EE6428 OPTICAL COMMUNICATIONS
1. Describe briefly why in multimode fibers, graded-index profile type will give a much
smaller pulse dispersion than step-index profile type. Hence give a typical
mathematical expression for the refractive index of such graded-index fibers as a
function of the radial distance, fiber core radius, fractional change of core-cladding
index, and other relevant parameters.
2. Compare the intermodal pulse dispersion and the bandwidth in a multimode fiber
whose core index, n1 = 1.50 and = 0.01 for the following three cases :
(a) the core index is a step-index function,
(b) the core index is graded-index governed by the power law index function with
(c) same as part (b) except = opt.
3. Design a multimode step-index fiber whose normalized V number is 100 and whose
numerical aperture NA is 0.30. The fiber will be used in a data link with a 0.85µm
LED source. Determine the fiber parameters a, n1 and n2 if fused quartz with
refractive index 1.457 is used (a) as the core material, and (b) as the cladding
4. Assuming plane mirrors are used in the laser resonant cavity and that we have the
following two cases :
(a) a gas laser with cavity length 50cm, and
(b) a GaAs laser diode with a cavity length of 300µm.
(i) Calculate the number of possible axial modes that can exist in both cases and
determine the frequency spacing between adjacent axial modes. Assuming
the operating wavelength is 0.8µm, the refractive index of the gas cavity is 1.0
and that of GaAs is 3.5 at such operating wavelength.
What will be the corresponding wavelength spacing between adjacent axial modes?
(ii) Estimate the maximum length of the cavity in both cases if the laser has to be
single mode and if the gain curve of the laser material covers a bandwidth of
5. (i) An LED has a circular emitting area of radius 400µm. The power emitted by
the diode is 1mW. Its radiation pattern is described by cosθ. This diode is buttjointed to a step index fiber of core diameter 50µm, numerical aperture 0.25, core
index 1.479 and attenuation 2dB/km. Calculate the optical power accepted by the
fiber, assuming 100% transmissivity between the source and the fiber.
(ii) If the optical source in (i) is now replaced by a laser diode with emitting area
of 0.2µm x 12µm and emitting power 10mW, radiation pattern cos200θ, calculate the
power accepted by the fiber.
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