Research On Broadband Optical True Time Delay Technology For Phased Array Antenna

With the fast development of fiber grating in the field of fiber communication, the optical properties of the fiber grating, such as bandwidth, reflectivity and time delay jitter, are required more and more stringently. Therefore, the simulation analysis and experimental verification on the optical properties of the fiber grating and the research associated with the practical application of the fiber communication become more and more important.With the fast development of phased array antenna and smart antenna technology, the core technology OBFNs(Optical controlled microwave beamforming networks) has been widely concerned, while the key components of the OBFNs:OTTD (Optical True Time Delay) module, especially these based on the fiber grating is simply structured and highly compact, with the property of easy to integrate and apply to practical, it is a hotspot of research. Now, the research on the OTTD module based on fiber grating are mostly concentrated in selecting different kind of fiber grating, such as uniform fiber bragg grating, chirped fiber grating, sampled fiber grating, or design different OTTD structure, such as combining uniform and chirped fiber grating, fiber bragg grating and photonic crystal fiber, aimed at achieving higher working bandwidth, smaller delay jitter, greater time delay range, higher time delay accuracy, higher reflectance, simpler structure, better scalability and other advantages. Uniform fiber bragg grating is the most common and simple structure which has the advantages of no time delay jitter, broad bandwidth, easy to product, but the minimum time delay which could achieve is usually10ps, only applicable in the OBFNs microwave operating at3GHz.In the paper, the basic theory and optical characteristics of uniform fiber bragg grating and chirped fiber grating is introduced, including how to use the transfer matrix method to analysis the time delay of chirped fiber grating, and the designation of the beamforming network that based on chirped fiber grating is given. The paper focuses on how to transform and optimize the OBFNs that based on fiber grating prism, starting from traditional structure, the paper explains its principle and points out the problems and the place where needs improving. A compact high frequency true time delay beamformer using bidirectional reflectance of the fiber gratings is proposed. The FGs can be a set of fiber Bragg gratings or chirped fiber gratings. The number and the length of the time delay lines are greatly reduced compared to the previous systems. A phase array antennas system using this TTD beamformer has many advantages including being compact, low requirement on the bandwidth of the tunable laser source and working at high microwave frequency. The proof-of-concept experiment results demonstrated the feasibility of the proposed scheme.The paper also presents a cascade beamforming structure, which is combined by two true time delay module. This structure combines the advantages of the two true time delay module, using a wavelength converter to cascade them, the first time delay module is responsible for large angle step, the second time delay module is responsible for small angle step.In the end, the paper introduces several mature true time delay technology, such as structure based on single chirped fiber grating and broadband optical source, structure based on photonic crystal fiber and superstructure fiber grating, structure based on single chirped fiber grating with adjustable chirped coefficient, and the paper pointes out the advantages and disadvantages of each structure.