Abstract: An optical network for the transfer of data between optical network units (ONU) connected to respective data terminal equipment including electro-optical interface for converting electrical signals to optical signals for transmission through the optical network and for converting optical signals to electrical signals for input to the terminal equipment, comprises a fiber optic line having first and second ends; first and second point-of-presence (POP) units connected to respective first and second ends of the fiber optic line, the first and second POP units for being connected to another optical network, the first and second POP units including optical multiple wavelength apparatus for optical signal generation and optical multiple wavelength apparatus for optical signal detection; first and second optical communicators connected to the fiber optic line at locations between the first and second POP units; first and second ONUs operably connected to respective the first and second optical communicators, the fi

Abstract: A discontinuity within an optical fiber is located using a reflectometry technique. A swept frequency sinusoidal signal is launched into the fiber, is reflected from the remote discontinuity, and is returned to the fiber input end. The signal consists of the combined outputs of two optical sources identically frequency modulated and having output wavelengths .lambda..sub.1, .lambda..sub.2 separated by several nanometers. The light at .lambda..sub.1 has a group velocity different from the light at .lambda..sub.2 and consequently, reflected light components at .lambda..sub.1 and .lambda..sub.2 interfere with each other at the fiber input end. The interference signal is frequency dependent and from the frequency dependence the distance from the fiber input end to the discontinuity can be calculated. The received signal is dependent also on the difference in group index at .lambda..sub.1 and .lambda..sub.2, this difference being relatively insensitive to temperature, stress and fiber composition.