Abstract: Apparatus for converting a frequency division multiplex (fdm) signal to a time domain multiplex (tdm) signal in which a voltage control oscillator whose output is a staircase of reference frequencies is mixed by a mixer with a received fdm signal. Each received frequency is output in turn at a common intermediate frequency from a bandpass filter thereby forming a tdm signal after passing through a demodulator and low pass filter. The conversion may take place in the optical domain by mixing optical reference frequencies with an optical fdm signal in, for example, a coherent heterodyne optical receiver.

Abstract: Shocks or sudden stresses caused by attempts to tap into an optical fiber network are detected by monitoring the "speckle pattern" produced by intermodal interference at the output of a multimode fiber. The fiber is a single mode optical fiber communication link through which a monitoring beam having a wavelength at which the fiber supports multiple propagation modes is also transmitted.

Abstract: A method and apparatus for locking the frequencies of a number of first beams of coherent radiation from respective sources. Typically, the sources will form local oscillators in respective transmitters. The apparatus includes a reference beam generator such as a HeNe laser which generates a primary reference beam fed to an interferometer which generates a second, reference beam composed of a number of reference frequencies. This second reference beam is caused to interfere with each of the beams from the local sources in an optical coupler and the resultant beam is detected with a photodiode. The electrical output signal of the photodiode is filtered and fed to a discriminator which controls the frequency of the local laser. The discriminator controls the laser to maintain the relationship between the frequencies of the laser and the reference laser substantially constant.

Abstract: An optical network comprises a plurality of transmitting (T.sub.n) and receiving (R.sub.n) terminals; and cross-coupling means (2), the transmitting (T.sub.n) and receiving (R.sub.n) terminals being optically coupled with the cross-coupling means whereby modulated carrier signals transmitted from each transmitting terminal (T.sub.n) are fed to each receiving terminal (R.sub.n) via the cross-coupling means (2). A plurality of first sources (4, 7) of optical signals are grouped together remotely from and optical coupled with the transmitting (T.sub.n) and receiving (R.sub.n) terminals to supply carrier signals and tuning signals to the transmitting and receiving terminals respectively.

Abstract: A system and method of referencing frequencies of radiation from a plurality of sources, for example lasers (1,2) includes filter element (11), such as a Fabry-Perot etalon, onto which the radiation is incident. The filter element has a replicated set of passbands spaced apart in frequency. Detection means (12, 13) detects radiation passing through the filter element (11) and provides corresponding output signals. Control means (16-18, 3, 4) is responsive to the output signals from the detection means (12, 13) to control the sources (1, 2) so that the radiation transmitted through the filter element (11) is maintained substantially constant. The bandwidths of the radiation are small compared with the corresponding passbands of the filter element. The invention is particularly applicable in a communications network in which a number of sets of remote sources of radiation are referenced to a central set of reference sources.