1. Field of the Invention
The present application relates to an optical transmitter, in particular, the application relates to a circuit for driving a semiconductor laser diode (hereafter denoted as LD).
2. Related Background Art
One type of drivers for an LD has been known as, what is called, a shunt-driver. Various prior arts have disclosed the shunt-driver circuits. The shunt-driver absorbs a primary portion of the bias current supplied to the LD responding to a driving signal provided to an input of the driver. Thus, the bias current flowing in an LD connected to the output of the shunt-driver in a primary portion thereof is shunted to the driver as leaving a rest portion in the LD. Thus, the LD is modulated by the driving signal. In order to adjust average power and an extinction ratio of an optical output of the LD, the driver often controls an input bias level thereof.
However, an optical transmitter including the shunt-driver leaves a subject that, in the start of the operation, the activation of the bias current for the LD precedes the powering of the driver circuit. Under such a situation, the driver circuit absorbs no bias current; that is a whole bias current is supplied to the LD, which results in an excess emission of the LD. Also, at stopping the optical transmitter, the power-down of the driver circuit occasionally precedes the cut-off the bias current to the LD. In such a case, the whole the bias current instantaneously flows in the LD to cause the excess emission. The present application is to provide a technique to prevent such an excess emission of the LD.