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Fiber Optics

Fiber Optics. By Matt Bayliss Jerome Carpenter. History and Background. Techniques of Total Internal Reflection first used by Greek and Venetian glass blowers centuries ago.

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Fiber Optics

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  1. Fiber Optics By Matt Bayliss Jerome Carpenter

  2. History and Background • Techniques of Total Internal Reflection first used by Greek and Venetian glass blowers centuries ago. • Rudimentary principle of “light pipes” formed the basis for the first attempts at guiding light through refractive tubes. • John Tyndall, England - 1870 - Demonstrated that light could be conducted along a curved stream of water. • 1920’s & 30’s - Scientists in England, Germany, and U.S. all experiment with using coated mineral fibers to transmit images. Their ideas are not pursued.

  3. Breaking Through • 1951 - A.C.S. van Heel in Holland theorizes the use of glass coated fibers, experiments with plastic coatings. • 1951 - In England, H.H. Hopkins and N.S. Kapany develop basic techniques for fiber alignment and transmit the first undistorted image through a bundle of uncoated glass fibers. • Kapany later coined the term “fiber optics”, and the use of glass-coated glass fibers is evolved to improve efficiency and reduce distortion. • 1961 - Laser and fiber optics fields are first integrated.

  4. Geometry of Total Internal Reflection • From Basic Electromagnetic theory we take Snell’s Law: • no*sin(thetao) = n’*sin(theta’)

  5. Critical Angle and Numerical Aperture • Define the Critical Angle, C by: sin(C) = n’/no • Snell’s Law + Critical Angle gives us: n*sin(theta) = no*sin(thetao) = no*sin[(pi/2) - C] = no[1-(n’/no)^2]^1/2 • We now define the Numerical Aperture: NA = n*sin(theta) = [no^2-n’^2]^1/2

  6. Fractional Index Difference • For a given fiber, there is a fractional index difference, D = (no - n’)/no • For weakly guiding fibers, D will be <<1, and therefore the critical angle will be large.

  7. Goals • Determine NA • Modulate the laser’s frequency and analyze the output signal

  8. Tools of the trade • HeNe Laser First the chamber is evacuated (high vacuum). Next He and Ne are added. 100% of desired wavelength reflected 99% of desired wavelength is reflected Each photon has an average of 100 passes before it leaves Diode Laser – Same concept, but uses A diode to generate light.

  9. Determining NA • Numerical Aperature – range of angles in which the light is “accepted by the fiber. Actually the sine of the angle at which the fiber only transmits 5% of the original power

  10. Determining NA Rough Determination of Na Better method of determining Na

  11. NA • HeNe Rough estimate = .342 determined to be .375 • NA vs log (power) gave a parabolic curve.

  12. Diode lasers • Differences between the Diode laser and the HeNe laser revisited. • Feed the diode some frequencies • Resistance

  13. Diode Laser and Freq. • Modulation depth fell of exponentially at high frequencies. • Waveform became distorted as frequency increased. • -3dB frequency was 750 Hz

  14. Resistance • 50 ohms –3dB 750 • 10k ohms –3dB ->0

  15. Applications • Telecom • Optronics?

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