Dispersion

Question 1

What is the speed of digital transmission?

Answer 1

~ >10^6 bits/sec

Question 2

When is the ability to distinguish an “on” bit from an “off” bit lost?

Answer 2

~ when the leading edge of one pulse can’t be discerned from the trailing edge of an adjoining pulse

Question 3

What is dispersion?

Answer 3

~ pulse-spreading mechanisms which limit the max transmission rate

Question 4

What is intermodal dispersion?

Answer 4

~ unique to multimode fibers

~ includes low order mode (ray traveling down center of the core) and higher order mode (second ray that zig-zags and has longer distance to travel to reach end of fiber)

~ low order modes reach detector first, high order modes reach it later, spreading the pulse

Question 5

What is a remedy?

Answer 5

~ the graded index profile fiber

~ glass compositional changes with progressively lower n

~ light propogating along the core center will travel slower than light extending to the outer core region

~ ideally, increased flight speed compensates for increased distance

Question 6

Why are single mode transmission rates set much faster?

Answer 6

~ there is no intermodal dispersion!

~ what remains is intramodal dispersion, which is orders of magnitude lower

Question 7

Are laser diodes truly single wavelength?

Answer 7

~ no, they typically have a 4-5nm spread

Question 8

What is the result of n decreasing with increasing wavelength?

Answer 8

~ the long wavelength portion of the pulse arrives earlier

~ leads to intramodal dispersion

Question 9

What is the result of a portion of the electromagnetic wave extending into the cladding?

Answer 9

~ the cladding has a lower n (evanescent field)

~ the wave is internally reflected

~ 30% of the wave can propogate into the cladding

Question 10

What is the result of differing speeds of light penetrating into the cladding?

Answer 10

~ waveguide intramodal dispersion

Question 11

When is the sum of material and waveguide dispersions equal to zero?

Answer 11

~at 1310 nm

~ this is the wavelength at which many long distance communication systems operate

Question 12

What wavelength is optimal based on the minimum attenuation from Rayleigh scattering and infrared absorption?

Answer 12

~ 1550nm

Question 13

What is a dispersion shifted fiber?

Answer 13

~ using a unique index of refraction radial profile, this point of cancellation may be shifted up to the optimal wavelength of 1550nm

Question 14

What do multimode fibers use?

Answer 14

~ less expensive LEDS to generate light signals

~ the fibers have wider core diameters and are used for short distance communication link

Question 15

When are single mode fibers used?

Answer 15

~ for communication links more than 1km

Question 16

What does wavelength division multiplexing do?

Answer 16

~ transmits a multitude of channels, each of a dedicated wavelength, using DIFFRACTION GRATINGS for multiplexing and de-multiplexing

~ typically 160 channels with a combined bandwidth of 1.6E12 bits/sec