Apparatus for amplitude modulating the output of a laser diode or L.E.D

The output of a laser diode or L.E.D is amplitude modulated by varying the forward current through it. This is done by applying a dc current component from a first current source and an ac current component from another part of the circuit. The ac current component is applied to the laser diode l via an ideal impedance matching transformer which comprises four capacitors l0, ll, l2, and l3. One of the capacitors l2 is the internal capacitance of the laser diode l such that when the frequency of the modulating signal is varied and hence the response of the laser diode l varies, ideal matching is still obtained since the capacitor l2 enables compensation to be made.