TEM Investigation of the Core/Cladding Interface of La 2 O 3 -Al 2 O 3 -SiO 2 Glasses for High Power Fiber Lasers
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TEM Investigation of the Core/Cladding Interface of La2O3-Al2O3-SiO2 Glasses for High Power Fiber Lasers Shang-Cong Cheng and Matthew J. Dejneka Science & Technology Division, Corning Incorporated Corning, New York 14831, USA ABSTRACT For fabricating Yb3+ doped La2O3-Al2O3-SiO2 glasses for high-powered fiber lasers, it is critical to choose the right core glass composition to obtain a high numerical aperture and to avoid phase separation. TEM techniques were used to study the relationship between the core composition and phase separation. In the study, inter-diffusion between the core and cladding glasses was found. The inter-diffusion caused large density fluctuation in the region of the core/cladding interface in fibers containing relatively high concentrations of La2O3. The TEM results were used to optimize the chemical composition of the core glass for high-power fiber lasers. INTRODUCTION Single mode semiconductor pumps have failed to keep pace with the increasing power requirements of Er-doped fiber amplifiers (EDFA), so there is a need for high-powered 980 nm sources [1]. Yb3+ doped tapered fiber lasers can provide high power output by conversion of a low brightness high powered 920 nm multimode broad stripe laser diode to high brightness 980 nm single mode output. The tapered fiber laser requires a fiber with high numerical aperture, NA, (>0.4), a rectangular core, and good Yb3+ spectroscopy for efficient operation. The core glass composition is a critical for maintaining a high NA fiber with good power extraction while avoiding phase separation, loss, and clustering. To search the best chemical contribution in La2O3-Al2O3-SiO2 glass system, several fiber samples with various chemical distributions were made and the optical properties of these fibers were measured. However, the experimental results of the optical properties of these fibers were opposite to expectation. The fiber made from a relatively high concentration of La2O3, which is expected to a have higher reflective index and higher optical coupling efficiency, in contrast, showed higher optical loss and low coupling efficiency than that of fibers made from relatively lower concentration of La2O3. In order to understand the results of optical measurements on these fibers, transmission electron microscopy (TEM) techniques were used to investigate the microstructures of the fibers, especially in the region of the core/cladding interface of the fibers. EXPERIMENTAL The fibers were made by a new method of “soft glass in tube”. The cladding tubes were made of SiO2 by chemical vapor deposition (CVD). First, a solid SiO2 rod was drawn down to 20 mm OD and 1 m length. The rods were then cut in half and a 12 mm ¥ 3.5 mm rectangular groove was machined from the center. The polished mating halves were then inserted into a 24 ¥
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60 overclad tube of the same composition and the assembly was drawn down to a 25 mm OD monolith with a 6 ¥ 2 rectangular bore. The core glass compositions were in the La2O3-Al2O3SiO2 system. The details of making the gl
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