LASER DRIVER CONTROL CIRCUIT

A control circuit for A laser diode 66 has an input 56 for receiving a current from a photodiode 58 which is optically coupled to the laser diode 66. An output 112 supplies a driving signal to the laser diode 66. A means 70 for example, a reference modulator, derives first and second measured values from the received current 76, 80, which correspond respectively to data ones and zeros in the laser output signal. The means 70 sets first and second reference values 78, 82 which corresponds respectively to a data one and a data zero. Feedback circuitry controls the driving signal such that the first and second measured values become equal to the first and second reference values respectively. The means for setting first and second reference values comprises, means 40 for example, a reference control block, which provides a reference current. A means selects a desired ratio between the first and second reference values. A means 60, 62 sets first and second reference values based on the reference current and a desired ratio between the first and second reference values. A signal processing element including the control circuit is also claimed. Preferably, a transimpedance amplifier converts the current received from the photodiode into a voltage signal.