High Linearity Wideband Coherent Amplitude Modulation (AM) Radio Frequency (RF)-Photonic Links

Abstract

Abstract : The objective of this project was to develop a fiber-optic RF link with high Spur Free Dynamic Range (SFDR). The approach taken was to utilize coherent AM modulation techniques which would, theoretically, provide a 6 dB advantage over Intensity Modulation/Direct Detection methods, and then insert a linearized electro-optic modulator in an attempt to achieve an SFDR of 130 dB Hz. In the coherent AM portion, locked homodyne detection and COTS components were used to achieve a measured SFDR of > 120 dB Hz at a 10 GHz operating frequency. The link, which used polarization maintaining components throughout and fusion splices instead of connectors, had none of the instabilities historically associated with homodyne systems. Efforts were made to raise the SFDR by using a Y-fed linearized directional coupler modulator; extensive measurements and theoretical analyses were made on these devices. The modulators showed the linearization predicted by theory; a logarithmic slope of 4.6 was demonstrated. Some shortcomings were also observed and analyzed that shed light on the difficulties of achieving wideband operation. The present devices can be designed to operate at specific center frequencies with maximum performance, but the linearized bandwidth is less than 2 GHz.