Abstract: The operation of laser diodes requires a preset operating point. Owing to the spread in the characteristic curves of laser diodes, a costly adjustment is required for accurately determining this operating point, more specifically after each exchange of a laser diode. By using a control circuit, the operating point is positioned in the portion of the characteristic curve in which the ratio of the slope of the characteristic curve to the slope of the characteristic curve in the linear portion of the characteristic curve is equal to a predetermined value. This ratio is obtained by measuring the change in the bias current, which compensates for a power change caused by a change of the modulation current.