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Optoelectronic Microwave Oscillators

David YooCenter for Microwave and Lightwave Engineering

Drexel University

ECE-E641 – Fiber Optics and Optical Communications I

2/20/03

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Electronic Oscillators

First developed by L. De Forest in 1912

Noise and stability limitations caused by ohmic and dispersive losses (e.g. in LC circuit)

But these limits can be overcome by combining oscillator with a high Q resonator

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Loaded Quality Factor of Resonators

General Definition of Q, the Loaded Quality Factor of a Resonator

Power Dissipated

Energy Stored0Q

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Optoelectronic Oscillators

Convert continuous light energy from a laser to RF and microwave signals

Use of fiber as an energy storage device results in signals with extremely low phase noise

Phase noise practically independent of absolute oscillation frequency

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OEO Block Diagram

LaserElectro-Optic

ModulatorOptical Fiber

SpoolOptical Line

Stretcher

Photodetector

RF AmplifierRF Coupler

Microwave

Spectrum Analyzer

RF Bandpass Filter

Microwave Output

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Conditions for Oscillation

Oscillation begins from noise In order to have self-sustained

oscillation :1. Open loop gain of system must be

greater than unity2. An integer number of half-

wavelengths must be in the loop

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Loop Length and Frequency of Oscillation

The OEO loop can produce a comb of frequencies that satisfy the conditions for oscillation

The Free Spectral Range determines the distance between these frequencies

FSR in loop

Co is speed of light in vacuum n is index of refraction Lfiber is physical length of loop

fibertravel L

nc

tFSR

0

1

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Phase Noise of OEOs

Significant noises in an OEO (Yao & Maleki, 1996)

1. Thermal noise2. Shot noise3. Intensity noise of laser (RIN)

For high laser optical power, the phase noise of an OEO is limited by the laser’s relative intensity noise

For low laser power, thermal noise tends to dominate

Phase noises as low as -143 dBc/Hz at 10 kHz offset have been achieved for a 10 GHz carrier

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OEWaves TIDALWave

Fixed frequency (up to 40 GHz)

-143 dBc/Hz @ 10 kHz offset 10 dBm minimum output 10” x 10” x 4”

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Recent and Current Work on OEOs

Development of Compact OEOs (1999) Use of semiconductor lasers and external modulators

Multiloop OEOs (2000) The 2 fiber loops essentially act as the short and long

cavities in a laser to select a single operation mode, and they also permit tunability

Miniaturization of OEOs (currently ongoing) Replace fiber delay length with fused silica

microspherical resonator Such resonators have Q’s of 107-1010 for optical

frequencies Self Mode-Locking OEO (2002)

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Important Points to Remember

Optoelectronic oscillators convert light energy from a laser into RF and microwave signals.

The fiber delays in OEOs are high Q because of the extremely low attenuation rate of fiber.

The phase noise of OEOs tends to be limited by the relative intensity noise of the system’s laser.