Optoelectronics Exam: Electronic Engineering, Summer 2008, Exams of Advanced Computer Architecture

Download Optoelectronics Exam: Electronic Engineering, Summer 2008 and more Exams Advanced Computer Architecture in PDF only on Docsity! Cork Institute of Technology Bachelor of Engineering (Honours) in Electronic Engineering – Award (EELXE_8_Y4) Summer 2008 Optoelectronic Devices and Communications Systems (Time: 2 Hours) Instructions Answer FOUR questions Examiners: Dr. O. Gough Prof. W.G. Hurley Dr. S. Foley Values for useful constants; electron charge q = 1.60 x 10-19 C mass of the electron me = 9.10 x 10-31 kg Planck constant: h = 6.63 x 10-34 J s Speed of light in a vacuum c = 2.998 x 108 m s-1 Boltzmann’s Constant k = 1.38 x 10-23JK-1 Q1. (a) Draw a diagram outlining the principal features of a diffraction grating tuned external cavity laser and briefly explain its operation. (7 Marks) (b) A InP based laser diode has a length of 600µm and a gain bandwidth under high injection of 24 nm. How many longitudinal modes can this support. The centre wavelength of the gain region is 1350 nm and the effective refractive index of the waveguide is 3.48. (5 Marks) (c) Write down the photon and carrier rate equations for a single mode laser diode and briefly explain the significance of each term. Show that in steady state the photon number in the cavity increases linearly with injected current. (13 Marks) - 2 - Q2. (a) Determine an expression for the SNR of an Avalanche Photodiode. Hence show why an avalanche photodiode will be preferable to a PIN photodiode in the thermally limited detection regime. (8 Marks) (b) A 1550 nm single mode fibre optic link is required to operate at 20 Gb/s. The receiver comprises a PIN detector with a quantum efficiency of 45% which has a dark current of 10 nA and is coupled to an amplifier with a noise figure of 3 dB. Determine the average optical power required to achieve a bit error rate of 10-9. (You may assume the load impedance to be 50Ω). (12 Marks) (c) The above system has a transmitter whose extinction ratio degrades to -10 dB. Determine the effect this has on the system design. (5 Marks) Q3. (a) Wavelength routed networks are one possible paradigm for next generation optical networks. Discuss the technological requirements for such networks. In particular address the design of optical cross connects and the use of wavelength conversion. (15 Marks) (b) One approach to dispersion compensation in optical communication systems is “dispersion mapping” with multiple fibre types used in spanning the link. A 3000 km link is to be constructed using standard single mode fibre and a dispersion compensating fibre. The standard fibre has D = 16.0 ps/nm-km at 1550 nm while the DCF has D = -178 ps/nm- km. It is planned to use low cost optical sources with a mean spectral width of 0.6 nm. The link requirements are as follows: Accumulated dispersion over the link length to be less than 5 ps for λ = 1550 nm, accumulated dispersion at any point along the link to be less than 100 ps at λ = 1550 nm. Design the dispersion map including tolerances to meet this requirement.. Estimate the worst case limiting bitrate for this design. (10 Marks)