7.2 AC Coupling

When trying to AC-couple the laser to the driver, the headroom of the driver is no longer a problem since it is DC-isolated from the laser with the coupling capacitor. At the output, the headroom of the driver is determined by the pull-up network. In Figure 7-2, the modulation current out of the driver is split between the pull-up network and the laser.

If, for example, the total pull-up resistor is twice the sum of the damping resistor and laser equivalent series resistance, then only $\frac{2}{3}$ of the modulation current will be used by the laser.

Therefore, to keep most of the modulation current going through the laser, the total pull-up resistor must be kept as high as possible. One solution involves using an inductor alone as pull-up, presenting a high impedance path for the modulation current and 0Ω path for the DC current offering headroom of the driver equal to V_CC and almost all the modulation current goes into the laser. The inductor alone will cause signal distortion. To improve this phenomenon, a combination of resistors and inductors can be used (as shown in Figure 7-2). In this case, the headroom of the drive is V_CC – R1 × ∝I_MOD, where ∝I_MOD is the portion of the modulation current that goes through the pull-up network.

When the laser is AC-coupled to the driver, the coupling capacitor creates a low-frequency cutoff in the circuit, and its value must be chosen to be as large as possible. If the value of the cap is too high, it will slow down the fast signals edges. Conversely, if its value is too small, it won’t be able to hold a constant change between the first bit and the last bit of a long string of identical bits in a low data rate application. This leads to higher pattern-dependent jitter in the transmitter signal—0.1 µF is found to be good for all applications from 155 Mbps to 1.25 Gbps.

AC-coupling the laser to the driver brings a solution to the driver headroom problem at the expense of extra components, loss of part of the modulation current wasted in the pull-up network, and additional power consumption.