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