Designing biasing circuits that are made to match both types of MZM technologies (LiNbO₃ and InP) requires high voltage and current ranges as shown in Table 9-1. The Optical Internetworking Forum (OIF) recommends four differential IQ bias and two differential phase bias inputs, as shown in Figure 9-1. This differential signaling scheme helps in minimizing the crosstalk and noise between channels, which may otherwise result in a complicated bias control algorithm. While an ideal dither tone should be a sine wave, generating a sine wave can be cumbersome in a largely digital circuit domain. A square wave is relatively easier to generate through digital circuits, and can also be used, provided that the bandwidth of this dither signal is lower than the low cutoff frequency of the receiver (that is, 100 kHz or 1 MHz as per OIF). Passive RC filters with cutoff frequency lower than 100 kHz can be used at the DAC output for LiNbO₃ modulators, which have very small bias current requirement. For InP modulators that are mainly used with optical modules, typically requiring a receiver low cutoff frequency of MHz, choose RC values so that the power dissipation across the resistors is small.

For smooth detection of the dither signal at the MZM output, use two orthogonal dither frequency sources for the I and Q arms. The amplitude of the dither waveform is typically 0.5% to 2.5% of the dc bias voltage, which is mainly governed by the design implementation.