Tunable Multiwavelength Fiber Laser Source For Optically Controlled Phased Array System

Optical true-time-delay(TTD)beamforming techniques for phased array antenna have been widely studied for it shows significant advantages such as small size,high bandwidth,low transmission loss,large tunability,and immunity to electromagnetic interference.A number of approaches have been proposed to achieve continuous TTD beam steering on optically carried ratio frequency signals.Among them A linear chirped fiber grating(LCFG)based system is commonly used,however it suffers a major disadvantage that a tunable multiwavelength laser source in which the wavelengths are tuned simultaneously with equally increased or decreased wavelength difference should be provided.This makes the TTD system cumbersome and expensive when it contains a large number of channels.In this thesis,a new approach to generate multiwavelength beam with equally increased or decreased wavelength difference is proposed and demonstrated.The SOA provides the C-band optical signal for the FPF,which is mounted on an electric one-dimensional translation stage providing high resolution linear motion.The cavity length of the FPF changes as the translation stage moves so that wavelength tuning is realized.Time delays with equally increased or decreased time delay difference for the equally spaced wavelengths are obtained when the modulated light signals are reflected at different locations of the LCFG.The sideband-mode suppression ratio(SMSR)of the output signal can be more than 35 dB.A true-time-delay system with five elements has been constructed and experimented at 10 GHz using the proposed laser source.The distance between neighboring elements is 0.015 m.The time delay can be as much as 50 ps and the scanning range is 10°to 90°.Thanks to the high position resolution of the translation stage,the minimum time delay step can be much smaller than 1 ps so that the directivity of the antenna is more accurate.