A thermally-sensitive recording medium having a protecting layer formed on a thermally-sensitive recording layer containing a colorless or pale-colored electron-donating leuco dye and electron-accepting color-developing agent. The PIVR is programmably configured relative to the disk drive’s mode of operation, including a read mode and a write mode. The cladding further includes at least one elongated side opening that preferably extends the entire length of the fiber and exposes said hollow core portion to the ambient environment, which provides broad and nearly immediate access of the core portion to gases and liquids in the ambient environment, thereby minimizing sensor response time.