1. Field of the Invention
The present invention relates to a method to driver a semiconductor laser diode (hereafter denoted as LD), and an optical transmitter using the same.
2. Related Background Art
Various driving circuit have been well known in the field of the optical communication to drive an LD. The U.S. Pat. No. 6,697,400 has disclosed a method where the circuit provides a plural memories each storing the driving current Iac and the bias current Idc with respect to the temperature. The circuit reads out two currents depending on the temperature and determines the driving current. The Japanese patent application published as JP 2005-019546A has discloses another circuit where not only the bias current is adjusted depending on the temperature, but the modulation current is adjusted based on the optical output of the LD detected by a photodiode (hereafter denoted as PD) to keep the optical output and the extinction ratio in constant.
Another Japanese patent application published as JP 2008-270287 has disclosed a driving circuit where the bias and modulation currents are adjusted based on the temperature but the output of the temperature sensor exponentially varies to the temperature which enhances the accuracy of the driving.
The method to control the bias and modulation currents by a look-up-table (hereafter denoted as LUT), which stores values of those currents linked with temperatures, requires a large number of data for higher temperatures. Or, when the number of data is limited, the accuracy of the control or adjust of the optical output degrades. Because the threshold current of the LD exponentially varies in higher temperatures which results in considerable change in the modulation current to keep the average power and the extinction ratio of the LD in constant; accordingly, a large number of data for the modulation and bias currents is necessary for higher temperatures.
On the other hand, because the change of the driving current becomes gentle at lower temperatures, the driving current is sometimes necessary to be fine. In other words, a precise control of the driving current for the LD requests a fine temperature step at higher operating temperatures; while, requests a fine values for the current in lower temperatures, which inevitably expands the bit width for both the temperature and the driving current when the LD is digitally controlled.