Abstract: An optical fibre has a light conducting core surrounded by first and second cladding layers of sequentially lower refractive index. A data-carrying signal can be propagated in the core and a monitoring signal--of a higher modal order than the data signal--can be propagated in the core and first cladding layer. Any attempt to rap the fibre and abstract data information results in aberration of the monitor signal, which can be detected. The monitor signal can be such that resolvable signals cannot be obtained from the data channel by a tap.

Abstract: The chromatic pulse dispersion of a light beam in an optical fibre can be equalized by dispersing the light beam into a spectrum, detecting different portions of the spectrum and producing signals related to each portion, applying different time delays to the signals and then combining the signals to provide an output. The spectrum can be divided into discrete portions having a range of wavelengths, with optical delay lines, optical filters, and multiple section photodetectors used. The spectrum can be formed on one surface of a PIN diode, having the I layer of a thickness which varies along the length, giving in effect division of the spectrum into an infinite number of portions.