High Performance Microwave Photonic Links
John Wyrwas and Ming C. Wu
Defense Advanced Research Projects Agency
Microwave photonic links have found applications in commercial and military antenna remoting, cable television and cellular communication links because of advantages in size, weight, immunity to electromagnetic interference, bandwidth and power over traditional coaxial links. The development of links with low distortion and low noise are critical for high-dynamic range sensing and communications applications. Optical frequency modulated (FM) photonic links are promising because FM distributed Bragg reflector (FM-DBR) lasers have been demonstrated with high RF-to-optical conversion efficiency, high linearity modulation and modulation bandwidths that are not limited by the laser relaxation frequency. However, the links' ultimate performance is limited by the FM discriminator, which converts FM to IM for direct photodetector. Our contribution is the design, fabrication and characterization of optimal FM discriminators for FM microwave photonic links. We have analytically showed that one can achieve nearly-ideal linear discrimination with two complementary optical filters that have linear phase and electric field transmission magnitude that ramps linearly with optical frequency. We are implementing this system using optical finite impulse response (FIR) filters fabricated in planar lightwave circuits (PLC). PLC offers integration with other optical components and allows for adaptive tuning of the filters. We are also applying research in optical injection locking to the design of optical limiting amplifiers to suppress residual intensity modulation of the laser, further reducing distortion.