Abstract: A non-linear optical switch includes a semiconductor optical gain medium. The switch is arranged to switch a signal at a first wavelength in response to a control signal at a second wavelength applied to the optical gain medium. The refractive index of the gain medium is modified in response to the control signal. An optical holding signal at a third wavelength is applied to the gain medium. The optical holding signal pumps the optical gain medium and thereby fixes the Fermi level. The switch may be embodied in OTDM demultiplexers, clock recovery circuits, wavelength converters and other devices.

Abstract: A waveguide carrying system signals at a first wavelength and OTDR signals at a second wavelength is provided with an optical amplifier and an isolator. The isolator has a high isolation factor at the first wavelength and a low isolation factor at the second wavelength.

Abstract: Apparatus and a method for increasing the optical measurement range in optical time domain reflectometers in which a fibre laser amplifier is used to amplify the optical time domain reflectometer pulses launched into a waveguide. This apparatus and method have particular usefulness for branched local networks. The signal returning to the optical time domain reflectometer is amplified as well as the outbound signal.

Abstract: An optical test apparatus includes an optical time domain reflectometer for launching light pulses into a test fibre. An optical amplifier is provided in the light path between the optical time domain reflectometer and the test fibre or amplifying the light pulses. An optical switch is positioned in the light path between the amplifier and the test fibre for preventing light reaching the test fibre between the light pulses launched by the OTDR. Timing structure is provided for synchronising the operation of the optical switch so as to open the switch as a light pulse reaches the switch and close the switch as a light pulse leaves the switch.

Abstract: Apparatus and a method for increasing the optical measurement range in OTDRs in which amplification is used to amplify the OTDR pulses launched into a waveguide. The waveguide may be part of a waveguide or fiber under test and the gain is distributed and substantially balanced by propagation losses so that the maximum signal level returning to the OTDR is not substantially greater than in the absence of the amplification, thereby not requiring enhanced electrical dynamic range of the receiver. Raman amplification is particularly suitable. The signal returning to the OTDR is amplified as well as the outbound signal.