A high performance integrated optic system to control RF phase and amplitudem for phased array antennas

Abstract

The practical application of phased arrays is limited by their need for complex corporate feed structures and active phase shifting elements which drive system cost, weight, and power consumption beyond acceptable limits for most potential space applications. Much of the development has focussed on MMIC technology in an attempt to minimize cost and weight.Although significant progress has been made on wideband phase and amplitude controllers, the RF peak and amplitude taper errors are far above the typical requirements of 3" for phase and 0.5 dB for amplitude Furthermore, coaxial cable is still required in the distribution of RF signals thus adding to the system weight. Photonic technologies have been extensively investigated for weight and power reduction and increased bandwidth. [ 1,2]. Our study has shown that photonic system can also significantly improve RF phase and amplitude performance. As a result of our investigations, we have developed a high performance RF vector modulators that can control RF phase amplitude and also convert the electrical signal to an optical signal. The modulator which is modulo-2.n in nature can be connected to optical time delay units to allow very wide instantaneous bandwidth.RF IN I In-phase component Figure 1. Photonic I-Q system block diagram. Figure 1 shows the block diagram for a Photonic In-Phase/Quadrature (I-Q) Microwave Phase and Amplitude control system. The microwave signal is split by a 90" hybrid into in-phase (I) and quadr,ature (Q) components which drive two integrated optic Mach-Zehnder Modulators (MZMs). The intensity modulated light from the I and Q legs is combined by the 2x2 optical coupler and directed to an optical detector. The detector performs the vector addition of the I and Q intensities and converts the optical signal into an electrical signal. Adjusting the relative and absolute I andl Q intensities hitting the detector results in RF phase and amplitude adjustment of the electrical signal out of the detector. By weighting the optical intensity modulated signal out of each MZM, it is possible to achieve 0" to 90" phase control as shown in the phasor diagram. The weighting can be accomplished by several means including adjusting the laser diode output powers.The Photonic I-Q system can achieve 360" phase control by utilizing a characteristics of the MZM. When biased at the -7c/4 point the modulated optical intensity will be in phase with the input RF signal. If the MZM is biased at the +7r/4 point, the intensity is 180" out of phase with the RF 59