An Automatic Bias Control (ABC) Circuit for Injection Lasers

The threshold current of an injection laser varies from device to device and is a function of the device age and temperature. This threshold variation causes the laser output to change when the drive current is held constant. One must therefore provide a bias control circuit to compensate for the threshold variations. Feedback circuits using photodetection have been successfully used for this purpose. 1 The output is monitored with an optical detector and compared with the input signal to generate an error signal that is fed back into the laser current. This paper introduces a new concept of compensating the change in the laser threshold by using the electrical characteristics of junction lasers. An electrical circuit monitors the ac voltage and the ac current of the laser and generates the bias current needed to operate the laser above the threshold level independently of the laser temperature and age. One of the benefits of using an electronic feedback scheme to stabilize the laser output is the elimination of an optical detector; this reduces the number of optical components required and may lead to a more economical and simpler solution than using a photodetector. The electronic bias control method is based on the fact that the laser junction voltage saturates at currents above threshold. 2-8 Figure 1 shows L and Vj as functions of I. L is the laser output light at one of the faces, Vj is the laser junction voltage, and I is the laser current. Figure 1 also shows the changes in L and Vj produced by varying I in the vicinity of the laser threshold, I t .