Practical assignment
Consult the prescribed textbook for the assignment.
1.1 An optical fibre communication system (NRZ coding) is operating at a data rate of 20 Mb/s and a bit-error-rate of 10-9 (at most one error can occur for every 109 bits sent). The receiver is a PIN-photodiode operating at a wavelength of 850 nm. The source is a GaAlAs LED that couples 50 μW optical power into a fibre (fibre loss is 3.5 db/km) with a 50 pm core diameter. Connector loss at the source and detector is 1 dB each and the safety/system margin is 6 dB. Determine the maximum transmission path length (link distance).
1.2 The LED together with its drive circuit has a rise time of 15 ns and a spectral width of 40 nm. The material-dispersion-related rise time is 3.5 ns/km over the link distance and the intermodal dispersion is 0 65 ns/km. The receiver bandwidth is 25 MHz, and the fibre has a 400 MHz.km bandwidth-length product. Calculate the total rise time of the link (system rise time).
1.3 If receiver is a transimpedance amplifier with an open loop gain of 200 and the average amplifier noise current is 2.7 pA/√Hz. The input resistance of the amplifier is 27 kΩ and the feedback resistance is 56 kΩ. The junction capacitance of the photodiode is 10 pF. Determine the photocurrent for the diode to maintain the required SNR as determined from 1.1.
1.3.1 Is the receiver and detector bandwidth larger than the required signal bandwidth?
1.3.2 Calculate the shot noise limited current for the detector (Assume dark current is small and T = 300 K).
1.3.3 The responsivity of the detector is 0.7 A/W. Calculate the received optical power for the detector.
2.1 Consider a 1550 nm laser diode with a rise time of 0.8 ns, spectral half width of 6 nm, that launches 2 mW optical power into a 60 km long single mode fiber with an attenuation of 0.3 dB/km. The intramodal dispersion is 3 ps/nm.km. An InGaAs APD with a -32 dBm sensitivity is used at 2.5 Gb/s (NRZ coding). A short optical jumper cable at each end of the cable with a loss of 3 dB is used to connect the fibre to the equipment rack. In addition, the connector loss at each joint between the jumper and fibre cable and jumper and equipment rack is 1 dB each. Determine the available safety margin for the system.
2.2 The same amplifier is used as in 1.3. The responsivity of the APD (x=1.0) with a junction capacitance of 10 pF, is 0.7 NW, the dark current is negligible, and the noise figure is 3 dB Assume the temperature is 300 K. Determine the optimum avalanche multiplication factor for the diode.
2.2.1 Calculate the shot noise limited current for the detector (Assume dark current is small and T = 300 K).
2.2.2 The responsivity of the detector is 0.7 AM. Calculate the received optical power for the detector.
2.3 Determine the temporal response of the system components and calculate the maximum transmission rate. Discuss if the system is bandwidth limited.