Linear response of an electric field sensing scheme based on electrode-less LiNbO3 asymmetric Mach-Zehnder interferometers and laser/LED light

LiNbO₃ integrated optics asymmetric Mach-Zehnder interferometers (AMZIs) can be used as electrode-less electric field sensors. As the electrooptic transfer function (EOTF) shows a cosinusoidal shape, linear sensing is ensured when the e-field modulates the light traveling through the optical waveguides around the quadrature point. As an electrode-less sensor cannot be electrically adjusted around such a point, a linear sensing is not ensured. When the modulated light is sent to a classical optical receiver, light is directly photodetected and the sensed e-field is recuperated as imprinted on the sensing side; linear detection is not ensured. To achieve a linear response when an electrode-less is used, a simple linearizing technique, consisting in cascading two matched AMZIs is proposed. In the linearizing scheme the first interferometer is the electrode-less sensor; the second one, which is provided of electrodes acts as an optical demodulator. The sensed e-field can be linearly detected when the EOTF of the demodulating AMZI is shifted to its quadrature point by applying a DC “tuning” voltage to its associated electrodes. The cascaded AMZI are illuminated either by coherent and incoherent light. The scheme is analyzed and this paper shows that a linear detection of the e-field is achieved when using incoherent light.