to be different to the target power required for quadrature bias the processor then determines whether the measured output is very different from the target power if it is determined that there is a large difference for example greater than 5 to 10 percent of the peak value between the measured output and the target power as might happen if the power of the laser were to suddenly change the processor determines that a recalibration of the modulator is required and processing reverts to the process depicted in fig4 if the difference between the measured output and the target power is determined to be relatively small then the processor is configured to adjust the voltage bias applied to the bias electrode to increase or reduce as required the bias voltage to return the modulator to the quadrature point in the event that adjustment should continue in one direction i e by increasing or decreasing the bias voltage to a limit point where the available control range has been exhausted at which point the controller would not be able to further adjust the bias to give quadrature the controller may either be configured to restart the process described above or be configured to reverse the direction of bias change and roll the bias voltage away from the limit point until another quadrature bias point with the desired slope is reached as aforementioned a particular advantage of the teachings of the present invention is that the controller is readily scaleable without having to duplicate all of the controller components this is of particular importance when constructing multi channel systems and a schematic representation of an illustrative bias controller suitable for a multi channel system is shown in fig6 as shown in fig6 the controller of.