Download New Techniques for Transmitting Microwave Signals Over Optical

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