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Compact Optical Coherent Receiver for Avionics Applications

An optical coherent receiver for the down conversion of radio frequency (RF) signals from 10-18 GHz to 2 GHz is presented. Light from a distributed feedback semiconductor laser is split between two lithium niobate Mach-Zehnder modulators driven either by a tunable local oscillator (LO) tone or a RF signal coming, for example, from a receiving antenna. The modulated light signals are combined with an optical coupler and filtered by two fiber Bragg gratings (FBG) that select one optical sideband from each signal. Detection of the filtered light by a balanced photo-detector produces an electrical signal at an intermediate frequency equal to the beat difference between the RF and LO frequencies. Most current RF photonic systems are made from individually packaged devices that are interconnected with fiber-optic cables. In order to reduce size and weight and make the coherent receiver suitable for use in smaller airborne and mobile platforms, optical and opto-electronic components are packaged within a common enclosure where light routing is performed by micro-optics. A printed circuit board (PCB) is included within the module. It comprises a micro-processor to control and monitor the laser, the FBGs and thermo-electric coolers to ensure a robust operation over time and fluctuating environmental conditions. The module including the PCB, laser, modulators, optics, optical filters and balanced detector has a size of 89 x 64 x 32 mm3.