ETM 4106 Optoelectronics and Optical Communication
Tutorial 4
Solve the following problems:
Q.1 (a) A multi-mode step index fiber gives a total pulse broadening of 95 ns over a 5 Km length. Estimate
the bandwidth-length product for the fiber when a non-return to zero code is used.
(b) A single-mode step-index fiber has a bandwidth-length product of 10 GHz-Km. Estimate the rms
pulse broadening over a 40 Km digital optical link without repeaters consisting of the fiber and
using return to zero code.
Ans. (a) 13.2 MHz-Km (b) 800 ps
Q.2 An optical fiber link utilises a multimode graded index fiber which exhibits a total pulse broadening of
0.15 s over a distance of 21 km. Determine:
(a) the maximum possible bandwidth on the link assuming no intersymbol interference
(b) the pulse dispersion per unit length
(c) the bandwidth- length product for fiber
Ans. (a) 3.33 MHz (b) 7.14 ns/km (c) 69.9 MHz-km
Q.3 A silica fiber exhibits a material dispersion of 0.029. Estimate;
(a) the material dispersion parameter at a wavelength of 0.83 m, and the rms pulse broading per km
for an LED source with an rms spectral width of 22nm at this wavelength.
(b) The rms pulse broading per km when an injection laser, with a relative spectral width of 0.0013 at
wavelength of 0.83 m is used.
Ans. (a) 116.5 psnm-1 km-1 , 2.56 nskm-1 (b) 0.13 nskm-1
Q.4 A multimode, optimum near parabolic profile graded index fiber has a material dispersion parameter of
30 psnm-1 km-1 when used with a good LED source of rms spectral width 25 nm. The fiber has a
numerical aperture of 0.4 and a core axis refractive index of 1.48. Determine the total rms pulse
broadening per km within the fiber assuming waveguide dispersion to be negligible. Also, estimate the
bandwidth-length product for the fiber.
Ans. 774 pskm-1 , 258 MHz km
Q.5 A multimode step index fiber has a numerical aperture of 0.2 and a core refractive index of 1.47.
Estimate the bandwidth-length product for the fiber assuming only intermodal dispersion and a return
to zero code when there is no mode coupling between the guided modes.
Ans. 11.0 MHz-Km
Q.6 An optical link which has a length of 6.3 km, consists of a multimode step index fiber with a core
refractive index of 1.49 and a relative refractive index of 1%. Determine:
(a) the delay difference between the slowest and fastest modes at the fiber output
(b) the rms pulse broadening due to intermodal dispersion on the link
(c) The maximum bit rate that may be obtained without substantial errors on the link assuming only
intermodal dispersion
(d) The bandwidth-length product
Ans. (a) 313 ns (b) 90.4 ns (c) 2.2 Mbits/s (d) 13.9 MHz km