Phase Noise Conversion In Switchable Optical Time Delay Networks For Microwave Signal Processing

It has become increasingly apparent that the next generation of electronically scanned array antennas will require smaller and higher performance signal distribution and time delay beamforming networks. The latter characterisics provides wide instantaneous bandwidth at each steering angle. This eliminates beam squint and enables narrow pulse operation on large antennas, multiple frequency operation and multi-function aperture operation [l , 21. Photonics technology has the potential for having a tremendous impact on the architecture and realization of these systems. Optical interconnects are recognized to provide wider bandwidth, lower loss, smaller size, lighter weight and higher signal isolation than electrical transmission lines. Photonic device and circuit technology can implement modulation and true time delay beamforming functions on the microwave-modulated lightwave signals and provide much wider bandwidth than is presently possible with MMlC technology [3]. In telecommunication applications variable short term memories for queing and packet retiming are needed. In this contribution a noise analysis of true time delay (TTD) optical signal processing systems for arbitrary delay differences is presented. Due to the crosstalk of the switches each stage forms an interferometer which converts the phase noise of the laser source into amplitude noise at the output. This additional noise reduces the signal-to-noise ratio of the system and therefore the phase accuracy of the microwave signal. In wideband beamforming networks consisting of more than 4-5 bit phase shifters the optical true time delay signal processing is carried out in the subgroup level of a phased array antenna. The optical phase shifter shall provide the coarse delay steps ranging from T to n.T (T: period of the microwave signal) while the fine differential delays are provided with electronic delay lines in the transmitheceive modules. Therefore the delay time can be of the order of the coherence time of the laser source. Both the coherent and incoherent region are considered in this contribution.