We describe a new method to keep an electrically isolated Mach-Zehnder electro-optic modulator biased in quadrature despite changes in temperature and optical coupling efficiency. The modulator is part of a system to measure electromagnetic waves in an outdoor environment. All data and control signals between the modulator and electronics control module are carried by analog optical-fiber links, and all of the bias signals and control procedures are implemented with an onboard computer and digital signal-processing unit.
Our method compares the average DC optical power in the complementary outputs of the modulator and adjusts the bias point by controlling the optical power to small photocells in the sensor head. A second control loop balances responses from a small dither signal applied to the modulator, consequently balancing the optical losses in each of the complementary outputs by way of a variable optical attenuator. This control system allows us to maintain the bias to within ±2 degrees of the quadrature point. The response time of the control loop is about 10 ms.