- August 14, 2020

PHTN4662 Midterm Examination Questions . \Go on to next page. 1 Question-1 (For students with the 3rd digit of StudentID, z: 0,1,2,3,4) a) The diagram below shows the typical parameters and their relations in step-index optical fibre design. 1) Please give a simple description of the red curve (2 marks) 2) Please briefly and respectively explain Zone 1 to Zone 4 in terms of their advantages or disadvantages in fibre fabrications and applications (8 marks) Core radius a vs relative index difference in step-index optical fibre design b) The cladding index of an optical fibre is 1.45. The fibre’s core radius is 5m and its cutoff wavelength has been measured to be 1.25m. Calculate the core index of the fibre and its spot size at =1.5m. (10 marks) Question-1 (For students with the 3rd digit of StudentID, z: 5,6,7,8,9) a) The rms pulse width at the end of an optical fibre is given by the equation 1) Please briefly explain the respective origin of each term in the equation. (4 marks) 2) Based on the equation, discuss and compare cases using different types of optical fibres (SMF, MMF, SMF and DCF) in optical fibre communication systems (6 marks) b) The core index of an optical fibre is 1.46. The fibre’s core radius is 5m and its cutoff wavelength has been measured to be 1.3m. Calculate the cladding index of the fibre and its spot size at =1.5m. (10 marks) 0 0.002 0.004 0.006 0.008 0.01 0 5 10 15 a ( m) c =1.28 m 1 2 3 4 222 wmin int PHTN4662 Midterm Examination Questions . \Go on to next page. 2 Question-2 (For students with the 4th digit of StudentID, z: 0,1,2,3,4) A laser diode emits an optical power of 20mW at 1550nm and has a spot size 4m and spectral width 1nm. It is joined to an optical fibre with a tilt angle of 2 and launches a rectangular pulse of absolute width 2ns into the fibre. The fibre has the following parameters: Cutoff wavelength = 1320nm Spot size = 4m at 1550nm Cladding refractive index = 1.457 Material dispersion = 17ps/nm-km at 1550nm Attenuation coefficient = 0.2dB/km at 1550nm The output end of the fibre is connected to an optical receiver of sensitivity of – 40dBm. (i) Without including any amplifier in the system, calculate the maximum length of fibre to be used between the laser diode and the optical receiver based on the power budget consideration. (10 marks) (ii) Calculate the maximum bit rate to be used in this system by taking both the material and waveguide dispersions of the fibre into account. (10 marks) Question-2 (For students with the 4th digit of StudentID, z: 5,6,7,8,9) A laser diode emits an optical power of 30mW at 1550nm and has a spot size 5m and spectral width 1nm. It is joined to an optical fibre with axial offset of 2m and launches a rectangular pulse of absolute width 4ns into the fibre. The fibre has the following parameters: Cutoff wavelength = 1280nm Spot size = 5m at 1550nm Cladding refractive index = 1.457 Material dispersion = 17ps/nm-km at 1550nm Attenuation coefficient = 0.2dB/km at 1550nm The output end of the fibre is connected to an optical receiver of sensitivity of – 30dBm. (i) Without including any amplifier in the system, calculate the maximum length of fibre to be used between the laser diode and the optical receiver based on the power budget consideration. (10 marks) (ii) Calculate the maximum bit rate to be used in this system by taking both the material and waveguide dispersions of the fibre into account. (10 marks) PHTN4662 Midterm Examination Questions . \Go on to next page. 3 Question-3 (For students with the 5th digit of StudentID, z: 0,1,2,3,4) a) What are fundamental processes involving light and matter interaction? Briefly explain the principles, advantages and applications of fibre amplifiers. Give one example of fibre amplifier design. (10 marks) ( b) What are the fundamental principles and key design issues in using a pn junction for light emission? (10 marks) Question-3 (For students with the 5th digit of StudentID, z: 5,6,7,8,9) a) What are fundamental processes involving light and matter interaction? Briefly explain the principles, advantages and applications of lasers. Give one example of fibre laser design. (10 marks) b) What are the fundamental principles and key design issues in using a pn junction for light detection? (10 marks) Question-4 (For students with the 6th digit of StudentID, z: 0,1,2,3,4) a) A semiconductor photodetector is made up of a simple p-n junction. If the band gap of the semiconductor material is 1eV, what is the likely wavelength range that the detector could work? (10 marks) b) A laser diode operates at 1.55m. It has an overall quantum efficiency of 90% when the injection current is 200mA. What optical power the laser emits in this case? (10 marks) Question-4 (For students with the 6th digit of StudentID, z: 5,6,7,8,9) a) A semiconductor diode laser is made up of a simple p-n junction. If the band gap of the semiconductor material is 1.2eV, what is the likely wavelength range that the laser could work? (10 marks) b) A photodetector has a quantum efficiency of 80%. What is its responsivity at 1.55m? (10 marks) Q Q uestion- The laser of th cavit Assu Dete (10 m uestion- The laser curre of th Assu Dete (10 m 5 (For stu coated mir is 0.5 and e cavity is y to lase. (1 me an ope rmine how arks) 5 (For stu coated mir is 0.75 an nt density, e cavity. (1 me an ope rmine how arks) PHTN46 dents with ror reflectiv 0.5. The lo 210-3 cm 0 marks) rating wav many long dents with ror reflectiv d 0.75. The Jth, require 0 marks) rating wav many long r1 r1 62 Midte \Go o the 7th d ity at eith ss coefficie /A, work elength of itudinal mo the 7th d ity at eith loss coeff d for the c elength of itudinal mo J J rm Exam n to next p igit of Stu er end of th nt of the op out the th 1550nm, des will las igit of Stu er end of th icient of th avity to las 1300nm, des will las L L ination Q age. dentID, z e 500m tical cavity reshold cur a spontane e if the cur dentID, z e 750m e optical ca e is 5103 a spontane e if the cur r r uestions long optica is 20 dB / rent densit ous emissio rent is twic long optica vity is 10 A/cm2, wo ous emissio rent is twic 2 2 : 0, l cavity of cm. If the g y, Jth, requ n linewid e the thresh : 5, l cavity of dB/cm. If t rk out the g n linewid e the thresh . 1,2,3,4) an injectio ain factor ired for th th of 30nm old curren 6,7,8,9) an injectio he threshol ain factor th of 30nm old curren 4 n e , t. n d , t.