ETM 4106 Optoelectronics and Optical Communication
Tutorial 3
Solve the following problems:
Q.1 The optical power launched into an 8 km length fiber is 120  W, while at the output power is reduced
to 3  W. Determine:
(a) the signal loss in dB through the fiber
(b) signal attenuation per km for the fiber
(c) the signal attenuation for a 10km optical link using the same fiber with splices at 1 Km intervals, each
giving an attenuation of 1 dB.
(d) Ans. (a) 16 dB, (b) 2.0 dB, (c) 29 dB
Q.2 The numerical input/ output mean optical power ratio in 1 km length of fiber is found to be 2.5.
Calculate the received mean optical power if 1 m W is launched into a fiber of 5 Km length (assuming
no joints and connectors).
Ans. 10.0  W.
Q.3 A 15 Km link uses fiber with a loss of 1.5 dB/ Km. The fiber is jointed every Km with connectors
which give an attenuation of 0.8 dB each. Determine the minimum mean optical power, which must be
launched into the fiber in order to maintain a mean optical power level of 0.30  W at the detector.
Ans. 703  W.
Q.4 Determine the theoretical attenuation in decibels per km due to the Rayleight scattering in silica at
optical wavelengths of 0.63, 100 and 1.30  m. It is given that the fricative temperature is 1400K, with
an isothermal compressibility of 7  10-11 m2 N-1 , refractive index n 1.46 and the photoelastic
coefficient of 0.286. The Boltzman constant is 1.381  10-23 JK-1 .
Ans. 5.2 dB/Km, 0.8 dB/Km, 0.3 dB/Km.
Q.5 A K2 O-SiO2 glass core optical fiber has an attenuation resulting from Rayleigh scattering 0.46 dB/Km
at a wavelength of 1 m. The glass has an estimated fricative temperature of 758 K, isothermal
compressibility of 8.4  10-11 m2 /N and a photoelastic coefficient of 0.245. Determine from
theoretical considerations the refractive index of glass.
Ans. 1.49
Q.6 The threshold optical powers for stimulated Brillouin scattering and Raman scattering in a long 8 m
diameter SM fiber are found to be 190 mW and 1.70 W respectively when using an injection laser
source of bandwidth 1 GHz. Calculate the operating wavelength of the laser and the attenuation in
decibels per kilometer of the fiber at this wavelength.
Ans. 1.5 m.
Q.7 Estimate the bending loss for a multimode fiber with a core refractive index of 1.5, a relative refractive
index difference of 3% and an operating wavelength of 0.82 m. Compare the result to an 8m
diameter single mode fiber with a core refractive index of 1.5, relative refractive index difference of
0.3% and operating at a 1.55 m wavelength.
Ans. 9 m., 34 mm.