Control Systems for Linearized External Modulators

Ortel proposes to investigate methods of performing closed loop control of optically linearized modulators for use in externally modulated analog fiberoptic links. These links are becoming more popular at 1550 nm because of lower optical loss at the longer wavelength, and because of the availability of low-noise all-optical amplifiers (EDFAs). Ordinary Mach Zehnder modulators have poor third-order intermodulation performance. Optically linearized modulators overcome this limitation at the expense of great sensitivity to operating parameters. In order to achieve the best possible linearity over the temperature range and lifetime of the transmitter, it is necessary to perform closed loop control of the splitting ratio between the RF inputs to the optically linearized modulator, as well as bias voltage control. A method is proposed to perform closed loop minimization of third-order distortion in these modulators by controlling the RF splitting ratio. We also propose to investigate methods of bias voltage control for minimization of second-order distortion.