Research On Novel Fiber RF Antenna With Coaxial Structure

With the sustained rapid development of the Internet, the large-scale popularization of cellular telephone systems and the continuing growth of the handhold personal communications equipment market, ultra-wideband wireless access technology has become a research focus in the wireless communication field. Photonic RF receiving technology utilizing the method of microwave photonics to receive microwave signal scan meet the needs of bandwidth and long-distance transmission in wireless communication, and are heading in the direction of integration, miniaturization, low power consumption and high-efficiency. However, the commercial microwave photonic devices are discrete currently. Hence how to integrate optical modulator and microwave receiver, taking into account the modulation efficiency is of great significance in the field of microwave photonics receiver,In this paper, a novel optical fiber RF receiving antenna is deeply explored, which combines the optical fiber, the RF waveguide and Surface Plasmon Polaritons (SPPs) waveguide together based on the coaxial structure. With the help of the surface plasmon resonance reinforced effect, the RF signal scan be effectively modulated to the optical regime and then be transmitted. A single-mode fiber with a long period fiber grating (LPFG) is coated with a10-60nm thick metal layer, which is used to excite the SPPs. EO polymer is deposited around it. Another metal layer is then coated outside this polymer layer to form a mini coaxial structure, in which quasi-TEM mode of RF signal can be distributed. The inner metal layer is connected to the ground, and this coaxial structure can work as the RF antenna. The LPFG in the design is to couple light from the core mode to forward-propagating cladding modes. Based on phase-matching properties of the LPFGs, it is possible to excite a particular SPPs coupled cladding mode on the metal coating around the fiber when the axial component of the propagation constant of a cladding mode matches that of SPPs. Due to the coupling with SPPs, the propagating constant is highly sensitive to refractive index of the surrounding polymer. When the RF signal is received by the structure and distributed over the mini coaxial cable, the refractive index of the EO polymer between the two layers of the coaxial cable will be changed, which will affect the plasmon resonance and change the SPPs coupled resonance wavelength of the LPFG. Since the spectral location of maximum coupling to the surface plasmon resonances is highly dependent upon the refractive index of the surrounding medium, RF signals can be modulated on the power transmission in fiber core.In this paper, this novel fiber RF antenna with coaxial structure is studied in both theoretical analysis and numerical simulations. Firstly, based on the theory of long-period fiber gratings to excite the optical fiber SPPs, the impact of changing the refractive index of external environment on the optical fiber transmission spectrum is analyzed. Secondly, transmission characteristics of the multi-coaxial structure are analyzed. The main mode and higher modes within the ultra-wideband range in the coaxial structure are studied by mode analysis, and the single mode propagation conditions are concluded. Finally, the structural parameters of this antenna are optimized by considering the electromagnetic properties of this novel fiber RF antenna in both the optical and electric regime, and the electro-optic modulation effect are comprehensively analyzed through considering the relationship of the received signal, the electro-optic characteristics of the EO polymer and transmission spectrum of the fiber.In conclusion, the feasibility of the novel fiber RF antenna with coaxial structure is verified via simulation and calculation in this paper. Compared with the conventional LiNbO3crystal modulator, this antenna can modulate radio frequency signal to the optical carrier effectively. Also the integration of the antenna and the modulator helps to reduce the complexity of the RF photonic receiving system. In the end of the paper, further improvement, the manufacture difficulties, and also the prospect of such optical fiber RF antenna are forwarded.