TUTORIAL
EKT 442 (20 OCT 2008)
1. Draw a block diagram of a typical digital optical link. List the various optical
components suitable to implement the link.
(1 mark)
2. Two reasons why optical fiber are replacing copper cables for long-haul
communication.
(2 marks)
3. Numerical aperture (NA) is an important parameter of an optical fiber. It is
related to the coupling efficiency of the fiber.
i.
Explain in brief what determines the NA of an optical fiber.
ii.
Discuss by using suitable diagram, how the NA is related to the coupling
efficiency of the fiber.
4.
(1 mark)
Explain why most optical communication link worldwide uses either 1300nm or
1550nm as the source wavelength.
5.
(1 mark)
(2 marks)
A multimode step index (SI) fiber has a a core diameter of 50-µm large enough
to be considered by ray theory. It also has a core refractive index of 1.50 and a
relative refractive index difference of 1%. If the operating wavelength is 850nm, estimate;
a. The critical angle at the core-cladding interface.
(2 marks)
b. The numerical aperture for the fiber
(1 mark)
c. The acceptance angle in air for the fiber
(1 mark)
d. The total number of guided modes
(2 marks)
e. New core size to decrease the total number of guide modes obtained above
by 50%.
(2 marks)
f. Determine the cut-off wavelength for the fiber to exhibit single mode
operation corresponding to the new core size.
(3 marks)
1
6.
a. Describe the mechanism for the transmission of light within an optical fiber
using simple ray theory. Then with the aid of a suitable diagram, briefly
discuss what is meant by the acceptance angle for an optical fiber. (5 marks)
b. With the aid of suitable diagram, show that the relationship of the fiber
numerical aperture (NA) and the refractive index for the fiber core and the
cladding is given by:
NA ≈n1(2Δ)1/2
where Δ is a relative index difference of the fiber core and the cladding.
(10 marks)
7. A 40/125 µm graded index (GRIN) fibre has a core axis refractive index of 1.5 and
a relative refractive index difference of 1.3 %. It was found that the fibre supports
approximately 94 guided modes for a light emitting diode (LED) with an emission
of 1.55 µm.
a. Estimate the characteristic index profile, 𝛼 for this fibre.
(5 marks)
b. Determine the cut-off value of the normalized frequency to support singlemode transmission in the fibre.
(3 marks)
8. An optical link is needed to transmit data between two points separated 10-m
apart. You are given a choice of two different fibers. (a) Glass fiber with numerical
aperture (NA) of 0.24 and attenuation of 5 dB/km and (b) Polymer fibers with NA
of 0.48 and attenuation of 𝛼 dB/km. It was found that BOTH the fibers are suitable
and allows same optical power to reach the output. Calculate the value of 𝛼. State
any assumption(s) made.
(10 marks)
9. If the loss in an optical fiber is 0.2 dB/km as it is good fiber operating at
wavelength 1550-nm, find the attenuation coefficient, 𝛼.
(5 marks)
2
10. Explain briefly the meaning of modal dispersion in an optical fiber and how it can
be reduced.
11.
(a)
(5 marks)
Describe how design verification of optical communication link or
network is normally carried out.
(b)
(5 marks)
A supercomputer operating at 1Gbits/sec (assume a non-return-tozero (NRZ) line code) uses an optical link to send data to two nodes,
A and B. the system uses a laser diode connected to 3-dB optical
coupler. The two nodes are connected to the optical coupler as
shown in figure below. All two nodes are 110km away from the
output point of the optical coupler. The optical power coupled from
the laser diode into the input port of the coupler is 3.0 dBm with a
spectral width of 0.1nm. The laser diode has a risetime of 100ps. The
detectors used at the two nodes are identical and has a sensitivity of
-30 dBm and response time of 0.2 ns. The design requires a safety
margin of 10 dB from the splitter output to each respective node. You
are required to carryout the verification of the design. (ALL
assumptions made in the verification process must be stated)
(10 marks)
Laser Drive
Circuit
Node A
INPUT
OUTPUT
LASER
Node B
*OPTICAL
COUPLER
110 km
*Optical Coupler
Split optical input equally to
the output ports
FC/PC connector with an
insertion loss of 1 dB each
3