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Question 2 (Total Marks: 24 marks) Your company has been commissioned by a council to design a 40 Gb/s optical link connecting two suburban towns (Town A and B) located 100 km away from each other. The council has conducted a quick research and has provided an initial design as shown below. They have also identified the following optical components that will be used in the link: - Transmitters: DFB laser with external modulator with an optical power of 5dBm at the output of the modulator - Receivers: PIN photodetector with a receiver sensitivity of −15dBm at BER=10−9 - Wavelength: 1550 nm - Fibre: Single-mode fibre (SMF) with an attenuation of 0.2 dB/km and D=17ps/(km.nm) - Maximum pulse spread the link can tolerant at the receiver, Δτ=0.5∗ bit-period (the council decides to relax the normal dispersion criteria from 0.25∗ bit-period to 0.5∗ bitperiod) - Assume the only loss in the link is from the fibre attenuation a. Based on the information given by the council, show that the designed link is dispersion limited. (2 marks) b. To overcome the dispersion, the council has decided to use dispersion compensating fibre (DCF) with D=−100ps/(km.nm) and an attenuation characteristic of 0.5 dB/km. Calculate the length of DCF to exactly compensate for the SMF dispersion (i.e.100\% compensation). (2 marks) c. Will the designed link work with the DCF in place? If not, why not? Justify your answer. (3 marks) d. To improve the link performance, the council decides to invest in an erbium-doped fibre amplifier. What is the minimum EDFA gain (in dB ) required to meet the receiver sensitivity at BER=10−9 ? Here you can assume a system margin of 3 dB is put aside for ageing, maintenance, repair, etc. (5 marks)
In addition to providing gain to the link, EDFA also contributes to amplified spontaneous noise (ASE) which will degrade the optical signal-noise-ratio (OSNR). The chosen EDFA has the following parameters: - nsp​=1.5 - Optical bandwidth, Bopt​=2THz - PASE​=λnc​nsp​(G−1)Bopt​ - 0SNR=PASE​Pave ​​ i. Calculate the ASE power (PASE​) in dBm, based on the gain obtained in part d. (2 marks) ii. Show that the resulting OSNR is ∼8 dB. (3 marks) iii. The council has notified that the minimum acceptable OSNR is 15 dB. To improve the OSNR, a narrower optical filter can be used. Find the optical bandwidth required to meet the acceptable OSNR of 15 dB. (5 marks) iv. Briefly explain why a narrower optical filter is able to improve the performance at the receiver. How does the ASE noise impact the electrical signal-to-noise ratio? (2 marks)