We experimentally demonstrate a few-mode erbium doped fiber amplifier (FM-EDFA) supporting 6 spatial modes with a cladding pumped architecture. Average modal gains are measured to be >20dB between 1534nm-1565nm with a differential modal gain of ~3dB among the mode groups and noise figures of 6-7dB. The cladding pumped FM-EDFA offers a cost effective alternative to core-pumped variant as low cost, high power multimode pumps can be used, and offers performance, scalability and simplicity to FM-EDFA design.

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Abstract-We report a highly versatile chemical-in-crucible preform fabrication technique suitable for gas-phase deposition of doped optical fibers. Aluminosilicate and ytterbium-doped phosphosilicate fibers are presented demonstrating the technique and its potential for realizing complex fiber designs which are suitable for the nextgeneration of high power fiber devices. The results show aluminum doped fiber with NA of 0.28 and ytterbium-doped fiber with a measured slope efficiency of 84 % with respect to pump launch power.

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...