(1) An argon ion laser, emitting light at a wavelength of 488nm with a beam divergence of 150 firad, is used to illuminate the moon.
(i) Assuming the earth-to-moon distance is 3:8_108 m, estimate the diameter of the spot on the moon.
(ii) Calculate the waist of the beam in the laser, assuming that the beam is a fundamental Gaussian mode.
(iii) Suggest a way by which the spot diameter on the moon could be reduced by a factor of 100.
(2) The bandgap of GaAs is 1.4 eV.
(i) Calculate the probability of a valence electron being in the conduction band by thermal excitation at 0°C.
(ii) The donor binding energy in GaAs is 5.8 meV. Calculate the probability of finding a donor electron in the conduction band at 0°C.
(iii) From an electronic point of view, what is the difference between undoped and n-type GaAs?
(3) An optical fibre has a core refractive index of 1.52, and a numerical aperture of 0.12.
(i) Critical angle.
(ii) Cladding refractive index.
(iii) Maximum acceptance angle in (a) air, and in (b) water (refractive index =1.3)
(4) (i) What is the primary reason that optical fibres are used for communicating information?
(ii) How can you maximise the amount of information transmitted?
(iii)Estimate the bandwidth of a fibre of core diameter 50°m, n1 = 1:50, n2 = 1:49, and length L = 50 km.
(iv) Identify 2 other things that optical fibres are useful for.
(5) The mean power launched into an optical fibre of length 10 km is 150 _W and the average power at the output is 5°W. Calculate the overall attenuation in dB, and the attenuation per kilometre, assuming there are no interface losses.
Each question is worth ten points.