Fiber Optic Communication Links for Millimeter Wave Phased Array Antennas

For large, complex millimeter wave systems, such as phased array antennas employing monolithic T/R modules, the signal distribution presents a formidable challenge. Conventional feeds using waveguides and coaxial cables are not practical. Fiber optic (FO) links offer an attractive alternative for the distribution of control and communication signals in a phased array system. The control signals are responsible for the frequency synchronization of the independent slave oscillators to the master, beam forming and beam steering. The synchronization of the distributed local oscillators is particularly important to assure coherent wave combining. This paper is concerned with a fiber optic communication link for a millimeter wave phased array antenna. The feed to each individual T/R module is comprised of two FO links. The first link is responsible for the synchronization of the local oscillator by indirect optical injection locking. A semiconductor laser is directly modulated in a large signal mode from a master oscillator at 3.246GHz. In this mode of operation the laser generates higher order harmonics which are transmitted through the fiber and detected by a high speed PIN diode. In particular in these experiments the fourth order harmonic, at 12.986GHz, is detected, amplified and used to subharmonically injection lock an IMPATT oscillator at 38.96GHz. A locking range of over 100MHz was achived. The second fiber carries the communication signal which has a bandwidth of 1GHz. The two signals are mixed resulting in a 1GHz communication signal on top of a 38.986GHz millimeter wave carrier.