M-z-type Polymer Electro-optic Modulator Theoretical Research

Mach-Zehnder (M-Z) type polymer electro-optic modulators have advantages of frequency chirp-free, low half-wave voltage, easy to achieve velocity matching and low driving voltage. It has a wide range of applications in many fields, such as high-speed optical communications, high-speed optical switch array, phased array radar systems and microwave photonics system. In microwave photonic system, the essence of micro- wave optical modulation is that microwave signal field applied microwave signal on the traveling wave electrode lead to change the refractive index of electro-optic polymer material due to the linear electro-optic effect. The change will ultimately influence the phase or intensity of light-wave. As the frequency of microwave signal is higher than electric signal, the optical modulator requires the characteristics of larger modulation bandwidth and lower loss. Polymer electro-optic materials with advantages of low dielectric constant, large electro-optic coefficient and low loss, is widely used in the high-speed broad bandwidth modulator.In this paper, based on the material characteristic and pole theory of polymer, the electro-optic effect has been analyzed. Using effective index method (EIM), the ridge polymer waveguide has been designed. The transfer function of M-Z modulator is sinusoidal. In the absence of bias, the dynamic range of modulated signal is small, which limits the modulation depth. Generally, M-Z modulators are required to be biased at quadrature or half-power point so as to operate in the most linear region. There are two main methods to establish the optimal bias point. One is dc bias, the other is optical bias. Two methods of optical bias have been introduced in this paper. They are the asymmetric path and the asymmetric refractive index of the two arms. Combined with effective index method (EIM) and finite difference beam propagation method (FD- BPM), the two kinds of M-Z optical modulator based on optical bias have been investigated. The results show that initial phase difference of the two arms can achieve aroundπ/2, and the extinction ratios are 26dB and 23dB respectively.The electrode system is another important part for electro-optic modulator design. A microstrip electrode has been placed on one arm of M-Z modulator. The modulation bandwidth has large relationship with velocity difference between them. The ideal situation is that the two velocities are equal, that is velocity matching. The efficiency of microwave signal power is characterized by impedance of the electrode. When achieving impedance matching, microwave signal power can completely go into the modulator, and the driving power can be reduced. In this paper, quasi-static method has been used to investigate the microstrip electrode. The characteristic impedance and microwave effective dielectric constant affected by structure parameters have been simulated. A compensation layer has been introduced to adjust the microwave effective dielectric constant and characteristic impedance. The electrode characteristic parameters affected by the thickness and the material dielectric of the layer have been studied. Finally, the velocity matching and impedance matching have been achieved at the same time.