Abstract: An optical signal modulation device comprises first and second optical waveguides (1, 2) fabricated on Z cut lithium niobate. The waveguides (1, 2) are connected at one end to an optical signal splitter (7) and at the other end to a Y coupler (11). The output of the Y coupler (11) is connected to a single mode optical fibre (13). A first pair of modulator electrodes (3, 4) are positioned across the waveguides (1, 2) to cause relative phase shifts between signals in the two optical waveguides (1, 2) in response to a first control signal V.sub.1. These relative phase shifts are caused in both TM mode and TE mode signals. The device also includes a second pair of bias electrodes (14, 15) arranged to cause a relative phase shift between signals of the TE mode only whereby the transfer characteristics of the two modes may be shifted relatively to one another in use in accordance with the second control function.

Abstract: An AGC for a laser amplifier uses a control tone modulated onto the optical signals. During use the modulation on the optical signals is superimposed on the drive current for the laser. The AGC derives a control signal by detecting the modulation in the drive current, multiplying it by the drive current. The drive current for the laser is adjusted to keep the control signal constant. In a preferred embodiment a low level sensor detects when the control tone falls below a threshold level and switches the circuit to a default configuration. The default configuration provides a distress signal to indicate a fault and to identify the affected amplifier. This preferred embodiment is particularly suitable for submarine telecommunications capable which include a plurality of inaccessible amplifiers.