## 1 Introduction  
High performance electro-optic (EO) modulators are highly demanded in wideband optics communication networks [1]. The lithium niobite-based modulators are mature but suffer from a large footprint [2]. Silicon modulators are compatible with CMOS technology, while facing the problem of low EO efficiency [3]. III-V based modulators offer a small footprint; however, the insertion loss and thermal stability are still to be improved [4]. SiC-based waveguides also show application potential [5]. Due to the unsatisfactory EO properties of the above underlying materials, modulators that meet all requires are still elusive. Graphene with excellent EO characteristics has been well studied and widely adopted in optical modulators [6, 7, 8, 9]. In 2011, [PERSON] et al. experimentally presented a graphene modulator with a modulation depth (MD), the difference between \"ON\" state and \"OFF\" state) of 0.1 dB/\(\upmu\)m [10]. A built-in p-oxide-n-like junction graphene modulator exhibited an MD of 0.16 dB/\(\upmu\)m and a power consumption of 1 pJ/bit [11]. The graphene EO modulator with a high 3 dB-bandwidth of 30 GHz has also been demonstrated [12]. However, the performance of above graphene modulators is limited by the weak interaction between the extremely thin graphene flake and the light field that is strongly bounded in the high index dielectric material. If the plasmonic effect is introduced, the light can propagate as a surface plasmon polarizations (SPPs) mode along the waveguide, which implies an enhanced optical field intensity. More optical power will be guided to the dielectric/graphene interface, facilitating the light-graphene interaction [13, 14, 15]. [PERSON] et al. proposed a waveguide-coupled hybrid plasmonic graphene modulator with a 3 dB-bandwidth of 0.48 THz and an energy consumption of 145 fJ/bit [16]. A hybrid plasmonic graphene modulator with a 3 dB-bandwidth of 0.662 THz and a power consumption of 118.7 fJ/bitpromises good potential for hybrid plasmonic graphene EO modulators [17]. Another issue for the graphene modulator is the polarization sensitivity, which originates from the low absorption rate of 2.3% for the electric field polarization perpendicular to the graphene flake surface. Most reported works can only operate in transverse magnetic (TM) or transverse electric (TE) polarization independently [18]. The introduction of polarization control functions will inevitably complexify the device structure [19]. Therefore, a compact and polarization-insensitive graphene modulator with characteristic of broadband operation is desirable.  
In this paper, we proposed a graphene modulator based on a dual built-in orthogonal slots waveguide (DBOSW). The geometric parameters of the hybrid plasmonic waveguide are investigated and optimized by finite element method (FEM). The mechanism of polarization insensitivity and broadband modulation is elaborated in detail. Our proposed waveguide structure overcomes the strict polarization dependence of plasmonic-based waveguides. Moreover, the DBOSW graphene modulator realizes broadband modulation with excellent polarization independence, which shows good potential in microwave photonic systems.