Ultra-short bilayer graphene optical fiber intensity modulator based on silver-semicircular hybrid plasmonic waveguide

Optical modulators play a crucial role in optical communication [1], [2], [3], [4], optical sensing [1], [4] and other optical systems. In recent years, all-fiber modulators have attracted more attention because of their fixed modulation bandwidth, low insert loss, and ability to improve the transmission efficiency of optical communication systems [1], [5]. There are a lot of research rising on improving the modulation efficiency of all-fiber modulator, but only by adding transition materials for absorption [6], [7] or phase change [8], [9] is difficult to achieve significant effect to avoid large footprint, strong loss, or high manufacturing cost [6], [7], [10]. To overcome these disadvantages, HPW is added to the all-fiber modulator, the light traveled in fiber core can enter to the plasmonic waveguide and inspire HSPP [1], [11], [12]. The modulation effect is greatly improved by appropriately adjusting the placement of the semicircles, the thickness of the Al2O3 and the spacing of the bilayer graphene. The structure combined HPW and MZM avoids the complex system and low extinction ratio of the photothermal modulator [9], the temperature instability of the electric absorption modulator [6], [7], and the low material–light interaction of the material coupling phase-shift modulator [8].

Graphene has been widely used in optical modulation systems due to its excellent electro-optic properties, high carrier mobility and operating speed [5], [9], [13], and the permittivity of graphene can be adjusted by its Fermi level which can be changed by driving voltage [7], [8], [9], [14], [15]. Therefore, graphene has been used in various light modulators in recent years: Liu et al. proposed a double-layer graphene optical modulator with a modulation depth of 0.16 dB/μm in 2011 [16]. In 2014, Zhou et al. applied graphene in phase modulation and explored the possibility of application of this two-dimensional material in all-fiber modulator [8]. In 2015, Phare et al. experimentally verified that graphene can be used for light intensity modulation and obtained the modulation depth of 15 dB [17]. Chu et al. experimentally explored the effect of graphene applied to optical fiber photothermal modulation in 2019 [9]. Thus, application of graphene in all-fiber modulators has a mature theoretical basis and has been verified by experiments, but it is still limited by the low graphene–light​ coupling efficiency, the not good enough extinction ratio of the modulator. Therefore, this paper combines bilayer graphene with HPW to obtain better phase modulation, allowing for a larger linear range of light intensity modulation, shorter modulation length and lower insert loss.

The application of HPW in optical systems has been the focus of recent research, HPW-based modulators have the excellent performance of small footprint and high modulation efficiency [18], [19], [20], [21], but it suffers from high insert loss. In 2017, Ayata et al. proposed a high-speed plasmonic modulator with a length of only 36 μm, which realized light modulation in multi-core fiber by carving MZM into gold plates [18]. Bian et al. proposed an optical waveguide based on hybrid plasmonic nanowire and greatly improved its optical transmission performance by placing the nanowire on the top of the waveguide [22]. In 2020, Su et al. proposed a hybrid plasmonic waveguide that consists of a metallic nanowire, which has the modulation depth of 26.2 dB/μm [23]. Ye et al. in 2021 proposed a nanowire-based graphene phase modulator that achieves satisfactory modulation with a modulation length of only 20 μm [1]. The hybrid plasmonic waveguide can bring excellent modulation efficiency, but the high insert loss is always a challenge. Thus, the modulator proposed in this paper applies the arrayed HPW to control the loss and enhance the phase modulation performance at the same time.

In this paper, an ultra-short bilayer graphene light intensity modulator based on hybrid plasmonic waveguide array is proposed. It combines the high modulation efficiency of the HPW and adjustable characteristic of the graphene, which achieves the purpose of linear modulation of light intensity in short modulation length by just adjusting the Fermi level of graphene through external voltage. As the optical intensity modulator is mainly used in fiber optic communication systems, the design is based on the central wavelength of the communication band, 1550 nm, which is also the loss-free wavelength of the communication band. Thanks to the efficient HSPP, the high extinction ratio of 39.39 dB and 47.77 dB for two ports and the modulation bandwidth of 59.945 GHz can be obtained with a modulator length of only 45 μm. Owing to appropriately adapted modulator structure and the arrayed HPW, the insert loss can be controlled below 0.0159 dB/μm, and the total loss is only 0.7155 dB owing to the short modulation length. The combination of graphene all-fiber modulator and HPW provides more possibilities for modulator design, this structure can be widely applied in optical modulation and sensing in the future.

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