Electro-optic modulators based on polymeric Y-fed directional couplers
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The performance of current commercially available electro-optical (EO) intensity modulators is adversely affected by the distortions caused by the nonlinearity of their modulation curves. Furthermore, the direct-current biases, required in order to bring these modulators to their best working points, result in more complicated circuit fabrications than are desirable. This current research investigated the feasibility of fabricating bias-free and/or high-linear EO modulators by incorporating the merits of the Y-fed directional couplers (YCMs) with those of the EO polymer materials. To this end, EO polymer-based conventional YCMs were employed to fabricate bias-free EO modulators. A low switching voltage of 3.6V and a high extinction ratio of 26dB were obtained, at a 1.34µm wavelength, with a conventional polymeric YCM having lumped electrodes. A high-speed intrinsic 3dB-biased EO modulator based on a conventional polymeric YCM operating at a 1.55µm wavelength was successfully demonstrated to provide a 22-GHz 3-dB bandwidth, with a performance similar to that of a quadrature-biased Mach-Zehnder modulator (MZM) in terms of nonlinear distortion suppression when no direct-current bias was applied. A broadband high-linear EO modulator based on a ∆β-inverted polymeric YCM was also demonstrated for the first time. This modulator had a 3-dB bandwidth of 20GHz, and a high nonlinear distortion suppression, which was approximately 20dB larger than that of a quadrature-biased MZM in the range between 0 and 8 GHz. The linearity properties of the high-speed ∆β-inverted YCMs were also investigated theoretically. The results show that broadband high-linear modulators can be achieved by taking advantage of the small dispersion in EO polymers’ refractive indexes at microwave and optic frequencies. A systematic study on traveling-wave (TW) electrodes for polymer-based directional couplers was conducted for the first time in order to find the optimal electrode structure and dimensions. A conformal-mapping approach to the optimum TW electrode design of a coplanar waveguide with ground (CPWG) was developed and employed to optimize the electrode dimensions.