An all-pass ring on a bus, driven by shifting its resonance. The laser sits at a fixed wavelength; the two drive levels put the notch in two different places, and the difference in transmitted power is your signal. The best bias is not the one that maximises extinction ratio.
A pure electro-refractive shift is assumed: the resonance moves but its depth and width do not change. Real depletion modulators also add free carrier absorption, which shallows the notch on the "on" level and costs you a little OMA on top of what is shown here. Drive shift is the full swing between the two levels, so a device with 25 pm/V under 2 Vpp is 50 pm. The "bias for max OMA" button scans the whole span, which is worth doing after every change to κ or loss, since the optimum moves.
Alex Sludds