Optical Modulator Bias Control for Analog Fiber Optic Link Applications

Typically, the electro-optical Mach-Zehnder interferometric intensity modulator is the key optical component that determines the ultimate performance of an externally-modulated high-performance analog RF fiber-optic link. Intrinsically, the device has a symmetric structure and has a transmission peak at zero bias. In order to operate at the maximum linearity quadrature point, the optical symmetry has to be broken and a DC bias voltage needs to be applied and maintained to create a differential 90 degree phase shift between the two arms of the MZ. The spurious free dynamic range (SFDR) of a wideband analog fiber optic link is critically dependent on the accuracy of the bias control. This DC bias point is dependent on a variety of issues such as operating wavelength and polarization crosstalk. Furthermore, lithium niobate exhibits undesirable properties in addition to the desired linear electro-optic effect including pyro-electric, piezo-electric and photo-refractive effects. These other effects can also cause the bias point of the modulator to change. Under this SBIR program, EOSPACE intends to significantly reduce this modulator bias control problem through two paths including both improved device designs and novel electronic feedback bias control techniques.