An optical fiber includes a glass fiber having a glass core and a cladding, and a hermetic layer having a high thermal stability disposed on the cladding. The glass core contains not more than about 1 mole % phosphorous. The optical fiber is adapted to operate under harsh conditions, such as elevated temperatures and/or hydrogen-containing environments. Methods for producing optical fibers, as well as methods for transmitting radiation in harsh environments using the optical fibers, are also provided.