With a CuBall coating made of a copper-based alloy, optical fibers that can be used at up to 700 °C for short periods and up to 500 °C for long periods have been released by Oxford Electronics Ltd. They have a 125-µm cladding diameter and are available in lengths of several kilometers and in single-mode and graded- and step-index types. The company says that the fibers exhibit high hermeticity and diffusion resistance, low luminescence and high light isolation that minimizes crosstalk and stray light. They are suitable for deep-well and mining applications and, because they can be soldered, can be used in vacuum applications. Multiple fibers can be soldered into bundles that do not require epoxy and that can be used to make low-fluorescence measurements.