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New Techniques for Transmitting Microwave Signals Over Optical Fiber Thomas E. Murphy Dept. of Electrical and Computer Engineering Institute for Research in Applied Physics ECE Research Review Day – October 9, 2009 Sponsors Participants and Collaborators • Dr. Bryan Haas (Laboratory for Physical Sciences) • Dr. Timothy Horton (Laboratory for Physical Sciences) Coax vs. Fiber (~ 5 mm diameter) http://www.phy.davidson.edu/stuhome/phstewart/ • High loss (750 dB/km at 5 GHz) • Large, heavy • Bandwidth: DC–20GHz http://www.actewagl.com.au/ • Low loss (0.2 dB/km at l = 1.5 µm) • Thin, lightweight • Bandwidth: 187-230 THz (or l = 1.3–1.6 µm) Microwave Photonics: Using optical fiber to transmit microwave signals Advantages: • Negligible attenuation in fiber • Lightweight • Immune to EMI Disadvantages: • Requires high-fidelity modulation & demodulation • Total RF to RF loss can still be high Application: Antenna Relay • High-speed A/D conversion and signal processing is done remotely Problem: How to modulate and demodulate optical carrier without distortion Conventional Microwave Optical Link Optical Modulation Nonlinearity and Distortion Intermodulation Distortion • 3rd order intermodulation distortion products: (2f2-f1) and (2f1-f2) • Doesn’t matter how close together f1 and f2 are What About Phase Modulation? A Linearized Phase Modulator • TE and TM experience different degrees of phase modulation Linearization Condition • Third order nonlinearity can be cancelled, provided we choose a and q to satisfy: • g = ratio of TE/TM phase modulation Linearized Phase Modulator • For LiNbO3, TE and TM modes experience different phase mod. r33 = 31 pm/V r13 = 10 pm/V • Similar ratio is seen in EO polymers Experiment Lithium Niobate Phase Modulator Reduced Distortion • Third order nonlinearity can be cancelled, provided we choose a and q to satisfy: • g = ratio of TE/TM phase modulation Summary • Modulation nonlinearity can introduce distortion in microwave photonic transmission systems • New technique for linearization by using both polarization states in a phase modulator Future • Integrated polymer phase modulators, with built-in antenna • Applications in next-generation picocell wireless networks