Fiber OpticsFiber Optics

By Matt BaylissBy Matt Bayliss

Jerome CarpenterJerome Carpenter

History and BackgroundHistory and Background

Techniques of Total Internal Reflection first used by Techniques of Total Internal Reflection first used by Greek and Venetian glass blowers centuries ago.Greek and Venetian glass blowers centuries ago.

Rudimentary principle of “light pipes” formed the basis Rudimentary principle of “light pipes” formed the basis for the first attempts at guiding light through refractive for the first attempts at guiding light through refractive tubes.tubes.

John Tyndall, England - 1870 - Demonstrated that light John Tyndall, England - 1870 - Demonstrated that light could be conducted along a curved stream of water.could be conducted along a curved stream of water.

1920’s & 30’s - Scientists in England, Germany, and 1920’s & 30’s - Scientists in England, Germany, and U.S. all experiment with using coated mineral fibers to U.S. all experiment with using coated mineral fibers to transmit images. Their ideas are not pursued.transmit images. Their ideas are not pursued.

Breaking ThroughBreaking Through

1951 - A.C.S. van Heel in Holland theorizes the use of 1951 - A.C.S. van Heel in Holland theorizes the use of glass coated fibers, experiments with plastic coatings.glass coated fibers, experiments with plastic coatings.

1951 - In England, H.H. Hopkins and N.S. Kapany 1951 - In England, H.H. Hopkins and N.S. Kapany develop basic techniques for fiber alignment and develop basic techniques for fiber alignment and transmit the first undistorted image through a bundle of transmit the first undistorted image through a bundle of uncoated glass fibers.uncoated glass fibers.

Kapany later coined the term “fiber optics”, and the use Kapany later coined the term “fiber optics”, and the use of glass-coated glass fibers is evolved to improve of glass-coated glass fibers is evolved to improve efficiency and reduce distortion.efficiency and reduce distortion.

1961 - Laser and fiber optics fields are first integrated.1961 - Laser and fiber optics fields are first integrated.

Geometry of Total Internal ReflectionGeometry of Total Internal Reflection

From Basic From Basic Electromagnetic Electromagnetic theory we take Snell’s theory we take Snell’s Law:Law:

nnoo*sin(theta*sin(thetaoo) = n’*sin(theta’)) = n’*sin(theta’)

Critical Angle and Numerical ApertureCritical Angle and Numerical Aperture

Define the Critical Angle, C by: sin(C) = n’/nDefine the Critical Angle, C by: sin(C) = n’/noo

Snell’s Law + Critical Angle gives us:Snell’s Law + Critical Angle gives us:n*sin(theta) = nn*sin(theta) = noo*sin(theta*sin(thetaoo))

= n = noo*sin[(pi/2) - C]*sin[(pi/2) - C] = n = noo[1-(n’/n[1-(n’/noo)^2]^1/2)^2]^1/2

We now define the Numerical Aperture:We now define the Numerical Aperture:NA = n*sin(theta) = [nNA = n*sin(theta) = [noo^2-n’^2]^1/2^2-n’^2]^1/2

Fractional Index DifferenceFractional Index Difference

For a given fiber, there is a fractional index For a given fiber, there is a fractional index difference, D = (ndifference, D = (no o - n’)/n- n’)/noo

For weakly guiding fibers, D will be <<1, and For weakly guiding fibers, D will be <<1, and therefore the critical angle will be large.therefore the critical angle will be large.

GoalsGoals

Determine NADetermine NA

Modulate the laser’s frequency and Modulate the laser’s frequency and analyze the output signalanalyze the output signal

Tools of the tradeTools of the trade

HeNe LaserHeNe Laser

100% of desired wavelength reflected 99% of desired wavelength is reflected

First the chamber is evacuated (high vacuum).Next He and Ne are added.

Each photon has an average of 100 passes before it leaves

Diode Laser – Same concept, but usesDiode Laser – Same concept, but uses

A diode to generate light. A diode to generate light.

Determining NADetermining NA

Numerical Aperature – range of angles in Numerical Aperature – range of angles in which the light is “accepted by the fiber.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

Determining NADetermining NA

Rough Determination of Na

Better method of determining Na

NANA

HeNe Rough estimate = .342HeNe Rough estimate = .342

determined to be .375determined to be .375

NA vs log (power) gave a parabolic curve. NA vs log (power) gave a parabolic curve.

Diode lasersDiode lasers

Differences between the Diode laser and Differences between the Diode laser and the HeNe laser revisited.the HeNe laser revisited.

Feed the diode some frequenciesFeed the diode some frequencies

ResistanceResistance

Diode Laser and Freq.Diode Laser and Freq.

Modulation depth fell of exponentially at Modulation depth fell of exponentially at high frequencies.high frequencies.

Waveform became distorted as frequency Waveform became distorted as frequency increased.increased.

-3dB frequency was 750 Hz-3dB frequency was 750 Hz

ResistanceResistance

50 ohms –3dB 75050 ohms –3dB 750

10k ohms –3dB ->010k ohms –3dB ->0

ApplicationsApplications

TelecomTelecom

Optronics?Optronics?