New Techniques for Transmitting Microwave Signals Over Optical Fiber
ECE Research Review Day – October 9, 2009
Thomas E. MurphyDept. of Electrical and Computer Engineering
Institute for Research in Applied Physics
Sponsors
Participants and Collaborators
• Dr. Bryan Haas (Laboratory for Physical Sciences)
• Dr. Timothy Horton (Laboratory for Physical Sciences)
Coax vs. Fiber
• High loss (750 dB/km at 5 GHz)
• Large, heavy• Bandwidth:
DC–20GHz
http://www.phy.davidson.edu/stuhome/phstewart/ http://www.actewagl.com.au/
• Low loss(0.2 dB/km at = 1.5 µm)
• Thin, lightweight• Bandwidth: 187-230 THz
(or = 1.3–1.6 µm)
(~ 5 mm diameter)
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?
• TE and TM experience different degrees of phase modulation
A Linearized Phase Modulator
Linearization Condition
• Third order nonlinearity can be cancelled, provided we choose and to satisfy:
• = ratio of TE/TM phase modulation
Linearized Phase Modulator
• For LiNbO3, TE and TMmodes experiencedifferent phase mod.r33 = 31 pm/Vr13 = 10 pm/V
• Similar ratio is seen in EO polymers
Lithium Niobate Phase Modulator
Experiment
Reduced Distortion
• Third order nonlinearity can be cancelled, provided we choose and to satisfy:
• = 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
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