Abstract: A system and method are provided for controlling the pre-emphasis applied to an optical signal, in which the output level of individual transmitters is controlled in order to reach a pre-defined desired value of a quality metric. Transmitters are able to adjust their output power without external control in such a way as to optimise the power distribution across the system.

Abstract: In the present invention, a method of controlling a tuneable optical dispersion compensating device to act on an optical signal by automatically controlling a plurality of dispersion control settings of the device in a systematic way using feedback, thereby to adapt freely the optical group delay for the optical signal within a predetermined wavelength range including that of the optical signal.

Abstract: An optical data receiver comprises an optical input for receiving optical data signals, an optical sputter for splitting the optical signals into first and second receiver arms, an optical filter in the first receiver arm, means for increasing an intensity ratio of optical signal strength in the first receiver arm to optical signal strength in the second receiver arm, means for adjusting a phase difference between the first and second receiver arms, and an optical coupler for coupling outputs of the first and second receiver arms to a photodetector. The receiver of the present invention selectively filters a carrier component of received optical data signals, adjusts the relative strength of the carrier component and the received signal and then recombines them. In this way efficient optical transmission can be achieved with direct detection at the receiver, without the need for a complex receiver design including a local oscillator.

Abstract: A branching unit, for use in optical communication systems, has three branches A, B and C each including a connection for a power feed and several inputs/outputs for optical fibers. Optical switches are provided for selectively coupling inputs/outputs of different branches to enable a routeing of optical signals through the unit. The routeing is effected in dependence upon the way in which each of the three branch connections are provided with an electrical power feed.

Abstract: An optical signal scrambler (18) has an optical phase modulator (22) which has a common input/output for an optical signal. The modulator also has an input for a modulating signal and a common output/input which is coupled to a 45° Faraday mirror (24). The scrambler (18) operates to modify an optical input signal received at the input/output of the modulator (22) so that it is returned as an output which is phase scrambled.

Abstract: A method of locating a submarine cable includes the steps of transmitting a toning signal along the submarine cable, the toning signal being a superposition of a first signal waveform to be detected and at least a second signal waveform such that the second signal waveform interferes with the first signal waveform to limit the peak to peak amplitude of the toning signal for transmission. Preferably, the toning signal is a square wave. The method provides a way of offsetting the loss of transmission distance due to the use of state of the art optical repeaters. Pre-emphasis of high frequency harmonics in the toning signal enables a detectable toning signal to propagate still further.

Abstract: A wavelength converter comprises a loop of optical fiber having an entry end (14) and an exit end (16). The entry end of the loop forms an input for a pump wavelength and the exit end forms an outlet for a converted signal. The loop is arranged to produce Stokes shifted wavelength components from a pump wavelength by Raman effect. A wavelength selective coupler (18) is provided between the entry and exit end (14, 16) of the loop. The coupler permits exit of a predetermined wanted Stokes shifted wavelength component but causes coupling of lower order Stokes wavelengths to the entry for recirculating in the loop to enhance generation of the predetermined wavelength component.

Abstract: An optical signal transmitter (10) is constituted by a coherent light generator (12) for generating a carrier wavelength and a modulator apparatus (16, 32) coupled with the output of the light generator. The modulator apparatus has a series arrangement of an optical intensity modulator (16) and an optical phase modulator (32) coupled between the input and an output of the modulator means. The intensity modulator (16) has an input (26) for a non return to zero (NRZ) digital signal of a predetermined data rate as defined by a clock generator. The phase modulator (32) has an input coupled with the clock generator which provides pulses to the phase modulator at the data rate, or at an even multiple of the data rate, but phase shifted relative to the data signal such that one end of each digital period is red shifted and the other end is blue shifted to facilitate detection.

Abstract: A branching unit for an optical fiber transmission system has a pair of inputs/outputs for receiving an end of a different section of trunk cable of the type having an electrical conductor and one or more pairs of optical fibres. A switching circuit (8) is effective to selectively route power between the electrical conductors (54, 56) of each trunk section or via an earth return (72) in response to a supervisory signal provided on the trunk.

Abstract: A branching unit for an underwater communications system is arranged so as to provide remote hot switching for electrical power. The unit has three terminations (1, 2, 3) each for a different one of three line cables and one or more power demand circuits (20) requiring an electrical power feed from the line cables. A first switch is actuable to complete a current path between an appropriate pair of the three terminations via one or more of the power demand circuits depending upon relative line voltage. A second switch is actuable after actuation of the first switch depending upon current supplied to the power demand circuit(s) rising above that which occurs upon action of the first switch. The second switch completes a circuit between the non-paired termination and a fourth termination for a sea-earth.

Abstract: An active branching unit for interconnecting three cables (two trunk cables and one spur cable) each including optical fibres. In a first interconnection configuration fibres of the trunk cables are connected together via the spur cable. Optical switch means, comprised for example by an erbium doped fibre amplifier (EDFA) (20) and an optical attenuator (21) in series, are disposed between optical fibres of the two trunk cables and are such as to pass telecommunications traffic between the trunk cables when, for example, there is fault in the spur cable and an alternative configuration is required. In this case a pump signal is applied to the EDFA (20) and the loss of the attenuator (21) overcome so that traffic can pass directly between the trunk cables and thus be diverted from the spur cable. This path switching can be achieved automatically.