Microwave photonics is among the fastest advancing technologies with potential applications in various areas such as satellite communication and cellular distribution networks. Recently, several attempts have been made to integrate microwave photonic systems on a single chip to meet the increasing demand for low power consumption, high stability, and low manufacturing cost. In particular, low-cost silicon photonics exhibit good integration with complementary metal-oxide semiconductors thus a promising solution. Nonlinear distortions greatly affect the performance of microwave photonic systems and thus should be suppressed using linear electro-optic modulators on silicon substrates. However, the realization of these electro-optic modulators remains the greatest challenge. This requires understanding of the different nonlinear distortions in microwave photonics systems such as second-order harmonic (SHD2) distortion.
Several approaches have been proposed to enhance the modulation linearity. These strategies are mainly based on eliminating intrinsic nonlinearity from Mach-Zehnder transfer function and suppressing the nonlinearity from the free carrier dispersion modulation effect of Si. This has led to the achievement of relatively high spurious-free dynamic range. The recent development in nanotechnology has resulted in the use of graphene in various optoelectronic applications owing to their excellent optical and electronic properties. For example, graphene-based silicon waveguide electro-absorption modulators have been developed with high modulation speed and efficiency. Unlike silicon modulators, graphene-based modulators are advantageous in terms of efficiency, cost and power consumption which could too be a good move in developing integrated microwave photonics applications. Unfortunately, the potential of graphene electro-refraction modulators has not been fully explored for integrated microwave photonics applications due to unavailable theoretical and experimental studies.
Recently, Yuansheng Tao, Haowen Shu, Ming Jin, Professor Xingjun Wang from Peking University in collaboration with Linjie Zhou and Weiwen Zou from Shanghai Jiao Tong University looked carefully at the modulation linearity of graphene-based silicon waveguide electro-absorption and electro-refraction modulators and their application in microwave photonics. The linearity performance was analyzed by a theoretically developed model and numerical simulation for the second-order harmonic distortions and third-order intermodulation (IMD3) distortions. The linearity performance of the graphene-based electro-absorption modulators were enhanced by suppressing the distortions through the bias optimization. By using graphene-based electro-refraction modulators, the authors encountered difficulty in simultaneously suppressing the SHD2 distortions and IMD3 distortions. This was however addressed by canceling the SHD2 distortions through push-pull modulation and quadrature biasing; for the remaining IMD3 distortions, it was necessary to adjust the bias voltage and phase shift length to linearize them. The work is published now in the journal, Optics Express.
Generally, the spurious-free dynamic range performance of graphene-based silicon waveguide modulator was influenced by various factors: the number of graphene layers, waveguide material, and oxide insulation layer. However, the graphene electro-refraction modulator designs did not influence the proposed distortions linearization schemes in any way. For graphene electro-refraction modulator, the maximum spurious-free dynamic range for third-order intermodulation distortions can reach to 130 dB·Hz2/3, comparable to commercial LiNbO3 modulator.
In summary, the research team successfully demonstrated the high linearity of graphene-based silicon waveguide electro-absorption and electro-refraction modulators in microwave photonics. The simulation results were slightly different from the realistic linearity performance results due to various reasons such as the fabrication errors and quality deterioration in the transferred graphene layer. The study paves way for the development of high-performance graphene-based modulators specifically for integrated microwave photonics applications.
Tao, Y., Shu, H., Jin, M., Wang, X., Zhou, L., & Zou, W. (2019). Numerical investigation of the linearity of graphene-based silicon waveguide modulator. Optics Express, 27(6), 9013.Go To Optics Express