We derive and numerically verify a simple analytical approximation for the calculation of Brillouin gain spectra (BGS) in acoustically antiguiding optical fibers. We use our approximation to find acoustic speed profiles that minimize the peak Brillouin gain. The optimized BGS are top-hat-shaped, regardless of the optical mode distribution, with peak value inversely proportional to the effective area of the optical mode. We further investigate acoustically guiding speed profiles, in which acoustic waveguide modes lead to peaks in the Brillouin gain spectrum. However, for large cores with diameters down to 50 microm, and often even smaller, these Brillouin gain peaks are spectrally dense, and can be smeared out due to broadening in many practical systems. The Brillouin gain spectrum is then similar for waveguiding and antiguiding acoustic profiles.
Abstract:We demonstrate an efficient, ytterbium-doped low-NA fiber with core glass containing high levels of Al 2 O 3 and P 2 O 5 in silica host that shows low-photodarkening and generated 175 W of continuous-wave output power with 80% laser efficiency.
IntroductionThe last decade has witnessed a remarkable growth in power scaling of fiber lasers and amplifiers. The CW output power of ytterbium (Yb) doped fiber lasers (YDFLs) have now reached several kilowatts [1]. High power fiber sources that maintain a good output beam quality are often realized using a large-mode-area (LMA) fiber (i.e., large core and low NA) to reduce optical intensity in the core whilst guiding only a small number of modes. This is combined with a high rare-earth (RE) ion concentration to reach sufficient pump absorption even in a claddingpumped configuration. This reduces the fiber length and increases the threshold for undesirable nonlinear effects in the fiber. In order to allow a higher RE doping level into the silica matrix, the fiber core is usually co-doped with high concentration of Al 2 O 3 (Al) or P 2 O 5 (P) to prevent clustering of the RE ions. Yb-doped fiber (YDF) with high levels of Al and P also exhibits low photo-darkening (PD) [2]. PD is a detrimental effect which limits the long term performance of YDFL in many applications. Silica doped with Yb-ions and with either Al or P increases the refractive index, and so the core NA, significantly. This is undesirable for some LMA fiber approaches. Although a pedestal geometry can be used to ensure a low effective core NA, this is a significant fabrication challenge.Lipatov et al. reported passive optical fiber with refractive index (RI) close to pure silica using a ternary glass system of Al 2 O 3 -P 2 O 5 -SiO 2 with an equimolar amount of Al and P [3]. The fiber preforms were fabricated by the modified chemical vapor deposition (MCVD) process adding AlCl 3 vapor to the reaction gas mixture during deposition, to maintain a precise control of the co-dopants. Recently, low PD in YDF with P/Al molar ratio of 1, fabricated using conventional MCVD and solution doping technique, was reported [2]. However, this report failed to demonstrate a low-NA YDF with step-like core RI profile, particularly for fiber containing equimolar amounts of P and Al. This makes it difficult to reach large mode areas in such fiber structures. In this work, we present an efficient Yb-doped LMA fiber by engineering the P and Al composition in the fiber. We observed a low PD in the fiber when tested under 965 irradiation at a Yb 3+ excitation level of 35%. As a free running laser, the fiber exhibited a slope efficiency of 80% with respect to launched pump power.
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