Maximizing modulation contrast of KTN electro-optic modulator

Abstract The KTa1-xNbxO3 (KTN) electro-optic modulator (EOM) is extensively applied to laser three-dimensional imaging and other fields, and optimizing its modulation contrast is essential for improving its modulation performance. It is generally considered that the modulator achieves optimal modulation contrast when its transmittance is 50%; however, the dielectric constant does not change with the electric field. Instead, the dielectric constant of a KTN crystal has a dynamic response under an electric field, which causes deviations in optimization. This study investigates the dynamic response of the dielectric constant to an electric field at low and high frequencies, and the maximizing modulation contrast method, which is based on the transmittance derivative function, is proposed. In the verification experiment, the proposed optimization method is used to achieve a maximum modulation contrast of 0.106 when the AC electric field is 320 V/m at a frequency of 1 kHz. The bias electric field is 95.4 kV/m, which is 3.5 times that of the 50% transmittance point. Additionally, when the AC electric field is 320 V/m at 500 kHz and the bias electric field is 87.8 kV/m, a maximum modulation contrast of 0.05 is achieved, which is double that of the 50% transmittance point.