EMI Reduction of Laser Modulation

The topic discussed here involves the solution of an EMI problem caused by modulating a laser at 40Mhz. In EMI testing there were failures at multiple harmonics of the 40Mhz. The root of the problem is that the laser is a two terminal device and the case is electrically tied to the anode. A simplified LTspice circuit model of the laser driver is shown in Figure 1 showing the stray capacitance.

Figure 1 - Original circuit

The stray capcitance, C1,  from the laser case to the chassis ground allows for a high frequncy AC current path in the chassis of the assembly flowing back to the common ground. This current path forms a large loop which radiates EMI. The option of finding a 3-terminal laser with a grounded case was not available. So instead the circuit was re-designed to mitigate the problem. The choice here was to use the principle of a common mode choke (CMC). The modfied circuit is shown in Figure 2 with the choke driving the laser diode.

Figure 2 - Circuit with CM choke

If the current flowing into the diode equals the current flowing out of the diode the flux changes in the CMC cancel and there is no impediment to the current flow. On the other hand if there is a current imbalance due to current flowing through the stray capacitance of the chassis then there is a differeintial current in the CMC. The differential current see's a high impedance of the choke and the amplitude of the current is diminished. There is no effect on the normal operation of the circuit, i.e., no reduction in the 40Mhz modulation of the laser current.

This solution very effectivley reduced the EMI problem and allowed the equipment to pass EMC testing.