This document discusses optical fiber sensors, including their principles and classifications. Optical fiber sensors have advantages like electromagnetic immunity, electrical isolation, compact size, and ability to do distributed or multiplexed configurations. They can sense and measure changes in light intensity, phase, polarization, wavelength or spectral distribution. Optical fiber sensors are classified as extrinsic or intrinsic. Extrinsic sensors have the light leave the fiber to be altered before returning, while intrinsic sensors alter the light within the fiber. The document gives examples of applications for different types of optical fiber sensors such as temperature, chemical, pressure, and military sensors.