomenon of photodarkening are described in detail. At the end of the paper, we summarize the means and methods to suppress photodarkening.

Transverse mode instability and thermal effects in thulium-doped fiber amplifiers under high thermal loads

We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 µm, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847 W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.

Thermal bleaching of photodarkening and heat-induced loss and spectral broadening in Tm3+-doped fibers

We demonstrate the thermal bleaching effect on a photodarkened thulium-doped fiber (TDF) in detail. The bleaching effect on visible transmission initiates at 250 °C and a complete recovery is achieved at 550 °C. Prior to the recovery, a post-irradiation heat-induced spectral loss is observed. It indicates that an intermediate energy state is generated in the TDF under exposure to near-infrared (NIR) radiation, exhibiting the spectral attenuation in visible (VIS) and NIR region as driven by color center after thermal activation. And, with thermal treatment, the bleached TDF shows a partial photodarkening (PD) resistance when it is subject to photoirradiation again. In addition, the temperature-dependent spectral broadening and red shift that may distort the measured decay curve of excess loss is observed and discussed.

Bleaching of photodarkening in Tm-doped silica fiber with deuterium loading

We demonstrate the rapid photodarkening (PD) phenomenon in Tm-doped fiber (TDF) core pumped by a laser at 1080 nm and the bleaching effect of deuterium (${{\rm D}_2}$D₂) on PD TDF. By ${{\rm D}_2}$D₂ loading for seven days, the PD-induced excess loss (PIEL) in the visible (VIS) and near-infrared (NIR) region have been largely eliminated, and no degradation was observed within 30 days. PD resistance of the ${{\rm D}_2}$D₂ pretreated TDF has been investigated as well. The formation of color centers based on defects and precursors in the silica matrix and the mechanism of ${{\rm D}_2}$D₂ bleaching are discussed.

Photobleaching of thulium-doped ZBLAN fibers with visible light

Photobleaching of darkened thulium-doped ZBLAN fibers is observed after short exposure to visible light. The bleaching process is characterized by a stretched-exponential function of time with an exponent beta independent of both the bleaching intensity and the thulium concentration. The bleaching rate 1/ is also shown to scale linearly with the bleaching intensity and to have a 1/3 -power dependence on the thulium concentration. Incomplete and slow recovery of the color centers is observed in previously bleached fibers, suggesting the presence of at least two types of defect.

Photodegradation of near-infrared-pumped Tm(3+)-doped ZBLAN fiber upconversion lasers

Photodegradation has been observed in Tm(3+)-doped ZBLAN fiber lasers pumped with laser diodes at 1135 nm. After upconversion lasing at 482 nm, the fiber develops color centers that absorb strongly at wavelengths below ~650 nm, affecting further upconversion lasing. The rate of damage formation is strongly dependent on the pump power level and on the thulium concentration. The color centers are bleached by intense blue light but recover with thermal excitation and can be removed by thermal annealing at temperature near 100 degrees C.