New robust bias control method for optical modulators

For analog signal applications, lithium niobate Mach-Zehnder modulators are biased at the V pi/2 operating point to achieve maximum dynamic range and minimum signal distortion. Even a slight deviation from this optimum bias point causes severe reduction in the available dynamic range. Because of device instabilities, the bias voltage for the optimum operating point can drift with time-requiring constant readjustments. The automatic bias controller described in this paper automatically generates the desired bias voltage to maintain the optimum operating point - even in the presence of modulator drift. The bias controller operates with only one optical tap from the modulator output and is insensitive to variations in the absolute-optical-intensity level.