7.1 DC Coupling

In addition to the low power consumption and high modulation current, the SY84782U offers a high compliance voltage. The minimum voltage needed at the output of the driver for proper operation is less than 600 mV, leaving a large headroom, V_CC – 600 mV, to the laser with the damping resistor. To show the importance of this high compliance voltage, consider the voltage drops along the path from V_CC to ground through the laser, damping resistor, and driver:

Ldi/dt is the voltage drop due to the laser parasitic inductance during I_MOD transitions. Assuming L = 1 nH, t_f = 80 ps (measured between 20% and 80% of I_MOD), and I_MOD = 70 mA (42 mA from 20% to 80%), then Ldi/dt will be equal to 525 mV. This number can be minimized by making the laser leads as short as possible and by using the RC compensation network between the cathode of the laser and ground or across the laser driver outputs, as shown in Figure 7-1.

To be able to drive the laser DC-coupled with a high current, it is necessary to keep the damping resistor as small as possible. For example, if the drop due to parasitic inductance of the laser is neglected (compensated for) and the maximum drop across the laser (1.6V) considered while keeping a minimum of 600 mV headroom for the driver, then the maximum damping resistor that allows a 70 mA modulation current into the laser is:

On the other hand, the smaller the value of R_d, the higher is the overshoot/undershoot on the optical signal from the laser. In the circuit shown in AC-coupled Laser, the RC compensation network across the driver outputs (MOD+ and MOD–) allows the user R_d = 10Ω, and C_Comp = 3 pF. The compensation network may change from one board to another and from one type of laser to another. An additional compensation network (RC) can be added at the laser cathode for further compensation and eye smoothing.