Measurement of photodarkening in Yb-doped aluminosilicate fibres at elevated temperature

Abstract

Photodarkening behavior in Yb-doped aluminosilicate fibers at elevated temperatures is reported. The fiber was core-pumped at 977 nm and the transmitted powers were monitored at 633 (probe) and 977 nm (pump) simultaneously with ~47% of population inversion of Yb 3+ ions. A saturated photodarkening loss was found, which is inversely proportional to temperature and, at ~573 K, the loss was negligible at the pump wavelength. From the decay curves at different temperatures, we found that the photodarkening involves second order kinetics to form color centers.Introduction: Yb-doped fibers (YDF) exhibit excess loss covering Yb band when the fiber is irradiated at pump or shorter wavelength [1-2,]. The induced loss is proportional to the inversion level of the Yb 3+ ions, [3] and also depends on host material [4][5][6]. Measurement of photodarkening, in most cases, relies on monitoring transmitted power of a visible probe beam located far away from the Yb-absorption band during pump irradiation [1,6]. A loss in Yb band can be scaled from the loss at the probe wavelength due to similar spectral shape of the induced loss in a wide range of Yb fibers [1]. However, thermal treatment sometimes results in distortion in the loss spectrum, which invalidates this assumption [7]. On the other hand, the photodarkening measurement at high temperatures was proposed to accelerate the process in low Yb concentration fibers [8], but the measurement was performed only at visible wavelength and the temperature range was not very wide.In this report, we present experimental results on photodarkening induced loss in Yb-doped aluminosilicate fibers at elevated temperatures. The induced loss was recorded at 633 nm (probe) and 977 nm (pump) simultaneously. The temperature dependence of the induced losses was related to kinetics of color center formation.Experimental results: The photodarkening of the YDF was evaluated by monitoring the transmitted output power through the YDF under 977 nm irradiation. We used fiber-coupled and Bragg grating (FBG) stabilized 977 nm laser diode as a pump source. The output end of the pump fiber was spliced to the YDF and the throughput power was measured with a power meter. A He-Ne laser at 633 nm was used as a probe beam which coupled to the YDF and propagated the same direction as the pump beam. The probe beam was chopped and the output was detected by photodiode and lock-in amplifier. The measured output powers were recorded by data logger. We used 1 cm of the YDF to suppress amplified spontaneous emission. A uniform population inversion of Yb 3+ around 47% was maintained throughout the YDF.The tested YDF had 0.15 of core numerical aperture (NA) and 8.4 μm /125 μm of core /cladding diameters. The Yb concentration was 17,000 ppm 2 wt% in aluminosilicate host. The YDF was placed on a hot plate and covered by a metal plate to prevent possible heat loss from the top of the fiber. Temperature was monitored with an external thermocouple placed by the YDF.The temporal characteristics of the transmitted power at di...

The three-electron bond =Si<O_2:∙Yb absorption center of pre-darkened ytterbium-doped silica

“…In [9] the PD process is found to demand 5 -6 eV in activation energy (E A ) or what correspond to 4 -5 excited Yb ions. In [10] dynamic color center bleaching is found to show 0.07 eV in E A with full recovery of the induced PD for fibers operating at 573 K. The photochemical reaction mechanisms for the active pumped fibers relies on the actual level of pump absorption, temperature, fiber geometry and Yb ion concentration. To distinguish the different mechanisms involved in the PD process it appears therefore advantageous to analyze the bleaching of pre-darkened fibers by annealing as the relaxation reaction hereby only relies on fiber temperature.…”

Comparative Experimental Analysis of Thermal Characteristics of Ytterbium-Doped Phosphosilicate and Aluminosilicate Fibers

“…However, thermal aspects with Yb-doped fibers (YDFs) cannot be simply overlooked if they operate in highpower regimes because their thermal properties are invariably linked to their long-term reliability as well as their overall efficiency [4][5][6][7][8][9][10][11]. For example, the efficiency of an YDF as a gain medium is dependent on the spectroscopy of Yb 3+ ions that changes with temperature [5][6][7], and, in particular, photo-darkening (PD) effects are significantly affected with temperature [8][9][10][11]. Consequently, thermal characteristics, together with nonlinear scattering characteristics, can eventually be limiting factors to the performance of an YDF [12][13].…”

“…Consequently, thermal characteristics, together with nonlinear scattering characteristics, can eventually be limiting factors to the performance of an YDF [12][13]. Thus, they have been intensively investigated from various prospects [4][5][6][7][8][9][10][11]. However, most of the related results have been limited to aluminosilicate (Al)-based YDFs [6,7] since they have predominantly been utilized in many high-power applications [1][2][3].…”

“…One of the reasons must be that they present higher transition cross-sections around the 1-µm region compared to those of YDFs based on other host materials like phosphosilicate [14][15][16] as well as offering relatively low refractive core indices [1]. However, a main problem with Al-based YDFs is their vulnerability to PD [8][9][10][11][16][17][18], particularly in fibers of high doping concentrations of Yb 3+ ions [18]. Therefore, other types of YDFs have been considered as a replacement for Al-based YDFs [ 15,19,20].…”

Analysis of temperature dependence of photodarkening in ytterbium-doped fibers

“…Additionally, photodarkening experiments performed at elevated temperatures have shown that photodarkening is accelerated by temperature, but result in a lower level of saturation of the losses. 11,12 In this paper, we report our studies on further improving the analysis of photodarkening. We have investigated the photodarkening kinetics and derived the parameters mostly at temperatures that, for the used measurement durations, are below the activation threshold of thermal bleaching.…”