Vacuum transport of transition metal-doped ZnSe fiber lasers

Abstract

Crystalline optical fibers have garnered high interest in recent years due to their high transparency in the near- to mid-IR when compared to conventional amorphous glass fibers. In addition, a uniform crystal field allows such materials to act as passive or active optical elements for lasing as well as nonlinear frequency converters and switches. Of these fiber materials, II-VI chalcogenides such as ZnSe possess among the widest transparency windows in the IR. Furthermore, when doped with transition metals such as Cr or Fe, ZnSe fibers display widely tunable lasing that covers mid-IR atmospheric transition windows and allows them to be used in remote sensing and defense. Here we report the transport and material refinement of Cr2+:ZnSe in microscale fibers.High pressure chemical vapor deposition (HPCVD), which utilizes pressures on the order of MPa to deposit solid materials into highly confined geometries via thermal decomposition of gaseous precursors, is thus far the only synthesis method capable of producing ZnSe fiber lasers. Although this technique has reliably deposited crystalline materials into hollow capillaries microns in diameter and centimeters in length, the HPCVD of ZnSe produces a hollow central pore due to the buildup of reaction byproducts. This pore is detrimental towards waveguiding and lasing of doped ZnSe fibers because it restricts the waveguide mode structure to higher order modes, increasing loss and disrupting optical feedback. To remove this pore, postprocessing has been applied to HPCVD ZnSe fibers in the form of vacuum transport, effectively removing the pore over lengths on the order of centimeters.The major drawback to vacuum transport is thermal expansion mismatch between ZnSe and the silica cladding, which leads to cracking and high optical loss. In this report, fully-filled ZnSe fibers on the order or several millimeters have been synthesized for the first time. By introducing hydrogen as a chemical transport agent, followed by applied vacuum as low as 10-4 Torr, rearrangement has been reduced to temperatures as low as 680°C based on the following reaction ZnSe(s) + H2(g) → Zn(g) + H2Se(g). Such fibers show low order waveguiding mode structures and increased crystallinity as demonstrated by Raman spectroscopy and X-ray diffraction. In addition, IR fluorescence is reported by end pumping with infrared light, opening the possibility of creating a low-loss ZnSe fiber laser with high lasing efficiency